On Measures of “decoupling”

– Robert U. Ayres. Paris. 232 October 2014

decoupling chainThe term “decoupling” is commonly used among ecological economists to express the notion that somehow economic activity can proceed without (or with much less) material resource consumption e.g. {Fischer-Kowalski, 2011 #7235}. The reasons for worrying about “decoupling” are three interconnected twin concerns: (1) environmental damages, (2) potential material scarcity problems, and (3) linkages to economic growth (or the lack of it). These three concerns are usually addressed separately, because different categories of material resources are involved in different ways. I will comment briefly on #2 and #3 later, but threats of environmental harm (especially climate change and bio-diversity loss)are the primary motivator of the “decoupling” proponents while #2 is somewhat supportive of #1 but #3 is, so to speak, “the elephant in the room”.

The phrase “absolute decoupling” – a phrase in the literature — implies that there is no link at all between material resource use and some economic activity. This is another way of saying that the economic activity is self-propelled. It is the economic analog of “perpetual motion”, long-since discredited in physics. Economic activity and growth, without activating resource inputs, is a physical (thermodynamic) impossibility even though it happens to be consistent with the neo-classical economic paradigm (in which materials and energy play a negligible role). In reality – as opposed to neo-classical theory, economic activity is very tightly linked to resource use and always will be as long as final services require material goods that must be produced via material extraction and transformation processes. The legitimate underlying question for ecological economics is not how to eliminate resource consumption per se which is impossible. It is, rather, to what extent economic activity can be shifted away from geologically scarce resources that have harmful by-products to much less scarce resources with less harmful by-products.

But, first things first: how should de-coupling be measured? The simplest procedure is to lump very heterogeneous materials into a single category (resources), which are then measured only in mass terms e.g. {Hagedorn, 1992 #2249;Schmidt-Bleek, 1993 #4500;Schmidt-Bleek, 1993 #4499;Adriaanse, 1997 #26} {Hinterberger, 1999 #2436} {Matthews, 2000 #6686} {Ibenholt, 2002 #2579} {Bringezu, 2002 #971} {OECD, 2002 #6545;Yong, 2008 #6536;Fischer-Kowalski, 2011 #7235}. But, while large scale mass-flows like floods or erosion often cause environmental harm, the potential for environmental harm in principle is not, in general, proportional to mass. What we seek, really, is a measure of the potential for causing harm, which might be termed (for lack of a better word) eco-toxicity {Ayres, 1995 #506}.

Some of the biggest solid mass flows, including mine overburden, soil erosion, soil displaced by construction, demolition waste, coal ash and municipal solid waste are nearly inert from a toxicity point of view. They are harmful insofar as they disturb the physical environment primarily because the masses involved are very large. For example, each person living in the US generates almost a ton (1700 lb) of municipal waste each year. It is estimated that a third of all food produced is wasted – much of it ending in “garbage” (i.e. municipal waste). But, worse, soil loss from erosion in the US is estimated to be 1.9 billion tons per year, which is about 6 times the weight of food crops produced. The magnitudes are even greater for fresh water. Irrigation water, cooling water and municipal water are moved in enormous quantities. In 2005 the US Geological Survey estimated that fresh water withdrawals were 415 billion tons, or 470 tons per person. But these large mass flows constitute only negligible environmental hazards on a per unit mass basis. In fact, mass flow analysis (MFA) routinely ignores most fresh water flows as such, and considers only the pollutants in the water {Ayres, 2002 #612}. Sewage, runoff from paved surfaces, acid mine waste, and textile washing and dyeing wastes are more problematic, but much smaller in volume. And those flows are still far less dangerous per unit mass than (for instance) nuclear reactor wastes, tannery wastes, metal-plating waste, chlorination wastes and other chemical process wastes.

Unfortunately “eco-toxicity” is not a general measure for potential harm because it depends on the specific characteristics of the affected organism. Carbon monoxide is toxic to air-breathing animals, because it replaces the oxygen in blood hemo-globin. But oxygen is toxic to anaerobic organisms that excrete oxygen as a waste product. Sugar is toxic to someone with diabetes. Hydrogen sulfide is toxic to us humans and most aerobic species, but there are deep-sea organisms that feed off it. There are some substances, mainly heavy metals, that are highly toxic to many organisms because they tend to disrupt essential metabolic processes, usually by displacing other compounds or elements like sodium, potassium, calcium or iron. Examples include arsenic, cadmium, copper salts, organic mercury compounds, chromium 6, lead salts, thallium, chlorinated aromatics, plutonium, etc. Some elements are essential at low doses and toxic at higher doses (e.g. chromium and selenium).

Despite the problems, there is a significant literature attempting to rank-order conventional toxic substances relative to (say) lead or arsenic, for purposes of life-cycle assessment {De Sesso, 1987 #1471} {Guinee, 1993 #2221;Hertwich, 1996 #2405;Huijbregts, 2000 #2542;Hertwich, 2001 #2407}. However, such comparisons are limited by the fact that some toxins have an immediate metabolic effect while others (like radiation poisoning) are cumulative; some (like snake venom or insect stings) can trigger a detoxification response or an immune system response, while others don’t. Should virus infections be regarded as toxins? Is tobacco smoke a toxin? What about alcohol? The “bottom line” is that there is still no satisfactory scale of relative or absolute toxicity for humans. The situation is still less clear for other species.

