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Technology and Environment 1989. Pp. 23~9. Washington, DC National Academy Press. Industrial Metabolism ROBERT U. AYRES We may think of both the biosphere and the industrial economy as systems for the transformation of materials. The biosphere as it now exists is very nearly a perfect system for recycling materials. This was not the case when life on earth began. The industrial system of today resembles the ear- liest stage of biological evolution, when the most primitive living organisms obtained their energy from a stock of organic molecules accumulated during prebiotic times. It is increasingly urgent for us to learn from the biosphere and modify our industrial metabolism, the energy- and value-yielding pro- cess essential to economic development. Modifications are needed both to increase reliance on regenerative (or sustainable) processes and to increase efficiency both in production and in the use of by-products. In this chapter, mass flows for key industrial materials of environ- mental significance, and the waste emissions associated with them, are reviewed along with the environmental impact of the waste residuals; eco- nomic and technical forces driving the evolution of industrial processes; long-range tendencies in the development of the industrial metabolism; and some applications of "materials-balance" principles to provide more reliable estimates of outputs of waste residuals. The Appendix contains theoretical explorations of the biosphere and the industrial economy as materials-transformation systems and lessons that might be learned from their comparison. Before presenting the main analytic framework, it is useful to begin with some positive examples of how industrial metabolism can shift in the direction of increased efficiency in materials flows and waste streams. It 23

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24 ROBERT U LYRES has been justly remarked that the history of the chemical industry is one of finding new uses for what were formerly waste products (Multhauf, 1967~. One of the most interesting early examples of such an innovation was the Leblanc process (the predecessor of the Solvay process) for manufacturing sodium carbonate circa 1800. As a raw material, it made use of sodium sulfate, an unwanted by-product of the eighteenth-centu~y process for manufacturing ammonium chloride (sal ammoniac). Sal ammoniac was used for cleaning metals and as a convenient source of ammonia, but sodium sulfate had no use. The Leblanc process reduced sodium sulfate to sodium sulfide by heating it in a furnace with charcoal. This, in turn, was heated with calcium carbonate (chalk), which induced a reaction producing the desirable sodium carbonate and a new waste product, calcium sulfide (Multhauf, 1967~. Meanwhile, the market for ammonium chloride failed, so sodium sulfate had to be produced in the same complex by reacting sulfuric acid with common salt. This yielded hydrogen chloride (hydrochloric acid) as another by-product. In this case, the hydrochloric acid quickly found a practical use in the manufacture of chloride of lime as a bleach for the rapidly growing textile industry. The other waste product, calcium sulfide, was not successfully used until the 1880s, as a source of sulfur for the manufacture of sulfuric acid. One of the most important waste products of the early nineteenth century was coal tar, which was generated in large amounts by gasworks making "town gas" for illumination. It was a systematic search for useful by- products, initiated by German chemists, that resulted in the creation of the modern organic chemical industry. The synthetic aniline dyes introduced after 1860 were all essentially derived from chemicals obtained from coal tar. Phenolic resins, aspirin, and the sulfa drugs are later examples of derivatives. Until recently, even natural gas was regarded as a by-product of petroleum production. Although it was used almost from the beginning as a fuel for refinery processes and illumination in local areas, much of it was wasted by "flaring" (indeed, this is still true in remote areas of the, world). It had no significant chemical uses until World War II, when natural gas became the feedstock for producing ethylene, and thence butadiene and styrene, the major ingredients of synthetic rubber. Chlorine is a final example. It is manufactured jointly with sodium or sodium hydroxide (lye) by electrolysis of salt or brine. When this process was introduced In the 1890s, it was the sodium hydroxide that was wanted for a variety of purposes, including petroleum refining and soap manufacturing. At the time, chlorine was a low-value by-product, which was luckily available for use in municipal water treatment The development of a number of valuable chlorine-based organic chemicals (e.g., the most

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INDUSTRIAL METABOLISM TABLE 1 Mass of Active Materials Extracted Commercially in the United States, 1960-1975 (million tons) Year Material1960196519701975 Food and feed crops267295314403 (excluding hay) Meat, fish, and dairy products82858584 Cotton, wool, hides, and tobacco5544 Timber (who moisture basis)256267271249 Fuels (coal, lignite, oil, gas)9901,1761,4581,392 Ores(Fe, Cu. Pb, Zn)400435528460 Nonmetallics2ooa24Oa266255 Total2,2~a2>ooa2,9262,847 25 - aEstimated value. NOTE: Vegetable material harvested directly by animals has been omitted for lack of data, along with some obviously minor agricultural and horticultural products. Figures for metal ores exclude mine tailings and gangue removed to uncover ore bodies. Inert construction materials such as stone, sand, and gravel have also been omitted. Inert materials account for enormous tonnages, but undergo no chemical or physical change except to the extent that they are incorporated in concrete or paved surfaces. The table also excludes soil and subsoil shifted during construction projects or lost by erosion. SOURCE: U.S. Bureau of the Census (1960 1975~. common industrial solvents and refrigerants, pesticides, herbicides, and plastics such as polyvinyl chloride) actually reversed the position. By the l950s and 1960s, chlorine was the primary product and sodium hydroxide was the less valuable by-product. MASS FLOWS AND WASTE EMISSIONS Our economic system depends on the extraction of large quantities of matter from the environment. Extraction is followed by processing and conversion into various forms, culminating in products for "consumption." The U.S economy extracts more than 10 tons of "active" mass per person (excluding atmospheric oxygen and fresh water) from U.S. territory each year. Of the active mass processed each year, roughly 75 percent is mineral and "nonrenewable," and 25 percent is, in principle, from renewable (i.e., biological) sources as shown in Bible 1. Of the latter, much is ultimately discarded as waste, although much of it could (in principle) be used for energy recovery. It is difficult to estimate the fraction of the total mass of processed

