climate justice: can we agree to disagree

CLIMATE JUSTICE: CAN WE AGREE TO DISAGREE? OPERATIONALISING COMPETING EQUITY PRINCIPLES TO MITIGATE GLOBAL WARMING

YANN ROBIOU DU PONT orcid.org/0000-0002-1275-9597

THESIS SUBMITTED IN TOTAL FULFILMENT OF THE REQUIREMENTS OF THE DEGREE OF DOCTOR OF PHILOSOPHY

OCTOBER 2017

AUSTRALIAN-GERMAN CLIMATE & ENERGY COLLEGE / DEPARTMENT OF EARTH SCIENCE FACULTY OF SCIENCE THE UNIVERSITY OF MELBOURNE

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ABSTRACT With the Paris Agreement, the international community has agreed to limit global warming to well below 2 °C and to pursue efforts to stay below 1.5 °C (UNFCCC 2015a) to avoid dangerous climate impacts. Staying within these boundaries requires important emissions mitigation efforts from all countries (Rogelj et al 2015). Equitable distribution across countries of mitigation efforts, or equivalently of emissions rights, consistent with global mitigation objectives is a contentious issue that involves divergent interpretations of distributive justice (Winkler and Rajamani 2014a). The latest Intergovernmental Panel on Climate Change (IPCC) report categorises equity approaches from the scientific literature in five groups (Clarke et al 2014). At climate negotiations, most countries tend to support the approach that requires the least efforts on their behalf (Fleurbaey et al 2014, Lange et al 2010). With the absence of consensus on an effort-sharing approach, current negotiations under the United Nations Framework Convention on Climate Change (UNFCCC) follow a self-interested, or ‘bottom-up’, approach to target setting (Andresen 2015, Bodansky 2016) where each country decides its own effort following its understanding of fairness. As a result, the sum of all parties’ announced contributions is not consistent with limiting global warming to 2 °C, let alone 1.5 °C (Rogelj et al 2016a). Under the Paris Agreement, countries committed to increase the ambition of their post-Kyoto climate pledges through a ratcheting-up process that begins in 2023. With the disagreement on effort-sharing approaches, the international community relies on diverging metrics to evaluate the adequacy of national pledges with the global warming thresholds. Since the beginning of climate negotiations under the United Nations, a rich literature has modelled allocations of emissions rights to countries using various effort-sharing approaches with uncoordinated parameterisation. At the start of this PhD work, no study modelled the effort-sharing categories presented in the last IPCC report under a common parameterisation. Additionally, the literature on the combination of effort-sharing approaches remained thin and consisted of averaging the emissions allocations of multiple effort-sharing approaches. This PhD thesis addresses these gaps with the modelling of a new emissions allocation framework, the ‘PRIMAPEquity’ framework, and with the suggestion of a new combination of effort-sharing approaches. Firstly, this thesis quantifies allocations of emissions rights to countries in a manner that reflects the existing literature on distributive justice. An emissions allocation framework is developed to derive national emissions allocations that reflect the five equity categories of the fifth IPCC report. This modelling framework is applied to derive emissions allocations, under each of the five equity categories, consistent with the emissions mitigation goals of the G7 Elmau agreement 1 signed in June 2015. The allocation framework is then used to derive national emissions trajectories aligned with the recent Paris Agreement goals of both well below 2 °C and 1.5 °C, consistently with the five equity categories 2 . This work represents the first quantification of equitable national trajectories to achieve 1.5 °C goal and informs scientists and government experts in the preparation of the IPCC Special Report on 1.5 °C (IPCC 2017). The Nationally Determined Contributions 1 2

published as Robiou du Pont et al., (2016) published as Robiou du Pont et al., (2017)

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(NDCs), countries’ national pledges, of 171 Parties are then evaluated in order to determine which, if any, categories of equity they are consistent with. As well, the thesis highlights the consistency of G20 countries’ pledges with equity allocations. This is discussed in the context of the statement on fairness contained in each pledge. This PhD thesis then addresses the apparent incompatibility between the global warming thresholds and countries’ self-interested visions of effort-sharing by suggesting a new quantitative approach. Doing so, this PhD thesis provides a new metric, inclusive of all international positions, to assess the ambition of the NDCs under the Paris Agreement. This new ‘hybrid’ allocation method reconciles the ‘bottom-up’ approach of equity with the ‘top-down’ climate threshold that they commonly agreed. Under this ‘hybrid’ approach, each country follows the least stringent effort-sharing approach – out of the five that reflect the equity categories presented in the last IPCC report – to achieve the Paris Agreement. The aggregation of current national pledges is found to align with such a ‘bottom-up’ combination of approaches and lead to a warming of up to 2.3 °C in 2100 (with a 50% chance). Conversely, an enhanced ‘bottom-up’ approach – ‘hybrid’ – of global emissions scenarios leading to 1.1 °C and 1.3 °C warmings results in the achievement of the Paris Agreement mitigation goals of 1.5 °C and well below 2 °C, respectively. Ultimately, this study quantifies a compromise where each country can choose an equity approach to determine its effort, but does directly use that approach to assess other countries’ pledges. Finally, the application of this ‘hybrid’ approach provides a temperature assessment for all countries’ climate pledges, indicating the consistency of countries’ ambition in light of the global temperature goals. The NDCs of India, the EU, the USA and China are in line with global ‘bottom-up’ situations leading to warmings of 2.6 °C, 3.2 °C, 4 °C and over 5.1 °C, respectively. The results of this thesis can inform public opinions and decision makers through the ratchetingup process on what constitutes fair and ambitious pledges to achieve the Paris Agreement following a range or combination of equity approaches. Additionally, the assessments of the adequacy of countries’ pledges with international agreements can inform courts when ruling ‘climate cases’ where governments are sued for their lack of ambition in mitigating emissions (Sabin Center for Climate Change Law 2018).

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DECLARATION This is to certify that: (i) (ii) (iii)

The thesis comprises only my original work towards the PhD except where indicated in the preface, Due acknowledgement has been made in the text to all other material used, and The thesis is fewer than 100,000 words for a PhD or doctoral thesis exclusive of tables, maps, bibliographies and appendices.

Yann Robiou du Pont, October 2017

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PREFACE Yann Robiou du Pont was the primary researcher for all chapters in this thesis, including the planning and execution of the research and included articles. Chapter 4 and Chapter 6 of this thesis consist of original publishes articles with Yann Robiou du Pont as the first author. The references of these articles are detailed below with the participation of the PhD candidate. Inclusion of these publications into the thesis has been approved by the advisory committee. The primary supervisor, Malte Meinshausen, has signed the University of Melbourne Declaration for a thesis with publication. All co-authors have signed the University of Melbourne co-author declarations form attesting that Yann Robiou du Pont contributed more than 50% of the work. In addition, Chapter 8 is intended for submission to Nature Climate Change following the submission of the thesis. Malte Meinshausen, Louise Jeffery, Peter Christoff and Johannes Gütschow provided feedback for the Chapter 8 analysis. Johannes Gütschow provided the temperature estimates of the global emissions scenarios derived in this chapter. Malte Meinshausen, Louise Jeffery and Johannes Gütschow updated and managed the composite PRIMAP database. The contribution of Yann Robiou du Pont to this Chapter 8 article is 75%. Yann’s candidature was supported by a Melbourne International Engagement Award.

Full title: Authors: Candidate’s contribution: Journal: DOI:

Full title: Authors: Candidate’s contribution: Journal: DOI:

National contributions for decarbonizing the world economy in line with the G7 agreement Yann Robiou du Pont, M. Louise Jeffery, Johannes Gütschow, Peter Christoff and Malte Meinshausen 65% Environmental Research Letters 10.1088/1748-9326/11/5/054005

Equitable mitigation to achieve the Paris Agreement goals Yann Robiou du Pont, M. Louise Jeffery, Johannes Gütschow, Joeri Rogelj, Peter Christoff and Malte Meinshausen 75% Nature Climate Change 10.1038/NCLIMATE3186

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ACKNOWLEDGEMENTS First, I would like to acknowledge the traditional owner of the land where I did most of my research, the Wurundjeri people of the Kulin Nation and pay my respect to their elders, past and present. It is an inspiring privilege to work on sustainability in the midst of one of the oldest living population. I am most grateful to all those who helped me in so many ways throughout my PhD. Deepest thanks to my main supervisor, Malte Meinshausen for his guidance by example and his genuine selflessness. Both his expertise and creativity greatly contributed to the design and outcomes of this project. I also wish to thank Louise Jeffery who closely supervised my PhD and provided me with the valuable opportunity to contribute to climate negotiations. I wish to thank all the coauthors of the articles included in this thesis for their time and contributions: Peter Christoff, Johannes Gütschow and Joeri Rogelj; as well as Claire Fyson, Anita Talberg, Martin Wainstein and Lena Boysen who have always been available to support my research. The research would not have been possible without the friendly students and staff of AustralianGerman Climate and Energy College and the Potsdam Institute for Climate Impact Research (PIK) who have always been available and helpful, and made it a joy to work on this PhD project. Heartfelt thanks to the Melbourne Sustainable Society Institute (MSSI) for supporting my work and its dissemination through the funding of the drafting a report and the construction of an interactive website. Importantly, I wish to thank the MSSI and the EU Centre for Shared Complex Challenges at the University of Melbourne (EU Centre) for providing me with support necessary to complete this PhD and for adding to the minimal living allowances that the Melbourne University provides its PhD students. Finally, deep thanks to all the scientists, lawyers, diplomats, activists and citizens who selflessly devote their time and efforts to protecting and increasing the wellbeing of those who are the most in need. The multifaceted motivations of such actors at climate change negotiations are both inspiring and have heavily influenced the shape and timely completing of this PhD project.

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ABBREVIATIONS

(I)NDC °C ADP ASIA BaU CAP CBDR-RC CDR CER CH4 CO2 CO2eq/CO2e COP CPC EIT EPC GDP GDPppp GDR GHG Gt GWP IAM IPCC IPCC-AR LAM LULUCF MAF Mt N/A N2O NGO OECD PhD ppm PRIMAP RCI RCP SSP UN UNFCCC

(Intended) Nationally Determined Contributions Degree Celsius Ad Hoc Working Group on the Durban Platform for Enhanced Action Region Asia Business as usual Capability Common but Differentiated Responsibilities and Respective Capabilities Carbon Dioxide Removal Constant Emissions Ratio Methane Carbon dioxide Carbon dioxide equivalents Conference of the Parties Equal Cumulative per Capita Region Economies in transition Equal per Capita Gross Domestic Product Gross Domestic Product Purchase Parity Power Greenhouse Development Rights Kyoto Global Greenhouse Gas Gigatonnes Global warming potential Integrated Assessment Model Intergovernmental Panel on Climate Change Intergovernmental Panel on Climate Change Assessment Report Region Latin America Land Use, Land-Use Change and Forestry Region Middle-East and Africa Megatonnes Not applicable Nitrous oxide Non-Governmental Organisation Organisation for Economic Co-operation and Development Philosophiae Doctor Parts per million Potsdam Real-time Integrated Model for the probabilistic Assessment of emission Paths Responsibility Capability Index Representative Concentration Pathway Shared Socioeconomic Pathways United Nations United Nations Framework Convention on Climate Change

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TABLE OF CONTENTS Abstract ................................................................................................................................................................. ii Declaration............................................................................................................................................................ iv Preface................................................................................................................................................................... v Acknowledgements ............................................................................................................................................... vi Abbreviations ....................................................................................................................................................... vii Table of contents..................................................................................................................................................viii List of figures......................................................................................................................................................... xi List of tables ........................................................................................................................................................ xvii List of third party copyright material ..................................................................................................................... xix Chapter 1 – Introduction ........................................................................................................................................ 1 1.1 Context ......................................................................................................................................................... 1 1.2 Literature gap and statement of the problems tackled ..................................................................................... 2 1.3 Aim and research questions ........................................................................................................................... 3 1.4 Relevance ..................................................................................................................................................... 3 1.5 Framework and limitations............................................................................................................................ 4 1.5.1 Disciplinary approach............................................................................................................................. 4 1.5.2 Definition of key terms ........................................................................................................................... 4 1.5.3 Framework scope ................................................................................................................................... 5 1.6 Thesis outline ............................................................................................................................................... 7 Chapter 2 – Background ......................................................................................................................................... 8 2.1 Global temperature and mitigation goals........................................................................................................ 8 2.1.1 Lead-up to the Paris global climate goals ................................................................................................ 8 2.1.2 The impacts of a 2 °C or 1.5 °C warmer world ........................................................................................ 9 2.1.3 Global emissions scenarios to reach the climate goals ........................................................................... 11 2.2 National contributions to reach global goals ................................................................................................ 14 2.2.1 Ratcheting-up global ambition .............................................................................................................. 14 2.2.2 Equity under the UNFCCC ................................................................................................................... 14 2.2.3 National positions on fairness ............................................................................................................... 15 Chapter 3 – Literature review on effort-sharing approaches.................................................................................. 18 3.1 Categorisation of concepts of distributive justice ......................................................................................... 18 3.2 Agreeing on climate equity ......................................................................................................................... 26 3.3 Research gaps in the current literature ......................................................................................................... 28 Chapter 4 – National contributions for decarbonizing the world economy in line with the G7 agreement.............. 30 4.1 Main study.................................................................................................................................................. 31

ix | P a g e 4.1.1 Introduction ......................................................................................................................................... 31 4.1.2 Methods ............................................................................................................................................... 33 4.1.3 Results ................................................................................................................................................. 34 4.1.4 Discussion............................................................................................................................................ 37 4.1.5 Conclusions ......................................................................................................................................... 38 4.1.7 References ........................................................................................................................................... 38 4.2 Supplementary information ......................................................................................................................... 40 4.2.1 Supplementary methods ....................................................................................................................... 42 4.2.2 Supplementary discussion..................................................................................................................... 56 4.2.3 Supplementary tables ........................................................................................................................... 57 4.2.4 References ........................................................................................................................................... 67 Chapter 5 – Additional methods ........................................................................................................................... 69 5.1 Creating a new modelling framework .......................................................................................................... 69 5.2 Influence of parameterisation on the allocation framework .......................................................................... 72 5.2.1 Parameterisation range ......................................................................................................................... 72 5.2.2 Results ................................................................................................................................................. 75 5.3 Conclusion.................................................................................................................................................. 78 Chapter 6 – Equitable mitigations to realize the Paris Agreement goals................................................................. 79 6.1 Main study.................................................................................................................................................. 80 6.1.1 Main text.............................................................................................................................................. 80 6.1.2 References ........................................................................................................................................... 85 6.1.3 Methods ............................................................................................................................................... 86 6.1.4 Methods’ references ............................................................................................................................. 87 6.1.5 Corrigendum ........................................................................................................................................ 88 6.2 Supplementary information ......................................................................................................................... 89 6.2.1 Methods’ supplementary material ......................................................................................................... 91 6.2.2 Supplementary discussion..................................................................................................................... 93 6.2.3 Supplementary tables ......................................................................................................................... 110 6.2.4 Bibliography ...................................................................................................................................... 111 Chapter 7 – Equitable G20 contributions towards achieving the Paris Global Goals ............................................. 113 7.1 Results for G20 countries .......................................................................................................................... 113 7.2 Consistency of equity performance with statements on fairness.................................................................. 118 7.3 Conclusion................................................................................................................................................ 121 Chapter 8 – Temperature assessment of national pledges under a bottom-up approach of equity ...................... 122 8.1 Study ........................................................................................................................................................ 122 8.2 Additional methods ................................................................................................................................... 132

x|P a g e Chapter 9 – Discussion and Conclusion ............................................................................................................... 147 9.1. Summary ................................................................................................................................................. 147 9.2 Limitations ............................................................................................................................................... 149 9.2.1 Numerical uncertainties ...................................................................................................................... 149 9.2.2 Limitations regarding countries considerations for relative and absolute gain....................................... 150 9.2.3 Limitations of combining equity concepts ........................................................................................... 150 9.3 Outlook for climate litigation .................................................................................................................... 152 9.4 Future research ......................................................................................................................................... 153 9.5 Conclusions .............................................................................................................................................. 155 Thesis output summary ...................................................................................................................................... 157 Publications as first author .............................................................................................................................. 157 Publications as co-author ................................................................................................................................ 157 Other outputs as first contributor ..................................................................................................................... 157 References ......................................................................................................................................................... 158

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LIST OF FIGURES The numbering of figures comprises two numbers separated by a period. The first number refers to the chapter, and the second indicates the order of the figure within the chapter. Figures contained in published chapters will are numbered following the journal’s system, preceded by the Chapter numbers. Throughout the thesis, they will be referred to by their chapter and figure name in the journal (either as “Figure X in Chapter Y” or as “Figure Y.X”). The names of the figures in the original articles and their supplementary material are left unchanged (i.e. the caption for Figure Y.X in Chapter Y starts simply with “Figure X”). Figures: Figure 2.1 | Key dates of the IPCC and UNFCCC agendas. ..................................................................................... 9 Figure 2.2 (reproduced from IPCC-AR5 WGII-Fig. 19-4) | The dependence of risk associated with the Reasons for Concern (RFCs) on the level of climate change. .................................................................................................... 10 Figure 3.1 (reproduced from IPCC-AR5 WGIII-Fig. 6.28) | Emission allowances in 2030 relative to 2010 emissions by effort-sharing category for mitigation scenarios. MAF is Middle-East and Africa, EIT is economies in transition, LAM stands for Latin America. ............................................................................................................................ 25 Figure 4.1 | IAM scenarios from the IPCCAR5 that meet the G7 agreement for global emissions mitigation. The ranges of IAM emissions scenarios, including LULUCF emissions, that match the G7 vision for GHG (in blue) and forCO2 (in green) are shown with RCP2.6 GHG (red line) andCO2 (yellow line) emissions scenarios. The Elmau agreement is interpreted as a GHG emissions reduction of 60%–70% below 2010 levels by 2050 (blue interval) and net zeroCO2 emissions (green dotted line) by the end of the century. The aggregate INDCs level (UNFCCC 2015c) is shown with the Paris decision 2030 goal and the UNEP recommendations for 2025, 2030 and 2050 (gray and black circles). The inset shows the seven selected scenarios (in blue), RCP2.6 (in red) selected out of the 846 IPCCAR5 database GHG scenarios (in gray). ....................................................................................................... 33 Figure 4.2 | Emissions allocations and INDCs of G7 and BRIC countries consistent with the G7 agreement according to the 5 effort sharing approaches: capability (dark blue), equal per capita (turquoise), Greenhouse Development Rights (green), equal cumulative per capita (yellow) and constant emissions ratio (orange). Emissions allocations consistent with RCP2.6 are shown for each approach (colored lines). Allocations, INDCs (black circles) and other pledges (gray circles) of each country are shown applied to ‘Kyoto-Annex A’ emissions. Side panels: 2030 target ranges consistent with each effort-sharing approach with INDCs (gray lines), the color shading is darker below each allocation consistent with aG7 scenario. The USA 2030 target is linearly interpolated between their 2025 and 2050 commitments. ........................................................................................................................................ 35 Figure 4.3 | Emissions allocations in 2030 consistent with the G7 agreement expressed as percentage of 2010 emissions in five world regions according to five equity approaches: capability (dark blue), equal per capita (turquoise), Greenhouse Development Rights (green), equal cumulative per capita (yellow) and constant emissions ratio (orange). The shading of the color patch is darker below each allocation consistent with aG7 scenario. The wider line shows results when considering RCP2.6. Allocations are applied to ‘Kyoto-Annex A’ emissions. ......... 37 Figure 4.4 | Selected GHG scenarios from the IPCCAR5 database. We selected scenarios that have net negative CO2 emissions by 2100 green), and 60% to 70% reduction compared below their 2010 levels with pre-2020 mitigation actions (blue). We removed scenarios with LULUCF sink greater than 15 GtCO2/y (red). We obtained seven scenarios (black) to which we added RCP2.6 (thick black). ................................................................................... 42 Figure 4.5 | Stacked national GHG emissions allocations of the RCP2.6 emissions according to five effort sharing approaches. ‘Capability’, ‘Equal cumulative per capita’, ‘Greenhouse Development Rights’, ‘Equal per capita’ and ‘Constant emissions ratio’. The allocation starts in 2011. The red line represents RCP2.6 emissions levels. LULUCF and bunker emissions are excluded. ...................................................................................................................... 46

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Figure 4.6 | Emissions allocations for the G7 group – including EU countries not part of the G7 – coherent with the Elmau agreement according to the five effort sharing approaches compared with 2009 L’Aquila target. National emissions scenarios (white lines in coloured patches) coherent with selected global scenarios are shown for five approaches: capability (dark blue), equal per capita (turquoise), Greenhouse Development Rights (green), equal cumulative per capita (yellow) and constant emissions ratio (orange). The target of 80% reduction is shown compared to the maximum and minimum emissions of the 1990-2010 period (black circles). Results are shown in percentage of their respective 2010 levels. LULUCF emissions are excluded......................................................... 58 Figure 4.7 | Emissions allocations in 2050 consistent with the G7 agreement expressed as percentage of 2010 emissions in five world regions according to five equity approaches: Capability, Equal Per Capita, Greenhouse Development Rights, Equal Cumulative Per Capita, Constant Emissions Ratio. The shading of the colour patch is darker below the allocation of each G7 scenario. The wider line shows results when considering RCP2.6. ............. 62 Figure 4.8 | Emissions allocations in 2030 consistent with the G7 agreement expressed as percentage of 2010 emissions in 10 world sub-regions according to five equity approaches: Capability, Equal Per Capita, Greenhouse Development Rights, Equal Cumulative Per Capita, Grandfathering. The shading of the colour patch is darker below the allocation of each G7 scenario. The wider line shows results when considering RCP2.6. .................................. 63 Figure 4.9 | Emissions allocations in 2050 consistent with the G7 agreement expressed as percentage of 2010 emissions in ten world sub-regions according to five equity approaches: Capability, Equal Per Capita, Greenhouse Development Rights, Equal Cumulative Per Capita, Grandfathering. The shading of the colour patch is darker below the allocation of each G7 scenario. The wider line shows results when considering RCP2.6. .................................. 64 Figure 4.10 | Emissions allocations in 2030 consistent with the G7 agreement as a percentage of 2010 levels for G20 members according to five equity approaches: Capability, Equal Per Capita, Greenhouse Development Rights, Equal Cumulative Per Capita, Grandfathering. The shading of the colour patch is darker below the allocation of each G7 scenario. The wider line shows results when considering RCP2.6. The grey lines show INDC mitigation targets applied to ‘Kyoto Annex A’ emissions. ................................................................................................................. 65 Figure 4.11 | Emissions allocations in 2050 consistent with the G7 agreement as a percentage of 2010 levels for G20 members according to five equity approaches: Capability, Equal Per Capita, Greenhouse Development Rights, Equal Cumulative Per Capita, Grandfathering. The shading of the colour patch is darker below the allocation of each G7 scenario. The wider line shows results when considering RCP2.6. The grey line shows Mexico’s pledge mitigation targets applied to ‘Kyoto Annex A’ emissions. ...................................................................................................... 66 Figure 5.1 | 2014 values of key indicators present in the modelling of the five equity approaches. a, Population (United Nations 2015). b, Emissions (Gütschow et al 2016). c, per capita GDP (World Bank 2015). d, per capita emissions. e, cumulative emissions, with a 1.5% discount rate, over cumulative population over the 1950-2014 period. f. same as e but over the 1990-2014 period. To preserve resolution between countries, the colour-scales of panels c, d, e and f are capped at five times the global average (orange line). Countries with missing data appear in grey. Emissions use the Global Warming Potential (GWP) of the IPCC-AR4, excluding LULUCF emissions. ....... 74 Figure 5.2 | GHG emission scenarios for China, the EU, India, Russia and the USA reflecting five effort sharing approaches: ‘Capability’, ‘Equal per capita’, ‘Greenhouse Development Rights’, ‘Equal cumulative per capita’ and ‘Constant emission ratios’. The allocation starts in 2010 (as for RCP2.6 emission scenario). The variability, calculated across all parameters from Table 5.1 with allocations starting in 2010, is represented with colour patches. The top panel uses a percentage scale indexed on 2010 levels, the bottom panel shows absolute emissions. ........... 77 Figure 5.3 | Emission allowances in 2030 expressed as a percentage of 2010 for ten world sub-regions following five equity approaches calculated across a range of parameterisations Table 5.1. The colour shading represents the quantiles of parameterisation runs. The default parameterisation is shown by a grey line and the median by a white line....................................................................................................................................................................... 78 Figure 6.1 | Global, national and regional emissions consistent with the Paris Agreement and five equity principles compared with current pledges. a, IAM scenarios consistent with the Paris Agreement under ‘1.5 °C-pre2020peak’ (red), ‘2 °C-pre2020peak’ (blue) and ‘2 °C-2030peak’ cases (purple), and their averages (thicker lines). Scenarios consistent with the 2030 Paris decision target (green circles) are more opaque. Inset, comparison with IPCC-AR5

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database scenarios (grey lines). b–f, National emissions allocations excluding LULUCF compared with (I)NDCs (black circles). Coloured patches and lines show allocation ranges of global ‘2 °C-pre2020peak’ scenarios, and averages over the range of global ‘1.5 °C-pre2020peak’ scenarios, respectively. g–k, Regionally aggregated 2030 allocations for ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’ scenarios compared with aggregated (I)NDCs......... 81 Figure 6.2 | Comparisons of national emissions change under different global goals. a–d, Relative changes between ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’, ‘2 °C-statedINDC’ and ‘2 °C-fairINDC’ cases over the 2010–2030 period (excluding LULUCF). e,f, Comparison of timing of first net-zero emissions and peaking national emissions averaged over the five equity approaches for the ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’ cases. g, Average of peaking emissions levels versus average peaking emissions years for ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’ cases. Disc sizes are proportional to 2010 emissions levels. Colours indicate world regions. G8CChina (larger disc) and the rest of the world (smaller disc) are shown in grey. ..................................................................................... 83 Figure 6.3 Gaps between equitable mitigation allocations and conditional (I)NDCs in 2030. a,b, Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2 °C (a) or 1.5 °C goals (b), reduce or increase the projected 2030 global missions levels (excluding LULUCF and bunker emissions) compared with aggregated conditional (I)NDCs. Countries are sorted left to right in decreasing order of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated conditional (I)NDCs and the average scenarios consistent with the Paris 2 °C or 1.5 °C goals (grey bar) are shown in each panel. ...................... 84 Figure 6.4 | Selected ‘bunker-free’ emission scenarios excluding LULUCF emissions. These scenarios are the same scenarios as presented with their LULUCF in Figure 1 in the main article, but exclude LULUCF and bunker emissions. Scenarios are shown for the 1.5 °C-pre2020peak category (red), 2 °C-pre2020peak (blue), 2 °C-2030peak (purple), and 2 °C-statedINDC/2 °C-fairINDC (average INDC assessment, black line). The average over all scenarios of each case are shown by a solid line. ................................................................................................... 92 Figure 6.5 | Comparisons of national emissions change by 2050 under different cases: ‘1.5 °C-pre2020peak’, ‘2 °Cpre2020peak’ and ‘2 °C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. .......................................................................................................................... 96 Figure 6.6 | Comparisons of national emissions change over the maximal global mitigation periods under different cases: ‘1.5 °C-pre2020peak’ over the 2010-2030 and ‘2 °C-statedINDC’ over 2030-2050 that are the periods of maximal mitigation of each of these cases. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. ........................................................................................................... 97 Figure 6.7 | Comparisons of national emissions change by 2030 for each equity approach under different cases: ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’ and ‘2 °C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. ............................................................................ 98 Figure 6.8 | Emissions-peaking timing and magnitude, and net-zero emissions timing compared between ‘1.5 °Cpre2020peak’ and ‘2 °C-pre2020peak’ under each equity approach. Average of peaking emissions levels versus average year of peaking emissions for ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’. Disks’ sizes are proportional to 2010 emissions levels. Peaking emissions are shown under a logarithmic scale in % of 2010 levels. Colours indicate countries’ world region. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey................................................................................................ 100 Figure 6.9 | Maximal annual mitigation rates over the century are compared between ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’ and between ‘1.5 °C-pre2020peak’ and ‘2 °C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. .............................................. 101 Figure 6.10 | Comparisons of national allocations minima under different cases. Minimal emissions allocated over the 2010-2100, 2010-2060 and 2010-2080 period for ‘1.5 °C-pre2020peak’ and ‘2 °C-pre2020peak’. Emissions minima are given in percent change to 2010 levels, which are represented by disks’ sizes. Colours indicate

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countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. .......................................................................................................................................................................... 102 Figure 6.11 | Effect of countries following equitable mitigation allocations on global 2030 emissions compared to averaged INDC assessment. Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2 °C (left panel) or 1.5 °C goals (right panel), reduce or increase the projected 2030 emissions levels (excluding LULUCF and bunker emissions) compared to aggregated INDCs (average of ‘highambition’ and ‘low-ambition’). Countries are ordered left to right by decreasing levels of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated INDCs and the average scenarios consistent with the Paris 2 °C or 1.5 °C goals (grey bar) are shown in each panel. ................................ 103 Figure 6.12 | Effect of countries following equitable mitigation allocations on global 2030 emissions compared with unconditional INDC assessment. Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2 °C (left panel) or 1.5 °C goals (right panel), reduce or increase the projected 2030 emissions levels (excluding LULUCF and bunker emissions) compared to aggregated ‘low-ambition’ INDCs. Countries are ordered left to right by decreasing levels of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated ‘low-ambition’ INDCs and the average scenarios consistent with the Paris 2 °C or 1.5 °C goals (grey bar) are shown in each panel. ............................................................................. 105 Figure 6.13 | Regional 2030 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Regional 2030 allocations under the ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’ or ‘2 °C-2030peak’ sets are shown, for the five equity approaches. .................................................................................................... 106 Figure 6.14 | Sub-regional 2030 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Sub-regional 2030 allocations under the ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’ or ‘2 °C2030peak’ sets are shown, for the five equity approaches, with the regionally aggregated INDCs (dashed lines). . 107 Figure 6.15 | Regional 2050 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Regional 2050 allocations under the ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’ or ‘2 °C-2030peak’ sets are shown, for the five equity approaches. .................................................................................................... 108 Figure 6.16 | Sub-regional 2050 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Sub-regional 2050 allocations under the ‘1.5 °C-pre2020peak’, ‘2 °C-pre2020peak’ or ‘2 °C2030peak’ sets are shown, for the five equity approaches. ................................................................................... 109 Figure 7.1 | Comparison of the G20 equitable emissions pathways under the 2 °C scenarios with the aggregation of their (I)NDCs. Emissions allocations, excluding land-use and bunker emissions, under five equity allocations representative of the five IPCC categories are compared the G20 sum of (I)NDCs average (black circles) and range (vertical black line). Coloured patches and lines show allocation ranges and averages, respectively, over global 2 °C scenarios. ........................................................................................................................................................... 114 Figure 7.2 | Comparisons of G20 countries’ emissions allocations under the 1.5 °C or 2 °C goals and with their (I)NDCs. a. Comparison of 1.5 °C and 2 °C allocations averaged over the five equity approaches in 2030 (as a percent change to 2010 levels). b, Comparison of 2 °C allocations averaged over the five equity approaches with (I)NDCs in 2030 (as a percent change to 2010 levels). Disk sizes are proportional to 2010 emissions levels. Colours indicate world regions. Aggregated results for G20 countries (larger disk) and the rest of the world (‘Non-G20’, smaller disk) are shown in grey. .......................................................................................................................... 115 Figure 7.3 | Gaps between equitable emissions allocations and conditional (I)NDCs in 2030. Countries following individual approaches (tip of coloured patches), or the average of the five approaches (white lines) under the 2 °C (left) or 1.5 °C goals (right), reduce or increase the projected 2030 global emissions levels (excluding LULUCF and bunker emissions) compared to aggregated conditional (I)NDCs. The global gaps (grey arrow) between current aggregated conditional (I)NDCs and the average of global scenarios consistent with the Paris 2 °C or 1.5 °C goals (grey bar) are shown in each panel. ..................................................................................................................... 116 Figure 7.4 | Comparison of equitable 2030 emissions allocations of G20 members with their respective 2030 (I)NDCs assessment under the 2 °C and 1.5 °C goals. Emissions allocations in 2030, excluding land-use and bunker

