- Email: [email protected]
FORUM: Consuming the earth: the biophysics of sustainability
Ecological Economics 29 (1999) 23 – 27
COMMENTARY FORUM
Consuming the earth: the biophysics of sustainability William E. Rees * School of Community and Regional Planning, Uni6ersity of British Columbia, Lasserre 433, 6333 Memorial Road, V6T 1Z2 Vancou6er, BC, Canada Received 5 June 1997; received in revised form 29 December 1997; accepted 10 March 1998
1. Premise and purpose The underlying premise of this paper is that much of economics is, or should be, human ecology (Rees and Wackernagel, 1994). The economy is that set of activities and relationships by which human beings acquire, process, and distribute the material necessities and wants of life. It therefore includes that subset of activities by which humankind interacts with the rest of the ecosphere. If we were dealing with any other species, these relationships would indisputably fall within the realm of ‘ecology’. To this extent, then, economists are arguably human ecologists. However, there is a problem. Ecologists study non-human species and the ecosystems that sustain them by measuring and analyzing the physical flows of energy, material, and information
* Tel.: +1 604 8222937; fax: + 1 604 8223787; e-mail: [email protected]
essential to the continuous restructuring and selforganization of those systems. By contrast, most economic analyses are money-based and totally ignore both physical reality and the behavioral dynamics of ecosystems. Economics (as presently structured) is therefore seriously flawed as (human) ecology. In the context of sustainability, its analytic blindness generates a false sense of societal well-being and leads to potentially disastrous economic prescriptions. To make matters worse, ecologists have themselves spent little effort on the study of humans. With neither discipline properly focused, it is little wonder that we still have such a poor understanding of the biophysical dimensions of the sustainability problem. In this light, a major goal of ecological economics is to reconstruct economics on a firm foundation of theory governing physical relationships and material transformations in nature. Thus, the specific purpose of this paper is to illustrate the critical role of such biophysical analyses in assessing progress toward ecologically/economically sustainable development.
0921-8009/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0921-8009(98)00074-3
24
W.E. Rees / Ecological Economics 29 (1999) 23–27
2. Lines of convergence and divergence One thing that both economists and ecologists agree upon is that human beings are consumer organisms. In fact, in todays increasingly marketbased society people are as likely to be called ‘consumers’ as they are citizens, even when the context is a non-economic one. The designated role of people in the economy is to consume the goods and services produced by businesses, which are deemed to constitute the only other major component of the economy. Ecologists would actually refer to humans as macro-consumers. In general, macro-consumers are large organisms, mainly animals, that depend on other organisms, either green plants or other animals, which they consume directly to satisfy their metabolic needs. However, a complete human ecology would also have to consider the consumption demands of our manufactured capital. Indeed, the major ecological difference between humans and other species is that in addition to our biological metabolism, the human enterprise is characterized by an industrial metabolism. All our toys and tools, factories and infrastructure, are ‘the exosomatic equivalent of organs’ (Sterrer, 1993) and, like bodily organs, require continuous flows of energy and material from and to ‘the environment’ for their production, maintenance, and operation. Economists and ecologists also both see humans as producers. We can only marvel at the enormous quantity of goods and services, both essential and frivolous, that advanced economies have spewed into a willing marketplace. However, there is a fundamental difference between production in nature and production in the economy. In nature, green plants are the factories. Using the simplest of low-grade inorganic chemicals (mainly water, carbon dioxide and a few mineral nutrients) and an extra-terrestrial source of relatively low-grade energy, light from the sun, plants assemble the high-grade fats, carbohydrates, proteins, and nucleic acids upon which most other life forms and the functioning of the ecosphere are dependent. Because they are essentially self-feeding and use only dispersed (high entropy) substances for their growth and maintenance, green plants are called primary producers.
By contrast, human beings and their economies are strictly secondary producers. The production and maintenance of our bodies and all the products of human factories require enormous inputs of high-grade energy and material resources from the rest of the ecosphere. That is, all production by the human enterprise, from the increase in population to the accumulation of manufactured capital, requires the consumption of a vastly larger quantity of available energy and material first produced by nature (typically at least 10 times more). This last point is critical when we consider that people and their economies are part of nature. Indeed, humans have become the major consumer organism in virtually all the significant ecosystem types on earth—in structural terms, the expanding human enterprise is positioned to consume the ecosphere from within.