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26 ROBERT U LYRES active materials that is annually embodied in long-lived products and capital goods (durables). None of the food or fuel is physically embodied in durable goods. Most timber is burned as fuel or made into pulp and paper products. At least 80 percent of the mass of "ores" consists of unwanted impurities (more than 99 percent in the case of copper). Of the final products made from metals, a large fraction is converted into "consumption goods," such as bottles, cans, and chemical products, and "throwaway" products such as batteries and light bulbs. Only in the case of nonmetallic minerals (if inert materials are ignored, as before) is as much as 50 percent of the mass embodied in durable goods (mainly Portland cement used for concrete and clays used for bricks and ceramics). The annual accumulation of active materials embodied in durables, after some allowance for discard and demolition, is probably not more than 150 million tons, or 6 percent of the total. The other 94 percent is converted into waste residuals as fast as it is extracted. Combustion and carbothe~mic reduction processes are the major sources of atmospheric pollutants today but by no means the only important ones; nor is the atmosphere the only vulnerable part of the environment. From a broader environmental perspective, the production and dispersal of thousands of synthetic chemicals many new to nature, and some highly toxic, carcinogenic, or mutagenic- and the mobilization of large tonnages of toxic heavy metals are of equally great concern. The complexity of the problem Is too great to permit any kind of short summary. However, two points are worthy of emphasis. First, as noted above, most materials "pass through" the economic system rather quickly. That is to say, the transformation from raw material to waste residual takes only a few months to a few years in most cases. Long-lived structures are very much the exception, and the more biologically "potent" materials are least likely to be embodied in a long-lived form. The second point is that many uses of materials are inherently dissipa- tive (Ayres, 1978~. That is, the materials are degraded, dispersed, and lost in the course of a single normal use. In addition to food and fuels (and additives such as preservatives), other materials that fall into this category include packaging materials, lubricants, solvents, flocculants, antifreezes, detergents, soaps, bleaches and cleaning agents, dyes, paints and pigments, most paper, cosmetics, pharmaceuticals, fertilizers, pesticides, herbicides, and germicides. Many of the current consumptive uses of toxic heavy met- als such as arsenic, cadmium, chromium, copper, lead, mercury, silver, and zinc are dissipative in the above strict sense. Other uses are dissipative in practice because of the difficulty of recycling such items as batteries and electronic devices. Bibles 2 and 3, which summarize estimates of emission coefficients for various heavy metals by use category and annual emissions attributable to

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INDUSTRIAL METABOLISM 27 dissipative uses, make clear the heterogeneity of the materials flows. In some cases the dissipation is slow and almost invisible. For instance, paints (often containing lead, zinc, or chromium) gradually crack, '~weather," and turn to powder, which is washed or blown away. Tires, which contain zinc (and cadmium) salts, are gradually worn away during use, leaving a residue on the roads and highways. Similarly, shoe leather, containing up to 2 percent chromium salts, is gradually worn away to powder during use. Incinerator ash contains fairly high concentrations of heavy metals from a variety of miscellaneous sources, ranging from used motor oil to plastics and pigments. On reflection, many dissipative uses (food and fuel again excepted) are generally seen to be nonessential in the sense that technologies are the- oretically available, or imaginable, that could eliminate the need for them. 1b take one example only: hydroponic agriculture in enclosed, atmospher- ically controlled greenhouses, with genetically engineered pest controls, would eliminate all losses of fertilizers and pesticides to watercourses by way of surface runoff. ENVIRONMENTAL IMPACT OF WASTE RESIDUALS Materials do not disappear after they are used up in the economic sense. They become waste residuals that can cause harm and must be disposed of. In fact, it is not difficult to show that the tonnages of waste residuals are actually greater than the tonnages of crops, timber, fuels, and minerals recorded by economic statistics. Although usually unpriced and unmeasured, both air and water are major inputs to industrial processes and they contribute mass to the residuals-especially combustion products. Residuals tend to disappear from the market domain, where everything has a price, but not from the real world in which the economic system is embedded. Many services provided by the environment derive ultimately from "common property," including the air, the oceans, the genetic pool of the biosphere, and the sun itself. Distortions in the market (i.e., prices) are unavoidably associated with the use of common property resources. Clearly, environmental resources such as air and water have been unpriced or (at best) significantly underpriced in the past. For this reason, such resources have generally been overused.2 As noted, the total mass of waste residuals produced each year by industrial metabolism far exceeds the mass of active inputs derived from economic activities. This is because nearly half of the inputs other than air and water are fossil fuels (hydrocarbons), which combine with atmospheric oxygen and form carbon dioxide and water vapor. The carbon fraction of hydrocarbons ranges from 75 percent in methane to about 90 percent

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28 TABLE2 Consumption-Related Emissions Factors: Heavy Metals ROBERT U LYRES Painting Paint Electron Other Metallic and and Tubes and Electrical Metal usea Coatingb PigmentsC Batteries d Equipment Silver 0.001 0.02 0.5 0.01 0.01 Arsenic 0.001 0 05 0.01 N^. Cadmium 0.001 0.15 05 0.02 N^. Chromium 0.001 0.02 05 N^. N^. Copper 0.005 0 1 N^. 0.10 Mercury 0.050 0.05 0.8 0.20 N^. Lead 0.005 0 0.5 0.01 NA. Zinc 0.001 0.02 05 0.01 N^. N^. = Not applicable. aAlloys or amalgams (in the case of mercury) not used in plating, electrical equipment, catalysts, or dental work. Losses can be assumed to be due primarily to corrosion, except for mercury, which volatilizes. b Protective surfaces deposited by dip coating (e.g., galvanizing), electroplating vacuum deposition, or chemical bath (e.g., chromic acid). The processes in question generally resulted in significant waterborne wastes until the 1970s. Cadmium-plating processes were particularly inefficient until recently. Losses in use are mainly due to wear and abrasion (e.g., silve~plate) or to flaking (decorative chrome trim). In the case of mercu~y-tin "silver" for mirrors, the loss is largely due to volatilization. Paints and pigments are lost primarily by weathering (e.g., for metal-protecting paints), wear, or disposal of painted dyes or pigmented objects, such as magazines. (::opper- and mercury-based paints volatilize slowly over time. A factor of 0.5 is assumed arbitrarily for all other paints and pigments. dIncludes all metals and chemicals (e.g., phosphorus) in tubes and primary and secondary battenes, but excludes copper wire. Losses in manufacturing may be significant. Mercury in mercury vapor lamps can escape to the air when tubes are broken. In all other cases it is assumed that discarded equipment goes mainly to landfills. Minor amounts are volatilized in fires or incinerators or lost by corrosion. Lead-acid batteries are recycled. Includes solders, contacts, semiconductors, and other special materials (but not copper wire) used in electrical equipment control devices, instruments, etc. Losses to the environment are primarily through discard of obsolete equipment to 1andfi11s. Mercury used in instruments may be lost through breakage and volatilization or spillage. in anthracite. Petroleum is intermediate. Every ton of fuel carbon is converted into 3.67 tons of carbon dioxide emitted to the atmosphere.3 It is thought that only about half of this amount remains in the atmosphere, but the carbon dioxide level of the earth's atmosphere has risen over the past century from about 290 parts per million (ppm) to around 344 ppm at present. Although the magnitude of the climatic warming (the "greenhouse