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emissions, under five equity allocations representative of the five IPCC categories are compared with countries’ (I)NDC averages (dashed black line). Emissions ranges of any colour represent countries’ allocation range under the 1.5 °C (left) and 2 °C (right) goals. ..................................................................................................................... 117 Figure 8.1 | ‘Bottom-up’ allocations of global emissions scenarios. a, Illustration of the bottom-up allocation and hybrid allocation. b, Least-stringent of five approaches, lowest cumulative emissions by 2100, for each country under the ‘bottom-up’ allocation of ‘2 °C-scenario’. Small island developing states are represented by their maritime zones. c, Scenarios towards 2 °C and 1.5 °C (excluding land-use and bunker emissions) shown with their corresponding ‘bottom-up’ allocation and aspirational scenarios and the (I)NDC assessment (Meinshausen and Alexander 2015) range (grey). ............................................................................................................................ 125 Figure 8.2 | Comparison of emissions changes by 2030 under the ‘bottom-up’, ‘hybrid’ and average allocations of global scenarios and with (I)NDCs. a, Comparison of the ‘bottom-up’ allocation of the 2°C-scenario and countries average (I)NDCs. b, Comparison of the ‘hybrid’ allocation of 2°C-scenario and the average of the five equity allocations. c, Comparison of the ‘hybrid’ allocation of 2°C-scenario and countries’ average (I)NDCs. d, Comparison of the ‘hybrid’ allocation of 2°C-scenario and 1.5°C-scenario. Disks’ sizes are proportional to 2010 emissions level. Colours indicate countries’ world regions, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. ........................................................................................................................ 128 Figure 8.3 | Global warming responses under the ‘CBDR-RC hybrid’ approach following NDC ambitions. a, For the top-fifteen emitters, 2030-emissions as a function of 2100 median warming above pre-industrial levels under the hybrid approach (coloured lines). Disks indicate the ‘average’ NDC assessment and their sizes are proportional to 2010 emissions. Vertical ranges (coloured rectangles) indicate NDC assessment ranges (Meinshausen and Alexander 2015). The horizontal uncertainty ranges (coloured rectangsles) over the warming of the IAM scenarios (coloured dots) that lie within the vertical NDC assessment range. Colours indicate countries’ world regions (see map inset). b, Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘average’ NDC ambitions for 169 countries as calculated in panel a. (maps with ‘high’ and ‘low’ NDC quantifications in Supplementary Figures 9 and 10). The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states are represented by their maritime zones. .................................................................................. 130 Figure 8.4 | Selected global scenarios’ 2030 emissions levels, excluding LULUCF, as a function of 2100 global warming. The 9 scenarios are selected (red filling) amongst the 85 scenarios from the SSP-database (blue circles) and the 36 scenarios from ref. (Rogelj et al 2015) (black circles) to align with the third-degree polynomial fit (red line). The 412 IPCC-AR5 scenarios with available data (grey circles) are shown with their third-degree polynomial fit (grey line). The ‘1.5 °C-scenario’, the ‘2 °C-scenario’ (RCP2.6, blue disk) and the business-as-usual scenario (RCP8.5, grey disk) are shown for comparison.................................................................................................... 133 Figure 8.5 | Selected global emissions scenarios. The scenario sub-selection (thick lines) from the 85 scenarios amongst the SSP-database (dashed blue lines) and the 36 scenarios from (Rogelj et al 2015) (dashed black lines) are shown with their 2100 warming assessments. The 412 IPCC-AR5 scenarios with available data are shown for comparison (grey dashed line). ........................................................................................................................... 134 Figure 8.6 | Scenarios towards 2 °C and 1.5 °C (thick solid blue and red lines) shown with their corresponding ‘bottom-up’ allocation (thin lines) and aspirational scenarios convergence runs (thin dashed lines). Unconditional, conditional and average (I)NDC assessment are shown in grey. LULUCF and bunker emissions are excluded. Converging aspirational scenarios (thin dotted lines) converge over 15 runs towards 2 °C and 1.5 °C (thin solid lines). ................................................................................................................................................................. 137 Figure 8.7 | Distribution of the aspirational pathway towards 2 °C according to the five equity approaches and under a ‘bottom-up’ approach. The ‘bottom-up’ distribution (based on largest cumulative emissions by 2100) of the aspirational 2 °C-scenario (black line) results in emissions matching that 2 °C-scenario (RCP2.6 excluding LULUCF emissions, red line). Each colour patch represents a country. ............................................................... 138 Figure 8.8 | Least-stringent of three approaches (CAP, EPC and CPC), by lowest 2030 emissions, under a ‘bottomup’ allocation (panel a), and hybrid allocation (panel b) of the ‘2°C-scenario’...................................................... 139

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Figure 8.9 | Comparison of emissions changes by 2030 under the ‘Complete Hybrid’; ‘CBDR-RC Hybrid’ leaststringent 2030 emissions with the allocations average and with (I)NDCs. a, Comparison of the ‘Complete Hybrid’ (with the five burden-sharing approaches) and the ‘CBDR-RC Hybrid’ (with the three equity approaches: CAP, EPC and CPC) emissions. b, Comparison of the ‘CBDR-RC Hybrid’ allocation of 2°C-scenario and the average of the three equity allocations CAP, EPC and CPC. c, Comparison of the ‘CBDR-RC Hybrid’ allocation of 2°C-scenario and countries’ average INDCs. d, Comparison of the ‘CBDR-RC Hybrid’ allocations of a 2°C-scenario and a 1.5°Cscenario. Disks’ sizes are proportional to 2010 emissions level. Colours indicate countries’ world regions, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey. .............................................. 140 Figure 8.10 | Global warming responses under a ‘CBDR-RC hybrid’ approach, following ‘high’ quantifications of NDC (Meinshausen and Alexander 2015). Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘high’ NDC quantifications for 169 countries, as calculated in Figure 8.3a using the quadratic curve fit. The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states by their maritime zones. ............................................................................................................................ 142 Figure 8.11 | Global warming responses under a ‘CBDR-RC hybrid’ approach, following ‘low’ quantifications of NDC accounting for conditional pledges (Meinshausen and Alexander 2015). Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘low’ NDC quantifications for 169 countries, as calculated in Figure 8.3a using the quadratic curve fit. The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states by their maritime zones. ................................................................ 142 Figure 8.12 | Global warming responses under a ‘complete’ hybrid approach following NDC ambitions without interpolation. Global warming responses (median assessment) following NDC ambitions. ................................... 143 Figure 8.13 | Global warming responses under a hybrid approach, including the GDR but excluding CER, following NDC ambitions. Global warming responses (median assessment) following NDC ambitions. .............................. 144 Figure 8.14 | Scenario selection for the determination of the GHG composition, including bunker emissions, of the 1.5 °C ‘aspirational’ scenarios (panels a.) and 2 °C ‘aspirational’ (panel b.). ........................................................ 146

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LIST OF TABLES The numbering of tables comprises two numbers separated by a period. The first number refers to the chapter, and the second indicates the order of the table within the chapter. Tables contained in published chapters will are numbered following the journal’s system, preceded by the Chapter numbers. Throughout the thesis, they will be referred to by their chapter and table name in the journal (either as “Table X in Chapter Y” or as “Table Y.X”). The names of the tables in the original articles and their supplementary material are left unchanged (i.e. the caption for Table Y.X in Chapter Y starts simply with “Table X”). Tables: Table 2.1 | Comparison of the characteristics of scenarios limiting global warming to either 2 °C or 1.5 °C, adapted from Rogelj et al. (2015)....................................................................................................................................... 13 Table 2.2 | Equity principles proposed by major emitters and their share of global GHG emissions (Nabel et al 2011). Sources are: (AWG-LCA 15) a UNFCCC workshop on equitable access to sustainable development (UNFCCC 2012c), submissions to the ADP in 2014 (UNFCCC 2014), and NDCs. Note: Sudan, the Democratic Republic of Congo and the Republic of Mali are members of both the LMDC and the LDC groups. Their emissions shares are accounted under the LDC group in this table. ........................................................................................ 16 Table 3.1 | Examples and categorisation of approaches of distributive justice applied to allocations of emissions rights. Prioritarian approaches using a utilitarian metric, as well as sufficientarian approaches fall under a ‘complete’ vision of climate justice (Knopf et al 2012). Other approaches follow an ‘isolated’ vision. .................................... 19 Table 4.1 | Emissions reductions consistent with the G7 agreement according the five effort sharing approaches. The INDCs and pledges, the average and complete range (in square brackets) of mitigation targets over the eight selected scenarios are given in percent of 2010 levels. ........................................................................................................ 36 Table 4.2 | Scenarios’ characteristics as from the IPCCAR5 database of the seven selected emissions scenarios. The columns ‘Max T(°C)’, which shows the maximum expected temperature before 2100, and ‘66% chance) to rise to 2.0 – 4.9 °C compared to pre-industrial levels (used as a reference for global warming figures mentioned hereafter), with a median of 3.2 °C by 2100 (Raftery et al 2017). While the magnitude and distribution of climate impacts from such warming is uncertain due to complex non-linear processes, the risks are deemed to be high to very high in relation to extreme weather events, large-scale singular events and global aggregate impacts (Figure 2.2). Informed by a growing scientific literature on the global warming processes, potential climate change impacts and the available measures to mitigate GHG emissions, the international community created the United Nations Framework Convention on Climate Change (UNFCCC) (UNFCCC 1992). Under the UNFCCC ratified by 197 Parties, the international community agreed to negotiate measures to mitigate global GHG emissions and adopted successive commitments of increasing ambition over the past 25 years (UNFCCC 1992, 1998, 2010, 2015a). With the Paris Agreement (UNFCCC 2015a), 195 states agreed to limit global warming to well below 2 °C and pursue efforts to limit the increase to 1.5 °C. In the lead up to the Paris Agreement, 189 Parties published Intended Nationally Determined Contributions (INDCs) to limit GHG emissions and 166 Parties ratified the Agreement 3 (UNFCCC 2017b). This unprecedented world-wide involvement represented a major progress towards achieving the goals of the Convention (Schellnhuber et al 2016). However, with the uncoordinated (or bottomup) nature of the Paris Agreement, each country decides what constitutes an ambitious pledge for itself and the aggregation of national pledges is insufficient to meet the global Paris mitigation goals (Rogelj et al 2016a, Robiou du Pont et al 2017). Global warming is forecasted to reach 2.9 °C (median likelihood) in 2100 under the current NDCs (median assessment) and 2.7 °C with additional conditional commitments (Rogelj et al 2016a). A ratcheting-up mechanism requires each ratifying member of the Paris Agreement to communicate successive NDCs that represent a progression and “reflect their highest possible ambition, reflecting their common but differentiated responsibilities and respective capabilities (CBDR-RC), in the light of different national circumstances” (UNFCCC 2015a). Generally, multilateral agreements are most likely to be reached and achieved when they are based on a common understanding of fairness (Ringius et al 2002, Keohane and Oppenheimer 2016). On the basis of reciprocity, the recognition of a country’s increase in effort by other countries is expected to lead to an increase of these countries’ efforts in return. For example, many countries have additional commitments conditional on greater efforts from other countries. An estimation of

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what countries’ efforts should be under the Convention should account for countries’ responsibilities and capabilities (Winkler and Rajamani 2014a). An equitable allocation to countries of mitigation efforts, or of the corresponding emissions rights, necessary to achieve the global goals would provide a scale to assess the fairness of countries’ pledges. Distributive fairness (often referred to as distributive justice in the literature) is generally relevant to the distribution of a scarce good to a community of parties (Tørstad and Sælen 2017). Academics and government experts have suggested multiple approaches to quantify equitable distribution of emissions rights, reflecting either specific equity aspects of the CBDR-RC, or of all of them combined (Chapter 3). However, current agreements do not define whether equity in the Convention should be understood as one or several principles (Winkler and Rajamani 2014a) and there is no commonly agreed effort-sharing formula to distribute emissions rights across countries.

In order to receive support from other Parties and strengthen their bargaining power, countries should invoke universal (or applicable to all) principles of fairness based on commonly accepted values (Risse 2000). At negotiations, or through national communications, countries’ experts tend to support the universal notions of distributive justice (Chapter 2) that are the least stringent for their country (Averchenkova et al 2014, Fleurbaey et al 2014, Lange et al 2010, Tørstad and Sælen 2017). The theory that negotiators tend to invoke fairness principles that align with their own interest was empirically observed at COP15, COP16 and COP17 (Tørstad and Sælen 2017). Consistently, support from countries’ negotiators to a principle of historical responsibility, and to a lesser extent to a principle of capability (or ability-to-pay), were inverse-correlated with their countries historical emissions and Gross Domestic Product (GDP) per capita, respectively (Tørstad and Sælen 2017). A rich literature suggested and quantified a wide range of approaches to share the effort to limit warming to 2 °C reflecting countries’ divergent views on distributive justice (Chapter 3). The latest report of the Intergovernmental Panel on Climate Change (IPCC) presented ranges of equitable 2030 emissions levels for the five world regions identified in over 40 studies grouped in five effortsharing categories (Clarke et al 2014). While the absence of a utopian agreement on a single effortsharing approach leaves decision makers and public opinions without a single metric, there is a plethora of viewpoints on how to assess the consistency of national contributions with the agreed mitigation goals and equity principle(s).

1.2 Literature gap and statement of the problems tackled Recent studies accounted for countries’ divergent preferences for equity and suggested allocations that numerically combine multiple effort sharing approaches (Chapter 3). However, the literature on the combination of effort-sharing approaches remains thin and mostly consists of averaging the emissions allocations of multiple approaches. This gap limits the relevance of ambition assessments of national pledges to the international community. Diverging assessments of national efforts are likely to limit the efficiency of the ratcheting-up process.

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The collective inadequacy of the aggregation of current national pledges with the globally agreed temperature thresholds has been pointed out and partly attributed to the bottom-up nature of the Paris Agreement (Bodansky 2016). However, a bottom-up approach of equity, where countries individually follow the most lenient equity approach and thereby collectively overshoot the commonly agreed warming threshold, has not been modelled. Therefore, the collective consistency of national pledges with a theory of self-interested national preferences for commonly accepted concepts of equity is not empirically verified. Additionally, there is currently no modelling of an emissions allocation where countries follow their own vision of equity while collectively aligning with the Paris Agreement goals. Such allocation would provide Parties and observers with a common metric to assess the ambition of countries pledges through the ratcheting-up process.

1.3 Aim and research quest ions To address these gaps, this study quantifies emissions allocation following countries’ divergent positions on equity to stay within global warming thresholds. Following the existing literature (Chapter 3), the current deadlock of negotiations on equity raises the following questions. 1) Under the current agreements, which countries' climate pledges match the commonly discussed equity principles? 2) What 2100 global warming would result from a self-interested world where each country follows the least stringent effort-sharing approach under 2 °C and 1.5 °C goals? 3) In order to stay below the agreed warming thresholds, what allocation of emissions rights reflects the most favourable application of climate justice to each country individually? By addressing these research questions, this thesis aims to contribute to the debate on the operationalisation of equity to determine national emissions trajectories.

1.4 Relevance The results and tools developed in this PhD will quantitatively sketch a potential way forward: Assessing the adequacy of a country’s mitigation contribution that both takes into account the global temperature threshold and the pluralism of viewpoints on equity. Specifically, this PhD thesis builds on the observation that countries tend to align their position with the equity principle that provides the least stringent emission reduction. Thus, this work attempts to define a constructive approach that circumvents the requirement to negotiate a globally agreed single equity principle. Whether fairness is a driver to increase ambition or is used as a bargaining tool to justify an existing commitment, a universally recognised metric to assess the fairness and ambition of countries’

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pledges would facilitate the ratcheting-up of global ambition. Due to this thesis’ outcomes (the articles as well as its products, like www.paris-equity-check.org), decision makers, NonGovernmental Organisations (NGOs), and the wider public has access to standardised quantifications of various metrics that can assist in assessing various governments’ targets and efforts.

1.5 Framework and limitat ions 1.5.1 Disciplinary approach This PhD thesis constitutes a modelling exercise intended to offer a quantitative suite of tools that can assist in evaluating national emissions against various equity criteria and the global temperature goals of the Paris Agreement - building on the existing literature in climate justice. The political interpretation of the quantitative results of this PhD, and the design of specific negotiation proposals on its basis, would represent an extensive body of analyses – which is however – outside the scope of this PhD thesis. The mathematical work undertaken to answer this PhD question will extend the knowledge of the climate and emissions scenarios modelling community. Further uses and analyses of the results’ implications could occur in other disciplines (e.g. policy, philosophy) but they remain outside the coverage of this PhD’s research question.

1.5.2 Definit io n of key terms This sub-section clarifies the understanding of some terms commonly used in this thesis and in the related literature. Throughout the thesis, and consistently with the literature, the term ‘emissions’ refers to annual emissions of Kyoto greenhouse gases – carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6), but not nitrogen trifluoride (NF3) included by the Doha Amendment (UNFCCC 2012a) – from anthropogenic activities described in the Annex A to the Kyoto Protocol (UNFCCC 1998) and from land use, land-use change and forestry (LULUCF) as accounted by the UNFCCC (UNFCCC 2008), unless specified otherwise. Anthropogenic emissions from land-use are part of the global emissions scenario used in this thesis but are excluded from countries’ allocations since the proposed accounting rules differ widely across countries (Chapter 4). The term ‘equity principles’ refer to the different concepts of distributive justice as defined in the literature (see Chapter 3) or presented in the international agreements (UNFCCC 1992). The use of the term ‘equity principle’, sometimes referred to as ‘equity concept’, is consistent with most of the literature (Rose 1990, Rose et al 1998, Ringius et al 2002, Höhne et al 2014, Clarke et al

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2014, Meinshausen et al 2015), although some studies depict equity solely as equality across individuals (Raupach et al 2014, Peters et al 2015). The term ‘effort-sharing’ refers to the partition across parties of a global effort necessary to reach a given common goal. This thesis addresses the sharing across countries of the effort necessary to limit global warming through the mitigation of global emissions to levels specified in the literature. The use of the term ‘effort-sharing’ is consistent with the literature (den Elzen et al 2008, Höhne et al 2014, Clarke et al 2014, Meinshausen et al 2015). In this thesis, an effort-sharing approach is named ‘equity approach’ when it is based on an ‘equity principle’. However, the perception of the equity of an approach may not be universally accepted, and authors may dispute the qualification of ‘equity approach’ to some approaches representing some understanding of distributive justice. What is presented as an equity approach in some texts may be qualified as an effort-sharing approach elsewhere. The distribution of emissions mitigation effort is quantified through the allocation of emissions ‘allocations’. The term ‘allocations’ refers to the emissions allowances distributed to countries or country groups through a given modelling of ‘effort-sharing’ approaches, but does not imply a top-down, political allocation of emissions. Some countries may put forward allocations (through their climate pledges) solely based on national considerations and circumstances and that do not rely on an effort-sharing scheme applicable to all under a given global goal. For example the emissions allocations implied by each country’s pledge do not add up to their commonly agreed goals (Rogelj et al 2016a). However, the allocations derived in this thesis are based on effort-sharing approaches that distribute across countries the emissions of a dynamic (over time) global trajectory. Countries emissions allocations are a proxy for the distribution of the global mitigation effort and can be met through a combination of domestic mitigation and the international trading of emissions permits. An adoption of the national emissions trajectories presented in this thesis does not require corresponding physical domestic emissions.

1.5.3 Framework scope This sub-section discusses the boundaries of this analysis and the limitations they imply in seeking to equitably avoid dangerous interference with the climate system, and to represent national positions on fairness. UNFCCC warming threshold The objective of the UNFCCC is to prevent dangerous interference with the climate system, on the basis of equity (UNFCCC 1992). The choice of what constitutes a threshold to dangerous interference, using global mean temperature as a proxy, implies equity considerations. This PhD will leave these considerations aside and use the commonly agreed thresholds as starting points to determine national allocations of emissions rights.

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The costs to mitigate emissions to a certain level and the impacts of warming at that level affect countries differently. For example, the existence of low-lying countries is directly threatened by sea-level rise under a 2 °C warming. The choice of a global warming threshold represents a different trade-off for every country and equity considerations are involved in the decision on a global warming threshold. Regardless of the chosen threshold, climate impacts are distributed unevenly, and depend both on countries’ physical geography and vulnerability. Equity considerations could therefore also influence the amount of support to provide to countries to cope with adaptation and losses, even if global warming is contained below 2 °C. Consequently, the emissions allocations derived in this thesis aim at representing equitable distributions of the mitigation effort to stay below the agreed threshold rather than the effort to avoid dangerous warming for each country.

Political motivations This section discusses the motivations behind countries’ positions at international negotiations. At climate negotiations, each country negotiates based on the ‘absolute’ gains or avoided losses expected from global warming mitigation. Indeed, countries invoke concepts of distributive justice applicable to all (Chapter 3). However, ‘relationist’ considerations also influence countries’ positions on fairness and the ambition of their pledge (Krasner 1991), and Parties will not cooperate without mutual assurance of some reciprocity by other parties. Some countries may only agree to take on mitigation efforts upon the assurance not to lose economic competitiveness or political influence to direct competitors. At the UNFCCC, the fairness of allocation schemes was discussed by countries on the basis of distributive justice (Chapter 2). In particular, at a workshop on equitable access to sustainable development (AWG-LCA 15), justice was discussed with notions of ‘egalitarianism’, ‘corrective justice’ and ‘sufficientarianism’ (Chapter 3). All equity principles publicly defended at the UNFCCC seek ‘absolute gain’. An ‘absolute gain’ agreement lies on the assumption that all parties could gain from an agreement (Keohane 1984). Each party negotiates to maximise its gain regardless of other parties’ gains. The preference of a country for an ‘absolute gain’ based approach over another may however be driven by ‘relative gain’ objectives. Additionally, countries’ perceptions of an ‘absolute’ gain approach may depend on another country’s gain, and therefore on considerations of ‘relative gain’. Despite the potential importance of ‘relative gain’ considerations in the perception of this PhD’s results, the analysis of countries motivation with respect to relative gains objectives is left outside of this work. The research question will explore the possibility of applying ‘absolute gain’ based allocations of emissions. Consistently with countries’ public positions on fairness, the assumption of this study will be that parties are cooperating for ‘absolute gains’ on the basis of reciprocal commitments (even if these are differentiated on the basis of different equity principles). This study therefore proposes solutions to overcome the disagreement on equitable effort-sharing as they are discussed publicly under the UNFCCC.

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However, the relevance of our results to decision makers depends on political motivations that are not always publicly accessible. For example, even an effort-sharing approach aligned with each party’s publicly declared vision of equity may not be followed in practice. Indeed, many parties may have other objectives, including ‘relative gain’ objectives, than achieving the equity principle they put forward. Such parties may have used an equity rationale as a negotiating tool. These considerations, which are not argued publicly, may yet be critical for parties to agree upon an allocation scheme and affect the relevance of this study.

Limitations of scale At the national level, countries are treated as sovereign entities that may use visions of equity at the sub-national level that differ from their negotiated principle. Each country following equitable allocations, may not decide to apply an equitable distribution internally or may use different visions of equity. Accordingly, this research derives emissions allocations for countries based on their population’s characteristics but not directly for individuals. This PhD does not discuss how equitable emissions allocations could be shared at the sub-national level.

1.6 Thesis out line The following chapters address the research questions presented earlier (Section 1.3). Chapter 2 provides background information on the internationally agreed global warming thresholds and on the current national emissions mitigation commitments. Chapter 3 reviews the literature on the modelling of emissions allocations following various concepts of distributive justice and details the gaps in the literature that this thesis addresses. Chapter 4 introduces the ‘PRIMAP-Equity’ modelling framework developed as part of this PhD and used to derive emissions allocations that reflect the five equity categories of the IPCC-AR5 report (Clarke et al 2014). In this chapter, this modelling framework is applied to derive emissions allocations for the five equity categories consistent with the G7 Elmau agreement signed in June 2015. Chapter 5 discusses in greater details the modelling of the ‘PRIMAP-Equity’ framework of Chapter 4. In particular, this chapter explores the influence of the choice of politically relevant parameters on countries’ emissions allocations. In Chapter 6, the ‘PRIMAP-Equity’ framework is used to derive emissions allocations consistent with the mitigation goals of the Paris Agreement signed in December 2015. The seventh chapter discusses the consistency of G20 countries’ climate pledges with the equitable emissions allocations derived in Chapter 6. These results are put in the perspective of the G20 countries’ statement on fairness contained in their climate pledge. Building on the ‘PRIMAP-Equity’ allocation framework introduced in Chapter 4, Chapter 8 addresses the second and third research questions (Section 1.3) and suggests a distribution of emissions across countries where each country can follow the most favourable equity principle while collectively limiting warming to the agreed thresholds. Finally, the combined results of all previous chapters are used to discuss the implications and draw conclusions. The relevance of the results, but also their numerical limitations, are presented in this concluding Chapter 9.

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CHAPTER 2 – BACKGROUND This chapter provides background information on international negotiations on climate change and highlights the importance of research in the field of modelling equitable allocation of emissions rights. It first provides historical background information on successive international agreements to mitigate global warming. The inclusion of equity in the international agreements is then discussed in light of the often-dissonant understandings of equity by countries. 2.1 Global temperature and mit igat ion goals 2.1.1 Lead-up to the Paris global climate goals The influence of anthropogenic GHG emissions on climate change has long been identified by scientists. The international community has accepted this finding and commissioned scientific research to assess the impacts of climate change and identify mitigation measures. In 1988 the United Nations (UN) and the World Meteorological Organization (WMO) jointly established (Figure 2.1) the IPCC to provide the international community with an impartial scientific assessment of climate change and its impacts. The IPCC’s first Assessment Report (AR), released two years later, called for a global treaty on anthropogenic GHG emissions. In response, in 1992, the international community agreed to the UNFCCC, with its long-term global goal of a ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system’ (UNFCCC 1992). With the continuous increase of global emissions and the acceleration of global warming, it became urgent to decide on a level at which to stabilise GHG concentrations to prevent dangerous climate impacts. Informed by the IPCC’s second and third assessment reports, in 2010 the Parties to the Convention adopted the goal of limiting global average warming to 2 °C, compared to preindustrial levels (UNFCCC 2010). The 2 °C threshold represents an economic challenge for most industrialised nations and current national pledges (targets) are collectively insufficient to stay below this limit. However, even this 2 °C threshold may be inadequate to avoid dangerous global warming impacts. Climate impacts are distributed unevenly around the globe and affect vulnerable countries the most and 2 °C warming represents major, sometimes existential, risks for many countries (UNFCCC 2015b). As a result, a group of 101 countries (Climate Analytics 2017) called for a 1.5 °C warming limit for the first time at the 15th Conference of the Parties (COP) to the UNFCCC in Copenhagen in 2009. The Paris Agreement (UNFCCC 2015a) strengthened the Convention’s objective with a commitment to limit warming to ‘well below 2 °C’ and to ‘pursue efforts to limit […] to 1.5 °C’. The Agreement also added the objective of reaching net-zero GHG emissions in the second half of the century. The adoption of the Paris Agreement’s global goals was considered as a global success both in the negotiation and in the subsequent literature (Schellnhuber et al 2016, Rogelj and Knutti 2016).

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Figure 2.1 | Key dates of the IPCC and UNFCCC agendas.

2.1.2 The impacts of a 2 °C or 1.5 °C warmer world Success in limiting global warming to 1.5 °C will depend on the estimated climate impacts of not reaching this goal versus the socio-economic-ecological costs incurred to attain it. While the implications of limiting global warming to 2 °C are well researched, in particular since the Copenhagen Accord in 2009, little scientific literature is available both on the economic requirements and on the climate impacts implied by the 1.5 °C limit. In order to fill that knowledge gap and not to miss the rapidly fading opportunity to limit warming to 1.5 °C (Rogelj et al 2013), through the Paris Agreement, the IPCC has been invited to provide the Conference of the Parties (COP) with a special report in 2018 (Figure 2.1) on the impacts of 1.5 °C global warming and related GHG emissions pathways (UNFCCC 2015a). Two years is a very brief period to research, write, review and publish new results that should then be gathered and put together in this special IPCC report. Fortunately, some studies have already identified specific climate risks that increase with global warming. The Fifth Assessment Report of the IPCC (IPCC-AR5) suggests that while the risks of ‘large-scale singular events’ and of ‘global aggregate impacts’ may not differ significantly under a 1.5 °C or 2 °C world, the risks to ‘unique and threatened systems’, the risk of ‘extreme weather events’ and the ‘distribution of impacts’ increase from ‘moderate’ to ‘high’ (Figure 2.2), reproduced from IPCC-AR5 WGII Fig.19-4. While no global comprehensive cost-estimate of the corresponding climate impacts is available, limiting warming to 1.5 °C would offer a greater chance to avoid costly climate impacts.

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Figure 2.2 (reproduced from IPCC-AR5 WGII-Fig. 19-4) | The dependence of risk associated with the Reasons for Concern (RFCs) on the level of climate change 4.

More recent work (Schleussner et al 2016a, 2016b) specifically studied the impacts expected at 1.5 °C and 2 °C on a selection of indicators. For example, the likelihood of a pre-industrial 1-ina-1000 day extreme-temperature event is 27 times higher in a 2 °C warmer world (Fischer and Knutti 2015, Schleussner et al 2016b). This probability is halved in a 1.5 °C warmer world 4

Figure 2.2 reproduced from IPCC-AR5 WGII, Fig. 19-4, p.1073, from Oppenheimer, M., M. Campos, R. Warren, J. Birkmann, G. Luber, B.C. O’Neill, and K. Takahashi, 2014: Emergent risks and key vulnerabilities. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy, S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Full legend: “The dependence of risk associated with the Reasons for Concern (RFCs) on the level of climate change, updated from the Third Assessment Report and Smith et al. (2009). The color shading indicates the additional risk due to climate change when a temperature level is reached and then sustained or exceeded. The shading of each ember provides a qualitative indication of the increase in risk with temperature for each individual ‘reason.’ Undetectable risk (white) indicates no associated impacts are detectable and attributable to climate change. Moderate risk (yellow) indicates that associated impacts are both detectable and attributable to climate change with at least medium confidence, also accounting for the other specific criteria for key risks. High risk (red) indicates severe and widespread impacts, also accounting for the other specific criteria for key risks. Purple, introduced in this assessment, shows that very high risk is indicated by all specific criteria for key risks. Comparison of the increase of risk across RFCs indicates the relative sensitivity of RFCs to increases in GMT. In general, assessment of RFCs takes autonomous adaptation into account, as was done previously (Smith et al., 2001, 2009; Schneider et al., 2007). In addition, this assessment took into account limits to adaptation in the case of RFC1, RFC3, and RFC5, independent of the development pathway. The rate and timing of climate change and physical impacts, not illustrated explicitly in this diagram, were taken into account in assessing RFC1 and RFC5. Comments superimposed on RFCs provide additional details that were factored into the assessment. The levels of risk illustrated reflect the judgments of Chapter 19 authors.” Available at: www.ipcc.ch/pdf/assessment-report/ar5/wg2/WGIIAR5-Chap19_FINAL.pdf

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compared to a 2 °C one. While the uncertainty associated with precipitation is considerably greater, the likelihood of extreme precipitation is expected to rise globally with warming temperatures (Schleussner et al 2016a), and in South-Asia in particular. At the same time, water availability is expected to decrease locally. The availability of water in the Mediterranean region is projected to decrease by 9% with 1.5 °C warming, and by 17% with 2 °C warming. With rapid 2 °C warming, all coral reefs are at risk of collapsing due to both warming and associated bleaching events, and ocean acidification (Hoegh-Guldberg et al 2007, Frieler et al 2012). Limiting warming to 1.5 °C is expected to save a fraction of the world’s corals from degradation. Sea-level rise is also significantly different in a 1.5 °C or 2 °C warmer world (Levermann et al 2013) and sea-level rise projections until 2300 strongly depend on emissions trajectories (Nauels et al 2016). Yields of maize, wheat, rice and soy in tropical regions are projected to decrease with global warming. Nevertheless, the increasing concentration of atmospheric CO2 attenuates the yield loss for maize and wheat, and even leads to a yield increase for rice and soy (Schleussner et al 2016a) – in the absence of impacts on extreme temperature and water availability. Overall, the negative effects on the economy are exacerbated by global warming (Burke et al 2015).

2.1.3 Glo bal emissions scenarios to reach the climate goals Mitigating global warming at minimal costs implies an optimal selection and deployment of lowcarbon options (e.g. policies, renewable energies, afforestation, reduction of consumption, recycling, etc.). The costs and effects, as well as the timing of these options are modelled using Integrated Assessment Models (IAMs). An IAM derives the cost-optimal implementation of mitigation measures and global scenarios of anthropogenic GHG emissions over time (see box ‘Emissions scenarios vs. carbon budgets’ below). Multiple research groups have developed IAMs under varying assumptions and explored a range of possible temperature outcomes. The analyses presented in the latest IPCC report are based on 846 scenarios 5 . Some of these emissions trajectories decrease rapidly and become negative soon after 2050. On the other hand, some emissions scenarios rise steadily and reflect business-as-usual trajectories in the absence of any climate policies. Only few scenarios of the IPCC-AR5 database offer a chance to return to 1.5 °C of warming by 2100 (Robiou du Pont et al 2017). Since the publication of the IPCC-AR5, a study (Rogelj et al 2015) derived 37 additional scenarios consistent with 1.5 °C and analysed the sectoral characteristics. According to this study, reaching emissions levels consistent with the 1.5 °C limit requires immediate action and a faster scale-up of lower carbon measures (Rogelj et al 2015). The differences in solutions to bend emissions from 2 °C trajectories to 1.5 °C ones are summarised in Table 2.1 (adapted from Rogelj et al. (2015)). Most importantly, lower energy demand and efficiency improvements, which substantially reduce the mitigation costs, are a key enabling factor in order to stay within the 1.5 °C limit. A large-scale deployment of Carbon Dioxide Removal is another requirement. Overall, costs associated with limiting warming to 1.5 °C are double that of 2 °C over the century, and even greater in the near term. However, the results of the study do not 5

The database is hosted at: tntcat.iiasa.ac.at/AR5DB/

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account for economic benefits of mitigating emissions towards 1.5 °C. Such co-benefits (for example: avoided climate impacts (Frieler et al 2016, Burke et al 2018), reduced air pollution and improved energy security) represent an important driver for emissions reduction (Averchenkova et al 2014).

EMISSIONS SCENARIOS VS. CARBON BUDGETS Carbon (CO2) budgets are often associated with national and global climate objectives. Due to the long lifetime of CO2 in the atmosphere, the temperature at the end of the century depends on the cumulative CO2 emissions rather than on the timing of those CO2 emissions. A linear relationship links the cumulative CO2 emissions to the maximal temperature over the century (IPCC 2014b, Meinshausen et al 2009) since the CO2 uptake from the atmosphere by natural sinks (land and ocean) balances almost exactly the global temperature inertia in response to CO2 emissions. Recent literature finds a wide a range of carbon budgets under a given temperature goal (Rogelj et al 2016b), and recent studies found a 1.5 °C budget several times larger than the IPCC (Peters 2018). The discrepancies arise from a variety of differences when assuming historical warming, non-CO2 climate forcing, modelled processes, and interpretation of the Paris temperature goal. The Paris Agreement does not specify timing of warming thresholds and whether or not they can be temporarily overshot. Following these interpretations, carbon budgets are categorised based on the properties of the underlying multi-gas emissions scenarios. A ‘threshold exceedance budget’ (TEB) refers to cumulative carbon emissions of a multi-gas emissions scenario until a temperature threshold is exceeded, and a ‘threshold avoidance budget’ (TAB) refers to cumulative carbon emissions over a given time period of a multi-gas emissions scenario that limits global-mean temperature below a specific threshold (Rogelj et al 2016b). Logically, TEBs provide larger budget estimates than TABs for a given temperature threshold. Carbon budgets relate global cumulative emissions over the century to temperature goals. However, using carbon budgets does not provide information on the cost-optimal timing on the mitigation measures to achieve the CO2 mitigation. Carbon budgets also leave out considerations on the other GHG. By contrast, the GHG emissions scenarios from Integrated Assessment Models that budgets are based on are associated with specific policies and technology deployment that minimise the emissions mitigation costs. Using directly using multi-gas emissions scenarios to derive national allocations preserves the information on countries' mitigation measures not just for CO2, but for all the main greenhouse-gases (as defined in section 1.5.2).