3. The hidden dimensions of consumption Since the beginning of the industrial revolution, human populations have been growing, material standards have been rising, and increasingly sophisticated methods have been developed for resource extraction and ‘harvesting’. Globally, ever greater quantities of energy and material are used for the manufacture and maintenance of productive capital and for the production of goods and services. On this basis, one might argue a priori that the rate of resource consumption by industrial economies will routinely exceed the rate of production by local ecosystems. In the absence of trade, such economies are fundamentally unsustainable.
4. Growing the economy, dissipating nature A recent study of human impacts on the carbon cycle in the Lower Fraser Basin of British Columbia, Canada, from the beginning of European settlement in the 1820s to 1990, provides ample support for this hypothesis (Boyle and Lavkulich, 1997). The Lower Fraser Basin is in temperate rainforest zone of coastal western North America, one of the most productive terrestrial ecosystems
W.E. Rees / Ecological Economics 29 (1999) 23–27
25
Table 1 Biomass carbon dissipated from Lower Fraser Basin ecosystems compared to global averages Net carbon releases since 1800 Total
Lower Fraser Basina World-wideb a b
231 million tonnes 60 billion tonnes
Releases per capita increase in population (tonnes)
Per ha (tonnes)
135
278
37
11
Fraser Basin to global ratio
P. capita
Per ha
3.6
25
Source Boyle and Lavkulich, 1997. Source Houghton and Skole, 1990.
on Earth. With an area of approximately 8300 km2, the study region had a 1990 population of 1.7 million. Its early formal economy was based on furs and fish, evolving rapidly through timber and agriculture, to light manufacturing and, most recently, high-end services. The region has always been heavily engaged in trade. This economic progression has been marked by intense resource exploitation, steady population growth, and rising incomes. Indeed, the Lower Fraser Basin is today one of the most economically privileged regions on the planet. The expansion of the human economy has, however, been at the expense of the economy of nature. Boyle and Lavkulich (1997) found that the human ‘development’ of the Lower Fraser has resulted in a net release of 231 million tonnes of biomass carbon into the atmosphere from exploited terrestrial ecosystems. A total of 43% of the carbon has come from soils, 42% from logged forests, and 14% from degraded wetlands. This is a carbon release rate of more than 130 tonnes per person increase in population, or an average of 278 tonnes/ha. For comparison, extrapolating from Houghton and Skole (1990), we can estimate a net carbon loss from terrestrial habitats worldwide of 160 billion tonnes since 1800, during which time the human population increased by about 4.3 billion (to 1990). This is a carbon release rate of 37 tonnes per person increase or 11 tonnes/terrestrial ha. Lower Fraser Basin carbon releases are therefore 3.6 times global releases per person increase since 1800, and more than 25 times the global contribution per ha (Table 1).
These data highlight the physical dimensions of (industrial) economic reality that are so completely hidden from conventional monetary analysis. We have already noted that secondary production necessarily consumes large quantities of high-grade energy and biomass. Only a small portion of this is converted to human population growth and the accumulation of manufactured capital. The rest is dissipated widely as low grade waste (e.g. carbon dioxide in the present example). Modern interpretations of the second law of thermodynamics suggest that such behavior is an invariable characteristic of all highly-ordered complex systems. The economy can grow and maintain its internal structure in a dynamic nonequilibrium state only through the continuous dissipation of available energy/matter ‘imported’ from its host environment. Thus, the material demands of our growth-addicted urban-industrial economies must inevitably exceed the fixed productive and waste assimilation capacities of regional ecosystems. The observed ‘dissipation’ of carbon from the ecosystems of the Lower Fraser Basin symbolizes both the depletion of the region’s natural capital stocks and the overtaxing of local waste sinks (at the expense of rising global CO2 levels and potential climate change).