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INDUSTRIAL METABOLISM 29 Chemical Agri- Nonagri Uses Not cultural cultural Medical, Miscella Metal Embodiedf Embodiedg Uses h Uses i Dental3 neous k Silver 1 0.40 N^. N^. 0.5 0.15 Arsenic N^. 0.05 0.5 0.8 0.8 0.15 Cadmium 1 0.15 NA. N^. N^. 0.15 Chromium 1 0.05 N^. 1 0.8 0.15 Copper 1 0.05 0.05 1 NA. 0.15 Mercury 1 NA. 0.80 0.9 0.2 0.50 Lead 1 0.75 0.05 0.1 NA. 0.15 Zinc 1 0.15 0.05 0.1 0.8 0.15 fuses not embodied in final products include catalysts, solvents, reagents, bleaches, etc. In some cases a chemical is embodied but there are some losses in processing. Losses in chemical manufacturing are included here. Major examples include copper and mercury catalysts (especially in chlorine manufacturing); copper, zinc, and chromium as mordants for dyes; mercury losses in felt manufacturing; chromium losses in tanning,; lead in desulfunzation of gasoline; and zinc in rayon spinning. In some cases, annual consumption is actually loss replacement and virtually all of the material is dissipated. Detonators such as mercury fulminate and lead azide (and explosives) are included in this category. "Uses embodied in final products other than paints or batteries include fuel additives (e.g., tetraethyl lead), anticorrosion agents (e.g., zinc dithiophosphate), initiators and plasticizers for plastics (e.g., zinc oxide), wood preservatives, and chromium salts embodied in leather. Losses to the environment occur when the embodying product is used, for example, gasoline containing tetraethyl lead is burned and largely (lasso) dispersed into the atmosphere. However, copper, chromium, and arsenic are used as wood preservatives and dispersed only if the wood is later burned or incinerated. In the case Of silver (photographic film), it is assumed that 60% is later recovered. Includes agricultural pesticides, herbicides, and fungicides. Uses are dissipative, but heavy metals are largely immobilized by soil. Arsenic and mercury are exceptions because of their volatility. i Biocides used in industrial, commercial, or residential applications. Loss rates are higher in some cases. j Includes primarily pharmaceuticals (including cosmetics), germicides, etc., as well as dental filling matenal. Most uses are dissipated to the environment through wastewater. Silver and mercury dental fillings are likely to be buried with cadavers. k Miscellaneous emissions not counted elsewhere. SOURCE: Ayres et al. (1988~. effect") due to the rising level of carbon dioxide is still quantitatively uncertain, the qualitative impact is likely to be adverse. Carbon monoxide is, of course, toxic to humans and has been impli- cated in health problems among urban populations. It is less well known that carbon monoxide plays an active, and not necessarily benign, role in a number of atmospheric chemical reactions before it is ultimately oxidized to carbon diomde. Inefficient combustion processes convert up to 10 percent

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30 ROBERT U LYRES TABLE 3 Emissions from Consumptive Uses: Heavy Metals, United States, 1980 (metnc tons) Metallic Uses Electrical (E!xcept Protective Coverings Other Coatings Plating Paints Batteries Electrical and and and and Uses, Instru Metal Electrical) Coating Pigments Equipment meets, etc. Silver 0.83 a o.o 0.15 0.67 Arsenic 0.04 0.0 0.0 1.97 0.0 Cadmiumb 0.04 136.1 116 7.81 0.0 Chromium 151.80 155.7 6,490 0.0 0.0 Copper c 11,074.00 0.0 0.0 0.0 0.0 Mercury 0.0 0.0 0.0 195.91 1752 Leadc 1,249.00 0.0 48,S00 8,510.00 0.0 Zinc 514.00 8,778.0 77,750 63.00 0.0 bIncluded in first column. cl979. 1977. SOURCE: Ayres et al. (1988). Of fuel carbon into carbon monoxide; carbothermic reduction of iron ore and other metals generates even higher percentages. However, the average percentage over all processes is much smaller. Actual emissions of carbon monoxide to the atmosphere in the United States were about 110 million tons in 1970 (mostly from automobiles and trucks), with a carbon content of 47 million tons. Emission controls reduced the net output of carbon monoxide in 1980 to about 85 million tons (U.S. Environmental Protection Agency, 1986~. The discovery of chlorofluorocarbons (CFCs) in the stratosphere has raised an even more frightening prospect: ozone depletion (see Glas, this volume). In apparent confirmation of this phenomenon, an "ozone hole" has recently appeared in the stratosphere over Antarctica. This "hole" has reappeared each spring for several years and seems to be growing (Clark, 1987; Miller and Mintzer, 1986~. Chlorofluorocarbons are chemically inert gases, discovered in 1928 and produced since the 1930s mainly as refrigerants and solvents and for "blowing" plastic foams (see Friedlander, this volume). The major use is for refrigeration and air conditioning. In most uses, CFCs are not released deliberately, but losses and leakage are inevitable. The problems revealed so far may be only the beginning. If (perhaps it is better to say "when") the ozone level in the stratosphere is depleted, the effect will be to let more of the sun's ultraviolet radiation through to