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Table 2.1 | Comparison of the characteristics of scenarios limiting global warming to either 2 °C or 1.5 °C, adapted from Rogelj et al. (2015).

Demand Technologies

Non-CO2 gases Carbon Dioxide Removal Costs

Global energy demand in 1.5 °C scenarios remains less than 40% above 2010 levels over the 21st century, and not more than 15% in most scenarios. In 2030, the contribution of low-carbon technologies in the electricity sector (renewable, nuclear, Carbon Capture and Storage) is 10% higher for 1.5 °C scenarios than for 2 °C. The transport sector has a similar share of electricity use for 1.5 °C and 2 °C scenarios, about 25% in 2050 (compared to 1% in 2013). In the industrial, transport and buildings (residential & commercial) sectors, emissions in 2050 are about 25%, 40% and 50% lower in 1.5 °C scenarios than in likely 2 °C scenarios, respectively. The full mitigation potential of non-CO2 gases is reached for 2 °C scenarios. The additional mitigation towards 1.5 °C scenarios mainly arises from additional CO2 mitigation. Carbon Dioxide Removal (CDR) compensates 60% to 85% of the total fossil fuel CO2 emissions over the 2010-2100 period under 1.5 °C scenarios, but only less than half under 2 °C scenarios. Large scale deployment of CDR technologies is a requirement of these 1.5 °C scenarios. Mitigation costs aggregated throughout the 21st century are up to twice as high for 1.5 °C scenarios than for 2 °C. Relatively, near-term costs are even greater.

The global warming thresholds of the Paris Agreement do not reflect the country-specific vulnerabilities and national interests in addressing climate change. The benefits of avoided climate change can influence national positions when negotiating treaties. Small Island Developing States, which include states whose existence is directly threatened by climate change, have formed the ‘High Ambition Coalition’ with developed countries to support limiting global warming to lower level even in the absence of a burden-sharing mechanism to equitably share the corresponding mitigation effort. Developed countries are less vulnerable to climate impacts than developing countries have, in that respect, a lesser interest in undertaking important mitigation efforts. However, under the Paris Agreement, developed countries shall provide resources to assist developing countries with respect to adaptation and should enhance support with respect to loss and damage (UNFCCC 2015a). With this greater recognition of loss and damage (Mace 2016), all countries should share some benefits of mitigating global warming. However, this recognition came with the explicit statement that is will no “involve or provide a basis for any liability or compensation” (Jacquet and Jamieson 2016). The benefits of avoided climate change do not account for the co-benefits of emissions mitigation measures. Most co-benefits are not captured by IAMs (Kriegler et al 2014, Rogelj et al 2015) and are not discussed under burden-sharing considerations. However, opportunities of co-benefits from mitigation measures (improved air quality, energy security, leading position sustainable technology…), much like efforts implied by mitigation measures, influence countries’ pledges and ambition (Averchenkova et al 2014, Zenghelis 2017). The emissions allocation derived in this PhD thesis represent shares of IAM emissions scenarios and thus do not reflect countries’ co-benefits. Considering that the emissions allocations can be met through a combination of domestic measures and ‘internationally transferred mitigation outcomes’ (UNFCCC 2015a), co-benefits represent incentives to mitigate domestic emissions beyond the levels implied by least-cost IAM scenarios.

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2.2 National contribut ions to reach global goals 2.2.1 Ratchet ing-up global ambit io n In the Preamble to the Paris Agreement (UNFCCC 2015a), the international community suggested global indicative emissions limit for the year 2030 of 40 GtCO2eq to stay below 2 °C of warming, or lower emissions levels consistent with the 1.5 °C threshold. The 40 GtCO2eq indicative limit is scientifically consistent with cost-optimal pathways selected to limit warming to 2 °C (Robiou du Pont et al 2017) and with the recent 42 GtCO2eq recommendation from the UNEP (2015). The adoption of a 2030 emissions limit consistent with the 1.5 °C threshold will be based on the upcoming IPCC special report on 1.5 °C to be published in 2018. The shift of the negotiation’s focus away from binding national emissions commitments since the Copenhagen Accord (UNFCCC 2009) played an important role in reaching high global ambition under the Paris Agreement (Oberthür and Groen 2017). However, current Nationally Determined Contributions (NDCs) aggregate to 52.5 GtCO2eq and do not match the 2030 levels consistent with cost-optimal trajectories (Rogelj et al 2016a, Robiou du Pont et al 2017, UNFCCC 2015a). The Paris Decision recognises the inadequacy between current pledges and the scenarios consistent with its long-term objectives (UNFCCC 2015a). Lacking sufficient ambition, GHG continue to accumulate in the atmosphere while both climate change impacts and the cost of emissions mitigation are growing. The probability to avoid dangerous warming is more affected by political inaction than by the factors at the root of this inaction: geophysical and technological uncertainties, and future energy demand (Rogelj et al 2013). In order to ratchet-up global ambition, the Paris Agreement binds countries to periodically take stock of progress, starting in 2018, ‘in light of equity and the best available science’ (UNFCCC 2015a), and each country’s successive pledge must represent a progression in ambition. Despite ongoing discussions on equitable effort-sharing mechanisms since the adoption of the UNFCCC, countries have not operationalised equity at the country level and the Paris Agreement contains only quantified emissions objectives at the global scale.

2.2.2 Equit y under the UNFCCC The principles of equity contained in the Paris Agreement were first included in the UNFCCC, where all countries agreed to share global mitigation efforts following their ‘common but differentiated responsibilities and respective capabilities’ (CBDR-RC) and to enable economic development (UNFCCC 1992). The Convention does not provide details regarding the operationalisation of equity (Winkler and Rajamani 2014a) and simply attributes differentiated commitments to specific countries. With developed countries required to take the lead in climate change mitigation and provide financial support to developing countries. In the lead-up to the Paris Agreement, rather than negotiating binding differentiated mitigation targets, countries agreed to submit their (Intended) Nationally Determined Contributions –

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(I)NDCs – to be accompanied by a justification of the underlying principle used to formulate each (I)NDC. Each country determines its emissions target presumably based on its perception of its national circumstances and of its own interpretation of CBDR-RC. To date, many countries failed to provide clarifications or justifications regarding the fairness of their pledge (Chapter 7). The ensuing Paris Agreement does not refer to explicit groups of countries, but rather binds countries individually to communicate successive NDCs with the ‘highest ambition’ reflecting their CBDRRC interpretation. The Agreement still requests developed countries, without listing them, to take the lead in reducing emissions economy-wide and mobilizing climate finance. In the absence of a common agreement on effort-sharing, the ambition of current pledges is often driven by self-interest and co-benefits (Averchenkova et al 2014, Lange et al 2010, Fleurbaey et al 2014). Since current pledges are insufficient to avoid the agreed warming thresholds, equity is still central for the ratcheting-up process towards the Paris goals or when discussing the adequate magnitude of climate finance and support (Mace 2016, Hare et al 2010). The determination of equitable national emissions targets that add up to scenarios consistent with the Paris Agreement would inform the process of ratcheting-up global ambition.

2.2.3 National posit io ns on fairness Since the adoption of the UNFCCC, many countries stated their interpretations of equity in national communication inside or outside UN climate negotiations. Before the Paris Agreement, many parties published their position on equity as submissions to the Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) from 2012 to 2014. In particular, several countries made submissions at the UNFCCC workshop on equitable access to sustainable development (AWG-LCA 15). Additionally, a paper from government experts of China, Brazil, India and South-Africa expressed the respective conceptions of equity as well as the methodology used to decide on every national target (BASIC experts 2011). Finally, countries agreed to provide information on how their (I)NDCs are fair and ambitious (UNFCCC 2017a). However, the UNFCCC does not require countries to state an effort-sharing approach or formula that they consider equitable, nor does it require countries to indicate the methodology used to calculate their pledge. In their communication, countries often indicate their preference for some parts of the basic equity dimensions of the UNFCCC principles regarding emissions mitigation: responsibility, capability or right to development (UNFCCC 1992). The concepts of responsibility and capability are directly mentioned in the CBDR-RC equity principle of the UNFCCC. Responsibility is most often discussed as historical responsibility for past emissions, or as historical contributions to global warming – the two understandings differ due to the non-linearity of the relationship between global warming to GHG emissions since a given year. Under this vision of corrective justice, high historical emitters should take greater efforts to mitigate future emissions than low historical emitters. Capability is commonly understood as requiring richer countries to pay for emissions mitigation. The right to economic development is not directly mentioned in the Principles of the

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UNFCCC, but is stated as an objective of the Convention (UNFCCC 1992). This right to equal development is often understood as mixing several dimensions of equity, such as capability or historical responsibility. Additional interpretations of equity and distributive justice in the context of climate change mitigation are presented and categorised in Chapter 3. Countries’ preferences for equity principles can be identified in their communication at an UNFCCC workshop AWG-LCA 15 (UNFCCC 2012c), through ADP submissions (UNFCCC 2014), in a paper by experts from BASIC countries (BASIC experts 2011) and in their NDC statements (Table 2.2). Some major emitters, such as the USA, Russia, Japan, Australia and Canada have not publicly supported any equity principle. Neither have they explained how the approach behind their pledge could be generalised to limit global warming. The USA stated that the 2007 COP in Bali did not asks for comparable targets (Pershing 2011) and that “equity should not be forced into one formula” (AWG-LCA 15). The European Union (EU) applies a capability-based approach to share mitigation efforts internally (CEC 2008), and suggested using notions of historical responsibility and capability to allocate emissions rights internationally (AWG-LCA 15). However, the EU did not refer to any equity principle in its NDC and only calls for a discussion on fairness (Robiou du Pont 2017).

Table 2.2 | Equity principles proposed by major emitters and their share of global GHG emissions (Nabel et al 2011). Sources are: (AWG-LCA 15) a UNFCCC workshop on equitable access to sustainable development (UNFCCC 2012c), submissions to the ADP in 2014 (UNFCCC 2014), and NDCs. Note: Sudan, the Democratic Republic of Congo and the Republic of Mali are members of both the LMDC and the LDC groups. Their emissions shares are accounted under the LDC group in this table.

Countries Like Minded Developing Countries USA Europe (28 countries) Russia Least Developed Countries Japan

Brazil Canada Australia

South Africa AILAC

% emissions

41.9 14.6 10.2 5.1 3.6 3.0

% emissions incl. LULUCF

Equity Principle

41.5 Responsibility 12.0 9.2 Responsibility, Capability 3.8 5.5 Right to development 2.7

2.5 1.6 1.2

3.7 Responsibility, Capability 1.6 1.2

1.0 0.9

Right to development, 1.0 Responsibility, Capability 1.3 Responsibility, Capability

References

ADP Submissions (AWG-LCA 15)+(CEC 2008) ADP + (AWG-LCA 15) (BASIC experts 2011) + (AWG-LCA 15) + NDC

(BASIC experts 2011) + (AWG-LCA 15) + NDC ADP Submissions

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The dissonance regarding the interpretation of the UNFCCC equity principle illustrates the difficulty to agree on binding mitigation targets. The national mitigation effort, or equivalently the emissions allocation, implied by the modelling of these equity principles depends on the modelling assumptions and parameterisation. Countries are more likely to support approaches that results in minimal additional effort compared to their current pledge (Averchenkova et al 2014, Fleurbaey et al 2014, Lange et al 2010). Firstly, an assessment of the multiple notions of equity under a consistent parameterisation could reveal that some countries’ dissonant positions on equity result in similar emissions allocations. Such assessment would also inform the civil society and decision makers to devise emissions target consistent with various visions of equity idealistically applied to all countries. Secondly, a ‘real-world’ evaluation of the necessary additional effort to reach the global goals would derive mitigation targets consistent with the current bottom-up situation where countries support dissonant equity approaches.

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CHAPTER 3 – LITERATURE REVIEW ON EFFORT-SHARING APPROACHES 3.1 Categorisat ion of concepts of distribut ive just ice This thesis discusses the distribution of emissions rights as a proxy to equitably distribute efforts to reduce GHG emissions and limit global warming. This section first presents some fundamental principles and metrics of distributive justice underlying all allocation methods. It then discusses the application of these principles to the distribution of emissions rights. The main allocation approaches presented in the literature are finally described in the light of the principles of distributive justice.

Under the UNFCCC, countries discuss the allocation of emissions rights on the basis of distributive justice (UNFCCC 2012b, BASIC experts 2011). However, the application of any concept of distributive justice to emissions rights is criticised on two grounds. First, most of the world’s limited resources (e.g. plutonium, aluminium or fossil fuels) are not shared under distributive principles (Caney 2009). Second, GHG emissions do not have intrinsic value and are only the byproduct of a valuable service in the form of energy. In that respect, it matters more to have a fair distribution of energy directly. Another criticism points that, except for some special cases, distributive justice should apply to a combined set of goods, and not to emissions (or energy) in isolation (Caney 2009). The right to emit GHG could be associated with the access to other goods or resources under a single distributive rule. Despite these criticisms, alternative frameworks, which sometimes consider the joint allocation of energy rights or addressing adaptation or loss and damage costs (Caney 2009), are left outside the scope of this project. Notions of distributive justice are applied to allocate emissions across countries generally using either an intrinsic or utilitarian metric (Table 3.1). An intrinsic metric considers the equitable allocation of emissions as an end goal, while a utilitarian metric considers the equitable allocation of the benefits from emissions rights (Brennan and Lo 2016). The most prominent conceptions of distributive justice are described as Egalitarianism, Prioritarianism, Sufficientarianism and, arguably, Libertarianism (Arneson 2013, Armstrong 2009). Egalitarianism allocates equally the remaining emissions rights or financial effort to individuals. A Prioritarian approach will aim at achieving justice, inclusive of external existing conditions (e.g. poverty, lower historical emissions, greater needs) by allocating emissions rights to those worse-off as priority (Arneson 2013, Armstrong 2009). Prioritarianism can overlap with corrective justice (Weinrib 2002) when the distribution corrects a prior unequitable appropriation of a good (e.g. correcting for unequal historical emissions). The allocation of emissions rights based on historical responsibility compensates unfair historical emissions by permitting wronged countries to commit equivalent tort. Alternatively, corrective justice can require reparation in the form of financial support for adaptation and compensation for loss and damage. A Sufficientarian approach will ensure that all countries have sufficient emissions space to develop sustainably. Finally, Libertarianism allows each country to emit freely as long as this does not have a negative

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impact on other countries. Given that additional emissions impact all countries to some degree, Libertarianism is not applicable to future allocation. An alternative categorisation of equity distinguishes the distribution of emissions that are ‘allocation-based’ or ‘outcome-based’ (Rose et al 1998). An ‘allocation-based’ approach seeks an equitable allocation of the remaining emissions or financial efforts. An ‘outcome-based’ approach applies equity concepts to achieve an equitable situation, e.g. inclusive of external or historical conditions (as prioritarianism does) or that equalises welfare changes.

Table 3.1 | Examples and categorisation of approaches of distributive justice applied to allocations of emissions rights. Prioritarian approaches using a utilitarian metric, as well as sufficientarian approaches fall under a ‘complete’ vision of climate justice (Knopf et al 2012). Other approaches follow an ‘isolated’ vision.

Goal\Metrics

Intrinsic (emissions)

Utilitarian

Egalitarianism (Allocationbased) Prioritarianism (Outcome-based)

Equal per capita, Dynamic per capita

Equal percentage of GDP

Equal cumulative per capita

Capability, Greenhouse Development Rights, Equal cumulative emissions benefits Sufficientarianism Global indicators Local indicators Libertarianism Sovereignty/Grandfathering (past emissions)

Corrective justice No

Inherent

Inherent Compatible

Finally, notions of climate justice can be characterised as ‘isolated’, when equitable mitigation is an end goal, or ‘complete’, when global justice is the purpose (Knopf et al 2012). Complete approaches use exogenous indicators (other than emissions rights) to derive emissions allocations. Indicators such as Gross Domestic Product (e.g. South-Africa (BASIC experts 2011)), basic needs (den Elzen et al 2013), capacity for domestic technology improvement (Höhne et al 2014), and development indices (Baer et al 2008, Kemp-Benedict 2010) all may be used to connect GHG emissions with economic or human development. Consequently, these approaches distribute the emissions in a way that minimises international per capita differences of these exogenous indicators. Drawing on the various approaches and metrics of distributive justice presented in Table 3.1, multiple normative allocation methods have been suggested in the literature to derive the distribution of emissions rights across countries.

Existing approaches Following the diverse categories of distributive justice, academics, experts from various countries (BASIC experts 2011), philosophers (Caney 2009, Fei and Singer 2013) and Non-Governmental Organisations (Müller and Mahadeva 2013, Kemp-Benedict 2010) modelled formulae that provide

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alternative approaches for allocating emissions right to countries, in line with their desired global warming thresholds (temperatures and likelihoods). The six following boxes present dynamic allocations of a global emissions scenario based on the most frequently modelled effort-sharing approaches categorised in Table 3.1. The implementation of effort-sharing approaches varies in the literature. In particular, the literature on effort-sharing approaches often discusses allocations of cumulative emissions (carbon budget or emissions budget) as it can be easier than allocating dynamic shares – as time-series – of a global scenario. However, the conversion of a national emissions budget into a national emissions trajectory or emissions targets remains at countries’ discretion. It is therefore difficult to assess compliance of countries’ pledges with allocated budgets, and to assess near-term global progress towards net-zero emissions. Instead, allocating emissions from an IAM scenario results in consistent national emissions scenarios and enables assessment of a country’s progress at any point in time. The allocated emissions scenarios can be met through a combination of domestic mitigation and support for mitigation overseas. Developing countries could potentially sell the allocated emissions rights in excess of their needs to other countries in order to finance their development.

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Sovereignty – Grandfathering approach – Inertia – Status quo (Müller 2005, Caney 2009, Raupach et al 2014, Peters et al 2015) Under this approach, all countries reduce their emissions (or possibly per capita emissions) at the same rate. Therefore, each country preserves its current share of global emissions as global emissions decline. The Paris Agreement states that its implementation should reflect equity in light of national circumstances. ‘National circumstances’ can refer to countries’ vulnerability to climate change, or arguably to the vulnerability to emissions mitigation (Saudi-Arabia’s NDC (UNFCCC 2017b)). This approach accounts for national circumstances regarding current emissions levels and favours high emitting countries. This approach is implicitly followed by many of the developed countries (Peters et al 2015, Robiou du Pont et al 2016). This approach is considered inequitable by many other parties (BASIC experts 2011) and academics (Caney 2009, Fei and Singer 2013), and does not represent any vision of distributive justice. Instead, it represents the absence of development on equity, or ‘inequity-as-usual’, under a global trajectory to net-zero emissions.

Capability This ‘complete’ vision of equity argues that mitigation efforts should be based on parties’ ability to pay for mitigation. The Jacoby Rule approach (Jacoby et al 1999) defines a per capita welfare threshold under which no contribution is required. The bigger the share of its population is above this welfare threshold, the more a party should contribute to the mitigation costs. Alternatively, a share of global emissions can be allocated proportionally to the inverse of per capita GDP and their population (Jacoby et al 2008).

Equal per capita (Meyer 2004, Raupach et al 2014, Tavoni et al 2014, Peters et al 2015) A dynamic interpretation of the equal per capita approach allocates global emissions to countries proportional to their population. Thus, it aims for each person in the world to have the same emission allowance at a given time. Given the great disparity on current per capita emissions, some countries argue for a convergence period that reflects current national circumstances. Implementing such convergence periods implies a near term ‘grandfathering’ influence, and its duration should be discussed between countries. This dynamic modelling, inclusive of a transition period is referred to as ‘Contraction and Convergence’ (Meyer 2004). The dynamic formulation of the equal per capita approach leaves little possibility for diverging modelling methodology. The critical modelling parameter is the duration of the convergence period until countries’ emissions are proportional to their population.

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Dynamic Per Capita (Höhne et al 2006, Meinshausen et al 2015, Nabel et al 2011) – Indian Proposal In this approach, per capita emissions for each country follow Business-as-usual pathways until they equal a reference level. This reference can be the per capita emissions level of another country or group of countries with the largest emissions. This high per capita emitting reference country should therefore reduce their emissions while the per capita emissions of other countries can still increase. After this convergence, all countries that reach this highest per capita level follow the same phasing-out pathway. This approach is similar to ‘equal per capita convergence’ but leaves smaller emitters unconstrained until they are amongst the biggest emitters themselves. Under this approach, developing countries could increase their emissions to enable their development, but they would not have spare emissions credit to sell to other countries. N.B. While this approach has been brought by the Indian government in 2008, it does not match the position put forward by Indian scholars in the 2011 BASIC publication (BASIC experts 2011).

Greenhouse Development Rights (Baer et al 2008, Kemp-Benedict 2010) – South African Proposal (BASIC experts 2011) This ‘complete’ approach seeks to ensure all countries a right to develop in a climate constrained world through the allocation of mitigation requirements. Countries with high GDP per capita and high historical emissions per capita have low emissions allocations. Conversely, some developing countries can have strictly positive allocations over the century. Under the GDR approach, these countries can either avoid the recourse to costly negative emissions or decide to sell their positive allocations to developed countries with negative allocations. The emissions budgets are calculated using a Responsibility and Capacity Index based on population, GDP (PPP – Purchasing Power Parity), historical emissions and the Gini index. Some variations can use other indicators such as the Human Development Index (South African approach (BASIC experts 2011)). The scheme shares the mitigation burden – the difference between the business as usual and the targeted mitigation scenarios – rather than the emissions of the targeted mitigation scenario directly. Therefore, countries’ allocations depend on assumed business as usual scenarios. A global agreement on national business as usual projections has not been discussed as part of the UNFCCC. Several parameterisations are used by Non-Governmental Organisations (NGOs) and some countries (South Africa).

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Equal cumulative per capita (Nabel et al 2011, Pan et al 2014, Bode 2004) – Chinese Proposal (BASIC experts 2011) The simplest modelling of the equal cumulative per capita approach allocates countries cumulative emissions proportional to cumulative population (for example using annual population data) over a period starting in the past. Countries with high historical per capita emissions will have low (potentially negative) per capita emissions in their future to compensate for the historical responsibility. Some critics object that a country should not be responsible for emissions before it knew of its actions’ climatic consequences (Caney 2009), whereas others argue that a country should take full responsibility for all past emissions because these past emissions have enabled the country to develop (BASIC experts 2011). Another philosophical objection is that a generation should not be held responsible for the actions of a previous one (Edenhofer et al 2012). This objection can be countered by the view that current generations benefit from these past emitting activities. Therefore, modelling this approach requires choosing a contentious starting date for accounting emissions. Suggestions include 1850 (first available data), 1900 as in the Chinese proposal (BASIC experts 2011), the date of discovery of anthropogenic contribution to global warming, 1990 (the date of the first IPCC report and second World Climate Conference), or starting when it is implemented. The end year, when equal cumulative per capita emissions are achieved, 2050 in the Chinese or Indian proposals (BASIC experts 2011) or 2100 (Robiou du Pont et al 2016), also has a strong influence on near term allocations. It is also possible to account for past emissions with a lower weight as technologies now emit much less to produce a comparable good. Alternatively, the Brazilian proposal (BASIC experts 2011) recommends accounting for emissions based on their impact on current climate. This approach therefore accounts for the non-linearity of global warming to cumulative emissions and the short-term impact of some non-CO2 GHG. Compared to a pure equal cumulative per capita approach, the Brazilian approach better reflects the polluter-payer principle, but also accounts for harm on a metric that was not accessible when it was caused.

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Global Cost Effectiveness – Equal Marginal abatement costs (Kriegler et al 2014, den Elzen et al 2008, Tavoni et al 2014) This geographical repartition of emissions following cost-optimal IAM scenarios does not reflect any equity concepts (Höhne et al 2014). The resulting national emissions trajectories indicates the market-based distribution of mitigation efforts following the economic and political assumptions of the IAM modelling. Emissions reductions are allocated to countries where mitigation is cheapest according to the IAM modelling. This approach (also referred as ‘equal marginal abatement costs’) corresponds to the emission mitigation allocated in Integrated Assessment Models. Global emissions scenarios from IAMs follow modelling achieving cost-effective emissions reduction, based on their respective assumptions. The associated emissions reduction comes at a price assessed by a Marginal Abatement Cost curve. This curve simply reflects the price of the technological improvements needed to reduce additional emissions. The unitary emission reduction price grows as the cheapest emissions reductions are successively achieved. In practice, emissions reductions are often the cheapest in developing countries where emissions intensity of the industry and energy production is the highest. Under a global emissions trading scheme, allocation following the equal marginal abatement costs would determine domestic emissions reductions irrespective of countries’ equitable allocations. The difference between equitable allocations and equal marginal abatement costs allocation would determine the amount of emissions that is profitable to buy or sell for each country. However, IAM scenarios are currently limited to a regional resolution.

Other approaches Some other proposal allocate mitigation on a sectoral basis (den Elzen et al 2008), or combine multiple approaches (Höhne et al 2014, Raupach et al 2014, Meinshausen et al 2015). Additionally, some countries suggested metrics based on GDP intensity or progress compared to business as usual but did not provide a formula applicable to all countries (den Elzen et al 2009, BASIC experts 2011).

In a recent study, over 20 burden-sharing approaches have been modelled and applied consistently to a global emissions scenario consistent with 2 °C of warming (Pan et al 2015). However, this study does not present results for each equity approach individually. Instead it only presents results aggregated over all equity approaches that give 2030 allocations that vary by several times 2010 national emissions levels. Some of the equity approaches are modelled using different parameters and results are presented indistinctively as a wide range of emissions allocations at a given time (Pan et al 2015). Another study compared ranges of regional 2030 and 2050 mitigation targets, identified in over 40 studies and in line with a 2 °C warming limit, and grouped them in five equity categories:

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'capability', 'equality', 'responsibility-capability-need', 'equal cumulative per capita' and 'staged approaches' (Höhne et al 2014), also providing the basis for the IPCC-AR5 assessment (Clarke et al 2014). The variability of results within a category is due partly to the modelling and parameterisation used to represent the equity concept, and partly due to the value of the diversity of global emissions scenarios that each study distributes across countries (Figure 3.1). Moreover, different sets of global scenarios are used across the different equity categories, making comparison across categories difficult. In a given region, the range of results for a category tends to increase with the number of studies in that category (Höhne et al 2014). As an exception, the capability category features a greater variability than the equality or staged approaches categories that include a similar number of studies’ results. Across regions, variability is the greatest in developing countries, in particular in the Middle East and Africa. Despite the absence of consensus on effort-sharing, government representative and experts refer to a limited number of equity categories. A convergence to a smaller number of concepts of equity is expected to lead to greater cooperation and thus to greater global achievements (Haites, E., Yamin, F., Höhne 2013, Kallbekken et al 2014, Kowarsch and Edenhofer 2016). However, a common definition of equity is unlikely to be adopted since countries generally endorse approaches that match their negotiating positions (Averchenkova et al 2014, Fleurbaey et al 2014, Lange et al 2010).

Figure 3.1 (reproduced from IPCC-AR5 WGIII-Fig. 6.28) | Emission allowances in 2030 relative to 2010 emissions by effort-sharing category for mitigation scenarios 6. MAF is Middle-East and Africa, EIT is economies in transition, LAM stands for Latin America.

6

Figure 3.1 reproduced from IPCC-AR5 WGIII, Fig. 6.28, p. 460, from Clarke L., K. Jiang, K. Akimoto, M. Babiker, G. Blanford, K. FisherVanden, J.-C. Hourcade, V. Krey, E. Kriegler, A. Löschel, D. McCollum, S. Paltsev, S. Rose, P. R. Shukla, M. Tavoni, B. C. C. van der Zwaan, and D.P. van Vuuren, 2014: Assessing Transformation Pathways. In: Climate Change 2014: Mitigation of Climate Change. Contribution of

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3.2 Agreeing on climate equit y Most of the literature has focused on quantifying the allocations from a commonly agreed, or topdown, equity concept in the perspective of a legally binding agreement on effort-sharing (Höhne et al 2014). However, the lack of consensus on an effort-sharing approach led to the avoidance of negotiations on a binding equitable mitigation allocation at COP21 in Paris. The new hybrid architecture, a bottom-up pledge and review approach (Keohane and Oppenheimer 2016) to achieve a top-down warming threshold, of the Paris Agreement (Bodansky 2016, Aldy and Pizer 2016) is drawing on the failures of the Kyoto Protocol (UNFCCC 1998) and the Copenhagen Accord (UNFCCC 2009) to achieve comprehensive and efficient mitigation across countries.

All agreements include some top-down and bottom-up features by nature, and an agreement cannot be purely top-down (Andresen 2015, Jacquet and Jamieson 2016). The Kyoto Protocol (UNFCCC 1998) is often considered more top-down than the Copenhagen Accord (UNFCCC 2009) or Paris Agreement due to the binding nature of its emissions objectives (Bodansky and Diringer 2014, Jacoby and Chen 2014, Bodansky 2016), even though countries nominated their preferred targets, which were then inscribed in the Protocol. Overall, the Kyoto Protocol only reflected the ambition that developed countries were willing to deliver, and did not include long terms targets to stabilise global emissions. The more inclusive Copenhagen Accord (UNFCCC 2009), later ratified though the Cancún Agreements (UNFCCC 2010), offered more flexibility and resulted in broader participation but lower ambition from each party (Bodansky and Diringer 2014, Stavins R. et al 2014, Hare et al 2010). The Paris Agreement is designed to allow a bottom-up determination of national ambition for all countries under top-down rules applicable to all, in order to promote accountability and ambition (Bodansky 2016). Consequently, the literature that will inform the ratcheting-up process must inform on effortsharing approaches that are both consistent with the Paris global goals and inclusive of the multiple countries’ views on equity. Currently, the absence of mutual appreciation of the efforts underlying NDCs hinders countries’ ambition. A contribution considered fair by (and for) one country is likely to be considered insufficient under the equity principles of another country given that countries tend to favour visions of equity that are the most favourable for them (Averchenkova et al 2014, Lange et al 2010, Fleurbaey et al 2014). Since a country’s ambition often reflects the perceived level of commitment from other countries (many countries condition additional effort to greater global commitments), a country’s pledge is likely to remain lower than what it considers fair. Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Full legend: “Emission allowances in 2030 relative to 2010 emissions by effort-sharing category for mitigation scenarios reaching 430–480 ppm CO2eq in 2100. GHG emissions (all gases and sectors) in GtCO2eq in 1990 and 2010 were 13.4 and 14.2 for OECD-1990, 8.4 and 5.6 for EIT, 10.7 and 19.9 for ASIA, 3.0 and 6.2 for MAF, 3.3 and 3.8 for LAM. Emissions allowances are shown compared to 2010 levels, but this does not imply a preference for a specific base-year. For the OECD-1990 in the category ‘responsibility, capability, need’ the emission allowances in 2030 is -106% to -128% (20th to 80th percentile) below 2010 level (therefore not shown here). The studies with the ‘Equal cumulative per capita emissions’ approaches do not have the regional representation MAF. For comparison in orange: ‘Equal marginal abatement cost’ (allocation based on the imposition of a global carbon price) and baseline scenarios. Source: Adapted from (Höhne et al 2014). Studies were placed in this CO2eq concentration range based on the level that the studies themselves indicate. The pathways of the studies were compared with the characteristics of the range, but concentration levels were not recalculated.”

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Moreover, the preference of countries for visions of equity that favours them over other Parties means that the sum of national emissions targets considered fair by each party will by default be inconsistent with the global goals (Meinshausen et al 2015, Robiou du Pont et al 2017).

Recent research has designed assessments that combine multiple visions of climate justice. The combination of two approaches, grandfathering and equal per capita, was modelled using weighting factors (Raupach et al 2014, Peters et al 2015, Höhne et al 2017). Using an average (equal weighting factors), as suggested in these studies, does not necessarily represent an ethical middle of the road. The choice of weighting factors requires a value judgement (Kowarsch and Edenhofer 2016) and would not substantially change the current negotiations on equity. Alternatively, the online equity assessment tool Climate Action Tracker (Climate Action Tracker 2017) has derived a multi-approach combination from the five equity categories’ assessment used by the IPCC (Figure 3.1) plus modelling a ‘responsibility’ and a ‘capability/cost’ approach. For each country separately, the least stringent approach is removed and the remaining range of what is then considered fair is divided into six levels of ambition 7. Countries are rated as ‘Role model’, ‘1.5 °C Paris Agreement compatible’, ‘2 °C compatible’, ‘Insufficient’, ‘Highly insufficient’ and ‘Critically insufficient’ based on the number of effort-sharing approaches that pledge aligns with. Although this framework is inclusive of all desired concepts of climate equity, countries are assessed on equity concepts that may not reflect their own vision and practice of such equity. To address this gap, a report from the PBL Netherlands Environmental Assessment Agency has calculated the budgets for each country under three possible approaches (convergence per capita, equal carbon tax, equal relative costs) if the EU reduces its emissions by 40% over the 1990-2030 period (Hof et al 2012). The report shows what the world could do to stay below 2 °C when the EU applies its own approach. However, while this report combines the EU pledge with other approaches, it does not investigate the systematic combination of multiple equity principles. More comprehensively, a recent study allocated emissions to each country using the least stringent of two equity approaches (Meinshausen et al 2015). In this study, the global level of ambition under each approach is set by a ‘diversity-aware’ leader so that the sum of all countries’ allocations matches 2 °C-consistent levels. Under that ‘diversity-aware’ approach, equity approaches are applied to budgets leading to a warming lower than 2 °C. Consequently, countries follow different equity approaches that are applied to different global temperature goals, which may be considered unfair by other countries.

