isms and industrial activities-indeed, the whole economic system-not only be- ... the whole integrated collection of physical processes that convert raw materials .... tained indefinitely only by a continuous flow of free energy. This follows imme~ diately from the second law of thermodynamics, which states that global entropy ·.
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A third way in which the analogy between biological metabolism and industrial metabolism is useful is to focus attention on the "Jife cycle" of individual "nutrients." The hydrological cycl~. the carbon cycle, and the nitrogen cycle are familiar concepts to earth scientists. The major way in which the industrial metabolic system differs from the natural metabolism of the earth is that the natural cycles (of water, carbon/oxygen, nitrogen, sulfur, etc.) are closed, whereas the industrial cycles are open. In other words, the industrial system does not generally recycle its nutrients. Rather, the industrial system starts with high-qualit)' materials (fossil fuels, ores) extracted from the earth, and retums them to nature in degraded forro. This point particular! y deserves clarification. The materials cycle, in general, can be visualized in terrns of a system of compartments containing stocks of one or more nutrients, linked by certainflows. For instance, in the case of the hydrological cycle, the glaciers; the oceans, the freshwater lakes, and the groundwater are stocks, while rainfall'and rivers are flows. A system is closed if there are no
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externa! sources or sinks. In this sense, the Earth as a whole is essentially a closed system, except for the occasional meteorite. A closed system becomes a closed cycle if the system is al so in steady-state, that is if the stocks in each compartment are constan! and unchanging, at least on the -average. The materials balance condition implies that the material inputs to each compartment must be exactly balanced (on the average) by the outputs. If this condition is not met, for a given compartment, then the stock in one or more compartments must be increasing, while the stocks in one or more other compartments must be decreasing. 2 It is· easy to see that a closed cycle of flows, in the above sense, can be sustained indefinitely only by a continuous flow of free energy. This follows imme~ diately from the second law of thermodynamics, which states that global entropy · incre.ases in every irreversible process. Thus, a closed cycle of flows can be sustained as long as its externa) energy supply lasts. An open system, on the contrary, is inherently unstable and unsustainable. It must either stabilize or collapse to a . thermal equilibrium state in which a\1 flows, that is, all physical and biological processes, cease. It is sometimes convenient to define a generalized four-box model to describe materials flows. The biological version is shown in Figure 2, while the analogous industrial version is shown in Figure 3. To revert to the point made at the beginning of 'his section, the natural system is characterized by closed cycles, at least
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FIGURE 2 Box scheme for bio-geo-chemical cycles.
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