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INDUSTRIAL METABOLISM Chemical _Biocidal Consumer Poison Uses Industrial Uses, Nonagri Catalysts, Additives, Agriculturalcultural Medical, Reagents, Extenders, Pesticides,Pesticides Dental, Explosives,Photography, Herbicides, (Except Pharma- Miscel etc.etc. Fungicides Medical) ceutical laneousTotal _ 7.648 0 0 15; 0.4560 492.00 2,950 5,901 19.70 09,364 029 0 0 0 0.98290 1,297.03,890 0 1,038 0 2,141.0011,659 4,222.00 1,560 0 0 015,452 412.10 16 236 8.36 6.96893 OO O O 0 1,329.0056,900 2,508.018,622 188 251 1,003.00 0109,670 31 the earth's surface. One likely impact on humans is a sharp increase in the incidence of skin cancer, especially among whites. The ecological impact on vulnerable plant or animal species cannot be estimated, at present, but could be severe. Methane, oxides of nitrogen (NO=), and sulfur oxides (SO=) are other residuals that have been seriously implicated in climatic or ecological effects. All three are generated by fossil fuel combustion, as well as other industrial processes. Like carbon monoxide, carbon dioxide, and the CFCs, they can be considered as metabolic products of industry. Methane is released in natural gas pipelines, petroleum drilling, coal mining, and several kinds of intensive agriculture (especially rice cultivation or cattle and sheep farming). Nitrogen oxides are also coproducts of combustion. In effect, at high temperatures, some of the atmospheric nitrogen is literally "burned." Nitrogen oxides are implicated in many atmospheric chemical reactions (including those that create smog) and eventually oxidize to nitric acid, which contributes to acid rain. In the stratosphere, where nitrogen oxides are decomposed by ultraviolet radiation, atomic nitrogen also contributes to the destruction of ozone. Finally, oxides of nitrogen are "greenhouse gases" that contribute to climatic warming (Miller and Mintzer, 1986~. Sulfur oxides are generated by the combustion of sulfur-containing fuels-especially bituminous coal-and by the smelting of sulfide ores. Most copper, lead, zinc, and nickel ores are of this kind. In principle, sulfur can be recovered for use from all these activities, and the recovery rate is rising. However, the costs of recovery, especially from coal-burning electric power plants, are still far higher than the market value of the potential products (e.g., dilute sulfuric acid). Hence, for the present,

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32 ROBERT U LYRES calcium sulfites and sulfates-as well as SOc-constitute a waste residual that must be disposed of. This is also true of fly ash.4 These examples show that although today's industrialized economic system may be in rough equilibrium in terms of supply and demand rela- tionships, it is far from equilibrium in thermodynamic terms. Enormous quantities of fossil fuels and high~uality minerals are extracted each year to drive the economic engine. The economic system is stable somewhat in the way a bicycle and its rider are stable: if forward motion stops, the sys- tem will collapse. Forward motion in the economic system is technological progress. EVOLUTION OF INDUSTRIAL PROCESSES It is generally accepted by economists that the mechanism that normally drives the evolution of industrial processes is technological innovation. The primary incentive for taking risks appears to be economic. A new process that saves one link in the chain between raw materials and finished ma- terials or final goods can usually be justified through savings in materials and energr inputs or capital requirements, if not both. Moreover, process technology is inherently easier to protect from piracy than product tech- nology. As noted, most chemical products are intermediates used in the production of other chemicals. Thus, final products result from chains, or sequences, of processes with an overall energy conversion efficiency that is the arithmetic product of the conversion efficiencies at each stage.5 If the typical chain has three steps, each of which has a conversion efficiency of 0.7, the overall conversion efficiency of the chain is about 0.34. A four-step chain would have an overall efficiency of around 0.24. That is, the available energy embodied in the final product might be somewhere between 25 and 35 percent of the available energy of the original feedstocks. Because primary feedstocks are essentially indistinguishable from fuels, efficiency improvements translate directly into cost savings. Clearly, a powerful long-term strategy for improving overall effective- ness in production is the development of new processes to shorten these process chains, bypassing as many intermediates as possible. In other words, one would like to be able to produce final products such as polyethylene or synthetic rubber directly from first-tier intermediates or even from primary feedstocks such as ethane and propane.6 Biological organisms differ from industrial organizations in that they are able to build complex molecules directly from elementary building blocks with relatively few intermediates. Thus, biotechnology offers a long-term prospect of radically higher produc- tion efficiencies and correspondingly lower costs (U.S. Office of Technology Assessment, 1982~. Another long-term strategy for increasing effectiveness is better use of

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INDUSTRIAL METABOLISM 33 by-products and wastes. When a process can be justified on the basis of the market for its primely product, by-product sales can be highly profitable. This positive motivation to seek new uses is compounded by the fact that, because of some of the environmental problems already noted, waste generation is being increasingly discouraged by environmental regulation. Moreover, waste disposal is increasingly expensive, and the cost is becoming more uncertain. Firms that buried toxic chemical wastes many years ago in landfills (methods that were regarded as acceptable at the time) are sometimes finding themselves saddled retroactively with heavy costs of digging up the same wastes and disposing of them again in a safer manner. The chemical waste dump used by Hooker Chemical Company at Love Canal, near Niagara Palls, is one example. Meanwhile, suitable sites for disposal of hazardous solid or liquid wastes are becoming scarce.7 Ma- terials that are recovered and reused internally or embodied in marketable products that can be readily and effectively recycled are less likely to cause this kind of problem.8 The incentives for technological innovation in the area of waste re- duction and disposal are not always operating, especially where there are massive market failures. In practice, the "unpaid" environmental damage costs have been deferred and many of them will have to be paid by later generations. But as these costs become larger and more visible, there will be growing political pressure to force the producers and users of fossil fuels (and other materials such as heavy metals) to pay the costs of abat- ing the resulting environmental damages.9 Notwithstanding resistance by energy users, it seems inevitable that in the long run these costs will be added to the prices of fuels and materials. This, in turn, will create sig- nificant economic opportunities for innovations in the area of "low-waste" technologies. EVOLUTIONARY ECONOMICS AND GAIA One view of evolution and probably the most common view among scientists-can be characterized as "the myopic drunkard's walk" The drunkard's walk is not exactly random, but it tends to follow the path of least resistance in the short run. If a mutation or an innovation offers short- term advantages, it will be adopted. In this view, there is no long-range force or tendency to approach a distant goal. I,here is no Aristotelian "final cause." Most scientists tend to regard more teleological theories, including Lovelock's Gala hypothesis ~velock 1988), with skepticism, because of their aroma of mysticism. In the case of biological evolution, indeed, it was difficult for a long time to suggest a likely mechanism whereby short-term advantages to the individual could in some cases be overridden in favor of longer-term benefits