7

with methodological update on 19 September 2017

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3.3 Research gaps in the current literature Following the discussion of the current literature, several research gaps can be identified. This section details the research gaps that will be addressed in this thesis, as well as their relevance to the scientific community and decision makers. Research gaps identified in the literature 1) Overall, the literature derived formulae based on a wide range of effort-sharing approaches reflecting countries’ divergent preferences to achieve the 2 °C objective. However, no literature details allocations consistent with the 1.5 °C objective, or the Paris Agreement mitigation goals specifically. 2) A categorisation of the literature has identified a vast range in 2030 and 2050 emissions allocations, but cannot attribute unequivocally that variability to the modelling choices. At the start of this PhD in 2013, the literature has not consistently compared the modelling of the prominent equity concepts under a unique modelling framework. 3) While the inadequacy of national pledges with the globally agreed goal has been pointed out, there is to date no temperature or emissions assessment of the current uncoordinated situation whereby each country can pick its favoured equity concepts. 4) Recent literature has suggested diverse frameworks to combine dissonant equity concepts, but none has suggested a way consistent with a given global goal to let each country apply its own vision of equity. 5) To date, the literature has not assessed the global warming resulting from each countries’ climate pledge. Relevance of research gaps 1) The Paris Agreement represents a major advancement in climate negotiations and is expected to be the working framework for the coming decade or more. Its overarching mitigation goals – to limit global warming to well below 2 °C, pursue efforts to stay below 1.5 °C and achieve net-zero emissions in the second half of the century – are unlikely to change in the near term. The UNFCCC has invited the IPCC to produce a special report on the 1.5 °C goal in 2018 as specific literature is missing. Research on the feasibility of the 1.5 °C goal is critical for the existence of coastal vulnerable livelihoods (Schleussner et al 2016b). The compatibility of this PhD with the 1.5 °C goal is especially relevant for the upcoming IPCC special report on 1.5 °C (IPCC 2017). 2) A modelling representative of the IPCC equity categories under a unique framework will help negotiators understand the convergences and divergences following from their viewpoints. Countries can also then discuss the approaches parameterisation following the factors they judge important (period of historical responsibility, capability thresholds). Countries can then consistently measure the implications of the various views of fairness in terms of emissions allocations and agree to converge to a limited set of emissions allocations.

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3) Assessing a situation where each country follows the most favourable equity concept can inform decision makers on the implications – both in terms of emissions and global warming – of the absence of progress of the negotiations on mitigation. 4) The definition of national emissions targets consistent with the current bottom-up, or uncoordinated, situation where countries follow multiple views on equity, will offer a pragmatic solution (Kowarsch and Edenhofer 2016) to reconcile the Paris agreement goals with its bottom-up ‘pledge and review’ (Keohane and Oppenheimer 2016) architecture. 5) A temperature assessment of countries’ climate pledges would provide a novel and easy to understand metric to judge all countries emissions. This temperature metric would provide a simple assessment of the ambition of countries against the temperature goals of the UNFCCC. The simplicity of that scale would allow experts, decision makers, NGOs and civil society to measure the ambition of different countries to foster greater ambition.

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CHAPTER 4 – NATIONAL CONTRIBUTIONS FOR DECARBONIZING THE WORLD ECONOMY IN LINE WITH THE G7 AGREEMENT This chapter is published in Environmental Research Letters and is accessible as: Robiou du Pont Y., Jeffery L., Gütschow J., Meinshausen, M. 2016 National contributions for decarbonizing the world economy in line with the G7 agreement Environ. Res. Lett. 11, 054005 Online: http://dx.doi.org/10.1088/1748-9326/11/5/054005

Environ. Res. Lett. 11 (2016) 054005

doi:10.1088/1748-9326/11/5/054005

LETTER

OPEN ACCESS

National contributions for decarbonizing the world economy in line with the G7 agreement

RECEIVED

21 January 2016 REVISED

Yann Robiou du Pont1, M Louise Jeffery2, Johannes Gütschow2, Peter Christoff3 and Malte Meinshausen1,2 1

8 April 2016 ACCEPTED FOR PUBLICATION

11 April 2016

2 3

Australian-German Climate & Energy College, University of Melbourne, Parkville 3010, Victoria, Australia Potsdam Institute for Climate Impact Research (PIK), Telegraphenberg, D-14412 Potsdam, Germany School of Geography, University of Melbourne, Parkville 3010, Victoria, Australia

E-mail: [email protected]

PUBLISHED

28 April 2016

Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Keywords: climate change mitigation, UNFCCC, burden sharing, G7, equity, emissions allocation, mitigation pathways Supplementary material for this article is available online

Abstract In June 2015, the G7 agreed to two global mitigation goals: ‘a decarbonization of the global economy over the course of this century’ and ‘the upper end of the latest Intergovernmental Panel on Climate Change (IPCC) recommendation of 40%–70% reductions by 2050 compared to 2010’. These IPCC recommendations aim to preserve a likely (>66%) chance of limiting global warming to 2 °C but are not necessarily consistent with the stronger ambition of the subsequent Paris Agreement of ‘holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels’. The G7 did not specify global or national emissions scenarios consistent with its own agreement. Here we identify global cost-optimal emissions scenarios from Integrated Assessment Models that match the G7 agreement. These scenarios have global 2030 emissions targets of 11%–43% below 2010, global net negative CO2 emissions starting between 2056 and 2080, and some exhibit net negative greenhouse gas emissions from 2080 onwards. We allocate emissions from these global scenarios to countries according to five equity approaches representative of the five equity categories presented in the Fifth Assessment Report of the IPCC (IPCCAR5): ‘capability’, ‘equality’, ‘responsibility-capability-need’, ‘equal cumulative per capita’ and ‘staged approaches’. Our results show that G7 members’ Intended Nationally Determined Contribution (INDCs) mitigation targets are in line with a grandfathering approach but lack ambition to meet various visions of climate justice. The INDCs of China and Russia fall short of meeting the requirements of any allocation approach. Depending on how their INDCs are evaluated, the INDCs of India and Brazil can match some equity approaches evaluated in this study.

1. Introduction The G7 includes the world’s seven largest advanced industrial economies (here we include Canada, Japan, the United States, and the 28 EU countries that are represented by the European Commission within the G7). As a group, the G7 produced over 31% of international greenhouse gas (GHG) emissions in 2010–over 27% including Land-Use, Land-Use Change and Forestry (LULUCF) emissions (Gütschow 2015). The G7’s domestic mitigation efforts can therefore have a significant impact on climate change. Moreover, in producing over 65% of © 2016 IOP Publishing Ltd

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current global Gross Domestic Product (GDP) (World Bank 2014), the G7 has considerable capacity to fund and lead the transition to a zero carbon global economy. The G8 (G7 plus Russia) first recognized the need for emissions mitigation (Kirton et al 2011) in 1979 and in 1992 strongly supported the creation of the United Nations Framework Convention on Climate Change (UNFCCC) (Kirton and Kokotsis 2015, p 107). Five months before the Copenhagen Accord in 2009, the G8 endorsed Intergovernmental Panel on Climate Change (IPCC) recommendations to limit global warming to 2 °C and supported a global

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emissions mitigation target of 50% below 1990 levels by 2050 (G8 2009, p 4). Following the Elmau agreement (G7 2015, p 12) in June 2015, the G7 now supports ‘the upper end of the latest IPCC recommendation of 40%–70% reductions by 2050 compared to 2010’ (G7 2015), intended to preserve a likely (>66%) chance of attaining the UNFCCC objective to limit global warming to 2 °C (Edenhofer et al 2014, table 6.3). In Elmau, the G7 also emphasized the requirement of a ‘decarbonization of the global economy over the course of the century’ and committed to do their part to achieve a ‘transformation of the energy sector by 2050’. The objective of the G7 agreement was later strengthened, in Paris, where G7 members and all other UNFCCC parties agreed to ‘achieve a balance between anthropogenic emissions by sources and removals by sinks of GHG emissions in the second half of the century’ (which basically means net zero GHG emissions sometime between 2050 and 2100) and to hold ‘the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5 °C’. In this study, we interpret the Elmau agreement as a requirement to reduce global GHG emissions by 60%–70% between 2010 and 2050 and fully decarbonize by 2100 at the latest. We note that the term ‘decarbonization’ is ambiguous, as it can be defined as the process of lowering carbon intensity (Edenhofer et al 2014, sec Annex I, Glossary) rather than the endpoint of net zero CO2 emissions as often interpreted by the public and some governments (Hendricks 2015). We identify the seven cost optimal Integrated Assessment Model (IAM) scenarios from the IPCCAR5 database (https://tntcat.iiasa.ac.at/AR5DB/) consistent with our interpretation of the Elmau agreement (figure 1 and SI). We add to this set RCP2.6, the only one of the four Representative Concentration Pathways employed by the IPCCAR5 that offers a likely chance of limiting global warming to 2 °C (van Vuuren et al 2011). While just outside our interpretation of the Elmau criteria, with GHG emissions 57% below 2010 in 2050, RCP2.6 lies within the range of our selected scenarios for most of the century. These eight selected economically optimal scenarios result from mitigation policies starting in 2010 and 2020 and show global GHG emissions between +1% and −28% in 2025 compared to 2010 levels, and between −11% and −43% in 2030 (or between −12% and −55% for CO2). These mitigation targets are slightly more ambitious than the targets recommended by UNEP, namely −4% in 2025 compared to 2010, −14% by 2030 and −55% by 2050 (UNEP 2014), and are in line with the Paris decision target of 40 GtCO2eq for 2030 (figure 1). The eight selected scenarios reach net negative CO2 emissions between 2056 and 2080 and some reach net negative GHG emissions after 2080 (see SI). If nearterm targets lie at the least ambitious end of the range presented here, achieving the climate objective would

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require long-term targets to lie at the most ambitious end of the range. The question of how mitigation effort, or equivalently emissions rights, should be distributed between countries invokes complex and competing value judgements (Rose et al 1998, Ringius et al 2002, Bode 2004, Höhne et al 2006, Baer et al 2008, Jacoby et al 2008, Nabel et al 2011, Winkler et al 2011, Höhne et al 2013, Edenhofer et al 2014, chapter 6, Raupach et al 2014, Winkler and Rajamani 2014, Meinshausen et al 2015). In 2009, the G8 supported a differentiated 2050 target for developed countries of at least 80% compared to 1990 or more recent years (G8 2009, p 4) (figure S3 in supplementary information). At Elmau, the G7 stated its determination to adopt an agreement with legal force ‘applicable to all parties that is ambitious, robust, inclusive and reflects evolving national circumstances’ (G7 2015, p 12) but it has not subsequently provided near-term global targets or national emissions allocations consistent with the 2 °C goal. In the absence of international consensus on an effort-sharing approach, scientists and government representatives have employed a range of equity principles when modeling international emissions distributions consistent with holding global warming below 2 °C (Rose et al 1998, Baer et al 2008, den Elzen et al 2008, Jacoby et al 2008, Nabel et al 2011, Winkler et al 2011, Höhne et al 2013, Tavoni et al 2014, Raupach et al 2014, Pan et al 2015, Peters et al 2015, Meinshausen et al 2015). The IPCCAR5 grouped the regional 2030 mitigation targets of over forty studies into five categories according to distributive justice concepts associated with ‘capability’, ‘equality’, ‘responsibility-capability-need’, ‘equal cumulative per capita (CPC)’ and ‘staged approaches’ (Höhne et al 2013, Edenhofer et al 2014, figure 6.28). While the global GHG emissions scenarios of these studies result in the stabilized concentration levels (425–485 ppm CO2eq) required to have a medium chance (50%– 66%) of limiting warming to 2 °C, they do not generally follow trajectories that are—under certain conditions—deemed technologically feasible and economically optimal within IAM modeling worlds (Höhne et al 2013). Under the strict implementation of such a nonoptimal allocation approach, countries would have to engage in inter-temporal trading of emission permits (borrowing or banking) in order to achieve realistic mitigation trajectories that minimize aggregate economic costs. Such inter-temporal trading relies on governments that are stable and accountable over time, and on an emission trading system that allows countries to use or sell their future or past emissions permits. Such an arrangement does not appear to be on the horizon in the new post-2020 regime. In this study, we allocate to countries, on the basis of equity approaches, emissions trajectories that add up to global IAM emissions scenarios at any point in time. In addition to domestic mitigation, countries can use

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Figure 1. IAM scenarios from the IPCCAR5 that meet the G7 agreement for global emissions mitigation. The ranges of IAM emissions scenarios, including LULUCF emissions, that match the G7 vision for GHG (in blue) and for CO2 (in green) are shown with RCP2.6 GHG (red line) and CO2 (yellow line) emissions scenarios. The Elmau agreement is interpreted as a GHG emissions reduction of 60%–70% below 2010 levels by 2050 (blue interval) and net zero CO2 emissions (green dotted line) by the end of the century. The aggregate INDCs level (UNFCCC 2015c) is shown with the Paris decision 2030 goal and the UNEP recommendations for 2025, 2030 and 2050 (gray and black circles). The inset shows the seven selected scenarios (in blue), RCP2.6 (in red) selected out of the 846 IPCCAR5 database GHG scenarios (in gray).

new mechanisms to match their emissions allocation. The recent Paris Agreement and decision recognized the voluntary ‘use of internationally transferred mitigation outcomes towards nationally determined contributions’ and encouraged the implementation of ‘results-based payments [K] for the implementation of policy approaches’ (UNFCCC 2015a). While no comprehensive global emissions trading scheme is currently in place, countries can match the emissions allocations derived in this study through a combination of domestic mitigation and financial contributions. These financial contributions could be purchases of mitigation outcomes as part of a global emissions trading scheme or as contributions to global climate finance. An assumption on the pricing, in terms of a contribution to climate finance, of a certain emissions allocation is necessary to compare currents efforts with the allocations derived in this study. The conversion of financial contributions to emissions reductions is beyond the scope of this study. Our study derives national targets that are consistent with various interpretations of equity and with the aggregate global decarbonization trajectory laid out in the G7 agreement. We model the five IPCC allocation categories as follows (see SI for further details). The ‘capability’ (CAP) approach, from Jacoby et al (2008), allocates to each country a share of global emissions proportional to its population divided by its per capita GDP—or proportional to its GDP when global net emissions become negative. The ‘equal per capita’ (EPC) approach, reflecting the ‘equality’ IPCC category, allocates global emissions shares that are proportional to each country’s population. The

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‘Greenhouse Development Rights’ (GDR) approach proposed by Baer et al (2008), reflects the ‘responsibility-capability-need’ IPCC category, and allocates emissions shares based on the historical and projected business-as-usual emissions, the population and the wealth distribution of each country. The ‘equal cumulative per capita’ (CPC) approach allocates each country with total cumulative emissions in proportion to its cumulative population over a chosen period. Finally, the ‘constant emissions ratio’ (CER) approach preserves the relative distribution of GHG emissions across countries from the start of the allocation onwards. This status-quo approach, also referred to as the ‘grandfathering’ approach (Rose et al 1998, Müller and Höhne 2013) or ‘inertia’ (Peters et al 2015), is included in the ‘equality’ category by the IPCC, but is often considered less equitable than other approaches found in the literature (Caney 2009, Peters et al 2015).

2. Methods We used the Potsdam Real-time Integrated Model for the probabilistic Assessment of emission Paths (PRIMAP) (Nabel et al 2011) to model allocations approaches. This model contains a database with historical and projected data of: national GHG emissions, population and GDP purchase power parity. We used GHG emissions data from the PRIMAP database (Nabel et al 2011) that combines UNFCCC CRF inventories for Annex I (UNFCCC 2014) countries and EDGAR42 data for non-Annex I countries (European Commission 2009). Incomplete historical datasets are extrapolated in the past using the growth rates

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of CDIAC data (Boden et al 2012) for CO2 and MATCH data (Höhne et al 2010) for other GHGs. Country level emissions projections are obtained from downscaled Regional RCP emissions using the Shared Socioeconomic Pathway Two (SSP2) socioeconomic data (O’Neill et al 2015) and the assumption of exponential convergence of emission intensities within a region. We combined Kyoto-GHG (carbon dioxide, methane, nitrous oxide, perfluorocarbons, hydrofluorocarbons and sulphur hexafluoride) emissions following the ‘SAR GWP-100’ (Global Warming Potential for a 100 year time horizon) introduced in the Second Assessment Report of the IPCC and used under the UNFCCC. We selected from the 846 IAM GHG scenarios available in the IPCCAR5 database those consistent with the G7 Elmau agreement. Scenarios were selected based on their dynamic capacity to achieve emissions reductions of 60%–70% between 2010 and 2050 (including emissions from LULUCF) as well as to reach net zero CO2 emissions by the end of the 21st century (figure S1 in supplementary information). We then removed seven scenarios that showed emissions values almost equal ( 0.4

< 0.4

> 0.4

> 0.4

< 0.4

< 0.4

< 0.4

Overshoot (W/m²)

< 20

< 20

< 20

< 20

< 20

< 20

< 20

Neg. emissions (GtCO2/yr)

Delay 2020

Delay 2020

Delay 2020

Immediate

Immediate

Immediate

Immediate

Policy

No restriction

No restriction

No restriction

Restrictions

Restrictions

Restrictions

No restriction

Neg. emissions technology

1.7

N/A

1.7

1.7

1.6

1.7

N/A

76

N/A

75

78

83

78

N/A

Table S1 | Scenarios’ characteristics as from the IPCCAR5 database of the seven selected emissions scenarios. The columns ‘Max T(°C)’, which shows the maximum expected temperature before 2100, and ‘66%) chance of limiting warming to 2 ◦ C (Methods). We explore five ‘sets’ of GHG emissions scenarios based on this selection (Table 1): (i) 32 scenarios peaking by 2020 (‘2 ◦ C-pre2020peak’), (ii) 39 peaking by 2020 with a more likely than not (>50%) chance of returning to 1.5 ◦ C in 2100 (‘1.5 ◦ C-pre2020peak’), (iii) 6 scenarios peaking in 2030 (‘2 ◦ C-2030peak’), (iv) a custom ‘2 ◦ C-statedINDC’ scenario with interpolated emissions between 2030 pledged INDC levels3 and, from 2050 onwards, the average of the ‘2 ◦ C-2030peak’ scenarios, and (v) a ‘2 ◦ C-fairINDC’ scenario equal to global scenario (iv) but with allocations starting in 2010 (Fig. 1a). The ‘2 ◦ C-2030peak’ scenarios are only loosely consistent with the Paris Agreement

1

Australian-German Climate & Energy College, The University of Melbourne, Parkville 3010, Victoria, Australia. 2 Potsdam Institute for Climate Impact Research (PIK), Telegraphenberg, 14412 Potsdam, Germany. 3 Energy Program, International Institute for Applied Systems Analysis (IIASA) Schlossplatz 1, A-2361 Laxenburg, Austria. 4 Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland. 5 School of Geography, The University of Melbourne, Parkville 3010, Victoria, Australia. *e-mail: [email protected]

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186

LETTERS a

100 Emissions in GtCO2eq

GHG emissions in GtCO2eq

60

40

60 40 20 0

20

2010 2020 2030 2040 2050 2060 2070 2080 2090 Year

IND C’ =

b

2010

2020

2030

c

2040

2050 Year

d

USA

100

150

ge ran e erag C av 1.5 °

CAP Capability

2 °C-pre2020peak

1.5 °C-pre2020peak

Emissions in 2030 as percentages of 2010 levels

2000

h

0 −20

−50

−40

−100

EPC Equal per capita

OECD 100 Guide to g−k: 80 60 Selected 40 scenarios 20 a b 0 Aggregated (I)NDCs −20 −40 −60 −80 −100 a b a b a b a b a b

20

50

−100 2100

40

0

2 °C

2050 Year

60

100

−50

2050 Year

2100

2000

2050 Year

GDR Greenhouse development rights

Economies in transition

i

2100

−150

2000

2050 Year

2100

CPC Equal cumulative per capita

Asia Aggregated (I)NDCs

j

Middle East and Africa

a b abab ab ab

2000

2050 Year

2100

CER Constant emissions ratio

k

Latin America

Aggregated (I)NDCs Aggregated (I)NDCs

Aggregated (I)NDCs

a b aba b ab ab

(I)NDC

80 NDC

150

−50

G8 + China

100

200

0

2000

f

India

250

NDC

0

Guide to b−f:

2090

300

50

0

2080

350

50

50

2070

e

EU

NDC

100

2060

100

NDC

C’

Unconditional (low ambition) (I)NDCs assessment Average (I)NDCs assessment Conditional (high ambition) (I)NDCs assessment

Aggregated (I)NDCs: 52.5 GtCO2eq Paris decision goal: 40 GtCO2eq 1.5 °C average in 2030: 32.6 GtCO2eq

China

−50

‘2 ° C-f airI

ND

−20

Emissions allocations as percentages of 2010 levels

80

‘2 ° C-s tate d

0

g

Selected GHG emissions pathways

a b a b ab ab a b

ab a b abab a b

Figure 1 | Global, national and regional emissions consistent with the Paris Agreement and five equity principles compared with current pledges. a, IAM scenarios consistent with the Paris Agreement under ‘1.5 ◦ C-pre2020peak’ (red), ‘2 ◦ C-pre2020peak’ (blue) and ‘2 ◦ C-2030peak’ cases (purple), and their averages (thicker lines). Scenarios consistent with the 2030 Paris decision target (green circles) are more opaque. Inset, comparison with IPCC-AR5 database scenarios (grey lines). b–f, National emissions allocations excluding LULUCF compared with (I)NDCs (black circles). Coloured patches and lines show allocation ranges of global ‘2 ◦ C-pre2020peak’ scenarios, and averages over the range of global ‘1.5 ◦ C-pre2020peak’ scenarios, respectively. g–k, Regionally aggregated 2030 allocations for ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’ scenarios compared with aggregated (I)NDCs.

(Methods). Emissions allocations of all sets start in 2010, except for (iv), which starts in 2030 at national (I)NDCs levels. The ‘2 ◦ C-pre2020peak’ scenario set has a 2030 average of 39.7 GtCO2 eq, similar to the Paris decision indicative target of 40 GtCO2 eq, and becomes net zero as early as 2080 (Fig. 1a). The ‘1.5 ◦ C-pre2020peak’ set averages at 32.6 GtCO2 eq in 2030 and becomes negative between 2059 and 2087. Average annual global emissions reduction rates over the 2030–2050 period, as a fraction of 2010 levels, are 1.6% yr−1 for early-action ‘2 ◦ C-pre2020peak’ 2

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scenarios (reaching 2.1% yr−1 in 2025), 2.2% yr−1 for 1.5 ◦ C scenarios (reaching 2.3% yr−1 in 2039), and 3.2% yr−1 for delayed-action ‘2 ◦ C-2030peak’ scenarios (reaching 3.5% yr−1 from 2040 to 2050). The selected cost-optimal scenarios rely on the IAM’s assumptions of harmonized international policies and emissions trading systems that are currently not in place. However, the Paris Agreement has recognized the voluntary ‘use of internationally transferred mitigation outcomes towards nationally determined contributions’1 . The emissions allocations determined here

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186

LETTERS

Table 1 | Allocation approaches and global scenario set descriptions. Allocation code

Allocation name

IPCC category

Allocation characteristics

CAP EPC GDR CPC

Capability Equal per capita Greenhouse development rights Equal cumulative per capita

Capability Equality Responsibility–capability– need Equal cumulative per capita

CER

Constant emissions ratio

Staged approaches

High mitigation for countries with high GDP per capita. Convergence towards equal annual emissions per person. High mitigation for countries with high GDP per capita and high historical per capita emissions. High mitigation for countries with high historical per capita emissions. Maintains current emissions ratios.

Scenario set 1.5 ◦ C-pre2020peak

Scenario type 1.5 ◦ C scenarios

IPCC category 39 P1P2 scenarios

2 ◦ C-pre2020peak

2 ◦ C early-action scenarios

32 P1P2 scenarios

2 ◦ C-2030peak

2 ◦ C delayed-action scenarios

6 P3 scenarios

2 ◦ C-statedINDC

2 ◦ C delayed-action scenario

1 P3 custom scenario

2 ◦ C-fairINDC

2 ◦ C delayed-action scenario

1 P3 custom scenario

Scenario characteristics More likely than not (>50%) chance of returning to 1.5 ◦ C in 2100. Global emissions peaking by 2020. National emissions allocated from 2010 onwards. Likely (>66%) chance of staying below 2 ◦ C by 2100. Global emissions peaking by 2020. National emissions allocated from 2010 onwards. Likely (>66%) chance of staying below 2 ◦ C by 2100. Global emissions peaking in 2030. National emissions allocated from 2010 onwards. De facto likely (>66%) chance of staying below 2 ◦ C by 2100. Global emissions peaking in 2030. National emissions allocated from 2030 (I)NDC levels onwards. De facto likely (>66%) chance of staying below 2 ◦ C by 2100. Global emissions peaking in 2030. National emissions allocated from 2010 onwards.

The allocation framework modelling and parameterization follow those of ref. 24. More details on the scenario selection are in the Supplementary Methods.

Table 2 | Mitigation targets, timing of peaking and net-zero emissions, and emissions budgets of selected countries for the ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’ cases, averaged over the five equity allocations. Country

Goal

World

2 ◦C 1.5 ◦ C 2 ◦C 1.5 ◦ C 2 ◦C 1.5 ◦ C 2 ◦C 1.5 ◦ C 2 ◦C 1.5 ◦ C

China USA EU India

2030 change to 2010 levels (in %) −5 −33 −27 (−59 to 6) −48 (−71 to −19) −44 (−66 to −5) −64 (−80 to −33) −38 (−62 to −5) −62 (−84 to −33) 72 (−5 to 155) 30 (−33 to 102)

2050 change to 2010 levels (in %) −47 −78 −70 (−95 to −44) −88 (−102 to −76) −89 (−119 to −47) −109 (−144 to −78) −86 (−122 to −47) −106 (−149 to −78) 40 (−47 to 152) −24 (−78 to 63)

Peaking year 2020 Immediate Immediate Immediate Immediate Immediate Immediate Immediate 2033 2022

Net-zero year 2082 2075 2075 2065 2067 2057 2068 2057 2087 2081

Budget to 2050 in GtCO2 eq 1,523 1,134 329 254 154 109 114 80 162 122

Budget to 2100 in GtCO2 eq 1,749 1,156 345 237 104 57 94 54 236 161

Target ranges indicate the extrema across the five approaches’ averages. Emissions from LULUCF and bunkers are excluded. Data for all countries are available in the Supplementary Tables. Emissions budgets are accounted from 2010.

could be met through a combination of domestic mitigation, internationally traded emissions mitigation1 and international financial contributions toward global mitigation24 . Under any of our modelled equity approaches, the national emissions scenarios are not cost-optimal if applied domestically. However, they are consistent with a global cost-optimal scenario if countries choose the right mix of domestic mitigation and transfer of support for additional mitigation elsewhere. National mitigation costs are allocated indirectly through the allocation of emissions allowances. A fair distribution of mitigation costs could be used to derive equitable emissions allocations when comprehensive national-level mitigation cost estimates are available. We allocate to all countries GHG emissions scenarios that add up, under each of the five equity approaches (Supplementary Tables), to global cost-optimal IAM scenarios—excluding emissions from Land Use, Land-Use Change and Forestry (LULUCF), and international shipping and aviation (Methods).

At the regional level (Fig. 1g–k), Middle East and Africa’s aggregated (I)NDCs are consistent with all approaches except the constant emissions ratio (CER) under all scenario sets. Asia’s aggregated (I)NDCs are not consistent with any allocation under early-action scenarios, while the Organisation for Economic Cooperation and Development’s (OECD’s) are consistent with the greenhouse development rights (GDR) and CER under the ‘2 ◦ Cpre2020peak’ and with none under ‘1.5 ◦ C-pre2020peak’. Only the aggregated (I)NDCs of the Middle East and Africa are consistent with some 1.5 ◦ C allocations (with great disparities at the subregional level, Supplementary Discussion). At the national level (Fig. 1b–f), all equity approaches require China’s emissions to peak earlier and lower than its current NDC. The USA’s and the EU’s NDCs are in line with the CER allocation and just within the ‘2 ◦ C-pre2020peak’ range under the GDR. The EU’s NDC is also within the equal per capita (EPC) range. India’s NDC is consistent with the equal cumulative per capita (CPC) and

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186

LETTERS

k’

0 −50 −100

2 °C-statedINDC: 2030

n

‘2

tio

an

ga

th

iti

rm

Emissions change in percentage of 2010 levels

e ng ea

p 20 20 re -p °C ’ 1.5 eak r‘ p de 20 un 20 re -p °C

0 50 −100 −50 100 Emissions change over 2010−2030 in percentage of 2010 levels

2 °C-statedINDC: 2030

0

50 100

Emissions change in percentage of 2010 levels

−22%

2 °C-statedINDC: 2030

2090 2080 2070

2030 2020 2010

1.5 °C-pre2020peak

g 2040

−12y

−3y

2 °C-pre2020peak

2010 2020 2030 2040 Average year of peaking national emissions

−5y

−10y

2060 2 °C-pre2020peak

2060 2070 2080 2090 Average year of first negative national emissions

Average peaking emissions in percentage of 2010 levels

f

1.5 °C-pre2020peak

Average year of first negative national emissions

2 °C-fairINDC: 2030

+39%

−100 −50 0 50 100 Emissions change in percentage of 2010 levels

Average year of peaking national emissions

−60%

e Emissions change in percentage of 2010 levels

d

−46%

−100 −50

k’

2 °C-pre2020peak: 2030

−39%

c

ro

Brazil (−31%) Iran (−26%) China (−21%) EU28 (−23%) USA (−20%) Korea (−18%)

Russia (−19%)

−8%

−100 −50 0 50 100 Emissions change in percentage of 2010 levels

Vietnam (−35%) Indonesia (−37%)

St

Mexico (−35%) South Africa (−21%) ‘Major economies’ (G8 + China): −21%

−100

50

India (−42%)

0

−50

Emissions change in percentage of 2010 levels

‘Other economies’ −39%

ea

1.5 °C-pre2020peak: 2030

Philippines (−55%)

0p

Emissions change over 2010−2030 in percentage of 2010 levels

50

2 20 re -p °C k’ 2 r ‘ pea 0 de un 202 n e tio pr ga °Citi m ‘1.5 er n ng tha ro St

100

100

2 °C-pre2020peak: 2030

b

Regional colours

1.5 °C-pre2020peak: 2030

a

700 600

2 °C-pre2020peak

1.5 °C-pre2020peak

500 400 300 200 100 2010 2020 2030 2040 Average year of peaking national emissions

Figure 2 | Comparisons of national emissions change under different global goals. a–d, Relative changes between ‘1.5 ◦ C-pre2020peak’, ‘2 ◦ C-pre2020peak’, ‘2 ◦ C-statedINDC’ and ‘2 ◦ C-fairINDC’ cases over the 2010–2030 period (excluding LULUCF). e,f, Comparison of timing of first net-zero emissions and peaking national emissions averaged over the five equity approaches for the ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’ cases. g, Average of peaking emissions levels versus average peaking emissions years for ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’ cases. Disc sizes are proportional to 2010 emissions levels. Colours indicate world regions. G8+China (larger disc) and the rest of the world (smaller disc) are shown in grey.

EPC allocations of ‘2 ◦ C-pre2020peak’ scenarios, and the CPC allocation averaged over the ‘1.5 ◦ C-pre2020peak’ scenarios lies within the NDC assessment’s uncertainty range (other countries in Supplementary Tables and provided at: www.paris-equity-check.org). Combining multiple visions of equity—using weighting factors20 or a leadership-based approach22 —is not necessarily equitable by design but can represent a political compromise20 , and is useful to compare national allocations under different global goals or scenario sets. The fairness of the CER, or ‘grandfathering’, approach is criticized in the literature23,27 and not supported as such by any Party. 4

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However, we include CER in the average because it represents one of the five IPCC equity categories, stressing national circumstances regarding current emissions levels, and is implicitly followed by many of the developed countries23,24 . The average allocation of the EU and the USA becomes negative soon after mid-century under both the ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’ sets. China’s average allocation becomes negative 10 years later, and India’s only at the end of the century (Table 2). Recent studies using alternative implementation24 or modelling21,22 of similar equity approaches towards 2 ◦ C find

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186 a

LETTERS b

2 °C-pre2020peak

1.5 °C-pre2020peak Gap between conditional (I)NDCs and allocations Average CER Constant emissions CPC Equal cumulative

ratio

per capita

EPC Equal per capita GDR Greenhouse development

Pakistan Bangladesh Ethiopia

China Rest of the world

G8 and China

China

Global 1.5 °C mitigation gap: 20.4 GtCO2eq

Japan

Global 2 °C mitigation gap: 8.8 GtCO2eq

Russia

Average of 2 °C-pre2020peak scenarios: 40.1 GtCO2eq

EU28 India Russia Japan

Aggregated conditional (I)NDCs : 48.9 GtCO2eq

Aggregated conditional (I)NDCs : 48.9 GtCO2eq

40

Mitigation gap between aggregated conditional (I)NDCs and averaged 1.5 °C/2 °C scenarios

USA

50

EU28 India

2030 emissions levels aggregated over all countries in GtCO2eq

Rest of the world (‘other economies’)

USA

rights

CAP Capability

G8 & China (‘major economies’) 30 Average of 1.5 °C-pre2020peak scenarios: 28.5 GtCO2eq

Countries bar width proportional to 2010 emissions

Figure 3 | Gaps between equitable mitigation allocations and conditional (I)NDCs in 2030. a,b, Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2 ◦ C (a) or 1.5 ◦ C goals (b), reduce or increase the projected 2030 global emissions levels (excluding LULUCF and bunker emissions) compared with aggregated conditional (I)NDCs. Countries are sorted left to right in decreasing order of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated conditional (I)NDCs and the average scenarios consistent with the Paris 2 ◦ C or 1.5 ◦ C goals (grey bar) are shown in each panel.

significant differences in some national emissions allocations, but generally reach similar conclusions (Supplementary Discussion). Overall, literature focusing on CO2 emissions de facto ignores other GHGs20,23 , and often allocates carbon budgets20 impossible to compare with single-year (I)NDCs. Reflecting the global goals, equitable national allocations towards 1.5 ◦ C require earlier mitigation than for 2 ◦ C (Fig. 2, results per-approach in the Supplementary Discussion). To achieve the 1.5 ◦ C goal ‘major economies’ (G8 and China as a group) need to lower their 2030 emissions targets by an additional 21 percentage points relative to 2010 emissions, compared with the ‘2 ◦ C-pre2020peak’ case, and other countries (‘other economies’) altogether by 39 additional percentage points (Fig. 2a). However, increasing current (I)NDCs by these additional percentages would not result in fair contributions towards the 1.5 ◦ C goal. Indeed, the aggregated (I)NDCs of the ‘major economies’ should already be 39 percentage points more stringent than they currently are to be in line with their averaged allocation under the ‘2 ◦ C-pre2020peak’

case (Fig. 2b). In contrast, the aggregated (I)NDCs of the ‘other economies’ are only 8 percentage points above ‘2 ◦ C-pre2020peak’ average allocations. Consequently, pledges in line with the 1.5 ◦ C goal should be respectively 60 and 46 percentage points more stringent than current (I)NDCs for ‘major economies’ and ‘other economies’ respectively (Fig. 2c). To compare the relative fairness of (I)NDCs under the current global ambition (52.5 GtCO2 eq for 2030), we compare (I)NDCs (‘2 ◦ C-statedINDC’ set) with the ‘2 ◦ C-fairINDC’ allocations (Fig. 2d). We find that the (I)NDCs of ‘other economies’, the USA, and the EU are more ambitious or aligned with their average allocation under current international 2030-ambition, while the (I)NDCs of Canada, Japan, and especially Russia and China are substantially less ambitious. Emissions budgets and timings of when peaking or net-zero emissions may constitute more easily actionable targets than temperature goals28 . Figure 2e–g compares the average timing of when emissions allocations peak or reach net zero under the five

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LETTERS

NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186

equity approaches for ‘1.5 ◦ C-pre2020peak’ and ‘2 ◦ C-pre2020peak’. Net-zero emissions are allocated five years earlier for 1.5 ◦ C for developing countries, and ten years earlier for developed countries (that is, around 2055–2060). Developing countries’ allocations peak about ten years earlier and up to 40% lower for 1.5 ◦ C than 2 ◦ C, which implies lower domestic emissions or lower revenues from emissions trading. Overall, aiming at 1.5 ◦ C rather than towards 2 ◦ C requires earlier but not faster or deeper mitigation at the national level (Supplementary Discussion). The lower emissions end of our (I)NDC quantification (‘highambition’ target) is set by the conditional targets and sometimes by the quantification uncertainty. Hence, in most countries, these ‘high-ambition’ targets have implicitly been identified as feasible. The implementation of these ‘high-ambition’ (I)NDCs3 would lead to 2030 emissions of 48.9 GtCO2 eq and leave an 8.8 GtCO2 eq gap with the average of ‘2 ◦ C-pre2020peak’ scenarios and a 20.4 GtCO2 eq gap with the ‘1.5 ◦ C-pre2020peak’ average (excluding LULUCF and bunkers emissions, Methods). The aggregated ‘highambition’ (I)NDCs of ‘other economies’ are collectively slightly more ambitious than the average of their 2 ◦ C allocations (Fig. 3 and Supplementary Discussion), although some individual (I)NDCs are less ambitious (for example, Iran, Saudi-Arabia and Turkey). Therefore, the ‘other economies’ altogether could meet their average ‘fair’ allocation by increasing their current unconditional contribution to the aggregate level of their conditional (I)NDCs. The average ‘fair’ allocations of ‘major economies’ is 9.6 GtCO2 eq below their current aggregated ‘high-ambition’ (I)NDCs. Put simply, the average ‘fair’ allocation of ‘major economies’ alone closes the global 2030 mitigation gap to 2 ◦ C, provided that other countries achieve their ‘high-ambition’ (I)NDC targets. Closing the 2030 gap to average ‘1.5 ◦ C-pre2020peak’ scenarios requires most countries to increase their ambition beyond their current conditional (I)NDCs. Current aggregate (I)NDCs fall substantially short of meeting either the 2 ◦ C or 1.5 ◦ C goals2,4 . The ratchet mechanisms established by the Paris Agreement1 need to achieve an additional 13 GtCO2 eq reduction in 2030 to align with 2 ◦ C cost-optimal scenarios, and 20 GtCO2 eq for 1.5 ◦ C (Fig. 1a). We derived ‘Equitably Determined Contributions’ consistent with the five IPCC equity approaches towards 2 ◦ C or 1.5 ◦ C goals (Supplementary Tables). Averaging across the five concepts of equity assigns the effort required to close the gap between current conditional (I)NDCs and the 2 ◦ C goal solely to the G8 and China. Equitably meeting the 1.5 ◦ C goal, and avoiding the additional climate impacts of a 2 ◦ C warmer world29 , means that almost all national contributions should be enhanced substantially, with key milestones, such as peaking or reaching netzero emissions, brought forward by a decade or more.