5. The invisible foot of the economy But the story does not end there. While Boyle and Lavkulich (1997) show that the rate of carbon
W.E. Rees / Ecological Economics 29 (1999) 23–27
26
Table 2 Ecological footprints of Vancouver and the Lower Fraser Basin Geographic unit
Population
Land area (ha)
Ecological footprint (ha)
Overshoot factor
Vancouver City L. Fraser Basin
472 000 2 000 000
11 400 830 000
2 360 600 10 000 000
207 12
release from the Lower Fraser has been declining since the 1930s, the regional population and economy are among the most rapidly growing in North America. These opposing trends are possible only because consumption in the region is now sustained largely by production outside its boundaries. A new tool I have developed with my students, ecological footprint analysis, shows that the average resident of the Lower Fraser currently appropriates through trade and natural flows the biophysical goods and services of about 5 ha of productive land and water (adjusted from Rees and Wackernagel, 1994, Rees, 1996, Wackernagel and Rees, 1996). Thus, the 472000 residents of the city of Vancouver, sitting on just 11400 ha, actually use the ecological output of 2.3 million ha. Similarly, the 2 million inhabitants (1997) of the basin as a whole require about 10 million ha of productive ecosystem to support their consumer lifestyles. In short, the true but invisible ‘ecological footprint’ of the region’s population is 12 times the geographic area of its home territory (Table 2). Having overused or drawn down their own natural capital stocks, the people of the Lower Fraser are now nearly wholly dependent on temporary surpluses elsewhere.
6. Conclusions: the economy as parasite Our technological prowess feeds the common belief that humans and their economies have become increasingly independent of nature. Indeed, the dollar contribution of the primary sector to many service- and knowledge-based economies is so small that economists now routinely omit land/ resources from their production functions in accounting for economic growth. This presumed economy – environment decoupling is pure illusion sustained by abstract mone-
tary models of economic process. The foregoing biophysical analysis shows that modern high income economies are actually becoming increasingly indebted to nature. Most are running massive unsustainable ecological deficits with the rest of the world (Rees, 1996; Wackernagel and Rees, 1996; Wackernagel et al., 1997). Technology and trade have merely obscured this relational truth by displacing the negative consequences of growth to distant ecosystems and the future. Rising productivity and incomes produce overweening confidence in human ingenuity while imposing ever-larger ecological footprints on the earth. The latter goes unnoticed by policy advisors and ordinary citizens alike. Meanwhile, the trophic relationship of industrial economies to their host environments remains that of parasite to host—the former gains vitality at the expense of the vitality and regenerative capacity of the latter (see Peacock, 1995; Rees, 1998). Temporarily shielded from the negative consequences of material profligacy, high-income countries continue blindly to consume the earth while actively encouraging the developing world to adopt the same driving values and lifestyles. This virtually guarantees that the entropic degradation of the environment will be raised to the level of the ecosphere, eventually affecting everyone more or less simultaneously (as is happening with ozone depletion). From this perspective, the prognosis for sustainability may seem bleak indeed. While the mainstream approach to sustainability requires a 5- to10-fold expansion of world industrial activity, the present analysis shows that throughput growth on anything like this scale is physically impossible. One can hardly imagine more diametrically opposed positions on humanity’s and the ecosphere’s future prospects. However, a clear understanding of the sustainability problem is a
W.E. Rees / Ecological Economics 29 (1999) 23–27
prerequisite for any effective solution. To the extent that the biophysically-based ecological economics advocated here approximates reality better than prevailing expansionist models, it may help to break the stalemate. In any event, it shows that economists can never become true human ecologists until they embrace the need for integrated biophysical analyses.
References Boyle, C., Lavkulich, L., 1997. Carbon pool dynamics in the Lower Fraser Basin from 1827 to 1990. Environ. Manag. 21, 443 – 445. Houghton, R.A., Skole, D.L., 1990. Carbon. In: Turner, L.B. II (Ed.), The Earth as Transformed by Human Action. Cambridge: Cambridge University Press, pp. 393–408.
.
27
Peacock, K.A., 1995. Sustainability as symbiosis. Alternatives 21 (4), 16 – 22. Rees, W.E., 1996. Revisiting carrying capacity: area-based indicators of sustainability. Popul. Environ. 17 (3), 195 – 215. Rees, W.E., 1998. How should a parasite value its host? Ecol. Econ. 25, 49 – 52. Rees, W.E., Wackernagel, M., 1994. Ecological footprints and appropriated carrying capacity: measuring the natural capital requirements of the human economy. In: Jansson, A.-M., Hammer, M., Folke, C., Costanza, R. (Eds.), Investing in Natural Capital: The Ecological Economics Approach to Sustainability. Island Press, Washington, DC., pp. 362 – 390. Sterrer, W., 1993. Human economics: a non-human perspective. Ecol. Econom. 7, 183 – 202. Wackernagel, M., Rees, W.E., 1996. Our Ecological Footprint: Reducing Human Impact on Earth. New Society Publishers, Gabriola Island, BC/Stony Creek, CT. Wackernagel, M. et al., 1997. Ecological Footprints of Nations. Report to the Earth Council, Costa Rica.