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INDU512IAL METABOLISM 39 cycle are familiar examples. The concept is also applicable, of course, to flows that are not truly cyclic, as in the case of arsenic (Figure 3~. This concept has been widely used by geochemists, hydrologists, ecologists, and environmental scientists to organize and systematize their work. Such a presentation helps specify geographical scales of analysis. It also facilitates such comparisons as the relative importance of natural and anthropogenic sources. Finally, and potentially most important, it provides a starting point for detailed analysis of the effect of anthropogenic emissions on natural processes. CONCLUSION It appears that we have methods to describe our industrial metabolism better, both qualitatively arid quantitatively. Initial analyses reveal several important points, for example, that many materials uses are inherently dissipative and thus pose difficulties for recycling. Analysis also shows that although residuals do not disappear from the real world of human health and environmental quality, they do tend to disappear from the market domain. Thus, many environmental resources are underpriced and overused. It is also clear that where the production and use of by-products are concerned, many industrial processes involve multiple steps, resulting in a low level of system efficiency, especially in comparison with biological systems. The sum of the argument here suggests that we should not only postulate, but indeed endorse, a long-run imperative favoring an industrial metabolism that results in reduced extraction of virgin materials, reduced loss of waste materials, and increased recycling of useful ones. APPENDIX The Biosphere as a Matenals-liansformation System Three salient characteristics mark the difference-between the natu- rally evolved biosphere and its human~esigned industrial counterpart, the "synthesphere." The first is that the metabolic processes of biological or- ganisms are derived (by photosynthesis) from a renewable source: sunlight. The second characteristic is that the metabolism of living organisms (cells) is executed by multistep regenerative chemical reactions in an aqueous medium at ambient temperatures and pressures. Most process intermedi- ates are regenerated within the cell. Reaction rates are controlled entirely by catalysts (enzymes). The energy transport function is performed in all living organisms by phosphate bonds, usually in the molecule adenosine triphosphate (ATP).~3

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40 Hydroxide H2SO4 | Hydroxide . . React with with Sulfurous A c i d fi n W e a k 2SO4 Ituric ~ //1 Sulfate Alum / / I OKpi0,)~ ~ i: ROBERT U AIRS Melt in I Reverberatory Fumace with Lime and Soda or _ Potash ~;~N Iron 0(e (Chromite) `F6Cr2O~ ~ \ ~1 ~ Alkaline Roast , and Leach ~~ - ~ / / Chromate ~ K2CrO4. J Norma / / ~ / ~ / 1\ / React ~ / ~ / with Electro / IN Sulfuric Iytic . / /; H2SO4 ~ Reduction \~ / ~/_ ~ ~ ,~tass~ ~\ ~ \ ~ Bichromate Bichromate i ~\ \L K2Cr2O7 Na2Cr207 Jr\ ~\\ ~1 1 ~ - Stainless | Steel and | Cloy 1 Steel | 1 1 1 Ferro- Mordants Corrosion Lead Chromate chrome (laxatives) Inhibitor Zinc Chromate Ugno- for Dyeing (Primer) sulfate FIGURE 2 Chromium process-product flows, circa 1968. Except for the bottom row, rectangles indicate processes; ellipses, products or reactions. SOURCE: Ayres et al. (1988~.

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INDUSTRIAL METABOLISM 41 The third salient characteristic differentiating the biosphere from the industrial synthesphere is that, although individual organisms do generate process wastes- primarily oxygen in the case of plants and carbon dioxide and urea in the case of animals the biosphere as a whole is extremely effi- cient at recycling the elements essential to life. Specialized organisms have evolved to capture nutrients in wastes (including dead organisms) and recy- cle them. A significant exception to this rule in the present geological epoch is the deposition of skeletal remains of zooplankton as sediments on the deep ocean floor. These remains are largely, but not entirely, calciferous.~4 Over geological time periods, some of this sedimentary material is likely to be recycled as chalk, limestone, or phosphate rock The biosphere as it now exists is a nearly perfect materials-regycling system, but this was not the case when life on earth began. The first and most critical evolutionary "invention," from which all else follows, was the process for replicating complex organic molecules. In effect, the information describing the entire living structure is stored as sequences of nucleic acids in the genetic substance known as deoxyribonucleic acid (DNA). The mechanism for storing, coding, transferring, and decoding that information apparently evolved some 4 billion years ago, before species differentiation. Both the code and the mechanism are common to all known living organisms. The first cellular organisms, which appeared about 3.5 billion years ago, were prokaryotes (i.e., cells without nuclei). They obtained the energy needed to sustain the reproduction cycle from the anaerobic fermentation of organic molecules previously created by natural geophysical processes in an atmosphere containing no free oxygen. If the primitive atmosphere had contained oxygen, organic molecules could not have survived long enough to achieve the degree of complexity needed to construct self-reproducing systemS.l5 In cellular fermentation a molecule of glucose is split into two mole- cules of pyruvate. Energy is captured in the form of high-energy phosphate bonds in ADP (adenosine diphosphate) and ATP. Fermentation of a glucose molecule has a net yield of available energy in the form of two molecules of h~gh-energy ATP, converted from the low-energy ADP form (Hinkle and McCarty, 1978~. Further reactions in the cell convert the pyruvate to ethyl alcohol, lactic acid, and carbon dioxide, all of which are excreted as wastes. No oxygen is required. The fermentation-based forms of life could not have been the foundation for a sustainable ecosystem, however, because they were using up a finite stockpile of exploitable organic molecules. The next great evolutionary innovation, about 3 billion years ago, was anaerobic photosynthesis. Me first photosynthesizers were prokaryotic pho- tobacteria. These organisms began to synthesize glucose from atmospheric carbon dioxide and sunlight, thus replacing the depleted organic "soup" of

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42 cn o cS 1 . ~ ~: c) o Z UJ ~n 1 o U) a, o C (D o ~ a a) 0~) o C~ '=o a' c C) o .. j ~ ~,, ~,. ,. ,. ~ = ,., A, ~iS, .,W, . ~. ~. ~, ~ ~, ~,.,iON, ti i, a, ~ ~ ~ ~ 3 ~ - D - c, a' _ C ~ _ ._ ~ O ~ a) c Q _ O ~ C) - ~n e~ - o - i . ~ ~ .0 ~ ~ ~ I G E =' ~ cS a, a) O == C C m ~ ~ ~S ~ .. Oo _ o, Co ". 0 - Ct - - ~C :: ._ C~ Ct P4 C) ._ Q) o o ._ Ct - CQ oo ~o C~ Ct C~t ~ ~i ~ CO