6. Averchenkova, A., Stern, N. & Zenghelis, D. Taming the Beasts of ‘Burden-Sharing’: An Analysis of Equitable Mitigation Actions and Approaches to 2030 Mitigation Pledges (Centre for Climate Change Economics and Policy, Grantham Research Institute on Climate Change and the Environment, 2014). 7. Mace, M. J. Mitigation commitments under the Paris Agreement and the way forward. Clim. Law 6, 21–39 (2016). 8. Voigt, C. & Ferreira, F. Differentiation in the Paris Agreement. Clim. Law 6, 58–74 (2016). 9. Impact Assessment: Document Accompanying the Package of Implementation Measures for the EU’s Objectives on Climate Change and Renewable Energy for 2020 Proposals (Commission of the European Communities, 2008). 10. Information, Views and Proposals on Matters Related to the Work of Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP) Workstream 1 (Submission by Japan, 2014). 11. Nepal on Behalf of the Least Developed Countries Group Views and Proposals on the Work of the Ad Hoc Working Group on the Durban Platform for Enhanced Action (ADP)(2014). 12. BASIC experts, Equitable Access to Sustainable Development: Contribution to the Body of Scientific Knowledge (BASIC Expert Group, 2011). 13. Submitted INDCs (UNFCCC, accessed 5 February 2016); http://www4.unfccc.int/submissions/indc 14. Baer, P., Fieldman, G., Athanasiou, T. & Kartha, S. Greenhouse Development Rights: towards an equitable framework for global climate policy. Camb. Rev. Int. Aff. 21, 649–669 (2008). 15. den Elzen, M., Höhne, N. & Moltmann, S. The Triptych approach revisited: a staged sectoral approach for climate mitigation. Energy Policy 36, 1107–1124 (2008). 16. Jacoby, H. D., Babiker, M. H., Paltsev, S. & Reilly, J. M. Sharing the Burden of GHG Reductions (MIT, 2008). 17. Nabel, J. E. M. S. et al. Decision support for international climate policy - The PRIMAP emission module. Environ. Model. Softw. 26, 1419–1433 (2011). 18. Höhne, N., den Elzen, M. & Escalante, D. Regional GHG reduction targets based on effort sharing: a comparison of studies. Clim. Policy 14, 122–147 (2013). 19. Tavoni, M. et al. Post-2020 climate agreements in the major economies assessed in the light of global models. Nat. Clim. Change 5, 119–126 (2014). 20. Raupach, M. R. et al. Sharing a quota on cumulative carbon emissions. Nat. Clim. Change 4, 873–879 (2014). 21. Pan, X., Teng, F., Tian, Y. & Wang, G. Countries’ emission allowances towards the low-carbon world: a consistent study. Appl. Energy 155, 218–228 (2015). 22. Meinshausen, M. et al. National post-2020 greenhouse gas targets and diversity-aware leadership. Nat. Clim. Change 5, 1098–1106 (2015). 23. Peters, G. P., Andrew, R. M., Solomon, S. & Friedlingstein, P. Measuring a fair and ambitious climate agreement using cumulative emissions. Environ. Res. Lett. 10, 105004 (2015). 24. Robiou du Pont, Y., Jeffery, M. L., Gütschow, J., Christoff, P. & Meinshausen, M. National contributions for decarbonizing the world economy in line with the G7 agreement. Environ. Res. Lett. 11, 054005 (2016). 25. Clarke, L. et al. in Climate Change 2014: Mitigation of Climate Change (eds Edenhofer, O. et al.) 456–462 (IPCC, Cambridge Univ. Press, 2014). 26. Rogelj, J. et al. Energy system transformations for limiting end-of-century warming to below 1.5 ◦ C. Nat. Clim. Change 5, 519–527 (2015). 27. Caney, S. Justice and the distribution of greenhouse gas emissions. J. Glob. Ethics 5, 125–146 (2009). 28. Geden, O. An actionable climate target. Nat. Geosci. 9, 340–342 (2016). 29. Schleussner, C.-F. et al. Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 ◦ C and 2 ◦ C. Earth Syst. Dyn. Discuss. 6, 2447–2505 (2016).

Methods Methods and any associated references are available in the online version of the paper. Received 4 July 2016; accepted 21 November 2016; published online 19 December 2016; corrected after print 1 February 2017

1. Adoption of the Paris Agreement FCCC/CP/2015/L.9/Rev.1 (UNFCCC, 2015); http://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf 2. Synthesis Report on the Aggregate Effect of the Intended Nationally Determined Contributions FCCC/CP (UNFCCC, 2015). 3. Meinshausen, M. & Alexander, R. INDC Factsheets. Australian-German Climate and Energy College (2015); http://climatecollege.unimelb.edu.au/ indc-factsheets 4. Rogelj, J. et al. Paris Agreement climate proposals need a boost to keep warming well below 2 ◦ C. Nature 534, 631–639 (2016). 5. United Nations Framework Convention on Climate Change FCCC/INFORMAL/84 (UNFCCC, 1992); https://unfccc.int/resource/docs/ convkp/conveng.pdf

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We gratefully acknowledge the work of modellers behind the IPCC-AR5 emissions scenarios. M. Meinshausen is supported by the Australian Research Council (ARC) Future Fellowship (grant number FT130100809). Deep thanks to A. Talberg for her comments on the manuscript.

Author contributions

References

6

Acknowledgements

All authors contributed to discussing the results and writing the manuscript. Y.R.d.P. led the study and performed the calculations. M.L.J. modelled the GDR approach. J.G. downscaled to the national-level global RCP8.5 emissions scenarios using SSP data. Y.R.d.P. and M.M. suggested the study. J.G., M.L.J. and M.M. updated and managed the composite PRIMAP database.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to Y.R.d.P.

Competing financial interests The authors declare no competing financial interests.

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186 Methods The equitable emissions allocations of all countries are included in the Supplementary Tables in the online version of the paper and can be visualized at: www.paris-equity-check.org. Scenario selection. We selected global emissions scenarios from the IPCC-AR5 database (hosted at the International Institute for Applied Systems Analysis and available at: https://tntcat.iiasa.ac.at/AR5DB) and ref. 26 that feature negative GHG emissions by the end of the century and a chance higher than 66% of limiting global warming to 2 ◦ C over the entire twenty-first century, or higher than 50% of returning to 1.5 ◦ C in 2100, compared with pre-industrial levels. IPCC-AR5 scenarios. The temperature likelihood response to 524 of these 846 Kyoto-GHG scenarios from the IPCC-AR5 database was projected using the simple carbon cycle and climate model MAGICC630,31 , under a probabilistic set-up32 (data visualization available at: https://www.pik-potsdam.de/ paris-reality-check/ar5-scenario-explorer). First, we selected from the database 155 scenarios that have net negative emissions in 2100. Of these 155 scenarios, a sub-selection was made of the 40 scenarios with a likely (>66%) chance of staying below 2 ◦ C throughout the twenty-first century. Of these 40 scenarios, 2 had a more likely than not (>50%) chance of resulting in a warming below 1.5 ◦ C in 2100. The numbers of scenarios matching each or a combination of these three criteria—negative emissions in 2100, 2 ◦ C (>66% over 2010–2100) and 1.5 ◦ C (>50% in 2100)—are shown in Supplementary Table 1 (Supplementary Information). All the selected scenarios that have a more likely than not chance of warming being below 1.5 ◦ C in 2100 also have a likely chance of remaining below 2 ◦ C over the 2010–2100 period. Only 2 of the 5 scenarios that have a more likely than not chance of being below 1.5 ◦ C in 2100 also have negative emissions in 2100. The model and study names of these scenarios are shown in Supplementary Table 2. The ‘2 ◦ C-2030peak’ scenarios have higher emissions levels than the ‘2 ◦ C-pre2020peak’ but still have a likely chance of limiting warming to 2 ◦ C and do not result in higher maximal temperatures over the century. However, these ‘2 ◦ C-2030peak’ scenarios are from the MERGE-ETL_ 2011 model (Supplementary Information) that uses exogenous sulfate forcing33 and feature higher SO2 —an aerosol with a cooling effect—concentrations than other IPCC-AR5 Working Group 3 scenarios34 . These aerosol emissions are outside the ranges consistent with the underlying CO2 path35 . Moreover, the ‘2 ◦ C-2030peak’ scenarios do not peak as soon as possible, as defined in Article 2 of the Paris Agreement1 . Additional 1.5 ◦ C scenarios. To this selection of 40 IPCC-AR5 scenarios, we added the 37 scenarios from ref. 26 that have a more likely than not (>50%) chance of having warming below 1.5 ◦ C in 2100. All of these scenarios have negative emissions in 2100. These 37 scenarios are from the MESSAGE or REMIND modelling frameworks and the scenario names and descriptions are available in Table 4 of the Supplementary Information of ref. 26. The average of all selected 1.5 ◦ C scenarios that peak between 2010 and 2020 is 32.6 GtCO2 eq in 2030. The UNEP gap report36 identified a 39 GtCO2 eq goal for 2030, which corresponds to the median of the 1.5 ◦ C scenarios (from the same source as our study) with emissions peaking only in 2020. (I)NDC scenario. In addition to the selected emissions scenarios, we construct a global emissions scenario that is in line with current aggregated (I)NDC targets. Between 2010 and 2030, this global ‘2 ◦ C-statedINDC’ scenario follows the global emissions from the ‘(I)NDC factsheets’3 (for ‘high-ambition’ or ‘low-ambition’ assessments, and the average of both), which include emissions projections of all countries, national Land Use, Land-Use Change and Forestry (LULUCF), and international shipping and aviation emissions (‘bunker emissions’) until 2030. Beyond 2030, the global ‘2 ◦ C-statedINDC’ emissions are a 20-year linear interpolation to reach the level of the average of the global ‘2 ◦ C-2030peak’ scenarios (including LULUCF emissions). Beyond 2050, the global ‘2 ◦ C-statedINDC’ scenario follows the average of global ‘2 ◦ C-2030peak’ scenarios. The ‘2 ◦ C-statedINDC’ scenario is expected to have a likely chance of limiting global warming to 2 ◦ C—with the same limitations regarding SO2 concentrations as the ‘2 ◦ C-2030peak’ scenarios. Indeed, the ‘2 ◦ C-statedINDC’ scenario (whether it follows the INDC’s ‘high-ambition’, ‘low-ambition’ assessments, or the average of both) has lower emissions than the average of ‘2 ◦ C-2030peak’ scenarios until 2050, and is equal to the average of ‘2 ◦ C-2030peak’ scenarios beyond 2050 (see Fig. 1). Scenario preparation. We used the Potsdam Real-time Integrated Model for the probabilistic Assessment of emission Paths (PRIMAP)17 to model allocations approaches. This model contains population, GDP, and GHG emissions historical and projected data from composite sources as detailed in ref. 24. Kyoto-GHG emissions are aggregated following the ‘SAR GWP-100’ (Global Warming Potential for a 100 year time horizon) as reported in the Second Assessment Report of the IPCC37 and used under the UNFCCC.

LETTERS All these global scenarios, shown in Fig. 1a, are harmonized to the PRIMAP17 database’s 2010 emissions of 47.7 GtCO2 eq (including LULUCF, and international shipping and aviation emissions). To do so, emissions are multiplied by a vector that is an interpolation between the 2010 PRIMAP emissions levels divided by the respective 2010 scenarios values, and 1 in 2040 (refs 24,37). In this study, we allocate emissions of ‘bunker-free’ scenarios that are in line with the global scenarios selected and constructed as described above, and that exclude LULUCF emissions as follows. Emissions of the LULUCF sector are not considered by all parties as part of the emissions scope to be negotiated. Moreover, no universal accounting method of positive or negative LULUCF emissions is currently in place. Therefore, we exclude LULUCF emissions from the global scenarios before allocating their emissions across countries. For the IPCC-AR5 scenarios, we excluded the corresponding LULUCF emissions. For the 37 1.5 ◦ C scenarios of ref. 26, where no specific LULUCF emissions were available, we excluded the CO2 emissions that do not come from fossil fuels combustion. We then subtracted from these IPCC-AR5 and ref. 26 scenarios international shipping and aviation bunker emissions from the QUANTIFY project38 coherent with the IPCC-SRESB1 scenario that limits global warming to 1.8 ◦ C compared with the 1980–1999 average24,39 . Shipping emissions are 3.9 times higher in 2100 compared with 2010 levels, and aviation emissions double over that same period, but peak in 2062. While the mitigation targets agreed in Article 4 apply to all GHGs, the Paris Agreement contains no specific reference to bunker emissions. The lack of current policies does not leave ground to project strong mitigation scenarios40,41 . Lower emissions from this sector would reduce the mitigation burden on all countries. We also constructed a version of the ‘2 ◦ C-statedINDC’ without bunker and LULUCF emissions following the methodology employed to construct the ‘2 ◦ C-statedINDC’ scenario that includes bunker and LULUCF emissions. This bunker-free ‘2 ◦ C-statedINDC’ emissions scenario is the sum of all national emissions from ref. 3 over the 2010–2030 period. Beyond 2030, the bunker-free ‘2 ◦ C-statedINDC’ emissions follow a 20-year linear interpolation to reach the level of the 2050 average of the bunker-free ‘2 ◦ C-2030peak’ scenarios (excluding bunker and LULUCF emissions). Beyond 2050, the bunker-free ‘2 ◦ C-statedINDC’ scenario follows the average of the bunker-free ‘2 ◦ C-2030peak’ scenarios. The bunker-free ‘2 ◦ C-statedINDC’ scenario is allocated across countries using our allocation framework from 2030 onwards, when countries have the emission level of their (I)NDC target3 . The ‘2 ◦ C-fairINDC’ global scenario is equal to the ‘2 ◦ C-statedINDC’ scenario, both with and without LULUCF and bunker emissions. At the national level, the emissions allocation of the ‘2 ◦ C-fairINDC’ scenario begins in 2010 and therefore differs from the national emissions of the ‘2 ◦ C-statedINDC’ scenario. All these bunker-free scenarios are harmonized to the PRIMAP17 database’s 2010 emissions of 42.5 GtCO2 eq (excluding LULUCF, international shipping and aviation emissions). To do so, national emissions are multiplied by a vector that is an interpolation between the 2010 PRIMAP national emissions levels divided by the respective 2010 bunker-free scenarios values, and 1 in 2040 (refs 24,37). These bunker-free scenarios, excluding LULUCF and international shipping and aviation bunker emissions, are shown in Supplementary Fig. 1 (Supplementary Information). The allocation of the scenarios’ bunker-free emissions follows the methodology and the parameterization described in the Supplementary Information of ref. 24. The only exception is the ‘2 ◦ C-statedINDC’ case whose allocation starts in 2030, starting at estimated national (I)NDC levels. All other cases have emissions allocations starting in 2010 at national historical levels17 . The GDR allocation approach requires business-as-usual emissions projections. We use Representative Concentration Pathway (RCP)8.5, downscaled using the SSP2 scenario (https://tntcat.iiasa.ac.at/SspDb) from the Shared Socioeconomic Pathways framework42,43 . More details are available in ref. 24. The business-as-usual emissions projections used in the ‘2 ◦ C-statedINDC’ case follow the growth rates of RCP8.5 over the period 2030–2100, starting at national (I)NDC levels in 2030. The modelling and the parameterization of the equity approaches follow those of a previous study24 . Notably, a 30-year linear transition period is implemented between national 2010 emissions and the allocations under the capability (CAP) and EPC approaches. Therefore, in 2030 this transition period still slightly favours countries with allocations lower than their 2010 levels—usually developed countries—and slightly disfavours countries with allocations higher than their 2010 levels. Historical emissions are accounted since 1990 under the GDR and CPC approaches. The CPC approach applies a 1.5% annual discount rate to emissions before 2010 and achieves equal cumulative per capita emissions in 2100. The GDR approach allocates emissions reduction, compared with business-as-usual scenarios, to country’s citizens earning over US$7,500 (in purchase power parity) annually. The distribution of regional mitigation action as represented in least-cost mitigation pathways is not necessarily equitable. Our results show how pathways that achieve the global Paris Agreement mitigation goals at lowest cost can be aligned with equity principles at the national scale.

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3186

LETTERS The 2030-(I)NDC assessment in this study is an average of the ‘high-ambition’ and ‘low-ambition’ cases from ref. 3, except in Fig. 3, which uses the ‘high-ambition’ (I)NDC assessment. The ‘high-ambition’ assessment uses conditional (I)NDCs when available as well as the most ambitious end of the uncertainty associated with the (I)NDC assessment (based on GDP, population, energy demand projections). The ‘low-ambition’ assessment reflects the lower ambitions end of the uncertainty associated with the assessment of unconditional (I)NDCs. The assessments used in this study2,3 are based on original (I)NDCs, before their conversion to NDCs. Countries with missing data. Deriving the CAP and GDR allocations requires national projections of GDP. The PRIMAP database does not contain such projections for all countries due to a lack of available data. Countries with some missing data (‘missing countries’ whose ISO-Alpha 3 country codes are: ‘AFG’, ‘AGO’, ‘ALB’, ‘AND’, ‘ARE’, ‘ATG’, ‘COK’, ‘DMA’, ‘FSM’, ‘GRD’, ‘KIR’, ‘KNA’, ‘LIE’, ‘MCO’, ‘MHL’, ‘MMR’, ‘MNE’, ‘NIU’, ‘NRU’, ‘PLW’, ‘PRK’, ‘QAT’, ‘SMR’, ‘SSD’, ‘SYC’, ‘TUV’, ‘ZWE’) are mostly developing countries whose emissions allocation could represent a significant fraction of global 2030 emissions, under the CAP allocation in particular given their low GDP per capita (https://www.imf.org/external/pubs/ft/ weo/2015/01/weodata/download.aspx). We excluded the countries with missing data from the allocations and the remaining countries share the global ‘bunker-free’ scenarios’ emissions. Figure 3 displays the aggregated conditional (I)NDCs excluding these ‘missing countries’. As a consequence, the mitigation gaps between the aggregated (I)NDCs and the aggregated average allocations are affected by the exclusion of countries’ 2030 (I)NDC emissions (and is greater or smaller depending on how the sum of average allocations of these countries would compare with the sum of their conditional (I)NDCs). The gap between that sum of all countries’ conditional (I)NDCs—49.8 GtCO2 eq including the ‘missing countries’ (51.4 GtCO2 eq with bunker emissions), excluding LULUCF emissions—and the sum of available average allocations—40.1 GtCO2 eq—would be 9.6 GtCO2 eq instead of 8.8 GtCO2 eq. As a reminder, the gap between the ‘major economies’ (G8 plus China) aggregated conditional (I)NDCs and their aggregated allocation is 9.6 GtCO2 eq. The conclusions derived from Fig. 3 are still valid in this configuration. Note that the aggregate level of all ‘high-ambition’ (I)NDCs including LULUCF emissions (including the ‘missing countries’) is 49.4 GtCO2 eq, and 47.8 GtCO2 eq excluding bunker emissions.

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References 30. Meinshausen, M., Raper, S. C. B. & Wigley, T. M. L. Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6 – Part 1: Model description and calibration. Atmos. Chem. Phys. 11, 1417–1456 (2011). 31. Meinshausen, M., Wigley, T. M. L. & Raper, S. C. B. Emulating atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6 Part 2: Applications. Atmos. Chem. Phys. 11, 1457–1471 (2011). 32. Meinshausen, M. et al. Greenhouse-gas emission targets for limiting global warming to 2 ◦ C. Nature 458, 1158–1162 (2009). 33. Harmsen, M. et al. How well do integrated assessment models represent non-CO2 radiative forcing? Climatic Change 565–582 (2015). 34. Bernie, D. & Lowe, J. Analysis of Climate Projections from the IPCC Working Group 3 Scenario Database (AVOID2, 2014). 35. Rogelj, J. et al. Air-pollution emission ranges consistent with the representative concentration pathways. Nat. Clim. Change 4, 446–450 (2014). 36. The Emission Gap Report 2015 A UNEP Synthesis Report (UNEP, 2015). 37. Meinshausen, M. et al. The RCP greenhouse gas concentrations and their extensions from 1765 to 2300. Climatic Change 109, 213–241 (2011). 38. Owen, B., Lee, D. S. & Lim, L. Flying into the future: aviation emissions scenarios to 2050. Environ. Sci. Technol. 44, 2255–2260 (2010). 39. IPCC, Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) (Cambridge Univ. Press, 2007). 40. Cames, M., Graichen, J., Siemons, A. & Cook, V. Emission Reduction Targets for International Aviation and Shipping (European Parliament - Policy Department, 2015). 41. Anderson, K. & Bows, A. Executing a Scharnow turn: reconciling shipping emissions with international commitments on climate change. Carbon Manag. 3, 615–628 (2012). 42. Samir, K. C. & Lutz, W. The human core of the shared socioeconomic pathways: population scenarios by age, sex and level of education for all countries to 2100. Glob. Environ. Change http://dx.doi.org/10.1016/j.gloenvcha.2014.06.004 (2014). 43. Crespo Cuaresma, J. Income projections for climate change research: a framework based on human capital dynamics. Glob. Environ. Change http://dx.doi.org/10.1016/j.gloenvcha.2015.02.012 (2015).

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NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3210

ARTICLES

Corrigendum: Equitable mitigation to achieve the Paris Agreement goals Yann Robiou du Pont, M. Louise Jeffery, Johannes Gütschow, Joeri Rogelj, Peter Christoff and Malte Meinshausen Nature Climate Change 7, 38–43 (2017); published online 19 December 2016; corrected after print 1 February 2017. In Fig. 1c of the original version of this Letter, the 2030 assessment of the NDC for the USA was misplotted. This changed the number of equity approaches that the USA’s NDC was in line with. The figure and text that referred to the findings of that figure have been updated.

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. d e v r e s e r s t h g i r l l A . e r u t a N r e g n i r p S f o t r a p , d e t i m i L s r e h s i l b u P n a l l i m c a M 7 1 0 2 ©

SUPPLEMENTARY INFORMATION DOI: 10.1038/NCLIMATE3186

In the format provided by the authors and unedited.

Equitable mitigation to achieve the Paris Agreement goals Yann Robiou du Pont1*, M. Louise Jeffery2, Johannes Gütschow2, Joeri Rogelj3,4, Peter Christoff5, Malte Meinshausen1,2

This supplementary Information provides figures and tables to support the Methods section of the study (section 1), includes a comparison with the literature and a discussion of additional results (section 2), and indicates how to read the Supplementary Tables (section 3).

An interactive data visualization is available online at: www.paris-equity-check.org

*Corresponding author: Yann Robiou du Pont, [email protected] 1 Australian-German Climate & Energy College, University of Melbourne, Parkville 3010, Victoria, Australia. 2 Potsdam Institute for Climate Impact Research (PIK), Telegraphenberg, 14412 Potsdam, Germany 3 Energy Program, International Institute for Applied Systems Analysis (IIASA) Schlossplatz 1, A-2361 Laxenburg, Austria. 4 Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland 5 School of Geography, University of Melbourne, Parkville 3010, Victoria, Australia.

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© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

1

Contents 1. Methods’ supplementary material ............................................................................................. 3 2. Supplementary Discussion .......................................................................................................... 5 2.1 Comparison with literature ................................................................................................... 5 2.2 Mid-century mitigation rates ................................................................................................ 7 2.3 Per approach mitigation rates............................................................................................... 9 2.4 Per approach peaking emissions and phase-out year ........................................................ 11 2.5 Annual mitigation rates and minimal allocations ............................................................... 13 2.6 Effect of national allocation on global 2030 levels ............................................................. 14 2.7 Near and mid-term regional and sub-regional ................................................................... 18 3. Supplementary Tables .............................................................................................................. 22

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1. Methods’ supplementary material Supplementary Table 1, Supplementary Table 2 and Supplementary Figure 2 support the content of the Methods section of the main article.

Supplementary Table 1 | Number of IPCCAR5 scenarios and combinations of features.

Scenario features Negative emissions in 2100

Negative emissions Below 2°C (≥66% Below 1.5°C (>50% in in 2100 over 2010-2100) 2100) 155 40 2

Below 2°C (>=66% over 20102100) Below 1.5°C (>50% in 2100)

77

5 5

Supplementary Table 2 | Number of IPCCAR5 scenarios, models, and studies used in each of the ‘1.5°C-pre2020peak’, ‘2°Cpre2020peak’ and ’2°C-2030peak’ cases.

Case

# of scenarios

Model

Study

2°C-2030peak

6

MERGE-ETL_2011

AMPERE2-450

2°C-pre2020peak

32

1.5°C-pre2020peak

2

GCAM 3.1, MERGE_EMF27, MERGEETL_2011, MESSAGE V.3, REMIND 1.5, GCAM 2.0, IMAGE 2.4 IMAGE 2.4, GCAM 2.0

LIMITS, AMPERE2, AMPERE3 AME

3

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EMF27, AME,

Supplementary Figure 1 | Selected ‘bunker-free’ emission scenarios excluding LULUCF emissions. These scenarios are the same scenarios as presented with their LULUCF in Figure 1 in the main article, but exclude LULUCF and bunker emissions. Scenarios are shown for the 1.5°C-pre2020peak category (red), 2°C-pre2020peak (blue), 2°C-2030peak (purple), and 2°C-statedINDC/2°CfairINDC (average INDC assessment, black line). The average over all scenarios of each case are shown by a solid line.

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2. Supplementary Discussion 2.1 Comparison with literature Sensitivity to the choice of global emissions scenarios The 2030 regional and national allocations of the ‘2°C-pre2020peak’ scenarios presented in Figure 1c-l of the main article are less stringent than the results derived using the same allocation modelling and parameterization applied to global emissions scenarios in line with the G7 Elmau agreement6. The Elmau Agreement of June 2015 stated two global mitigation goals: 'a decarbonisation of the global economy over the course of this century' and 'the upper end of the latest Intergovernmental Panel on Climate Change recommendation of 40%–70% reductions by 2050 compared to 2010'. These two goals were interpreted in ref.6 as a 60-70% global emissions reduction below 2010 by 2050 and a net-zero CO2 emissions by 2100. While the Paris Agreement is more stringent than the Elmau agreement at the end of a century with a net-zero GHG target, it doesn’t provide near- or mid-term mitigation targets. The global emissions scenarios consistent with Elmau agreement are in average lower in 2030 and 2050 than the global emissions scenarios consistent with the Paris Agreement 2°C goal. The regional ranges presented in Figure 1h-l are therefore slightly less stringent for each equity approach compared to the regional results presented in Figure 3 of ref.6.

Comparison with multi-approach allocations of 2°C scenarios One study13 previously used the same modelling and parameterization of the GDR approach as our study and applied it to the RCP2.6 global emissions scenarios14. These GDR allocations of RCP2.6 emissions are slightly more stringent than the GDR allocations of the ‘2°C-pre2020peak’ scenarios of our study for China and India, and much more stringent for the USA and the EU with 2030 targets of 83% and 88% below 2010 levels respectively. The overall greater stringency of the GDR allocations in line with RCP2.6 results from the lower global emissions of the RCP2.6 in 2030 compared the average of the ‘2°C-pre2020peak’ scenarios, excluding LULUCF and bunker emissions. The ‘equal cumulative per-Capita’ modelling of ref13 uses a ‘straight-line’ approach, rather than a ‘spline-line’ approach4, and restricts allocation to business-as-usual emissions at maximum, and the sum of national allocations does not add up to a least-cost mitigation scenario. The results of ref.13 have more stringent 2030 targets compared to our ‘2°C-pre2020peak’ CPC results, for India with +98% of 2010 levels, less stringent for China with -23% and similar for the EU with -43% and the USA with -57%. In general, the convergence date used for the ‘equal cumulative per-Capita’ modelling of ref.13 ranges from 2060 to 2100 at the latest, which is overall earlier than the 2100 convergence of our study. We choose 2100 as the convergence date to achieve equal cumulative per capita emissions as 2100 is the date of the temperature goals of the Paris Agreement and the latest date to achieve net-zero emissions. Per capita convergence at an earlier date results in more stringent near term allocations for high historical emitters as there is less time to compensate for the historical debt they have “towards other nations”. 5

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The ‘Common-but-differentiated Per-Capita Convergence’ (CDC) of ref.13 reflects the ‘equality’ category of the IPCCAR5 Figure 6.28 adapted from ref.15, as does our EPC approach. However, the modelling of the transition to strict equal per capita emissions of the CDC approach differs substantially from the transition of our EPC approach. The CDC approach allows countries’ percapita emissions to grow following BaU projections until they reach the value of a linear interpolation between the highest national per-capita 2013 level and zero at some date in the future. The CDC 2030 allocation is similar for China (-32% of 2010 levels), slightly less stringent for the USA (-41%), slightly more stringent for the EU (-41%) and a lot more stringent for India (+84%), compared the EPC allocations of the ‘2°C-pre2020peak’ scenarios. The population is expected to decline in the EU but to increase in the USA and can explain the relative difference of stringency of the studies during the transition period. Moreover, the INDC of India, and then possibly the BaU emissions projection used in the ref.13, is below the average of our EPC allocation, which is in agreement with the greater stringency of the 2030 CDC allocation compared to our study. Generally, the stringency of the CDC approach increases for countries as they reach the threshold value that constrains them to decrease their emissions. As a result, the CDC approach is initially less stringent than the EPC approach for most countries but more stringent at a later time that depends on the convergence date selected for the CDC approach. Aside from ref.6, the study of Pan et al.16 is the only one to apply a wide range of equity approaches to allocate at the national levels the GHG emissions of a unique scenario. We compare our results, the range over the five equity approaches averages in the ‘2°Cpre2020peak’ case, to the interquartile range of allocations in line with the 2°C objective of ref.16. Comparison with our results are limited since ref.16 only presents results aggregated over all equity approaches and gives very limited details on the modelling and parameterization of these 25 allocations. Looking at 2030 results, our results are more stringent for China and India, and less stringent for the EU and the USA. For 2050, our results are in line with ref. 16 for these four countries.