COMMENTARY FORUM
Consuming the earth: the biophysics of sustainability William E. Rees * School of Community and Regional Planning, Uni6ersity of British Columbia, Lasserre 433, 6333 Memorial Road, V6T 1Z2 Vancou6er, BC, Canada Received 5 June 1997; received in revised form 29 December 1997; accepted 10 March 1998
1. Premise and purpose The underlying premise of this paper is that much of economics is, or should be, human ecology (Rees and Wackernagel, 1994). The economy is that set of activities and relationships by which human beings acquire, process, and distribute the material necessities and wants of life. It therefore includes that subset of activities by which humankind interacts with the rest of the ecosphere. If we were dealing with any other species, these relationships would indisputably fall within the realm of ‘ecology’. To this extent, then, economists are arguably human ecologists. However, there is a problem. Ecologists study non-human species and the ecosystems that sustain them by measuring and analyzing the physical flows of energy, material, and information
* Tel.: +1 604 8222937; fax: + 1 604 8223787; e-mail: [email protected]
essential to the continuous restructuring and selforganization of those systems. By contrast, most economic analyses are money-based and totally ignore both physical reality and the behavioral dynamics of ecosystems. Economics (as presently structured) is therefore seriously flawed as (human) ecology. In the context of sustainability, its analytic blindness generates a false sense of societal well-being and leads to potentially disastrous economic prescriptions. To make matters worse, ecologists have themselves spent little effort on the study of humans. With neither discipline properly focused, it is little wonder that we still have such a poor understanding of the biophysical dimensions of the sustainability problem. In this light, a major goal of ecological economics is to reconstruct economics on a firm foundation of theory governing physical relationships and material transformations in nature. Thus, the specific purpose of this paper is to illustrate the critical role of such biophysical analyses in assessing progress toward ecologically/economically sustainable development.
0921-8009/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0921-8009(98)00074-3
24
W.E. Rees / Ecological Economics 29 (1999) 23–27
2. Lines of convergence and divergence One thing that both economists and ecologists agree upon is that human beings are consumer organisms. In fact, in todays increasingly marketbased society people are as likely to be called ‘consumers’ as they are citizens, even when the context is a non-economic one. The designated role of people in the economy is to consume the goods and services produced by businesses, which are deemed to constitute the only other major component of the economy. Ecologists would actually refer to humans as macro-consumers. In general, macro-consumers are large organisms, mainly animals, that depend on other organisms, either green plants or other animals, which they consume directly to satisfy their metabolic needs. However, a complete human ecology would also have to consider the consumption demands of our manufactured capital. Indeed, the major ecological difference between humans and other species is that in addition to our biological metabolism, the human enterprise is characterized by an industrial metabolism. All our toys and tools, factories and infrastructure, are ‘the exosomatic equivalent of organs’ (Sterrer, 1993) and, like bodily organs, require continuous flows of energy and material from and to ‘the environment’ for their production, maintenance, and operation. Economists and ecologists also both see humans as producers. We can only marvel at the enormous quantity of goods and services, both essential and frivolous, that advanced economies have spewed into a willing marketplace. However, there is a fundamental difference between production in nature and production in the economy. In nature, green plants are the factories. Using the simplest of low-grade inorganic chemicals (mainly water, carbon dioxide and a few mineral nutrients) and an extra-terrestrial source of relatively low-grade energy, light from the sun, plants assemble the high-grade fats, carbohydrates, proteins, and nucleic acids upon which most other life forms and the functioning of the ecosphere are dependent. Because they are essentially self-feeding and use only dispersed (high entropy) substances for their growth and maintenance, green plants are called primary producers.