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INDUSDUAL METABOLISM 43 the primitive oceans. They also produced oxygen as a waste product. At first, the free oxygen was quickly removed by stromatolites, organisms that combined oxygen with iron dissolved in the oceans and precipitated as iron- rich reefs. This seems to have been the origin of hematite deposits being exploited as iron ores today. However, when the dissolved iron was used up, about 1.8 billion years ago, the oxygen level in the atmosphere began to rise, thereby increasing the rate of dissolution of all macromolecules. Thus, the ecosystem was still unsustainable, because it could neither tolerate nor recycle its own toxic wastes. The oxygen toleration problem was partially solved by a new class of aerobic photosynthesizers, the cyanobacteria (or blue-green algae). These appeared a little more than 2 billion years ago. A third great evolutionary innovation the substitution of respiration (using oxygen) for fermentation to obtain energy from organic molecules- solved the recycling problem. The respiration process begins the same way as fermentation, with the splitting of glucose into pyruvate (called glycoly- sis). However, in respiration, glycolysis is followed by a longer sequence of reactions, known collectively as the citric acid cycle. In effect, the pyruvates are oxidized enzymatically to carbon dioxide, with the formation of many more high-energy ATP molecules. In fact, each glucose molecule, when fully oxidized, yields 36 molecules of ATP, whereas the glycolysis stage alone yields only 2. The yield of available energy for further metabolic processes is, therefore, 18 times that of the fermentation process. Because the respiration process is far more efficient than its precursor, aerobic respirators required much less organic material to sustain them. Thus, anaerobic organisms could not effectively compete with aerobic or- ganisms in the presence of oxygen. (They still fill an environmental niche in sediments and deep oceans lacking free oxygen.) With the "invention" of the citric acid circle, the biosphere became sustainable within more self-defined system boundaries. Evolutionary de- velopments since then-of which the most important were the develop- ment of the eukaryotes (cells with true nuclei) and the advent of sexual reproduction made the system more diverse and more efficient. It is in- teresting to note that, despite the radical changes in energy metabolism which occurred, the basic scheme of macromolecular reproduction seems to have remained essentially unchanged for 4 billion years. Industrial liar~sfonnation Processes In contrast to modern biological processes, industrial processes are almost exclusively energized by the combustion of fossil fuels, which (by definition) are not regenerated within the system. In this sense, the indus- tnal system of today resembles the earliest stage of biological evolution, when the most primitive living organisms obtained their energy from a

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44 ROBERT U LYRES stock of organic molecules accumulated during prebiotic times. Instead of regenerative cycles powered by solar (or nuclear) energy, industrial pro- cesses are linear sequences of discrete, irreversible transformations (Figure 4~. The sequence begins with extraction of raw matenals, followed by phys- ical separation and elimination of impunties, and subsequent reduction or recombination into convenient "first-tier" intermediates. This category includes prunary metals and other elements In pure form, cellulose, sodium carbonate, ammonia, methane, ethane, propane, butane, benzene, xylene, methanol, ethanol, acetylene, ethylene, propylene, and some others. These materials, in turn, are subsequently recombined into desired chemical and physical forms. Almost all of the processes for reducing metals from ores or producing first-tier intermediates are endothermic, that is, driven by externally sup- plied heat. Processes often use catalysts, and rates and directions are fine tuned by controlled variation of temperatures, pressures, and flow rates or dwell times. There are five major categories of endothermic processes: (1) dehydration; (2) calcination; (3) "reducing" processes for splitting metal (or other) oxides into their constituents; (4) dehydrogenation processes, of which the simplest is the splitting of the water molecule;~7 and (5) processes for combining or synthesizing molecules that do not combine spontaneously at ambient temperatures or pressures. Energy is obtained initially from combustion (oxidation) and delivered either by process steam or by direct contact with the oxidation products. Comparatively few indus- trial processes are electrolytic: the production of aluminum, sodium, and chlorine and the refining of blister copper are the primary examples. The most familiar example of dehydration is the production of plaster of parts from the mineral gypsum. Brick and ceramic manufacturing are also based to some extent on this process. The major example of calcination (and the origin of the name) is the production of calcium oxide (quicklime) from calcium carbonate (limestone) by driving off carbon dioxide. This is a major element in the manufacture of Portland cement. Both dehydration and calcination are accomplished by the simple application of heat. The so-called carbothermic reduction process by which iron ore is converted to pig iron is a typical example of the third category. In this reaction, coke is partially oxidized to carbon monoxide, which in turn reacts with the ore-at appropriate temperatures to reduce the iron oxides, while converting the carbon monoxide to carbon dioxide.~9 The temperature determines which way the reaction goes. If the temperature rises too high (above 1300 C), any pure iron that is present will reoxidize. Ammonia production exemplifies both the fourth and the fifth cate- gories. Dehydrogenation (water splitting) is an example of a process by which synthesis gas for ammonia is produced. In a first stage of the reac- tion, steam reacts with natural gas, in the presence of a catalyst, to produce

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INDUS17UAL METABOLISM Embodied Negentropy (Increase in Information) Separation of ore from crust Chemical processes that increase "availability" (or free energy) Matching of specifications for trace elements, crystal size, dislocations, etc. Matching of dimensions and shape specifications 45 Materials Transformation Lost Negentropy (Increase in Entropy) Extraction (Mining, Drilling, Harvesting, etc.) . mew Materia:3 Physical Separation and Refining | (Beneficiation, Smelting, etc.) termedia~ Pure ) Recombination | (Alloying, Synthesis, Heat Treatment, Hot/Cold Working, etc.) ~ Finished \~ Materials J Macro-Forming (Casting/Molding, Machining, _ Fording, etc.) (nished Pas) Work done on environment Heat loss to environment Design | Fabrication and Construction 1 Informationlossas J finished products 1 `( Finished ~ Products J Use junk. Refuse ] Information loss with wear Disposal ~Dispersal of materials I I FIGURE 4 Representation of the economic system as a multistage system for the extraction, physical separation, recombination, formation, and consumption of materials. At the end of the sequence, materials are returned to the environment in a degraded form as waste. The process of creating a finished product (center column) concentrates information, or negentropy, in the product at the expense of increased entropy in the environment.