Results comparison with IPCC results The equity modelling of this study follows the categorization presented in Figure 6.28 of the IPCCAR517 adapted from ref.15. Figure 6.28 displays a range of 2030 regional emissions allocations over the 40 studies included for each of the five equity categories. All these 40 studies allocate emissions consistent with a likely chance to limit global warming to 2°C over the century, but follow different global emissions scenarios (sometimes derived from CO2 budgets). The ranges of 2030 targets displayed in Figure 6.28 of the IPCCAR5 are therefore reflecting both the different modelling of an equity concept and the choice of global scenarios to which the allocation approach is applied. To the contrary, the emissions ranges of our study presented in Figure 1h-l only result from the range of global scenarios selected to match the goals of the Paris Agreement. We compare the results of Figure 6.28 of the IPCCAR5 our results in the 2°C-pre2020peak of Figure 1h-l. For the ‘capability’ category, we find more stringent targets for the OECD and Economies In Transition (EIT) regions, a similar target range for Asia but less stringent for Latin America (LAM) 6

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and in particular for Middle East and Africa (MAF). These differences arise from our modelling of the capability approach18 that does not assume BaU emissions scenarios, as detailed in ref.6. Our results for the ‘equality’ category are similar to the IPCC figure for the OECD and EIT, but less stringent for AISA, LAM and in particular for MAF. The modelling of the transition period and the choice of global scenarios are the main factors to the variability of the results of the ‘equality’ category. Our results for the GDR approach are significantly less stringent for the OECD, EIT MAF and LAM, but similar for ASIA, compared to the ‘Responsibility-capability-need’ category of the IPCCAR5 figure. The choice of the global target and BaU emissions scenarios has a strong impact on the GDR allocation. Our results for the CPC approach are slightly less stringent for the OECD, EIT and ASIA and significantly less stringent for LAM and MAF, compared to the ‘equal cumulative per capita’ category of the IPCC figure. Unlike the studies included in the IPCC figure, our study directly attributes GHG emissions adding up to a cost-optimal scenario. Our approach achieves strict equal cumulative per capita over the 1990-2100 period6, including an emission discount rate or 1.5% for emissions before 2010. Some studies of the IPCCAR5 figure do not allow for negative emissions, and therefore allocate more stringent near-term mitigation targets4. The ‘staged approaches’ category of the IPCC figure groups a wide range of equity approaches15, including but not restricted to the grandfathering approach. The CER approach modelled here, representative of the grandfathering approach, is less stringent for OECD and EIT, but similar for ASIA, MAF and LAM.

2.2 Mid-century mitigation rates By 2050, more stringent mitigation is required from almost all countries to aim at 1.5°C than to aim at 2°C under ‘2°C-pre2020peak’ or ‘2°C-statedINDC’ cases (Supplementary Figure 2). In particular, the averaged allocation of the ‘major economies’ under the ‘1.5°C-pre2020peak’ case is 20 percentage-points lower than the ‘2°C-pre2020peak’ case and 16 percentage-points lower than the ‘2°C-statedINDC’ case, while allocations of ‘other economies’ (all countries other than the G8 and China) are respectively 50 and 60 percentage-points lower (Supplementary Figure 2). Reaching 1.5°C requires less additional effort, compared with ‘2°C-pre2020peak’ or ‘2°CstatedINDC’ cases, in 2050 from developed countries than developing countries, which mitigate less by 2030. Many Southern-Asian (in yellow and orange) and African countries (in green) have higher 2050 allocations than their respective 2010 levels under each of the three cases. Over this period, India’s allocations are higher in 2050 than its 2010 level under all cases but ‘1.5°Cpre2020peak’. In contrast, developed countries, but also China or Russia have allocations at least 60% below their 2010 levels.

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Supplementary Figure 2 | Comparisons of national emissions change by 2050 under different cases: ‘1.5°C-pre2020peak’, ‘2°Cpre2020peak’ and ‘2°C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

Average annual mitigation rates over the 2030-2050 period are similar under the ‘1.5°Cpre2020peak’ and ‘2°C-pre2020peak’ cases on average for all countries. Delayed action towards 2°C from current INDC levels requires stronger mitigation over 2030-2050 from the ‘major economies’ group compared to early action towards 1.5°C or 2°C (Supplementary Figure 3).

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Supplementary Figure 3 | Comparisons of national emissions change over the maximal global mitigation periods under different cases: ‘1.5°C-pre2020peak’ over the 2010-2030 and ‘2°C-statedINDC’ over 2030-2050 that are the periods of maximal mitigation of each of these cases. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

2.3 Per approach mitigation rates We compare allocations in 2030 of each approach, as a fraction of 2010 levels, under the ‘1.5°Cpre2020peak’, ‘2°C-pre2020peak’ and ‘2°C-statedINDC’ cases. In 2030 for the CAP, the EPC and the CPC approaches, developed countries have almost similar allocations under ‘1.5°Cpre2020peak’ and ‘2°C-pre2020peak’ (left hand side panels of Supplementary Figure 4). For these three approaches, developing countries have significantly lower allocations under the ‘1.5°Cpre2020peak’ case compared to the ‘2°C-pre2020peak’. It is worth noting that the transition period of CAP and EPC approaches extends until 2040. The GDR allocation, which strongly depends on the difference between the business-as-usual and the targeted scenarios, results in similar allocation differences between the ‘1.5°C-pre2020peak’’ and ‘2°C-pre2020peak’ for most countries irrespectively of their development. Finally, the CER approach allocates the same relative emissions changes to all countries, and the differences between the ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’ cases is therefore the same for all countries. The right hand side panels of Supplementary Figure 4 compare the INDCs of all countries to the allocations of each equity approach. Under all approaches, the aggregated INDCs of the ‘major economies’ fall short of meeting their aggregated allocations under the ‘2°C-pre2020peak’ case (only 8 percentage-points short of the CER approach). The aggregated INDCs of the ‘other economies’, overshoot their aggregated ‘2°C-pre2020peak’ allocations of the CAP approach (by 39 percentage-points), the EPC (by 2 percentage-points), but fall short to meet the aggregated allocations of the GDR (by 30 percentages-points) and the CER (66 percentages-points) approaches. The overachievement of ‘other economies’ under the CAP, EPC and CPC approaches is mostly resulting from the relatively high ambition of the developing countries.

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Supplementary Figure 4 | Comparisons of national emissions change by 2030 for each equity approach under different cases: ‘1.5°C-pre2020peak’, ‘2°C-pre2020peak’ and ‘2°C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

2.4 Per approach peaking emissions and phase-out year In the manuscript, Figure 2e-g shows the comparison of emissions peaking timing and magnitude, and net-zero emissions timing averaged over the five concepts of equity for the ‘1.5°Cpre2020peak’ and ‘2°C-pre2020peak’ cases. In Supplementary Figure 5, peaking timing and magnitude, as well as net-zero emissions timing are compared for each of the five equity approaches individually. The peaking emissions timing of ‘other economies’ under ‘1.5°Cpre2020peak’ is ten years earlier than under ‘2°C-pre2020peak’ for all approaches (except for the GDR that is 18 years earlier). Developed countries should peak emissions immediately for both ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’ for all approaches but the CER. The timing difference of net-zero emissions between ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’ varies across approaches. Under the CER and EPC approaches, all countries should reach net-zero together in 2083 towards 2°C, and 2075 towards 1.5°C. The CAP approach – that allocates per capita shares of global emissions inversely proportional to per capita GDP when global emissions are positive, and proportional to GDP per capita when global emissions are negative 6 – requires developed countries to phase out up to 18 years earlier under ‘1.5°C-pre2020peak’ compared to ‘2°Cpre2020peak’, while developing countries have similar phase-out dates in both cases. The CPC approach allocates negative emissions to some countries over the century and net-zero emissions in 2100 to the others countries, depending on the parameterisation. Most countries that have negative emissions under both ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’ (e.g. developed countries, China…) should reach net-zero emissions 9 years earlier towards 1.5°C. The GDR approach allocates negative emissions towards 1.5°C about ten years earlier than towards 2°C and as early as 2025 for ‘major economies’. Many developing countries have strictly positive allocations over the course of the century under the GDR approach.

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Supplementary Figure 5 | Emissions-peaking timing and magnitude, and net-zero emissions timing compared between ‘1.5°Cpre2020peak’ and ‘2°C-pre2020peak’ under each equity approach. Average of peaking emissions levels versus average year of peaking emissions for ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’. Disks’ sizes are proportional to 2010 emissions levels. Peaking

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emissions are shown under a logarithmic scale in % of 2010 levels. Colours indicate countries’ world region. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

2.5 Annual mitigation rates and minimal allocations The annual mitigation rate of a national allocation reflects the additional effort (domestic mitigation or international support) that a country is allocated from year to year. We compare the maximal annual mitigation rates under the different scenario sets. The maximal mitigation rate over the century is 0.5 additional percent of 2010 levels (percentage-points) more stringent under ‘1.5°C-pre2020peak’ than under ‘2°C-pre2020peak’ for ‘major economies’ but 0.4 percentage-points less stringent for ‘other economies’ (Supplementary Figure 6). However, delaying mitigation towards the 2°C goal until 2030 requires greater maximal mitigation rates than immediate action towards 1.5°C, both at the global (Figure 1a and ref.2) and national (Supplementary Figure 6) levels for most countries.

Supplementary Figure 6 | Maximal annual mitigation rates over the century are compared between ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’ and between ‘1.5°C-pre2020peak’ and ‘2°C-statedINDC’. Emissions changes are given in percent of 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

Furthermore, national allocations towards 1.5°C do not have lower minima over the 21 st century than towards 2°C (Supplementary Figure 7). Over the 2010-2060 and 2010-2080 periods allocations minima are lower in the ‘1.5°C-pre2020peak’ case for almost all countries (Supplementary Figure 7). Allocations minima of ‘major economies’ do not vary much between the 2010-2060, 2010-2080 or 2010-2100 periods. The average-allocations minimum of ‘other economies’ under the ‘2°C-pre2020peak’ are in 50 percentage-point (of 2010 levels) lower over the 2010-2080 period compared to the 2010-2060 period, and 30 percentage points lower under 13

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the ‘1.5°C-pre2020peak’. Over the 2010-2100 period (Supplementary Figure 7), the average allocation minimum of the ‘other economies’ is 40 percentage-points lower than over the 20102080 period in the ‘2°C-pre2020peak’ case, and 21 percentage-points lower in the ‘1.5°Cpre2020peak’.

Supplementary Figure 7 | Comparisons of national allocations minima under different cases. Minimal emissions allocated over the 2010-2100, 2010-2060 and 2010-2080 period for ‘1.5°C-pre2020peak’ and ‘2°C-pre2020peak’. Emissions minima are given in percent change to 2010 levels, which are represented by disks’ sizes. Colours indicate countries’ world region, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

2.6 Effect of national allocation on global 2030 levels Average ambition INDC assessment In the main article, Figure 3 presented the effect on global 2030 emissions of a country (or country group) following equitable allocations derived in this study when the rest of the world follows the ‘high-ambition’ INDC assessment3 (which uses conditional targets when available). In Supplementary Figure 8 we show the effect on global 2030 emissions of a country (or country group) following equitable allocations when the rest of the world follows their INDCs (average of ‘high-ambition’ and ‘low-ambition’ assessments’). The aggregate INDCs, excluding LULUCF and bunker emissions in 2030 is 51.8 GtCO2eq (or 52.7 GtCO2eq when including the ‘missing countries’) and the gap to the average of cost-optimal ‘2°C-pre2020peak’ scenarios is then 11.7 GtCO2eq, and 23.3 GtCO2eq to the average of ‘1.5°C-pre2020peak’ scenarios. In this configuration, the G8, China and India as a group could close that mitigation gap by accepting the average of their five equitable allocations, provided that the rest of the world reaches its aggregate INDC level. Closing the mitigation gap to the average of the ‘1.5°C-pre2020peak’ scenarios requires more than the G8, China and India following the average of their equitable allocation.

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Supplementary Figure 8 | Effect of countries following equitable mitigation allocations on global 2030 emissions compared to averaged INDC assessment. Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2°C (left panel) or 1.5°C goals (right panel), reduce or increase the projected 2030 emissions levels (excluding LULUCF and bunker emissions) compared to aggregated INDCs (average of ‘high-ambition’ and ‘low-ambition’). Countries are ordered left to right by decreasing levels of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated INDCs and the average scenarios consistent with the Paris 2°C or 1.5°C goals (grey bar) are shown in each panel.

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‘Low-ambition’ INDC assessment In Supplementary Figure 9 we show the effect on global 2030 emissions of a country (or country group) following equitable allocations when the rest of the world follows the ‘low-ambition’ INDCs (that is unconditional INDC and higher emissions end of the range of the INDC assessment uncertainty). The aggregate ‘low-ambition’ INDCs, excluding LULUCF and bunker emissions in 2030 is 54.8 GtCO2eq (or 55.7 GtCO2eq when including the ‘missing countries’) and the gap to the average of cost-optimal 2°C-pre2020peak scenarios is then 14.7 GtCO2eq, and 26.3 GtCO2eq to the average of ‘1.5°C-pre2020peak’ scenarios. In this configuration the G8 and China (even with India) cannot close the mitigation gap to the average of ‘2°C-pre2020peak’ scenarios by following the average of their five equitable allocations.

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Supplementary Figure 9 | Effect of countries following equitable mitigation allocations on global 2030 emissions compared with unconditional INDC assessment. Countries following individual approaches (tip of coloured patches), or their average (black lines) under the 2°C (left panel) or 1.5°C goals (right panel), reduce or increase the projected 2030 emissions levels (excluding LULUCF and bunker emissions) compared to aggregated ‘low-ambition’ INDCs. Countries are ordered left to right by decreasing levels of 2010 emissions (proportional to bar width). The global gaps (grey arrow) between current aggregated ‘low-ambition’ INDCs and the average scenarios consistent with the Paris 2°C or 1.5°C goals (grey bar) are shown in each panel.

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2.7 Near and mid-term regional and sub-regional Regional and sub-regional results in 2030 In the manuscript, Figure 1h-l shows the 2030 mitigation target for 5 world region, under the five equity approaches, for 1.5°C, 2°C. In Supplementary Figure 10, we add to the comparison scenarios towards 2°C with delayed action. As mentioned in the main article and in the Methods, these scenarios peak in 2030 and have high SO2 concentration and are therefore only loosely consistent with the Paris Agreement. Emissions allocations in 2030 under ‘2°C-2030peak’ scenarios are obviously less stringent for any region under any approach than under the ‘2°Cpre2020peak’ or ‘1.5°C-pre2020peak’ scenarios. In Supplementary Figure 11, we compare 2030 results at the sub-regional level to their aggregated INDCs. In OECD sub-regions, 2030 results, as well as the ambition of aggregated INDCs, (‘North America’, ‘Western Europe’ and ‘Japan, Australia, New Zealand’) are similar. The OECD sub-regions also have similar allocations as ‘Economies in transition’ and ‘East Asia’, but their aggregated INDCs are more ambitious by about 40 percentage points of 2010 levels. The allocation of the sub-regions of ‘Middle East and Africa’ (‘Sub-Saharan Africa’ and ‘MiddleEast and North Africa’) differ greatly. The 2030 allocations for ‘Middle-East and North Africa’ a much more stringent than for ‘Sub-Saharan Africa’. The aggregated INDCs of ‘Middle-East and North Africa’ are more stringent, in absolute terms, than those of ‘Sub-Saharan Africa’ but are in line with a lot fewer equity approaches. The 2030 allocations of the ‘Middle-east, North Africa’ sub-regions are very similar to those of Latin America, but their aggregated INDCs are less ambitious by 40% of 2010 levels. In the Asia region, the aggregated INDCs of ‘East Asia’ are only in line with two equity approaches applied to the delayed 2°C scenarios. On the other hand, the INDCs of ‘Pacific Asia’ are in line with two equity approaches towards 2°C, and the INDCs of ‘South Asia’ are in line with three equity approaches towards both 2°C both 1.5°C.

Supplementary Figure 10 | Regional 2030 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Regional 2030 allocations under the ‘1.5°C-pre2020peak’, ‘2°C-pre2020peak’ or ‘2°C-2030peak’ sets are shown, for the five equity approaches.

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Supplementary Figure 11 | Sub-regional 2030 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Sub-regional 2030 allocations under the ‘1.5°C-pre2020peak’, ‘2°C-pre2020peak’ or ‘2°C2030peak’ sets are shown, for the five equity approaches, with the regionally aggregated INDCs (dashed lines).

Regional and sub-regional results in 2050 In 2050, OECD or ‘Economies in transition’ emissions allocations of most equity approaches are around zero emissions or lower for 1.5°C, 2°C and 2°C with delayed action – only the CER approach for 2°C scenarios, and the GDR approach for the Economies in transition are substantially higher (Supplementary Figure 12). In Asia and Latin America, emissions allocations under all equity approaches and each global scenario have decreased by an additional 40-50 percent of 2010 levels. The emissions allocations of Middle East and Africa region have a large range (up to a few times their 2010 levels). That is a consequence or their 2010 emissions levels that are relatively low compared to the maximum of their equity approaches’ allocations, but also a consequence of some global scenario reaching net-zero emissions in 2050 (Supplementary Figure 1).

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Supplementary Figure 12 | Regional 2050 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Regional 2050 allocations under the ‘1.5°C-pre2020peak’, ‘2°C-pre2020peak’ or ‘2°C-2030peak’ sets are shown, for the five equity approaches.

For the same reasons, at the sub-regional level, ‘Sub-Saharan Africa’ and ‘South Asia’ have the greatest variability in their 2050 allocations (Supplementary Figure 13). The allocations of the OECD region are still consistent with those of its sub-regions, but also with those of ‘East Asia’. The allocation results of the ‘Middle-east, North Africa’ and Latin America sub-regions are still similar in 2050. Overall, most equity approaches of the ‘Sub-Saharan Africa’ and ‘South Asia’ subregions under all global scenarios, as well as two equity approaches of the ‘Pacific Asia’ subregion towards 2°C, are above their 2010 levels.

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Supplementary Figure 13 | Sub-regional 2050 emissions consistent with the Paris Agreement and five equity principles compared to current pledges. Sub-regional 2050 allocations under the ‘1.5°C-pre2020peak’, ‘2°C-pre2020peak’ or ‘2°C2030peak’ sets are shown, for the five equity approaches.

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3. Supplementary Tables The supplementary tables provide similar information as Table 2 in the main article for all countries and the European Union (which still includes the United-Kingdom), based on respectively the ‘2°C-pre2020peak’ (Supplementary Table 3) and ‘1.5°C-pre2020peak’ (Supplementary Table 4) scenarios sets.



Column A indicates countries’ ISO Alpha-3 codes,



Column B indicates the full name of countries,



Column C indicates the approach’s name (see Table 1 in the main article),



Columns D to F indicate whether or not a country's INDC (depending on the ambition of its assessment: high, average or low ambition, see Methods) is consistent with the corresponding equity approach,

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Columns G to N indicate the average and the ranges of allocations over all the ‘2°Cpre2020peak’ scenarios in years 2025, 2030, 2040, 2050,



Column O indicates the peaking date of the average of each allocation,



Column P indicates the date of first net-negative emissions of the average of each allocation,



Columns Q and R indicate the cumulative GHG emissions over respectively the 20102050 and 2010-2100 periods of the average of each allocation.

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CHAPTER 7 – EQUITABLE G20 CONTRIBUTIONS TOWARDS ACHIEVING THE PARIS GLOBAL GOALS 7.1 Result s for G20 countries This chapter 9 further discusses the results, for G20 members specifically, initially presented in Chapter 6. In this chapter, results for the G20 (which officially includes 20 of the world’s largest economies: 19 countries and the EU) aggregate those of 44 countries with the 28 EU members that are represented by the European Commission and negotiate as a group under the UNFCCC. Together, the G20 emitted around 76% of global emissions in 2010 – over 74% when including land-use emissions (Gütschow et al 2016). Domestic mitigation action from the G20 can therefore have a crucial impact on climate change. Moreover, with 78% of the global Gross Domestic Product (GDP) (World Bank 2015), the G20 has the capacity to fund and lead international action to achieve the Paris Agreement goals. Following the communiqué of the G20 summit in Hangzhou in 2016 calling for an entry into force of the Paris Agreement as soon as possible, rapid ratification by G20 members enabled the Paris Agreement to become effective in less than a year. Of G20 members, only Russia and Turkey have not yet ratified the agreement. The European Union has ratified, but not all its members. Looking at the current G20 mitigation ambition under the 2 °C goal, the aggregated (I)NDCs of the G20’s members states appear inconsistent with any of the equity approaches employed by the IPCC (Figure 7.1). The mitigation gap with 1.5 °C equitable pathways is even greater. Collectively, G20 countries reach net-zero GHG emissions between 2065 and 2083, depending on the equity concept, on average over the 2 °C scenarios (Figure 7.1), and between 2050 and 2075 over the 1.5 °C scenarios. Note that the national equitable emissions pathways presented in this chapter can be met using a combination of domestic mitigation, internationally traded emissions mitigation (UNFCCC 2015a) or financial support towards global mitigation (Robiou du Pont et al 2017) as described in Chapter 6.

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Figure 7.1 | Comparison of the G20 equitable emissions pathways under the 2 °C scenarios with the aggregation of their (I)NDCs. Emissions allocations, excluding land-use and bunker emissions, under five equity allocations representative of the five IPCC categories are compared the G20 sum of (I)NDCs average (black circles) and range (vertical black line). Coloured patches and lines show allocation ranges and averages, respectively, over global 2 °C scenarios.

By definition, combining multiple visions of equity – using weighting factors (Raupach et al 2014) or a leadership based approach (Meinshausen et al 2015) – does not correspond to any vision of equity but can represent a political compromise (Raupach et al 2014), and is useful to compare national allocations under different temperature goals. To reach the 1.5 °C goal rather than the 2 °C goal, the G20 should reduce its 2030 emissions by an additional 24 percentage-points from 2010 levels, on average over the five equity allocations, (7.2a). This additional collective reduction by G20 members towards the 1.5 °C goal is comparable, in percent of 2010 levels, to that of the G8+China as a group or as individual Parties (Figure 7.2a and Figure 2a of Chapter 6). Non-G20 countries’ collective emissions, but also India’s emissions, should be around 45 percentage-points lower in 2030 to reach the 1.5 °C goal compared to the 2 °C goal. Comparing current (I)NDCs to the average of the five equity allocations, the G20 (I)NDCs should be 39 percentage-points lower collectively to align with the 2 °C goal. In contrast, non-G20 countries collectively overachieve the average of their allocations by 27 percentage-points (Figure 7.2b). Turkey’s INDC should be 143 percentage-points lower than currently, Saudi Arabia’s NDC 134 percentage-points, Russia’s INDC 71 percentage-points and China’s NDC 61 percentage points lower to align with average 2 °C allocations.

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Figure 7.2 | Comparisons of G20 countries’ emissions allocations under the 1.5 °C or 2 °C goals and with their (I)NDCs. a. Comparison of 1.5 °C and 2 °C allocations averaged over the five equity approaches in 2030 (as a percent change to 2010 levels). b, Comparison of 2 °C allocations averaged over the five equity approaches with (I)NDCs in 2030 (as a percent change to 2010 levels). Disk sizes are proportional to 2010 emissions levels. Colours indicate world regions. Aggregated results for G20 countries (larger disk) and the rest of the world (‘Non-G20’, smaller disk) are shown in grey.

The gap between conditional (I)NDCs (‘high-ambition’ assessment of (Meinshausen and Alexander 2015)) and 2 °C consistent pathways could be closed if the G8 and China together adopt the average of the five equity approaches (Robiou du Pont et al 2017). If the G20 countries collectively adopt the average of the five approaches, the gap between conditional (I)NDCs and 1.5 °C consistent pathways could almost be closed (Figure 7.3). The rest of the world (non-G20 countries) would then only have to collectively reduce emissions by an additional 510 MtCO2eq, compared to their conditional (I)NDCs, to meet the average of their five allocations towards 1.5 °C.

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Figure 7.3 | Gaps between equitable emissions allocations and conditional (I)NDCs in 2030. Countries following individual approaches (tip of coloured patches), or the average of the five approaches (white lines) under the 2 °C (left) or 1.5 °C goals (right), reduce or increase the projected 2030 global emissions levels (excluding LULUCF and bunker emissions) compared to aggregated conditional (I)NDCs. The global gaps (grey arrow) between current aggregated conditional (I)NDCs and the average of global scenarios consistent with the Paris 2 °C or 1.5 °C goals (grey bar) are shown in each panel.

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The consistency of (I)NDCs with the five equity approaches varies greatly among individual G20 members (Figure 7.4). The INDC of Turkey, and the (I)NDCs of Saudi Arabia, Russia and China imply 2030 emissions level much higher than any equity allocations, even under the 2 °C goal. Considering the 2 °C goal across G20 countries, Brazil and Mexico have NDCs that are within the ranges of four equity allocations. The EU’s NDC is within the range of three equity allocations. Under the 1.5 °C perspective, Australia’s NDC is within the range of two equity allocations (a grandfathering approach and the GDR that uses contentious business-as-usual projections), and Indonesia’s NDC is within the range of the equal cumulative per capita approach and just within the range of the equal per capita approach.

Figure 7.4 | Comparison of equitable 2030 emissions allocations of G20 members with their respective 2030 (I)NDCs assessment under the 2 °C and 1.5 °C goals. Emissions allocations in 2030, excluding land-use and bunker emissions, under five equity allocations representative of the five IPCC categories are compared with countries’ (I)NDC averages (dashed black line). Emissions ranges of any colour represent countries’ allocation range under the 1.5 °C (left) and 2 °C (right) goals.

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7.2 Consistency of equit y performance wit h statements on fairness Although all countries have formally recognised the CBDR-RC principle, most have rarely explained their position with a guiding principle or an effort-sharing formula. In their (I)NDCs, Parties were invited to communicate a description of “fairness and ambition” and a description of “how it contributes towards achieving the objective of the Convention” (UNFCCC 2017a). The statements of the G20 members (Robiou du Pont and Jeffery 2017) can be assessed with different perspectives (Table 7.1). Some statements invoke notions of fairness that are applicable to all Parties and can constitute a burden sharing approach (‘Fairness principle’ column). Some countries also presented achievements or targets without relating them to a burden-sharing scale, for example, progress in ambition, progress compared to business-as-usual (BaU), progress in GDP emissions intensity or the sole reduction of emissions (‘Additional metrics’ column). Additionally, some (I)NDCs refer to national circumstances, which are recognised by the Paris Agreement, to justify of the national-specific limits to reducing emissions domestically (National circumstances’ column). Some countries also stated that their national emissions represent a small share of the global emissions. These countries may imply that delivering a fair emissions reduction is either less important than for bigger emitters, which goes against any fairness principle applicable to all, or costlier due to the low economy of scale. Finally, some countries’ contributions are conditional upon other countries’ ambition in order to avoid relative losses. However, contributions can also be conditional on technology or finance support, which is inherent to the capability based principle. Amongst G20 countries, six did not discuss the fairness of their (I)NDCs, including Turkey who used the historical responsibility principle only to justify its small contribution to global warming, but not to explain its current INDC. The EU and its members did not discuss the fairness of their NDC either, but simply “look forward to discussing with other parties the fairness and ambition of INDCs”. Many Parties implicitly define the fairness of their contribution as the ambition of their commitment. However, some Parties indicated elements to determine fair mitigation contributions. Amongst G20 countries, ten parties declared their (I)NDCs fair and ambitious but only six of them mentioned a principle of fairness applicable to all. Brazil and India discussed the fairness of their NDC under notions of equality, historical responsibility and capability. The assessment of the Brazilian NDC under the 2 °C goal, which excludes land-use emissions, is consistent with all the equity allocations, except the capability approach (Robiou du Pont et al 2016) (Figure 7.4). India’s NDC is consistent with equity approaches representative of concepts of equality and historical responsibility, but not with the capability approach. Mexico’s NDC is in line with the equal per capita approach, consistently with its NDC declaration. South Africa calls for a framework applicable to all and raises concepts of historical responsibility and capability. While the South African NDC is aligned with the carbon budget derived by its own national experts (see NDC of South Africa), it is not aligned with the equal cumulative per capita approach of Figure 7.4. Prior to the INDC process, some countries showed support for some concepts of equity at a workshop under the UNFCCC (2012) or through national communication. Japan supports the consideration for “the impacts of GHG emissions of each Party on global warming” (Japan 2014, p 2). The EU distributes mitigation efforts across its members states following their historical emissions and capabilities (CEC 2008, p 9, UNFCCC 2012b). However, the NDCs of Japan and

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the EU are not in line with either the capability or the equal cumulative per capita approaches (Figure 7.4). China stressed the importance of historical responsibility to determine burden sharing efforts (BASIC experts 2011) but its NDC is not consistent with any of equity approaches including the equal cumulative per capita. Finally, the G7 INDCs were already found insufficient to equitably meet the ambition of the Elmau agreement signed by the G7 in June 2015 (Robiou du Pont et al 2016).

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Table 7.1 | Categorisation of arguments contained in the (I)NDCs’ of G20 countries justifications for fairness and ambition (UNFCCC 2017a, Robiou du Pont and Jeffery 2017). ‘Fairness principles’ relate to equitable burden sharing approaches; ‘Additional metrics’ refer to ambition stated by countries without a relation to a burden sharing approach (for example: progression in ambition, comparison with developed countries’ contributions, progress to Business-as-Usual); ‘National circumstance’ refers to elements invoked by Parties to justify their specific efforts to mitigate domestic emissions; ‘Conditionality’ indicates whether the commitments of a Party are unconditional, conditional or both. The conditionality can be on financial or technological support, on further agreements, or on economic growth. NDC declared “fair and ambitious”

Fairness principle

Argentina

Australia X

‘Very ambitious’

Equal per capita, historical responsibility, capability

Canada X China India

Additional metrics

National circumstances

Conditionality

GDR, CER

Progression, comparison

Both (conditions on support) Unconditional only

EPC, GDR, CPC, CER GDR, CER

Progression, comparison

Need for development, food security, small emitter National circumstances, resource provider, high abatement costs, Need for development,

Large landmass, resource provider, extreme temperatures, small emitter Need for development Need for development

Not specified

Need for development

Both (conditions on support) Not specified

BaU, Progression

X X

Equal per capita, historical responsibility, capability

Indonesia

Japan

No fairness justification

GDR, CER

X

Brazil

2 °C Equity check

EPC, CPC

EPC, CPC

Mexico

X

CER

X Equal per capita ‘Highly ambitious’

Transparency, GDP intensity Progression

EPC, GDR, CPC, CER

Russia X Saudi Arabia

X

EU (28)

Calls for equity framework, historical responsibility, capability

‘To the extent of possible’

Turkey

USA

Declining emissions, GDP intensity BaU

X

South Africa

South Korea

Utmost effort Utmost ambition

GDR

BaU

CER

Historical responsibility X Calls for discussion on fairness

X

Both (conditions on support and agreement on carbon pricing) Not specified

Fossil fuel dependent economy, vulnerable to emissions mitigation Need for development

Conditional only (on economic growth) Both (conditions on the delivery of existing support commitments) Not specified

‘Experiences constraints’, small emitter

Progression, declining emissions, GDP intensity

Not specified Conditional only (on support)

Need for development, small emitter

Fukushima halt to nuclear, small emitter

X GDR, CER EPC, GDR, CER

High mitigation costs,

Unconditional only

Both (conditions on support) Not specified Not specified

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7.3 Conclusio n The G20 members played a key role in the success of the Paris Agreement, which have most now ratified. However, the ambition of the G20’s aggregated contributions is insufficient to meet its fair share under any of the five equity allocations derived from the IPCC fifth assessment report. The G20 can close the 2030 mitigation gap towards 2 °C and considerably reduce the gap towards 1.5 °C by adopting the average of these five equity allocations. The mitigation ambitions of individual G20 members differ greatly. While the NDCs of Brazil and Mexico align with three concepts of equity, the (I)NDCs of China, Russia, Turkey and Saudi Arabia are not consistent with any. These four Parties also did not invoke any concept of equity when describing the fairness of their (I)NDCs. Starting in 2018, the UNFCCC stocktake process will provide all Parties with the opportunity to increase and harmonise the ambition contribution under the CBDR-RC equity principle of the convention. Considering the lack of ambition of their current pledges and the absence of reference to an equity concepts to guide the definition of their efforts, it appears unlikely that Argentina, Australia, Canada, China, Russia, South Korea, Saudi Arabia Turkey and the USA lead the ratcheting up process at the next stocktake. It remains possible that countries submitted (I)NDCs purposefully unambitious prior to the Paris Agreement in order to easily ratchet up in case of an agreement for greater ambition. Some of the current unambitious countries could increase their effort to compensate for their initial prudence.

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CHAPTER 8 – TEMPERATURE ASSESSMENT OF NATIONAL PLEDGES UNDER A BOTTOM-UP APPROACH OF EQUITY 8.1 Study Abstract Under the bottom-up architecture of the Paris Agreement, countries pledge Nationally Determined Contributions (NDCs). Current NDCs individually align, at best, with divergent concepts of equity and are in aggregation inconsistent with emissions scenarios to achieve the warming thresholds of the Paris Agreement. We find that the global 2030-emissions of current NDCs match the sum of each country adopting the least-stringent of five effort-sharing allocations of greenhouse-gas (GHG) emissions scenarios to achieve the Paris Agreement. We estimate that extending such a self-interested ‘bottom-up’ aggregation of equity might lead to a median (>50% likelihood) warming of 2.3°C in 2100. We find that ratcheting-up the warming goal of all the individual ‘bottom-up’ effort-sharing allocations to hypothetical levels of 1.1°C and 1.3°C could achieve the Paris Agreement’s warming thresholds of 1.5°C and well below 2°C, respectively. This new ‘hybrid’ allocation that reconciles the ‘bottom-up’ pledging nature of the Paris Agreement with its ‘top-down’ warming threshold, provides a temperature metric to assess NDCs. When taken as benchmark by other countries, the NDCs of India, the EU, the USA and China lead to warmings of 2.6°C, 3.2°C, 4°C and over 5.1°C respectively, under the current bottom-up regime.