However, there is one measurable characteristic of all wastes that seems to correspond (very roughly) to toxicity. If the internal environment of the cells of an organism can be characterized in terms of chemical composition (assuming temperature and pressure are ambient), the likelihood that a “strange” substance would disrupt normal metabolic activity is surely a function of its thermodynamic “distance” from the equilibrium (reference) state of the cell. The best available proxy for such a measure must be exergy, which is the amount of work that can be done as the “strange” substance approaches thermodynamic equilibrium. That happens when all possible exothermic (spontaneous) reactions have occurred. If the reactions in the cell are harmful i.e. they weaken the physical structure of the cell or alter the structure of the enzymes within it, the “strange” substance is a toxin; and conversely.[1]

The reference environment for most cells in most living organisms is similar to the ocean, from which most or all living organisms originally evolved. (The major difference between the internal cellular compositions of terrestrial vs aquatic organisms is probably just the salinity). The physical units of exergy are energy units (joules, kwh, btu). All materials (and material flows) embody exergy, not just fuels and biomass . (Energy reserves are really exergy reserves (Wall 1977, 1986; Wall, Sciubba, and Naso 1994; Ayres and Martinás 1995; Ayres, Martinás, and Ayres 1998) reserves). There are tables in several textbooks where the exergy content of a material, with respect to any reference environment, can be looked up (Szargut, Morris, and Steward 1988; Dincer and Rosen 2007). It can also be calculated from first principles, if necessary.

Evidently the exergy (useful energy) content of a material, per unit of mass, may be infinitesimal (as with non-moving air or water) or very large (as with fossil fuels or U235).

Of course, natural exergy flows (such as ocean and air currents) are also far greater in magnitude than industrial, municipal or household exergy flows, and are also routinely ignored. Bottom line: anthropogenic exergy flows are not proportional to, or determined by anthropogenic mass flows,

In some sense, the GHGs are “toxic” to the earth’s atmosphere and, indirectly, to the earth’s climate in exactly the same way that a “strange” substance may be toxic in a cell. Think of the earth as an organism with a functional equilibrium corresponding to a stable benign climate. That equilibrium is disturbed by GHGs that reflect infra-red radiation from the surface – as a greenhouse does — and thus raise the average temperature of the earth, as well as altering climate dynamics. Carbon dioxide, the most important GHG, is a by-product of combustion of carbon-based fuels. Combustion is an exergy-conversion process.  Atmospheric methane – apart from natural gas leaks – is mostly generated by anaerobic decay or digestion, which are also exergy conversion processes.)

While it is true that exergy content is not the same as toxicity, exergy and toxicity are both measures of disequilibrium or, more precisely, disturbances resulting from thermodynamic disequlibrium. But thermodynamic disequilibrium is not proportional to mass. Clearly GHG emissions need to be sharply reduced. That imperative is certainly one of the underlying drivers of interest in ‘decoupling” . But it would be a serious mistake to focus on reducing GHG emissions per se, as the IEA’s 2009 study “Energy Technology Transitions for Industry” has (very unfortunately) done. The reason is that it is too easy to assume, as most studies do (without ever saying so), that economic growth is actually independent of (i.e. is absolutely decoupled from) exergy consumption. The key point that needs to be understood more widely is that economic growth and exergy consumption (and to a lesser extent, consumption of other material resources) are really inseparable. The reason, in brief, is that both machines and humans (or animals) must be activated by an exergy input in order to produce anything, useful or otherwise. Machines without fuel or electric power are just inert mass. Humans and animals also require Calories (a measure of exergy) to survive, never mind to do useful work. I will say more about this in the next several paragraphs below.

[1] Of course there are specialized cells (mainly in the liver of higher animals) that can “detoxify” potentially harmful organic substances, such as alcohol. However, the detoxification capabilities of different organism vary depending on the external environment in which they live. This introduces a new level of complexity to the analysis and is probably the main reason that the toxicity of a given substance is not always proportional to its exergy content.


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Denison, Edward F. 1984. Accounting for slower economic growth: An update. In International Comparisons of Productivity and Causes of Slowdown, edited by J. W. Kendrick. Cambridge, UK: Ballinger.

Dincer, Ibrahim, and Marc A. Rosen. 2007. Exergy: energy, environment and sustainable

development: Elsevier. Hamilton, James D. 2003. What is an oil shock? Journal of Econometrics 113:363-398. ———. 2005. Oil and the macroeconomy. In The New Palgrave: A Dictionary of Economics,

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creativity: modeling innovation diffusion. Structural Change and Economic Dynamics 13:415-433.

Kuemmel, Reiner, Robert U. Ayres, and Dietmar Lindenberger. 2010. Thermodynamic laws, economic methods and the productive power of energy. Journal of Non-equilibrium Thermodynamics.

Lindenberger, Dietmar, Robert U. Ayres, Joerg Schmid, and Reiner Kuemmel. 2008. Macro-and micro- economic production functions. Wuerzburg, Germany. Meric, Jean-Paul. 2010. Chief Operating Officer, Italcementi Group. Paris.

Strahan, David. 2007. The last oil shock. London: John Murray Ltd.

Szargut, Jan, David R. II Morris, and Frank R. Steward. 1988. Exergy analysis of thermal, chemical, and metallurgical processes. New York: Hemisphere Publishing Corporation.

Wall, Goran. 1977. Exergy: A useful concept within resource accounting. Goteborg, Sweden: Institute of Theoretical Physics, Chalmers University of Technology and University of Goteborg.

———. 1986. Exergy conversion in the Swedish society. Energy 11:435-444.

Wall, Goran, Enrico Sciubba, and Vincenzo Naso. 1994. Exergy use in the Italian society. Energy – The International Journal 19 (12):1267-1274.

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About the author:

ayres bw smallRobert Ayres’s career has focused on the application of physical ideas, especially the laws of thermodynamics, to economics; a long-standing pioneering interest in material flows and transformations (Industrial Ecology or Industrial Metabolism); and most recently to challenging held ideas on the economic theory of growth.

[More at https://en.wikipedia.org/wiki/Robert_Ayres_(scientist)





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