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46 ROBERT U AYRES carbon monoxide and hydrogen. In a second stage, known as the water-gas shift reaction, carbon monoxide reacts with added steam to yield more hy- drogen and carbon dioxide, which must be removed by a scrubber such as potassium carbonate. The hydrogen is then mixed with nitrogen gas (from the air) in a 3:1 ratio.20 At very high pressures and temperatures, these in- gredients combine endothermicall~again in the presence of a catalyst to form ammonia gas, the basis of virtually all nitrogen-containing compounds used by our industrial civilization. Most of the reactions by which ammonia, chlorine, lime, sulfur, methanol, ethanol, acetylene, ethylene, propylene, or other first-tier inter- mediates are converted to other "downstream" compounds are exothermic and in effect-self-energizing. For example, many second-tier intermedi- ates are produced by controlled oxidation (e.g., sulfuric acid from sulfur, nitric acid from ammonia, acetaldehyde from ethanol or propane, acetic acid from acetaldebyde or butane, acetic anhydride from acetaldehyde, ethylene oxide from ethylene, propylene oxide from propylene, and so on). Most hydrogenation, chlorination, and hydrochlorination reactions are also exothermic, as are most reactions between strong acids and metals or hydroxides. In effect, the first-tier intermediates are the energy carriers for subse- quent reactions. They play a role somewhat analogous to that of ATP in biochemical systems. However, whereas the ATP is cyclically regenerated within the same cell, the first-tier intermediates are not regenerated but are physically embodied in downstream products. This is another funda- mental difference between industrial metabolism in its present form and its biological analogue. NOTES The three major German firms Bayer, Hoechst, and BASE were merged in the 1920s into a single giant under the name I.G. Farl~enindustrie. Farben is the German word for "color." The name reflected their common origin as synthetic dye (color) manufacturers. 2. An important corollary is that the underpricing of environmental resources corresponds to an underpricing of those exhaustible mineral resources whose subsequent disposal as waste residuals causes harm to the environment. This is because of the lack of any link between the market puce paid (for coal, oil, or whatever) and the subsequent cost of waste disposal or more important-of uncompensated environmental or health damages such as bronchitis, asthma, emphysema, cancer, soil acidification, the greenhouse effect, and so on. Here the distinction between renewable and nonrenewable resources is critical: although renewable resources can obviously create pollution problems, such as sewage, they are almost invariably localized in nature and can be abated at moderate cost. This is emphatically not the case for combustion products of fossil fuels or dispersion of toxic heavy metals. floe total world output of carbon dioxide from fuels and cement manufacturing has been estimated to be 5.1 billion metric tons for 1982, of which 26.7 percent was attributable to North America McFarland and Rotty, 1984~.

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INDUSTIUAL METABOLISM 47 4. Fly ash is primarily a by-product of coal combustion. At present it is being recovered fairly efficiently, from stack gases of large utility boilers and industrial furnaces, by means of electrostatic precipitatom However, the ash itself has become a large-scale nuisance because there exists no use or market for it. Ibe amounts are large: over 50 million tons are generated annually in the United States alone. Several possible remedies exist. Fly ash is a potential "ore" for several metals, especially iron, aluminum, and silicon. These metals could probably be recovered commercially if, for example, bauxite became unavailable (Ayres, 19823. Alternatively, fly ash could be used as a substitute, or more likely as a supplement, for Portland cement in the manufacture of concrete and concretelike products. fits major disadvantage in this application is that concrete made with fly ash does not harden and set as rapidly as the commercial variety. This has obvious economic costs, but so does the disposal of fly ash in landfills.) Another use of fly ash- already demonstrated in France is as a means for the permanent disposal of toxic liquid wastes in the folm of a hard, impermeable rocklike material suitable for long-term storage. The "chain" analogy is an oversimplification, of course, because many processes yield more than one useful product (the chlor-alkali industry is an obvious example) and many products also require two or more inputs. Thus, the structure of the system as a whole is more like a "tree." 6. For example, the first process to manufacture acetylene proceeded by way of calcium carbide production (from coke and limestone) but was displaced by direct dehydro genation of hydrocarbon feedstocks. The first production of ac~y'onitrile involved a reaction between ethylene oxide (itself the third step in a chain beginning with ethane) and hydrogen cyanide (made from ammonia). This was replaced by an acetylene cyanation process (acetylene being made from methane) and finally by a propylene ammoxidation process (propylene reacting directly with ammonia). Similarly, the fimt process in the manufacture of acetaldehyde started with acetylene (from calcium carbide) or ethanol (from ethylene). A newer process made acetaldehyde directly from ethylene. ~us, in the first example the calcium carbide stage was bypassed. In the second example the oxidation of ethylene was avoided, and in the third example the conversion of ethylene to alcohol was avoided. 7. The problem has been dramatized by several recent episodes in which cities have attempted to dispose of solid wastes by using private contractors who, in turn, thought to transport them to countries where disposal rules are nonexistent or wealcly enforced. For example, the city of Philadelphia contracted with Joseph Paolino & Sons to dispose of incinerator ash. Paolino, in turn, contracted with Amalgamated Shipping, a Bahamian concern, to transport the ash to the Bahamas. However, the Bahamian government bawd the dumping, and the ship carrying the ash was subsequently turned away from ports in the Dominican Republic, Haiti (after it had dumped 2,000 tons of ash), Honduras, Costa Rica, Guinea-Bissau, and the Cape Verde Islands. The ship apparently succeeded in dumping its load of ash somewhere in the Indian Ocean, after more than two years (New York Tones, November 10, 1988~. 8. Admittedly, it can still happen. Asbestos and polychlorinated biphenyls are two examples of materials that were once thought to be safe but have subsequently come to be regarded as hazardous and for which the major producers or users have had to spend billions of dollars to collect and dispose of them safely. Nevertheless, there would seem to be an economic opportunity for a "high-tech" resource-recove~y firm to go into business reconverting fly ash and incinerator ash into its most valuable components, light metals such as aluminum, iron, potash, and titanium (Ayres, 1982~.