Introduction Since the adoption of the United Nations Framework on Climate Change Convention (UNFCCC) (UNFCCC 1992) and its objective to stabilize GHG concentrations and avoid dangerous anthropogenic interference with the climate system, most countries have committed to limiting GHG emissions through domestic measures or support of mitigation action abroad. Informed by literature on effort-sharing approaches, the international community has long discussed the operationalization of equity following the UNFCCC principle of Common But Differentiated Responsibilities and Respective Capabilities (CBDR-RC) to drive national emissions allocations (UNFCCC 2012b, Winkler and Rajamani 2014b). The failure to agree on a top-down mechanism to derive binding national emissions targets for all countries led to a bottom-up situation where countries should pledge NDCs of “highest possible ambition” (UNFCCC 2015a, Mace 2016). While the quest for a common understanding of what is a fair effort-sharing continues, strongly falling technology costs of renewables and increasing mitigation co-benefits shift the attention away from burden-sharing considerations (Zenghelis 2017). However, current bottom-up NDCs do not yet add up to a global ambition that would be consistent with the joint temperature goals (Rogelj et al 2016a, Robiou du Pont et al 2017, UNFCCC 2015a, 2015d). A five-year stocktake requires all countries to pledge enhanced actions and support (UNFCCC 2015a). The quantification of national emissions levels consistent with both Paris Agreement’s mitigation and equity goals relies on contentious interpretations of distributive justice (Robiou du Pont et al

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2017, Winkler et al 2017, UNFCCC 2012c). Scientists, Non-Governmental Organizations and government experts have suggested multiple effort-sharing approaches to derive equitable national emissions allocations (BASIC experts 2011, Baer et al 2008, den Elzen et al 2008, Jacoby et al 2008, Nabel et al 2011, Höhne et al 2014, Tavoni et al 2014, Raupach et al 2014, Meinshausen et al 2015, Peters et al 2015, Robiou du Pont et al 2017, Pan et al 2017, Holz et al 2017, UNFCCC 2012c). While some countries do not use indicators that favour their equity argument in their communication (Winkler et al 2017), a common definition of equity is unlikely to be adopted since countries generally tend to support interpretations of distributive justice that best serves their selfinterest and likewise justify their negotiating positions (Averchenkova et al 2014, Fleurbaey et al 2014, Lange et al 2010, Tørstad and Sælen 2017). Furthermore, the commitments of many major emitters, including developed countries who committed to take the lead in reducing emissions and mobilizing finance to support mitigation in developing countries (UNFCCC 2015a), do not match concepts of equity that they publicly supported (Robiou du Pont et al 2016). The UNFCCC does not specify whether the equity and the CBDR-RC principles refer to distinct principles or to a single operationalization of equity (Winkler and Rajamani 2014a). One way to reconcile this ambiguity is to combine multiple dimensions of equity using weighting factors (Baer et al 2008, BASIC experts 2011, den Elzen et al 2008, Raupach et al 2014, Peters et al 2015, Holz et al 2017) and per-capita income thresholds (Baer et al 2008, Holz et al 2017) in a single effortsharing approach applicable to all countries. Alternatively, effort-sharing approaches can be combined in a differentiated manner where countries follow different equity principles. A recent study allocated emissions to each country using the least-stringent of two equity allocations (Meinshausen et al 2015). The global level of ambition of each equity allocation is then set by a ‘diversity-aware’ leader so that the sum of all countries’ allocations matches 2°C-consistent levels (Meinshausen et al 2015). Under that approach, equity approaches are applied to trajectories leading to warmings lower than 2°C. Consequently, countries follow different equity approaches that are applied under different temperature thresholds, which may be considered unfair by Parties. In the present study, we use a unique temperature threshold that is lowered consistently across all equity approaches until the ‘bottom-up’ allocation, where the least-stringent equity approach is attributed to each country individually (Meinshausen et al 2015), aligns with 2°C-consistent levels (Figure 8.1a). Hypothetically, countries could then follow different equity approaches applied to a common global aspirational temperature goal. Ultimately, the ‘hybrid’ approach follows a ‘bottom-up’ equity allocation consistent with a common ‘top-down’ warming threshold, which arguably reflects the hybrid nature of the Paris Agreement (Winkler et al 2017, Bodansky 2016). The key characteristic of this ‘hybrid’ approach is that a hypothetical warming threshold is adjusted downwards until the common warming target, either 2°C or 1.5°C, is achieved when the hypothetical warming threshold is pursued by each country under its most favourable equity allocation. The ‘hybrid’ approach does not constitute an ‘equitable’ operationalization of the CBDR-RC principle where all countries seek to maximize ‘absolute gain (Keohane 1984)’ by agreeing on a common approach of equity. Rather, it reflects national preferences for ‘relative gain (Krasner 1991)’, i.e. a country’s inclination to measure the fairness of its contribution to the global

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mitigation effort by looking at other countries’ efforts, rather than to domestic indicators alone. Despite claims that discussions of justice are “irrelevant or dangerous in a post-Paris world” (Robert Keohane quoted in (Klinsky et al 2017)), equity is fundamental for climate policy research (Klinsky et al 2017, Dooley et al 2018) and scientific analyses on equitable burden-sharing is influential on the UNFCCC processes (Mace 2016). However, the absence of agreement on an unanimous operationalization of the CBDR-RC should not be used as an excuse for inaction (Winkler and Rajamani 2014b) and should not leave the international community without a metric reflective of current agreements to assess the ratcheting-up process. The multiplicity of equity concepts results in a wide range of emissions allocations for countries (Robiou du Pont et al 2017, Pan et al 2017) and regions (Clarke et al 2014) that is sometimes used as an uncertainty range by non-experts. In a recent climate case, the District Court of The Hague ruled (The Hague District Court 2015, Schiermeier 2015) that the Dutch government has to reduce 2020 emissions by at least the least-ambitious end of the range presented in the IPCC-AR4 for the Annex I country group based on multiple equity allocations from 16 studies (Gupta et al 2007). The court did not pick an approach of equity and ruled for the minimum effort consistent with international treaties in light of commonly reviewed science. While the multiplication of climate litigations cases against governments (Sabin Center for Climate Change Law 2018) contributes to the ratcheting-up process, systematic court decisions that governments must follow the least-ambitious end of an equity range is unlikely to achieve the Paris Agreement. As a first step, this manuscript models such a ‘bottom-up’ situation where each country follows the least-ambitious effort-sharing approach representing the quantified IPCC categories. As a second step it models the ‘hybrid’ approach, to represent the current compromise where each country chooses an equity approach to determine its effort but does not directly use that approach to influence other countries’ effort. Overall, this study presents an operationalization of the current ‘agreement to disagree’ on equity concepts to achieve a common temperature goal. The ‘bottom-up’ allocation of emissions under the ‘hybrid’ approach is consistent with the pledge-and-review nature of the Paris Agreement and its mitigation goals and provides a metric for the ratchetting-up process.

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Figure 8.1 | ‘Bottom-up’ allocations of global emissions scenarios. a, Illustration of the bottom-up allocation and hybrid allocation. b, Least-stringent of five approaches, lowest cumulative emissions by 2100, for each country under the ‘bottom-up’ allocation of ‘2 °C-scenario’. Small island developing states are represented by their maritime zones. c, Scenarios towards 2 °C and 1.5 °C (excluding land-use and bunker emissions) shown with their corresponding ‘bottom-up’ allocation and aspirational scenarios and the (I)NDC assessment (Meinshausen and Alexander 2015) range (grey).

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Results Projecting a self-interested decarbonization, a ‘bottom-up’ approach of effort-sharing. The first step of this study is to model a self-interested approach of the Paris Agreement goals. We derive the ‘bottom-up’ allocation of two emissions pathways reflective of the Paris Agreement, excluding emissions from land-use and international shipping and aviation (section 8.2 Additional methods), the ‘2°C-scenario’ – with a likely (>66%) chance to stay below 2°C until 2100 (RCP2.6 (Rogelj et al 2012b)) and the ‘1.5°C-scenario’ – with a median 2100 warming below 1.5°C (section 8.2 Additional methods) – using five equity emissions allocations representative of the five effortsharing categories quantified in the latest IPCC report (Robiou du Pont et al 2017, 2016, Clarke et al 2014). These five categories include notions of: capability (CAP approach), equality with the Equal Per Capita (EPC) approach, responsibility-capability-need with the Greenhouse Development Rights (GDR), historical responsibility with the Equal Cumulative Per Capita (CPC), and national circumstances regarding current emissions levels with the Constant Emissions Ratio (CER). The CER, an approach considered unfair (Caney 2009, Peters et al 2015) that allocates equal emissions mitigation rates to all countries, is not openly supported by any country but implicitly matches many developed countries’ targets (Robiou du Pont et al 2017) , which they often declare as fair (Robiou du Pont 2017). Under the ‘bottom-up’ modelling, each country follows the equity approach that yields the greatest cumulative emissions by 2100 (Figure 8.1b, Figure 8.6). The ‘bottom-up’ attribution of the least-stringent equity allocation to each country aims at representing the currently discordant equity debate (Ringius et al 2002, Lange et al 2010, Averchenkova et al 2014, Meinshausen et al 2015). Coincidentally, the trajectories of these ‘bottom-up’ 1.5°C- and 2°C-scenarios align with the aggregated ‘high’ and ‘average’ (average of ‘high’ and ‘low’) NDCs assessments (Meinshausen and Alexander 2015) respectively (Figure 8.1c). Global pledged effort currently matches that of a world where each country follows the least-stringent vision of fairness for their circumstance. Extending such a ‘bottom-up’ situation throughout the century would result in median warmings of 2.0°C and 2.3°C in 2100, respectively higher than the 1.5°C and well below 2°C thresholds. We then increase the ambition of the ‘bottom-up’ allocation to be consistent with a top-down temperature threshold. A ‘bottom-up’ allocation consistent with more stringent ‘aspirational’ global scenarios that limit warming to 1.1°C and 1.3°C results in global emissions consistent with 1.5°C and 2°C respectively (Figure 8.1, section 8.2 Additional methods). Under such a selfinterested approach of effort-sharing, the international community needs to aim at an aspirational target of 1.3°C to effectively stay well below 2°C, and 1.1°C to effectively return to 1.5°C. The resulting temperature gaps between the aspirational target and effective warming reflect the necessary strengthening of global temperature aspirations to compensate for the disagreement on effort-sharing (Figure 8.1). At the national level, the current NDCs of China and Russia imply higher 2030-emissions than even the most favourable effort-sharing approach applied to a 2°C scenario. In other words, their NDCs are higher than a ‘bottom-up’ allocation of the ‘2°C-scenario’ (Figure 8.2a). G7 countries’ and India’s NDCs are aligned with such a ‘bottom-up’ allocation, i.e. the weakest of five allocation under a global 2°C warming goal constraint.

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The question is here to combine multiple approaches of equity to achieve the Paris Agreement long term mitigation goals. A method commonly used is to average or ‘blend’ (Raupach et al 2014) multiple equity approaches, which postulates a joint agreement on a common yardstick of fairness that does not reflect individual countries’ views on equity. The ‘hybrid’ approach modelled here avoids the use of subjective weighting factors across multiple equity approaches. The national allocations of the ‘hybrid’ approach happens to yield similar results to the ‘average’ of the five effort-sharing approaches’ allocations (Robiou du Pont et al 2017) for most G8 countries (including the 28 EU countries), and China (Figure 8.2b). Compared to using an average, the ‘hybrid’ allocations are greater for most Least Developed Countries (LDC), but lower by 47 percentage-points of 2010 emissions for Brazil, by 38 percentage-points for Mexico, and over 20 percentage-points for Indonesia and Egypt. The variability across national results does not show strong regional trends. Reaching the ‘2°C-hybrid’ allocation, including the unfair CER approach, implies raising the NDC’s ambition by around 30 percentage-points for the USA and the EU28, and 77 percentage points for China (Figure 8.2c). Aiming at 1.5°C, rather than 2°C, under the hybrid approach requires five additional percentage-points from the G8 countries and China, and 20 for the ‘other countries’ altogether, and more for LDC (Figure 8.2d).

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Figure 8.2 | Comparison of emissions changes by 2030 under the ‘bottom-up’, ‘hybrid’ and average allocations of global scenarios and with (I)NDCs. a, Comparison of the ‘bottom-up’ allocation of the 2°C-scenario and countries average (I)NDCs. b, Comparison of the ‘hybrid’ allocation of 2°C-scenario and the average of the five equity allocations. c, Comparison of the ‘hybrid’ allocation of 2°C-scenario and countries’ average (I)NDCs. d, Comparison of the ‘hybrid’ allocation of 2°C-scenario and 1.5°C-scenario. Disks’ sizes are proportional to 2010 emissions level. Colours indicate countries’ world regions, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

Ratcheting-up self-interest to meet the Paris Agreement, the ‘hybrid’ approach. The second step of this study derives a metric to assess the ambition of current NDCs and their future ratcheting-up, under the Paris Agreement mitigation goals and the CBDR-RC principle (UNFCCC 1992, Winkler and Rajamani 2014b, UNFCCC 2015a). We quantify the ‘hybrid’ approach to reflect countries’ preferences, in 2030, for equity allocations based on dimensions of equality,

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responsibility and capability (Winkler and Rajamani 2014b). Each country is attributed the equity approach with the least-stringent 2030-emissions, excluding the CER and GDR approaches (section 8.2 Additional methods). Current national emissions ratios, at the root of the CER approach (Caney 2009, Robiou du Pont et al 2017, Peters et al 2015), also influence near-term allocations of any continuous allocations, reflecting some of the national circumstances mentioned in the Paris Agreement (UNFCCC 2015a). The GDR approach is based on historical emissions and GDP per capita that are covered in the CPC and CAP approaches, respectively. The GDR approach relies on hypothetical projections of Gini indices and emissions (here downscaled from RCP8.5, see section 8.2 Additional methods) that can lead to large variations in emission allowances. These variations can be more determined by counterfactual input assumptions than by the effort-sharing principles themselves (http://paris-equity-check.org/). We therefore present a synthesis of the EPC, CPC and CAP approaches in a ‘CBDR-RC hybrid’ setup that enables the derivation of an NDC-warming assessment tool ‘applicable to all’. We also provide in the Supplementary Information results under a ‘hybrid’ setup that includes the GDR, which represents a ‘right to development’, and that uses the five effort-sharing approaches. To assess the ambition of national climate pledges, we compare countries’ NDCs (using the ‘average’ assessment of ref. (Meinshausen and Alexander 2015)) to the ‘hybrid’ allocations under global scenarios with 2100 median warmings ranging from 1.2°C to 5.1°C (Figure 8.3, section 8.2 Additional methods). We find that the NDCs of Canada, China, and Russia are less ambitious than their ‘hybrid’ allocations even under the least ambitious global emissions scenario we analyzed. Japan’s NDC is consistent with a 4.3°C median warming, the USA’s with 4°C, Brazil’s with 3.7°C, the EU28’s with 3.2°C, and India’s with 2.6°C. The aggregated emissions of ‘other countries’ are aligned with a 1.7°C warming. An assessment for 32 countries (Climate Action Tracker 2017) finds similar results for the NDCs of USA, Russia, and Indonesia, higher warming assessments for Ethiopia and the Philippines, and lower warming assessments for Australia, Brazil, Canada, China, Japan, India and the EU. However, the progresses of Brazil on deforestation are not accounted in this study as land-use emissions are excluded. Discussion As a result of the range of modelling assumptions in IAMs, multiple global scenarios with similar 2030-emissions values feature a range of 2100-warmings. Combining this scenario-related uncertainty of the link between global 2030-emissions and 2100-warming with that of the NDC assessment, we obtain a range of possible temperature assessments for each NDC with the ‘hybrid’ approach (Figure 8.3a). Here, we use second-degree fit to convey the relationship between the 2030-emissions allocations under the ‘hybrid’ approach and the 2100-warming of the corresponding global scenario. Choosing alternative scenarios subsets, or alternative representations of the scenario-induced uncertainty range would therefore affect the NDC temperature assessments but would not substantially change countries’ NDC-temperatures ordering.

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Figure 8.3 | Global warming responses under the ‘CBDR-RC hybrid’ approach following NDC ambitions. a, For the top-fifteen emitters, 2030-emissions as a function of 2100 median warming above pre-industrial levels under the hybrid approach (coloured lines). Disks indicate the ‘average’ NDC assessment and their sizes are proportional to 2010 emissions. Vertical ranges (coloured rectangles) indicate NDC assessment ranges (Meinshausen and Alexander 2015). The horizontal uncertainty ranges (coloured rectangsles) over the warming of the IAM scenarios (coloured dots) that lie within the vertical NDC assessment range. Colours indicate countries’ world regions (see map inset). b, Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘average’ NDC ambitions for 169 countries as calculated in panel a. (maps with ‘high’ and ‘low’ NDC quantifications in Supplementary Figures 9 and 10). The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states are represented by their maritime zones.

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Under the ‘hybrid’ combination, the equity approach followed by a country influences the amount of emissions available for other countries. Results therefore strongly depend on the set of effortsharing approaches included in ‘hybrid’ approach modelling. Using five approaches representative of the five effort-sharing categories quantified in the IPCC-AR5 results in lower temperature assessments of developed countries’ NDCs and consequently in higher temperature assessments of other countries’ NDCs (section 8.2 Additional methods). The inclusion of the GDR favours mostly Eastern-European countries, Australia and South Africa, partly due to the influence of relatively high business-as-usual emissions (section 8.2 Additional methods). Conclusion The UNFCCC and Paris Agreement do not indicate how to operationalize the CBDR-RC and countries supposedly build their pledge based on their own understanding of fairness, often selfinterested (Averchenkova et al 2014, Fleurbaey et al 2014, Lange et al 2010, Tørstad and Sælen 2017). As no single definition of fairness emerges from current NDCs (Winkler et al 2017), we quantify a combination of concepts of equity that reconciles the bottom-up ‘pledge and review’ architecture of the Paris Agreement’s with its top-down mitigation goals. The resulting metric provides a temperature assessment of countries’ NDCs under the current regime and can inform the ratchetting-up process, without hypothesizing an international agreement on a single approach of equity. This ‘hybrid’ combination of countries’ least-stringent equity approaches is also relevant to climate cases where the court can only rule for the least-ambitious end of an equity based range (Urgenda 2017). We find that most of the least-developed countries have NDCs consistent with the Paris Agreement goals. However, the NDCs of most developing countries appear insufficient, as do those of developed countries who agreed to take the lead in reducing emissions and mobilizing finance to support mitigation in developing countries (UNFCCC 2015a).

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8.2 Addit io nal met hods Global scenario selection The ‘2 °C-scenario’ is RCP2.6 (van Vuuren et al 2011), the only of the four IPCC-AR5 Representative Concentration Pathways that offers a likely chance of limiting global warming to 2 °C. The ‘1.5 °C-scenario’ in this study is the average of the 39 scenarios selected in ref. (Robiou du Pont et al 2017) to have both net-zero GHG emissions before 2100, including emissions from Land Use, Land-Use Change and Forestry (LULUCF) and international shipping and aviation, and result a median chance to result in global warming below 1.5 °C in 2100. Two scenarios are from the IPCC-AR5 database (hosted at the International Institute for Applied Systems Analysis 10 ) complemented by 37 scenarios of refs. (Rogelj et al 2013, 2012a, 2015). The global emissions scenarios used to derive the range of 2030 allocations under the ‘hybrid’ approach are from the Shared Socioeconomic Pathways (SSP) database (85 emissions scenarios with temperature assessment, hosted at the International Institute for Applied Systems Analysis 11). The 2100 temperature median assessment of these SSP-scenarios range from 1.7 °C to 5.1 °C. These are complemented by lower emissions scenarios from ref. (Rogelj et al 2015) (36 emissions scenarios) whose 2100 temperature median assessment ranges from 1.2 °C to 1.5 °C. The relationship between 2030 national emissions levels and the 2100 temperature responses presented in Figure 8.3 is derived from a representative sub-selection of global emissions scenarios. We standardise the data across both dimensions (2030 emissions excluding LULUCF and bunkers and 2100 warming) and derive the third-degree polynomial fit (Figure 8.4). Using a second-degree polynomial fit would result in a plateau where high global warming hardly depends on 2030 emissions levels. We then select scenarios with the least standardised distance to the fit, starting at the lowest 2100 temperature and every 0.5 °C (9 scenarios, Table 8.1, Figure 8.5). The USA’s 2030 policy projection of 6.74 GtCO2eq without the Clean Power Plan taken from ref. (Climate Action Tracker 2017).

10 11

available at: https://tntcat.iiasa.ac.at/AR5DB available at: https://tntcat.iiasa.ac.at/SspDb

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Figure 8.4 | Selected global scenarios’ 2030 emissions levels, excluding LULUCF, as a function of 2100 global warming. The 9 scenarios are selected (red filling) amongst the 85 scenarios from the SSP-database (blue circles) and the 36 scenarios from ref. (Rogelj et al 2015) (black circles) to align with the third-degree polynomial fit (red line). The 412 IPCC-AR5 scenarios with available data (grey circles) are shown with their third-degree polynomial fit (grey line). The ‘1.5 °C-scenario’, the ‘2 °C-scenario’ (RCP2.6, blue disk) and the business-as-usual scenario (RCP8.5, grey disk) are shown for comparison.

Table 8.1| Selected scenarios representative of the relationship between 2030 emissions and 2100 warming.

Source

Model

Scenario

2100 warming

Ref. (Rogelj et al 2015) SSP SSP

REMIND AIM/CGE MESSAGEGLOBIOM AIM/CGE IMAGE IMAGE AIM/CGE AIM/CGE WITCH-GLOBIOM

Scen135 SSP1-26 SSP2-34

1.2 °C 1.8 °C

2030 emissions (in GtCO2eq) 25.7 37.4

2.2 °C 2.7 °C 3.3 °C 3.9 °C 4.1 °C 4.7 °C 5.1 °C

47.1 54.0 57.9 62.1 66.6 67.4 73.9

SSP SSP SSP SSP SSP SSP

SSP5-45 SSP2-60 SSP3-Baseline SSP3-Baseline SSP5-Baseline SSP5-Baseline

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Figure 8.5 | Selected global emissions scenarios. The scenario sub-selection (thick lines) from the 85 scenarios amongst the SSP-database (dashed blue lines) and the 36 scenarios from (Rogelj et al 2015) (dashed black lines) are shown with their 2100 warming assessments. The 412 IPCC-AR5 scenarios with available data are shown for comparison (grey dashed line).

Global scenario preparation We used and extended the Potsdam Real-time Integrated Model for the probabilistic Assessment of emission Paths (Nabel et al 2011) (PRIMAP) to model allocations approaches. The database contains population, GDP, and GHG emissions historical and projected data from composite sources as detailed in ref. (Robiou du Pont et al 2016). The aggregation of Kyoto-GHG emissions follows the ‘SAR GWP-100’ (Global Warming Potential for a 100-year time horizon), consistently with the reporting under UNFCCC 12. The national emissions allocations derived in this study do not cover the LULUCF sector. Emissions from the LULUCF and from international shipping and aviation are removed from the global scenarios before allocating their emissions across countries using the methods and data indicated in ref. (Robiou du Pont et al 2017).

12

available at : (http://unfccc.int/ghg_data/items/3825.php)

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For RCP2.6 and the 85 SSP scenarios, we subtracted CO2 emissions from LULUCF. For the 36 scenarios of ref. (Rogelj et al 2015) (including the ‘1.5°C-scenario’) where no specific LULUCF emissions were available, we subtracted the CO2 emissions that do not come from fossil fuels combustion. The historical emissions of Figure 8.1b are from PRIMAP (Nabel et al 2011) until 2010 and follow the growth rates of ref. (Gütschow et al 2016) until 2014.

Hybrid allocation We name ‘bottom-up’ allocation of a global scenario the allocation to each country of the leaststringent of the scenarios calculated under the CAP, EPC, CPC, GDR and CER approaches. The modelling and parametrization of these five approaches follows that of ref. (Robiou du Pont et al 2016, 2017) (and their Supplementary Information) with the same limitations regarding the data missing for 27 countries and territories. Under the ‘hybrid’ approach, every country picks the least-stringent approach, in terms of cumulative emissions over 2010-2100 (Figure 8.1 and Figure 8.2) or 2030 emissions levels (Figure 8.3), while staying below a warming threshold. The modelling of the hybrid allocation consists in iterative steps to derive a global ‘aspirational pathway’ whose ‘bottom-up’ allocation matches any chosen emissions scenarios from IAM. The iterative process starts by calculating the difference ‘D(1)’ between the chosen IAM scenario (‘IAMscenario’) and the ‘bottom-up’ allocation of that chosen IAM scenario ‘BU(IAMscenario)’. We then build a first aspirational emissions scenario ‘A(1)’ that is ‘IAMscenario’ discounted by half the calculated difference ‘D(1)/2’. A(1) = IAMscenario – (BU(IAMscenario) – IAMscenario)/2. The following step consists in calculating the difference ‘D(2)’ between ‘IAMscenario’ and the ‘bottom-up’ allocation of the new aspirational pathway ‘A(1)’. We then build a new aspirational emissions pathways ‘A(2)’ that is A(1) discounted by the difference ‘D(2)’/2: A(2) = A(1) – (BU(A(1)) – IAMscenario)/2. These steps are repeated iteratively until BU(A(n)) = IAMscenario or until A(n+1) = A(n): A(n+1) = A(n) – (BU(A(n)) – IAMscenario)/2. Note that A(0) = IAMscenario. Figure 8.7 shows the national emissions allocations of the aspirational 2°C-scenario under the five selected equity allocation approaches (Figure 8.7a-e) and under its ‘bottom-up’ allocation which is also the ‘hybrid’ allocation of the 2°C-scenario (Figure 8.7f). The most favourable approach for a country may differ when considering a different global scenario, and therefore may also change over the iterative process.

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The allocations of country-groups presented in this study (EU28, ‘G8+China’ or the ‘rest of the world’) are calculated based on the least-stringent approach of each of their members individually, rather than the least-stringent approach for the country group.

Table 8.2 | Equity approaches allocated to countries under the ‘bottom-up’ allocation of the 2 °C-scenario used in Figure 8.1. Here, the ‘bottom-up’ approach attributes to countries equity approaches that provide the greatest cumulative emissions between 2010 and 2100.

Least stringent approach Capability: Countries with high GDP per capita have low emissions allocations.

Equal per Capita: Convergence towards equal annual emissions per person in 2040. Greenhouse Development Rights: Countries with high GDP per capita and high historical emissions per capita have low emissions allocation (Baer et al 2008, Kemp-Benedict 2010, Meinshausen et al 2015). Equal cumulative per capita: Populations with high historical emissions have low emissions allocations.

Constant emissions ratios: Maintains current emissions ratios, preserves status quo.


Country ISO ALPHA-3 codes 35 countries: BDI, BEN, BFA, BGD, CIV, CMR, COM, DJI, ERI, ETH, GHA, GIN, GNB, GRD, HTI, LBR, LSO, MDG, MLI, MOZ, MWI, NER, NPL, RWA, SEN, SLE, SOM, STP, TCD, TGO, TON, TZA, UGA, ZMB, ZWE 5 countries: BRB, CUB, LCA, VCT, WSM

41 countries: ARM, AZE, BGR, BHR, BIH, BLR, BOL, CAF, COD, COG, CYP, DZA, GEO, GUY, HRV, HUN, IRQ, JOR, KAZ, KGZ, LVA, MDA, MKD, MLT, MNG, MRT, NAM, POL, PRY, ROU, RUS, SDN, SLB, SRB, TJK, TKM, UKR, UZB, VEN, YEM, ZAF 44 countries: BLZ, BRA, BTN, BWA, COL, CPV, CRI, DOM, ECU, EGY, FJI, GMB, GNQ, GTM, HND, IDN, IND, JAM, KEN, KHM, LAO, LBN, LKA, MAR, MDV, MEX, MUS, NGA, NIC, PAK, PAN, PER, PHL, PNG, PRI, SLV, SUR, SWZ, SYR, TLS, TUN, TUR, VNM, VUT 47 countries: ARG, AUS, AUT, BEL, BHS, BRN, CAN, CHE, CHL, CHN, CZE, DEU, DNK, ESP, EST, FIN, FRA, GAB, GBR, GRC, HKG, IRL, IRN, ISL, ISR, ITA, JPN, KOR, KWT, LBY, LTU, LUX, MYS, NLD, NOR, NZL, OMN, PRT, SAU, SGP, SVK, SVN, SWE47, THA, TTO, URY, USA

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Table 8.3 | Equity approaches attributed to countries under the ‘bottom-up’ allocation of the 2 °C-scenario used in Figure 8.3. Here, the ‘bottom-up’ approach attributes to countries equity approaches that provide the highest 2030 emissions levels.

Least stringent approach Capability

Equal per Capita

Equal cumulative per capita

Country ISO ALPHA-3 codes 48 countries: BDI, BEN, BFA, BGD, CAF, CIV, CMR, COD, COM, CPV, DJI, ERI, ETH, GHA, GIN, GMB, GNB, GRD, GUY, HND, HTI, KEN, LAO, LBR, LKA, LSO, MDG, MLI, MNG, MOZ, MRT, MWI, NER, NIC, NPL, PAK, RWA, SEN, SLE, SOM, STP, TCD, TGO, TON, TZA, UGA, ZMB, ZWE 61 countries: ARG, AUS, AUT, BEL, BGR, BHR, BHS, BIH, BLR, BRB, CAN, CHE, CHN, CYP, CZE, DEU, DNK, ESP, EST, FIN, FJI, FRA, GBR, GRC, HRV, HUN, IRL, ISL, ITA, JPN, KAZ, KOR, KWT, LBY, LCA, LTU, LUX, LVA, MDA, MLT, NLD, NOR, NZL, OMN, POL, PRT, ROU, RUS, SAU, SLB, SRB, SVK, SVN, TKM, UKR, URY, USA, VCT, VEN, WSM, ZAF 63 countries: ARM, AZE, BLZ, BOL, BRA, BRN, BTN, BWA, CHL, COG, COL, CRI, CUB, DOM, DZA, ECU, EGY, GAB, GEO, GNQ, GTM, HKG, IDN, IND, IRN, IRQ, ISR, JAM, JOR, KGZ, KHM, LBN, MAR, MDV, MEX, MKD, MUS, MYS, NAM, NGA, PAN, PER, PHL, PNG, PRI, PRY, SDN, SGP, SLV, SUR, SWE, SWZ, SYR, THA, TJK, TLS, TTO, TUN, TUR, UZB, VNM, VUT, YEM

Figure 8.6 | Scenarios towards 2 °C and 1.5 °C (thick solid blue and red lines) shown with their corresponding ‘bottom-up’ allocation (thin lines) and aspirational scenarios convergence runs (thin dashed lines). Unconditional, conditional and average (I)NDC assessment are shown in grey. LULUCF and bunker emissions are excluded. Converging aspirational scenarios (thin dotted lines) converge over 15 runs towards 2 °C and 1.5 °C (thin solid lines).

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Figure 8.7 | Distribution of the aspirational pathway towards 2 °C according to the five equity approaches and under a ‘bottom-up’ approach. The ‘bottom-up’ distribution (based on largest cumulative emissions by 2100) of the aspirational 2 °C-scenario (black line) results in emissions matching that 2 °C-scenario (RCP2.6 excluding LULUCF emissions, red line). Each colour patch represents a country.

We update the CPC modelling approach when applied to global emissions scenarios with positive emissions in 2100 to avoid national positive emissions after a period of negative emissions. The CPC approach then derives national ratios of the global emissions scenarios that are positive in 2100. These national ratios are a linear interpolation between 2010 emissions ratios and the 2100 ratios that result in equal cumulative per capita emissions. The impact of high historical per capita emissions has therefore a lower impact on 2030 emissions than under the CPC modelling applied to global scenarios with negative 2100 emissions. The lesser ‘equity’ stringency on 2030 emissions aligns with the lesser global stringency. For example, the influence and importance of equitable allocation is lower when applied to business-as-usual scenarios. The accounting of historical emissions since 1990 and the ‘autonomous energy efficiency improvement’ index are similar in both CPC setups. Under the ‘CBDR-RC hybrid’ setup used in Figure 8.3, the ‘hybrid’ approach is based on countries’ least-stringent of three approaches only (CAP, EPC and CPC), following their 2030 allocations. Using this methodology, the least-stringent approaches applied to the 2°C-scenario (RCP3PD), corresponding to a ‘CBDR-RC bottom-up’ situation, is show in Figure 8.8a. The leaststringent approaches of the aspirational 2°C-scenario, corresponding to a ‘CBDR-RC hybrid’ approach, is shown in Figure 8.8b. Only few countries have different least-stringent approaches under the ‘CBDR-RC bottom-up’ and ‘CBDR-RC hybrid’ cases.

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Figure 8.8 | Least-stringent of three approaches (CAP, EPC and CPC), by lowest 2030 emissions, under a ‘bottom-up’ allocation (panel a), and hybrid allocation (panel b) of the ‘2°C-scenario’.

The comparison between a ‘complete hybrid’ (using all five burden-sharing approaches) and a ‘CBDR-RC hybrid’ (using only the CAP, EPC and CPC equity approaches) based on leaststringent 2030 emissions, is show in Figure 8.9a. Allocations under the ‘CBDR-RC hybrid’ are lower for the G8 and China, and higher for other countries altogether, compared to the ‘complete hybrid’. Allocations are similar under a ‘CBDR-RC hybrid’ and under the average of the CAP, EPC and CPC allocations for both the G8 and China as a group, and for other countries altogether (Figure 8.9b). However, differences arise at the national level (Figure 8.9c). The (I)NDCs of the G8 and China group are 64 percentage-points of 2010 emissions short to meet the allocation of the ‘CBDR-RC hybrid’ applied to the 2°C-scenario. Other countries altogether are 8 percentage-points short. Compared to the 2°C-scenario, reaching the 1.5°C-scenario results in less additional effort, relative to 2010 emissions, for the G8 and China group then for all other countries (Figure 8.9d).

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Figure 8.9 | Comparison of emissions changes by 2030 under the ‘Complete Hybrid’; ‘CBDR-RC Hybrid’ least-stringent 2030 emissions with the allocations average and with (I)NDCs. a, Comparison of the ‘Complete Hybrid’ (with the five burden-sharing approaches) and the ‘CBDR-RC Hybrid’ (with the three equity approaches: CAP, EPC and CPC) emissions. b, Comparison of the ‘CBDR-RC Hybrid’ allocation of 2°C-scenario and the average of the three equity allocations CAP, EPC and CPC. c, Comparison of the ‘CBDR-RC Hybrid’ allocation of 2°C-scenario and countries’ average INDCs. d, Comparison of the ‘CBDR-RC Hybrid’ allocations of a 2°Cscenario and a 1.5°C-scenario. Disks’ sizes are proportional to 2010 emissions level. Colours indicate countries’ world regions, and the G8+China (larger disk) and the rest of the world (smaller disk) are shown in grey.

Discussion on the monotony and uncertainty The bijectivity between NDCs ambition and their temperature assessment relies on the strict monotony of the relationship between global scenario’s 2030 emissions and the 2100 warming. We selected nine global scenarios every 0.5°C to achieve such strict monotony at the global level.

