The materials cycle
A third way in which the analogy between biological metabolism and industrial metabolism is useful is to focus attention on the "life cycle" of individual "nutrients."
The hydrological cycle, 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, other words, the industrial system does not generally recycle it trients. Rather, the industrial system starts with high-quality mat' (fossil fuels, ores) extracted from the earth, and returns them to nature in degraded form.
This point particularly deserves clarification. The materials c in general, can be visualized in terms of a system of compartments containing stocks of one or more nutrients, linked by certain flows. For instance, in the case of the hydrological cycle, the glaciers oceans, the fresh water lakes, and the groundwater are stocks, while rainfall and rivers are flows. A system is closed if there are no e nal sources or sinks. In this sense, the earth as a whole is essentially, closed system, except for the occasional meteorite.
A closed system becomes a closed cycle if the system is al steady state, i.e. the stocks in each compartment are constant an changing, at least on average. The materials balance condition plies that the material inputs to each compartment must be e, balanced (on average) by the outputs. If this condition is not m. a given compartment, then the stock in one or more compartments must be increasing, while the stocks in one or more other compartments meets must be decreasing.²
It is easy to see that a closed cycle of flows, in the above sense only be sustained indefinitely by a continuous flow of free en This follows immediately from the second law of thermodynamics, which states that global entropy increases in every irreversible process. Thus, a closed cycle of flows can be sustained as long external energy supply lasts. An open system, on the contrary, herently unstable and unsustainable. It must either stabilize or lapse to a thermal equilibrium state in which all flows, i.e. 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. Reverting to the point made at the beginning of this section, the nature tem is characterized by closed cycles, at least for the major nutrients (carbon, oxygen, nitrogen, sulphur) - in which biological processes play a major role in closing the cycle. By contrast, the industrial system is an open one in which "nutrients" are transformed "wastes," but not significantly recycled. The industrial system, exists today, is therefore ipso facto unsustainable.
At this stage, it should be noted that nothing can be said at least) with respect to any of the really critical questions. These are as follows:
It is fairly important to try to find answers to these questions.
Needless to say, we do not aspire to answer all these questions in the present volume.
It should also be pointed out that the bio-geosphere was not always a stable system of closed cycles. Far from it. The earliest living cells on earth obtained their nutrients, by fermentation, from nonliving organic molecules whose origin is still not completely understood. At that time the atmosphere contained no free oxygen or nitrogen; it probably consisted mostly of water vapour plus some hydrogen, and hydrogen-rich gases such as methane, hydrogen sulphide, and ammonia. The fermentation process yields ethanol and carbon dioxide. The system could only have continued until the fermentation organisms used up the original stock of "food" molecules or choked on the carbon dioxide buildup. The system stabilized temporarily when a new organism (blue-green algae, or cyano-bacteria) appeared that was capable of recycling carbon dioxide into sugars and cellulose, thus again closing the carbon cycle. This new process was anaerobic photosynthesis.
However, the photosynthesis process also had a waste product: namely, oxygen. For a long time (over a billion years) the oxygen generated by anaerobic photosynthesis was captured by dissolved ferrous iron molecules, and sequestered as insoluble ferric oxide or magnetite, with the help of another primitive organism, the Stromatolites. The resulting insoluble iron oxide was precipitated on the ocean bottoms. (The result is the large deposits of high-grade iron ore we exploit today.) The system was still unstable at this point. It was only the evolutionary invention of two more biological processes, aerobic respiration and aerobic photosynthesis, that closed the oxygen cycle as well. Still other biological processes - nitrification and denitrification, for instance - had to appear to close the nitrogen cycle and others.
Evidently, biological evolution responded to inherently unstable situations (open cycles) by "inventing" new processes (organisms) to stabilize the system by closing the cycles. This self-organizing capability is the essence of what has been called "Gaia." However, the instabilities in question were slow to develop, and the evolutionary responses were also slow to evolve. It took several billion years before the biosphere reached its present degree of stability.
In the case of the industrial system, the time scales have been drastically shortened. Human activity already dominates and excels natural processes in many respects. While cumulative anthropogenic changes to most natural nutrient stocks still remain fairly small in most cases, the rate of nutrient mobilization by human industrial activity is already comparable to the natural rate in many cases. Table 1 shows the natural and anthropogenic mobilization (flow) rates for the four major biological nutrients, carbon, nitrogen, phosphorus and sulphur. In all cases, with the possible exception of nitrogen, the anthropogenic contributions exceed the natural flows by a considerable margin. The same is true for most of the toxic heavy metals, as shown in table 2.
On the basis of relatively crude materials cycle analyses, at least, it would appear that industrialization has already drastically disturbed, and ipso facto destabilized, the natural system.
Table 1 Anthropogenic nutrient fluxes (teragams/year)
a. Assuming 10 per cent loss of synthetic ammonia-based fertilizers applied to land surface (75 tg/yr).
b. Total production (= use) less fertilizer use, allocated to landfill. The remainder is assumed to be disposed of via waterways.
Table 2 Worldwide atmospheric emissions of trace metals (1,000 tonnes per year)
Source: Nriagu, 1990.
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