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48 ROBERT U LYRES A mineralized glassy residue of sodium-silica and heavy metals would, of course, remain for disposal (though it might also find uses as a construction material). 9. This is a straightforward implication of the widely accepted, but sporadically enforced, "polluter must pay" principle. A number of possible processes for large-scale thermal decomposition of water have been suggested and studied in some detail, e.g., by the European Atomic Energy Commission (EURATOM). See, for example, Marchetti (19733. 11. Ammonia, hydrazine, and other compounds have also been suggested, but there is no clear leader at this time. A new study of the economic and technical feasibility of non~arbon-based liquid fuels would be helpful in clarifying the choices. 1Z It is true that some organisms have evolved to function in the deep oceans under conditions of high pressure and salinity, others have evolved to function in surface waters, saline or fresh; still others have evolved to function in environments with very little water. Nevertheless, the internal environment of every cell is aqueous and the pressure inside each cell is essentially the same as the external pressure of the environment in which the organism lives. 13. For a concise summary of the biochemistry of energy transport in cells, see Schopf (1978~. 14. The so-called manganese nodules, which are accretions of iron, manganese, copper, cobalt, and other transition elements, are evidently the result of some combination (as yet imperfectly understood) of biological, chemical, and geological processes (see, for example, Morgenstein, 1973~. 15. Even in the absence of oxygen, it would seem that dissolution of macromolecules should proceed faster than synthesis (Wald, 1954~. The exact evolutionary mechanism leading to self-reproduction is still obscure. Fixation of nitrogen must have been accomplished during this early period, because the early atmosphere was nearly transparent to ultraviolet radiation and any free ammonia in the atmosphere would have been quickly destroyed. 16. The obvious exceptions are elements occurring naturally (such as sulfur) and hy- drocarbons that can be obtained By physical separation from natural gas (methane, ethane, propane, butane) or coal tar (benzene, xylene, toluene). Cellulose occurs naturally in some very pure forms (e.g., cotton), but it is usually obtained from wood pulp lay a chemical digestion process. 17. Other major examples of dehydrogenation include the splitting of methane, ethane, propane, and butane to produce acetylene, ethylene, propylene, butylene, and butadi- ene. 18. Pig iron is a solution of iron carbide in iron, with a typical carbon content of approximately 6 percent. The conversion of pig iron to pure (wrought) iron or steel requires removing this carbon and then adding any desired alloying elements. 19. Multistage reactions such as the reduction of iron ore and the synthesis of ammonia, involving an intermediate (carbon monoxide) that is produced by the reaction and later consumed, are quite common in industry. Less common are reactions involving an intermediate that is not produced within the process but is recycled. One of the first examples of such a process was the Solvay (ammonia-soda) process for manufacturing synthetic sodium carbonate from sodium chloride and calcium carbonate. In this process, ammonium hydroxide reacts with calcium carbonate to yield ammonium carbonate and calcium hydroxide. Ammonium carbonate is converted to ammonium bicarbonate. When this reacts with sodium chloride, sodium carbonate and ammonium chloride are produced. Finally, calcium hydroxide and ammonium chloride are reacted to recover ammonia (gas) for recycling and calcium chloride. Ibe latter is a low-value by-product.

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INDUSTRIAL METABOLISM 49 20. In principle, the nitrogen gas and steam could be produced together lay partial oxidation of natural gas (or any hydrocarbon) in air to yield a mixture of nitrogen, steam, and carbon monoxide. The steam in the hot combustion products could then be reacted with additional natural gas to generate hydrogen and more carbon monoxide to be used as feedstock for the shift reaction. However, the presence of nitrogen before it is wanted complicates the engineering unreasonably. REFERENCES Ayres, R. U. 1978. Resources, Environment and Economics: Applications of the Materi- als/Energy Balance Principle. New York: John Wiley & Sons. Ayres, R U. 198Z Coalplex: An integrated enerp,y/resources system concept. United Nations Environment Program Seminar on Environmental Aspects of Technology Assessment, United Nations, Geneva, November-December 1980. Ayres, R. U., J. Cummings-Saxton, and E. Weinstein. 1978. Assessment of methodolo- gies for indirect impact assessment. Anal Report (IRT~68-R/a) prepared for U.S. Environmental Protection Agency. Washington, D.C.: International Research and Technology Corporation. Ayres, R. U., L W. Ayres, J. A. Tarr, and R. C. W~dge~y. 1988. An historical reconstruction of major pollutant levels in the Hudson-Raritan Basin: 188~1980. National Oceanic and Atmospheric Administration Technical Memorandum NOS OMA 43, 3 vols. Rockville, Md. Clark, R., ed. 1987. The Ozone Layer [Series: UNEP/GEMS Environment Library], Vol. 2. United Nations Environment Program, Nairobi. Gschwandtner, G., K C. Gschwandtner, and K. Eldridge. 1983. Historic Emissions of Sulfur and Nitrogen Oxides in the U.S. 1900 1980. Report to U.S. Environmental Protection Agency. Durham, N.C.: Pacific Environmental Services, Inc. Hinkle, P. C., and R. E. McCarty. 1978. How cells make ATP: The prevailing theory is the "chemiosomotic" one. Scientific American 23~3~: 104 123. Lovelock, J. 1988. The Ages of Gala: A Biography of Our Living Earth. New York Norton. Marchetti, C. 1973. Hydrogen and energy. Chemical Economy and Engineering Review 5~1~:7-25. Marland, G., and R. M. Rotty. 1984. Carbon dioxide emissions from fossil fuels: 195~1982. Tellus 36B(4~:23~261. Miller, ~ S., and I. M. Mintzer. 1986. The Sky is the Limit: Strategies for Protecting the Ozone I>yer, Research Report (3~. Washington, D.C: World Resources Institute. Morgenstein, M., ed. 1973. Papers on the Origin and Distribution of Manganese Nodules in the Pacific and Propects for Exploration. Symposium, Hawaii Institute of Geophysics, Honolulu. Multhauf, R. P. 1967. Industrial chemistry in the nineteenth century. Pp. 4~ in Technology in Western Civilization: The Emergence of Modern Industrial Society Earliest Times to 1900, M. Kranzberg and C W. Pursell, Jr., eds. New York: Oxford University Press. New York limes. November 10, 1988. After two years at sea, ship dumps U.S. ash. Vol. 138, lO(N), c4(L3. Schopf, J. W. 1978. The evolution of the earliest cells. Scientific American ~9~3~:11~138. U.S. Bureau of the Census. 1960 1975. Statistical Abstract of the United States. Washing- ton, D.C.: U.S. Government Printing Office. U.S. Environmental Protection Agency. 1986. National Air Pollution Emission Estimates, 194~1984 (EPA~50/4~5~14~. Research Triangle Park, N.C: Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency. U.S. Office of Technology Assessment. 1983 Genetic Technology: A New Frontier. Boulder, Colo.: Westview Press. Mild, G. 1954. The origin of life. Scientific American 191(August):45-53.