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The 2030 emissions levels dependency to the NDC temperature assessment of Figure 8.3 is a second-degree polynomial fit based the allocations derived from the selected nine global scenarios. A second-degree polynomial approach smoothens the variability while preserving the greater sensitivity of national 2030 emissions allocations at lower 2100 warmings. For 36 countries, relationships between 2030 emissions and 2100 warming non-strictly monotonous and oscillate. In each case, the inflection points are at 4.8°C or more, higher than their NDC assessments of the corresponding countries. The national emissions allocations at high temperature are only indicative for these countries. In the absence of interpolation, inflection points are found for 14 countries, and are at lower temperature than the NDCs of: Iraq, Trinidad and Tobago, Jordan, Brunei Darussalam and the Maldives. The allocation of high global emissions pathways using equity approaches reflects equitable contributions to high global warming. However, the allocation of global BaU scenarios results in national scenarios that are, de facto, no longer business-as-usual. The range of 2030 emissions levels from global BaU scenarios reflect a range of modelling assumptions rather than a range of ambitions. The high-warming allocations derived in this study serve a metric to assess NDCs. The equity allocations of BaU scenarios, and facing the impacts of global warming of 3.9°C or more cannot represent equitable objectives (Althor et al 2016). The uncertainty resulting from the various NDC quantification is shown in Figure 8.3 by the height of the NDC ranges. The ‘high’ assessments assumes the most optimistic end of the assessment, including the application of conditional targets (Meinshausen and Alexander 2015). The ‘low’ NDC assessment takes the least optimistic assumptions of unconditional targets. The maps of global warming responses under ta ‘CBDR-RC hybrid’ approach associated with ‘high and ‘low’ NDC assessments are shown in Figure 8.10 and Figure 8.11.

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Figure 8.10 | Global warming responses under a ‘CBDR-RC hybrid’ approach, following ‘high’ quantifications of NDC (Meinshausen and Alexander 2015). Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘high’ NDC quantifications for 169 countries, as calculated in Figure 8.3a using the quadratic curve fit. The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states by their maritime zones.

Figure 8.11 | Global warming responses under a ‘CBDR-RC hybrid’ approach, following ‘low’ quantifications of NDC accounting for conditional pledges (Meinshausen and Alexander 2015). Global warming assessment (50% likelihood, compared to pre-industrial levels) of ‘low’ NDC quantifications for 169 countries, as calculated in Figure 8.3a using the quadratic curve fit. The assessment ranges from 1.2°C to 5.1°C, NDCs outside this range are not differentiated. Small island developing states by their maritime zones.

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Choice of equity approaches The IPCC-AR5 presented quantifications of emissions reduction following five effort-sharing categories (Chapter 6, Figure 6.28 ref. (Clarke et al 2014)), which reflect combinations of three underlying equity principles: responsibility, equality and capability. In addition, the IPCC-AR5 presented a category (but did not present a quantification) based on responsibility only as proposed by Brazil in 1997 that derives emissions goals without allocation (IPCC-AR5 Chapter 6, table 6.5, ref. (Clarke et al 2014, Höhne et al 2014)). The historical responsibility of countries for their past emissions is modelled here with the equal cumulative per capita approach (CPC) that uses historical emissions and population data to calculate countries’ emissions allocations. The GDR approach also uses historical emissions and accounts for countries historical responsibility. Other approaches of distributive justice (for example based on sufficiencatrian approach (Arneson 2013), or using the Human Development Index) and other metrics (for example accounting for consumption-based emissions or exported emissions) not currently used in the IPCC report or under the UNFCCC are not modelled here but would bring useful perspectives that could be integrated in the ‘hybrid’ approach. The selection of equity approaches to derive countries’ least-stringent allocations directly influences the hybrid allocations of all countries. Removing the least-stringent approach of a country group would penalize these countries that would have to follow a more stringent approach, and would consequently favour all other countries. The ‘hybrid’ combination of five equity approaches is representative of the five burden-sharing categories quantified in the IPCC-AR5. The ‘complete’ hybrid allocation, which uses the five effort-sharing approaches, results in lower temperature assessments of developed countries’ NDCs and consequently in higher temperature assessments of other countries’ NDCs (Figure 8.12). The assessment of China’s NDC is still higher than the temperature scale range.

Figure 8.12 | Global warming responses under a ‘complete’ hybrid approach following NDC ambitions without interpolation. Global warming responses (median assessment) following NDC ambitions.

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The choice of the five equity approaches includes a contentious grandfathering approach (CER) that is only implicitly supported by some countries through their pledges. The CER approach represents a status-quo in terms of equity where all countries conserve their share of global emissions and mitigate at the unique rate, that of the global scenario. The GDR approach, categorized as a ‘responsibility-capability-need’ (Clarke et al 2014), preserves a ‘right to development’ through the allocation of mitigation requirements (Baer et al 2008, Kemp-Benedict 2010, Meinshausen et al 2015). Excluding only the CER approach, and including the GDR, represents four equity approaches representative of the four key equity principles dimensions: “responsibility, capacity, equality, and the right to sustainable development” (IPCC-AR5 WG3 Chapter 4, Section 4.6.2.1., ref. (Fleurbaey et al 2014)). The modelling of the GDR approach relies on business-as-usual (BaU) emissions that countries do not mutually recognize. The BaU emissions used here are downscaled from RCP8.5 resulting in allocations substantially higher than other allocations for Eastern-European countries and Australia. Compared to the ‘hybrid’ setup presented in Figure 8.3, the inclusion of the GDR approach in the ‘hybrid’ setup results in a lower temperature assessment in favour of the NDCs of Eastern-European countries, Australia, and South Africa, and higher temperature assessment disfavouring India, Brazil, Mexico and Indonesia (Figure 8.11). Removing both the GDR and the CER approaches from the hybrid allocation (Figure 8.3) leaves equity approaches that rely on measurable population and GDP data that can be updated over time.

Figure 8.13 | Global warming responses under a hybrid approach, including the GDR but excluding CER, following NDC ambitions. Global warming responses (median assessment) following NDC ambitions.

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Temperature assessment of the global ‘bottom-up’ and ‘aspirational’ scenarios The evaluation of the warming resulting from the ‘bottom-up’ scenarios requires the calculation of their GHG compositions. The Equal Quantile Walk (EQW) method (Meinshausen et al 2006) is used to derive a multi-gas scenario that is needed by the simple climate model MAGICC (Meinshausen et al 2011a, 2011b) for the evaluation of the temperature response. Land-use CO2 is taken directly from the target 2°C-scenario and 1.5°C-scenario. The probabilistic temperature projections of the ‘bottom-up’ multi-gas scenario is constrained by historical global mean temperature observations and a priori estimates of uncertain model parameters, such as climate sensitivity (Meinshausen et al 2009). We use MAGICC version 6.8 and climate sensitivity distribution from ref. (Rogelj et al 2012b). The ‘aspirational’ scenarios are of purely numerical nature and have no underlying economic assumptions. These scenarios, which include bunkers but exclude land-use emissions, show a steep decline in the first half of the century with minima in 2070 (‘aspirational’ 2°C-scenario, Figure 8.14a) and 2060 (‘aspirational’ 1.5°C-scenario, Figure 8.14b) and very low emissions throughout the second half of the century. The EQW is based on older scenarios that did not have negative CO2 emissions. The EQW thus cannot model negative fossil CO2 emissions. However, these negative emissions are necessary to reach the low emissions levels in the second half of the century. We calculate multi-gas emissions scenarios consistent with global ‘aspirational’ scenarios given in CO2-equivalent units as the aggregation of all Kyoto-GHG. A dataset of full-gas 1.5°C scenarios (Rogelj et al 2015) is harmonized to 2010 emissions (Nabel et al 2011). Harmonization is carried out for the global aggregate Kyoto-GHG value excluding land-use emissions but including bunkers emissions. Bunkers emissions are included because for most IAM scenarios they are not available as independent time series but included in the world total and regional time series. The harmonization factor linearly converges to unity in 2040, and scenarios are unchanged onwards. The Kyoto-GHG harmonization factor is used for all substances. From the harmonized database, we select the ten scenarios with the least mean-square distance to the ‘aspirational scenario’ over the 2010-2100 period. Absolute distances are used because relative distances are not meaningful to compare positive and negative values, which is inevitable when using low emissions scenarios with negative emissions. The average across the ten selected scenarios is taken for each gas individually. In order to match the ‘aspirational’ scenario, only the CO2 emissions levels are reduced, which corresponds to additional mitigation implemented in the fossil CO2 sector. Indeed, fossil CO2 emissions can be mitigated more profoundly than other GHG (e.g. methane from agriculture). Carbon dioxide is the only gas where prototypes for large scale negative emissions technologies exist (even though cost and acceptability of large scale projects is uncertain). Conversely, additional fossil CO2 emissions are assumed where the ‘aspirational’ scenario is higher than the average of the selected scenarios. Land-use CO2 is taken directly from the target 2°C-scenarios and 1.5°C-scenarios. The resulting multi-gas ‘aspirational’ scenarios are then used for probabilistic temperature assessment as described above for the ‘bottom-up’ scenarios.

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Figure 8.14 | Scenario selection for the determination of the GHG composition, including bunker emissions, of the 1.5 °C ‘aspirational’ scenarios (panels a.) and 2 °C ‘aspirational’ (panel b.).

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CHAPTER 9 – DISCUSSION AND CONCLUSION 9.1. Summary This thesis aimed to take into consideration countries’ divergent positions on equity when quantifying emissions allocation for staying within global warming thresholds. This section summarises the contributions of this thesis towards that aim. First, a presentation of the current state of international negotiations resulted in the identification and justification of the research questions addressed in this thesis (Chapter 1). Second, a review of the evolution of negotiations and the national position regarding climate justice (Chapter 2) provided a framework for understanding the relevance of, as well as the gaps in, the literature on the quantification of climate justice concepts (Chapter 3). Third, a modelling framework – the ‘PRIMAP-Equity’ framework – was introduced to model emissions allocations that are representative of the IPCC’s five equity categories (Chapter 4, Chapter 5 and Chapter 6). From this, multiple views on climate change were compared with consistency under a given global cost-optimal emissions scenario. This framework was used to assess equitable contributions for G7 and BRIC members (Chapter 4). Results at the regional scale were compared by equity category to the results in the literature, in line with the categories introduced in IPCC-AR5. The ‘PRIMAP-Equity’ framework was applied specifically to the Paris Agreement goal (Chapter 6) and produced the first assessment of national emissions trajectories consistent with the newly adopted goal of limiting warming to 1.5 °C. The influence of global ambition on national emissions allocations was studied consistently across almost all countries and systematically compared to (I)NDCs. Specific milestones were identified at the national level, such as the dates and levels for peaking emissions, and the dates for emissions phase-out. The influence of countries (or country-groups) adopting equitable approaches on the global 2030 emissions gaps was discussed under both the 2 °C and 1.5 °C goals. Using the results of Chapter 6 for G20 members across the five equity metrics, the ambition of G20 members’ pledges was compared to the concept of fairness supported in their NDC, if any (Chapter 7). The multi-equity allocation framework indicates the level of mitigation required for each country to align with its preferred concept of fairness. However, given each country’s tendency to support the least stringent concept of equity, even if each country aligns its pledges with its preferred concept of equity, the aggregate is insufficient to limit warming to the agreed threshold. The lack of progress towards a common operationalisation of the CBDR-RC concept for allocating emissions has steered negotiations away from an agreement on effort-sharing. It has been argued that equity is no longer the drive for mitigation ratcheting-up (Averchenkova et al 2014). Yet, the current lack of ambition in existing pledges and policy projections (Climate Action Tracker 2017) indicates that additional efforts will be necessary beyond those opportunities that are created from

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the reduced costs of some mitigation technologies. An equity metric helps to identify which countries should make additional efforts to stay below the warming threshold. The ‘uncoordinated’ or ‘bottom-up’ architecture structure of the Paris Agreement requires every country to set its own mitigation targets. The Agreement also asks Parties to describe how they consider their mitigation commitment to be fair and ambitious. The method introduced in Chapter 8, taking into consideration the dissonant national views of equity, calculated the national mitigation targets that are needed to ratchet up ambition consistently within a bottom-up architecture. This novel ‘hybrid’ method is a metric that allows countries individually to opt for the least stringent of a set of equity concepts, and collectively to stay within the Paris Agreement thresholds. This ‘real-world’ combination of equity is used to derive a temperature scale for assessing national pledges.

The results derived in this thesis help to inform public opinion and decision makers on emissions trajectories consistent with their own, or other countries’, views of equity. Countries and civil society can use these results to review and judge countries’ ambition in the light publicly expressed concepts of fairness. The results of this thesis also inform countries in the ratcheting-up of their 2030 pledges (on emissions levels that could be considered ‘fair and ambitious’) and in the determination of longer-term targets (e.g. fair and ambitious 2050 emissions targets or when to reach net-zero emissions – Chapter 6). However, even the alignment of each country’s effort with their publicly stated concept of fairness is insufficient for achieving the Paris Agreement goals. Countries tend to support concepts of equity that are most favourable to their own national interests. The aggregation of each countries’ most favourable allocation of a global 2 °C global scenario results in a warming of 2.3 °C (median assessment), and likely below 2.5 °C in 2100, if that bottom-up situation is continued throughout the century (Chapter 8). The ‘hybrid’ method introduced in this PhD project combines equity concepts relevant to the uncoordinated ‘pledge-and-review’ architecture of the Paris Agreement and avoids the need to weight the importance of equity principles against each other. This distribution, which could potentially include of all concepts of equity, is a solution to avoid negotiating one equity concept applicable to all countries. As such, this approach does not require any equity concept to win the debate over others. Instead, it offers a way forward that includes all countries’ concepts of fairness, where each country’s concept of fairness is applied to that country, but is not used to judge another country ambition.

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9.2 Limit at ions This section presents the numerical and conceptual limitations associated with the modelling choices made in the design of the ‘PRIMAP-Equity’ framework, or of the ‘hybrid’ approach.

9.2.1 Numerical uncertaint ies Beyond the limitations inherent to the conceptual framework discussed in Section 1.5.2, the methodologies and results have specific numerical limitations. Two key sources of numerical variability are (1) historical data, future projections of population, GDP and emissions, and (2) parameterisation of the model. A source of uncertainties of the results of this thesis lies in the data used to quantify the equity allocations. The modelling of the equity allocations presented in Chapter 4 relies on historical data of: emissions and land-use emissions from 1990, and population data. It also relies on projections to 2100 of national business-as-usual emissions, global emissions mitigation scenarios (including land-use and bunker emissions), population figures, GDP data, and Gini indices. While the uncertainty of historical data is relatively low from 1990 in particular for developed countries (Gütschow et al 2016), projections to 2100 have much greater uncertainties affecting uncertainties in the equity allocations. In particular, hypothetical business-as-usual emissions and GDP projections comprise the greatest uncertainties. This was explored across a range of parameters and data sources in Chapter 5. A more systematic analysis, describing the uncertainty linked to each data source would inform on the greatest sources of uncertainties. The results of this PhD project can also be compared to similar modelling using different projections – e.g. alternative GDP and business-as-usual projections (CSO Equity Review Coalition 2015). A recent study models similar equity concepts using different underlying data scenarios and finds overall similar NDC assessments (Pan et al 2017). Beyond the influence of the underlying data, the influence of a range of parameters on the five equity allocations is explored in Chapter 5. Comparison with additional modelling or parameterisation choices under similar global scenarios would inform on the influence of specific modelling choices. Recent studies provide national equitable emissions allocations that add up to cost-optimal global scenarios (Pan et al 2015, 2017). The results of Chapter 6 are compared to alternative modelling of similar equity concepts under close, but not equal, global scenarios (Pan et al 2017). An in-depth analysis of the multiple modelling, parameterisation and underlying databases would indicate which choices are the most critical for decision makers to discuss. The most significant source of direct physical uncertainties affects the relationship between emissions and temperature response. While part of the perceived uncertainty regarding the consistency of emissions trajectories with temperature goals arises from the definition of climate goals (Chapter 2) and societal choices (Peters 2018, Rogelj et al 2016b) (Chapter 8), the modelling of the earth system adds a layer of uncertainty to all results presented in this thesis. The simple carbon-cycle model MAGICC used here deviates from the mean of more complex Atmosphere-

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Ocean Global Circulation Model (AOGCM) by 2.2% (Meinshausen et al 2011a) – noting that uncertainties from parametric and structural projections in climate models are at least a magnitude larger. For example, the 2100 temperature range estimated by IPCC-AR5 based on multiple lines of evidence for scenario RCP8.5 is 3.2 °C to 5.5 °C warmer than 1850-1900 levels (IPCC 2014a). The ability of these climate models to simulate global-mean surface temperature is improving and they reproduce many features of the variance observed on interannual to centennial time scale (Flato et al 2013). The simulation of clouds remains challenging and results in much of the spread of climate sensitivity across AOGCMs.

9.2.2 Limitat io ns regarding countries co nsideratio ns for relat ive and abso lute gain The emissions allocations presented in this PhD thesis are based on principles of equity applicable to all and therefore ignore the motivation of a country to preserve or gain a relative advantage with respect to another (Chapter 2). These allocations therefore inform on the question of how countries’ pledges are “fair and ambitious”, which should be answered in NDCs under the Paris Agreement (UNFCCC 2015a). However, considerations of relative gains are fundamental drivers for national positions and it is unclear to what extent countries evaluated their pledges relative to others (Rose et al 2017). The position on fairness that many countries expressed in their NDCs (submitted before the Paris Agreement) can be explained more by their categorisations as Annex I or non-Annex I, than by their situation on the fairness indicators such as those developed in this PhD thesis (Tørstad and Sælen 2017). Prior to the Paris Agreement, advanced economies agreed to take the lead in fighting climate change (UNFCCC 1992) but did not want to sign in Paris an agreement that would erode their advantage over developing countries. The agreement that all nations should put forward increasingly ambitious climate policies was key to achieving the Paris Agreement (Obama 2017). Considerations for relative gains are particularly important amongst the largest economies, which are also the largest emitters. The absence of such considerations of relative gains brings limitations when using the material developed in this PhD thesis to explain countries’ positions at negotiations often contrasting with their statement of fairness. However, the results of this thesis remain relevant to inform on ‘fair and ambitious’ sharing of mitigation efforts based on equity principles applicable to all.

9.2.3 Limitat ions of combining equit y co ncepts Another limitation relates to the philosophical interpretation of blending multiple equity principles. Different interpretations of distributive justice have been modelled independently in the literature (Chapter 3), and in this thesis (Chapter 4 and Chapter 6). Previous studies have operationalised the CBDR-RC principle by ‘blending’ multiple visions of distributive justice into one approach applicable to all countries. Such ‘blending’ of multiple equity principles was done using weighting factors (Baer et al 2008, Raupach et al 2014, Peters et al 2015), and an average was used to

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compare national results under various global goals in Chapter 6. Weighting factors affect on the results depending on how they are implemented. For example, weighting the distribution of an effort – the difference between a business-as-usual scenario and the targeted scenario – differs from weighting emissions allocation. The implications and acceptability of using weighting factors as an ethical or political compromise are not discussed in this study.

The ‘hybrid’ approach introduced in Chapter 8 presents an alternative to weighting factors. While the approach is applicable to all countries, it attributes different equity principles to different countries. Therefore, the hybrid approach appears inconsistent with the idea of a global equity principle applicable to all countries. Rather, this perspective matches the current ‘bottom-up’ architecture of the current international climate governance reflective of the multiple equity perspectives of sovereign countries. The philosophical or ethical implications of a differentiated approach are not discussed in this work, which focuses on the actual quantification of approaches. More generally, this thesis informs the discussion on the operationalisation of the CBDR-RC as distinct, blended or differentiated equity principles. The equity concepts modelled in this thesis are consistent with a maximisation of ‘absolute gains’ for all countries (Chapter 2). However, the preference stated by countries for an equity principle over another, may be driven by ‘relative gain’ perspectives. The ‘hybrid’ approach, with the attribution to each country of the most favourable equity concept, is consistent with countries’ preferences, in terms of ‘relative gain’ over other countries, for an ‘absolute gain’ equity concept over another. The ‘hybrid’ approach accounts for countries’ ‘relative gain’ considerations in general, but not towards specific countries. Overall, the inclusion of each country’ preference in terms of ‘relative gain’ for an equity concept results for each country in national emissions allocations less favourable than some of the underlying equity concepts. The combination of multiple equity concepts also raises the issue of the overlap of the included equity approaches. Indeed, ‘blending’ or ‘hybridising’ equity approaches that are not orthogonal results in an overrepresentation of the redundant equity dimensions. For example, the dimensions of capability and historical responsibility are each included in two of the five allocations averaged in Chapter 4 and Chapter 6. Furthermore, the inclusion of an approach that is reflective of some countries’ implicit positions but not deemed fair by the ethical literature, and not even advocated as such by government representatives is controversial. While it represents the political landscape (Chapter 6), a combination that includes an ‘unfair’ approach is unlikely to be widely accepted. A recognition by the UNFCCC of a limited set of equity dimensions following – for example the five equity categories introduced in the government approved IPCC report – would enable experts to design politically relevant combinations. Convergence by governments to a limited set of equity approaches in the IPCC reports or under the UNFCCC was recognised in the Urgenda climate (The Hague District Court 2015). A convergence on equity approaches reduces the range of equitable emissions targets, and increases the effort enforceable by the court case (see section 9.3).

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9.3 Outlook for climate lit igat ion Concerned by the lack of ambition from their government, a group of 886 Dutch citizens and the Urgenda Foundation filed a climate case against the Dutch Government. On 24 June 2015, the District Court of The Hague ruled that the Dutch government has to reduce emissions by at least the lower end of the 25-40% reduction range, compared to 1990 levels (The Hague District Court 2015, Schiermeier 2015), as quantified the IPCC-AR4 for Annex I country group. This range encompasses the results of 16 studies reflecting multiple equity allocations. The Court therefore decided that the insufficient measures of the Dutch government to prevent dangerous climate change is a breach of its duty of care towards the plaintiffs. Specifically, the Court judged unlawful the adoption of mitigation measures less ambitious than the range resulting from a scientific modelling of equitable mitigation. “The court agrees with Urgenda that by choosing this reduction path, even though it is also aimed at realising the 2 °C target, will in fact make significant contributions to the risk of hazardous climate change and can therefore not be deemed as a sufficient and acceptable alternative to the scientifically proven and acknowledged higher reduction path of 25-40% in 2020” (The Hague District Court 2015, Schiermeier 2015), Point 4.85 “Based on the foregoing, the court concludes that the State – apart from the defence to be discussed below – has acted negligently and therefore unlawfully towards Urgenda by starting from a reduction target for 2020 of less than 25% compared to the year 1990.” Point 4.93 The Court recognised the relevance of scientific literature on effort-sharing, in particular that included in the IPCC reports. The Court also referred to a principle of fairness from Dutch literature (Goote and Hey 1996). This statement supports notions of transgenerational justice, historical responsibility, egalitarianism and capability: “The principle of fairness (i) means that the policy should not only start from what is most beneficial to the current generation at this moment, but also what this means for future generations, so that future generations are not exclusively and disproportionately burdened with the consequences of climate change. The principle of fairness also expresses that industrialised countries have to take the lead in combating climate change and its negative impact. The justification for this, and this is also noted in literature, lies first and foremost in the fact that from a historical perspective the current industrialised countries are the main causers of the current high greenhouse gas concentration in the atmosphere and that these countries also benefited from the use of fossil fuels, in the form of economic growth and prosperity. Their prosperity also means that these countries have the most means available to take measures to combat climate change.” Point 4.57, and “Here too, the court takes into account that in view of a fair distribution the Netherlands, like the other Annex I countries, has taken the lead in taking mitigation measures and has therefore committed to a more than proportionate contribution to reduction. Moreover, it

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is beyond dispute that the Dutch per capita emissions are one of the highest in the world.” Point 4.79. The principle of fairness stated by the Court implicitly rules out some of the notions of equity presented in the IPCC-AR4 and narrows the reduction range. A narrower reduction range would result in a more ambition emissions reductions target for the Netherlands. However, the Court’s power to ensure the Government’s duty of care does not obstruct the Government’s discretionary power to adopt measures consistent with a fair effort-sharing to avoid dangerous warming (Point 4.53 and 4.74). Therefore, the Court decided not to rule which concept of equity the government should follow, in spite of the above-mentioned equity principle, and only imposed the lower end of the reduction range as a minimal threshold. “On the contrary: the State also argues that a higher reduction target is one of the possibilities. This leads the court to the conclusion regarding this issue of the dispute that the State, given the limitation of its discretionary power discussed here, in case of a reduction below 25-40% fails to fulfil its duty of care and therefore acts unlawfully. Although it has been established that the State in the past committed to a 30% reduction target and it has not been established that this higher reduction target is not feasible, the court sees insufficient grounds to compel the State to adopt a higher level than the minimum level of 25%. According to the scientific standard, a reduction target of this magnitude is the absolute minimum and sufficiently effective, for the Netherlands, to avert the danger of hazardous climate change, but the obligation to adhere to a higher percentage clashes with the discretionary power vested in the State, also with due regard for the limitation discussed here.” Point 4.86 An update on the IPCC results to provide national allocations – rather than regional allocations – consistent with the Paris Agreement would inform actors of such legal cases. The ‘hybrid’ approach developed in Chapter 8 offers a way to reduce the fairness range to a single value. This value results from attributing the least-stringent approach to each country. Such a value could be imposed without contradicting the discretionary power of the Government, and preserve consistency with an approach applicable to all under the Paris Agreement goals.

9.4 Future research The ‘hybrid’ approach created in this project maintains a level of flexibility in how it is implemented. For example, an alternative implementation of the hybrid approach could attribute to each country the equity approach that best matches their publicly expressed preferences, rather than the approach that produces the most generous allocation. While countries tend to promote the concept of equity that is the most favourable to them, some countries have shown greater ambition in their public statements (Chapter 6). This alternative implementation of the hybrid approach might better represent countries’ positions. However, it would certainly result in more stringent targets, compared to modelling in Chapter 6, for countries who promote an equity approach that is not the least ambitious. This alternative implementation of the hybrid approach could be more or

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less relevant to legal challenges (such as the Climate Case described in the previous section). It would depend on whether the judges found that the country should be accountable to its own claims regarding equitable allocations, rather than a range of acceptable equitable allocations. Another level of flexibility to this research is that the modelling of distributive approaches developed in this thesis is applicable, or adaptable, to the distribution of many different types of goods other than greenhouse gas emissions. In particular, the application of justice concepts to manage solely the negative impacts the use of fossil fuels, and not to the allocation of their benefits, reflects a libertarian approach. Goods are acquired freely, as long as this does not harm other parties. Globally, the benefits of fossil fuels as a cheap or practical energy carrier could fuel the transition to clean energy. However, the control of these reserves by a few countries influences their ambition in combatting climate change as their wealth is more vulnerable to climate change mitigations than to climate change impacts, compared to other countries (in its INDC, Saudi Arabia declares itself to be vulnerable to plans to address climate change). Such countries have major influence over UNFCCC negotiations where participation is voluntary and no majority can adopt an agreement applicable to all parties. The distributive approaches modelled in this thesis can also be extended to deal with the allocation of costs related to adaptation or loss and damages. The concepts of distributive justice modelled in this thesis were applied to distribute emissions rights across countries independently of the consequences of global warming related to these emissions. With rising temperatures and the increase of climate impacts, questions regarding the distribution of costs of adaptation and loss and damage become more pressing. The CBDR-RC principle applies to all sections of the UNFCCC and of the Paris Agreement, not only to mitigation. The notions of equality, capability and historical responsibility could be used to attribute climate impact related costs. The equality and capability approaches could be adapted to attribute costs proportionally to a country’s population or GDP. Modelling an approach based on historical responsibility implies a linkage with historical emissions. Addressing the interconnected issues on the distribution of costs of both mitigation and impacts simultaneously would provide information critical to the most vulnerable countries. For many countries efforts to mitigate emissions are conditional upon sufficient support to adapt to or compensate for losses due to climate change. However, linking considerations of distributive justice regarding the domain of emissions mitigation and the domain of climate impacts related costs raises specific questions. Should different concepts of distributive justice drive effort-sharing in each of these two domains? If not, how can historical responsibility be used fairly across these domains? Allocating an equal right to pollute to all countries over time while attributing a differentiated responsibility to cover climate change costs is contradictory. Further research could provide relevant information to decision makers and facilitate progress towards the ultimate objective of the UNFCCC: “to prevent dangerous anthropogenic interference with the climate system […] and to enable economic development in a sustainable manner”.

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9.5 Conclusio ns Despite irreconcilable differences between the almost-200 nations of the world, the UNFCCC is founded on a consensus basis (UNFCCC 1992). At the heart of these irreconcilable differences lie national interests that inevitably compete with global interests and concepts of a ‘common good’ (Hardin 1968). Yet, find a way to co-exist and safeguard our common future we must! This project develops a modus vivendi – a way to accommodate incompatible cultural and ethical perspectives for the sake of agreement. It is a methodology that allows countries to agree to disagree on what is a fair allocation of global emissions trajectories to avoid a temperature increase in excess of 1.5 °C or well below 2 °C above pre-industrial levels. The major contribution of this project is a novel modelling framework for quantifying national emissions scenarios consistent with global cost-optimal emissions trajectories. The framework – ‘PRIMAP-Equity’ – was created for, and then used to, quantify national emissions allocations under the Paris Agreement goals in line with commonly discussed equity concepts, and a combination of them. The novel framework and methodology developed through this project directly addresses the three research questions of section 1.3, producing key outcomes that extend the literature. The first outcome, addressing Research Question 1, is a tool to measure national climate pledges and advise on emissions trajectories following commonly discussed equity concepts to realise the latest international climate agreements. This provides policymakers, negotiators and civil society with a transparent tool to assess the ambition of countries from the perspective of commonly discussed concepts of equity. Stakeholders from different countries can discuss and compare their understanding of fairness and ambition using a common and transparent platform. Civil societies of all countries nations can use this tool and the platform it creates to pressure their governments into aligning with the concept of equity that they publicly support. The second outcome is a projection of the current tragedy of the commons (Hardin 1968) that we face in the climate space and which has not been adequately addressed in the Paris Agreement. In this projection, each country follows the least stringent equity concept. The result is a world that is 2.3 °C warmer. The aggregation of current NDCs aligns with that projection and its 2.3 °C warming – modelling a collection of self-interested nations is thus a defensible approach. However, the result indicates that a self-interested approach to effort sharing that aims to stay below 2 °C is insufficient for achieving that goal. The third outcome takes this approach of modelling a collection of self-interested nations and fits it to the Paris Agreement temperature threshold – that is, it allocates emissions to countries based on the least stringent equity concept for each country and does this in line with a global warming cap. This ‘real world’ combination of countries’ equity concepts circumvents the need for a utopic agreement on a global definition for equity. Civil society and courts across the world can support, and order the adoption of mitigation targets without singling out a unique concept of equity. Finally, and importantly, the equivalence derived between national effort and global warming outcomes offers a novel and easy-to-understand metric for communicating levels of national

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ambition. That is, each country’s target can, through this ‘hybrid’ approach, be equated to a temperature outcome. This can form the basis of country-comparison and provides a transparent starting point for the ratcheting-up process. Such a common metric can help countries seeking rapid progress under the UNFCCC to track mutual progress towards the Paris goals. So, can we agree to disagree? Yes, we can, and this thesis offers a way forward. However, what this thesis does not answer is whether we should agree to disagree or whether we should seek consensus on our understanding of equity. The idea at the core of this thesis, to associate divergent equity concepts, raises philosophical questions. Equity is commonly understood as unique and applicable to all. Accepting divergent concepts of equity could therefore be considered oxymoronic. The aim of this project was not to engage in, nor to solve, this oxymoron. Instead, the project aimed to deliver a modus vivendi, a way forward when an agreement seems out-of-reach; and this is what it has delivered. This

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THESIS OUTPUT SUMMARY Publicat ions as first author Robiou du Pont Y., Jeffery L., Gütschow J., Meinshausen, M. 2016 National contributions for decarbonizing the world economy in line with the G7 agreement Environmental Research Letters. 11, 054005 Online: http://dx.doi.org/10.1088/1748-9326/11/5/054005 Robiou du Pont Y., Jeffery M. L., Gütschow J., Rogelj J., Christoff P., & Meinshausen M. (2017). Equitable mitigation to achieve the Paris Agreement goals. Nature Climate Change, 7 (January), 38–43. http://doi.org/10.1038/NCLIMATE3186 Robiou du Pont Y., Meinshausen M. Temperature assessment of the bottom-up Paris emissions pledges. Under revisions with Nature Communications (June 2018), submitted in October 2017: Robiou du Pont Y., Proudlove R., Jeffery M. L., & Meinshausen M. Ratcheting up climate ambition: comparisons of G20 countries’ statements, pledges and policy projections. Submitted to Climate Policy in June 2018. Publicat ions as co-author Meinshausen M., Jeffery L., Guetschow J., Robiou du Pont Y., Rogelj J., Schaeffer M., Höhne N., den Elzen M., Oberthür S. and Meinshausen N. 2015 National post-2020 greenhouse gas targets and diversity-aware leadership Nature Climate Change 1–10 Online: http://www.nature.com/doifinder/10.1038/nclimate2826 Submission to Australian government: Annabelle Workman, Zebedee Nicholls, Yann Robiou du Pont, Graham Palmer Seb Rattansen, Poomphan Chanvittayanuchit (2017). Submission to 2017 Review of Australia's climate change policies, http://climate-energycollege.org/submission-2017-review-australias-climate-change-policies Other outputs as first contributor Report: Robiou du Pont, Y., 2017. The Paris Agreement global goals: What does a fair share for G20 countries look like?, Melbourne. Available at: http://climate-energy-college.org/parisagreement-global-goals-what-does-fair-share-g20-countries-look Website: How fair are countries' climate pledges?, Robiou du Pont, Y., Jeffery, M. L. (2016) Paris-Equity-Check.org, Website, http://paris-equity-check.org Conversation article: Robiou du Pont, Y., Talberg A. (2016). Election FactCheck: is Australia among the only major advanced economies where pollution levels are going up? The Conversation. https://theconversation.com/election-factcheck-is-australia-among-the-only-majoradvanced-economies-where-pollution-levels-are-going-up-59731 Oral presentation: Equitably Determined Contributions to reach the Paris Agreement goals – the 1.5 °C perspective, 2016, “1.5 °C conference”, Oxford https://vimeo.com/193622738

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