By contrast, human beings and their economies are strictly secondary producers. The production and maintenance of our bodies and all the products of human factories require enormous inputs of high-grade energy and material resources from the rest of the ecosphere. That is, all production by the human enterprise, from the increase in population to the accumulation of manufactured capital, requires the consumption of a vastly larger quantity of available energy and material first produced by nature (typically at least 10 times more). This last point is critical when we consider that people and their economies are part of nature. Indeed, humans have become the major consumer organism in virtually all the significant ecosystem types on earth—in structural terms, the expanding human enterprise is positioned to consume the ecosphere from within.
3. The hidden dimensions of consumption Since the beginning of the industrial revolution, human populations have been growing, material standards have been rising, and increasingly sophisticated methods have been developed for resource extraction and ‘harvesting’. Globally, ever greater quantities of energy and material are used for the manufacture and maintenance of productive capital and for the production of goods and services. On this basis, one might argue a priori that the rate of resource consumption by industrial economies will routinely exceed the rate of production by local ecosystems. In the absence of trade, such economies are fundamentally unsustainable.
4. Growing the economy, dissipating nature A recent study of human impacts on the carbon cycle in the Lower Fraser Basin of British Columbia, Canada, from the beginning of European settlement in the 1820s to 1990, provides ample support for this hypothesis (Boyle and Lavkulich, 1997). The Lower Fraser Basin is in temperate rainforest zone of coastal western North America, one of the most productive terrestrial ecosystems
W.E. Rees / Ecological Economics 29 (1999) 23–27
25
Table 1 Biomass carbon dissipated from Lower Fraser Basin ecosystems compared to global averages Net carbon releases since 1800 Total
Lower Fraser Basina World-wideb a b
231 million tonnes 60 billion tonnes
Releases per capita increase in population (tonnes)
Per ha (tonnes)
135
278
37
11
Fraser Basin to global ratio
P. capita
Per ha
3.6
25
Source Boyle and Lavkulich, 1997. Source Houghton and Skole, 1990.
on Earth. With an area of approximately 8300 km2, the study region had a 1990 population of 1.7 million. Its early formal economy was based on furs and fish, evolving rapidly through timber and agriculture, to light manufacturing and, most recently, high-end services. The region has always been heavily engaged in trade. This economic progression has been marked by intense resource exploitation, steady population growth, and rising incomes. Indeed, the Lower Fraser Basin is today one of the most economically privileged regions on the planet. The expansion of the human economy has, however, been at the expense of the economy of nature. Boyle and Lavkulich (1997) found that the human ‘development’ of the Lower Fraser has resulted in a net release of 231 million tonnes of biomass carbon into the atmosphere from exploited terrestrial ecosystems. A total of 43% of the carbon has come from soils, 42% from logged forests, and 14% from degraded wetlands. This is a carbon release rate of more than 130 tonnes per person increase in population, or an average of 278 tonnes/ha. For comparison, extrapolating from Houghton and Skole (1990), we can estimate a net carbon loss from terrestrial habitats worldwide of 160 billion tonnes since 1800, during which time the human population increased by about 4.3 billion (to 1990). This is a carbon release rate of 37 tonnes per person increase or 11 tonnes/terrestrial ha. Lower Fraser Basin carbon releases are therefore 3.6 times global releases per person increase since 1800, and more than 25 times the global contribution per ha (Table 1).
These data highlight the physical dimensions of (industrial) economic reality that are so completely hidden from conventional monetary analysis. We have already noted that secondary production necessarily consumes large quantities of high-grade energy and biomass. Only a small portion of this is converted to human population growth and the accumulation of manufactured capital. The rest is dissipated widely as low grade waste (e.g. carbon dioxide in the present example). Modern interpretations of the second law of thermodynamics suggest that such behavior is an invariable characteristic of all highly-ordered complex systems. The economy can grow and maintain its internal structure in a dynamic nonequilibrium state only through the continuous dissipation of available energy/matter ‘imported’ from its host environment. Thus, the material demands of our growth-addicted urban-industrial economies must inevitably exceed the fixed productive and waste assimilation capacities of regional ecosystems. The observed ‘dissipation’ of carbon from the ecosystems of the Lower Fraser Basin symbolizes both the depletion of the region’s natural capital stocks and the overtaxing of local waste sinks (at the expense of rising global CO2 levels and potential climate change).
5. The invisible foot of the economy But the story does not end there. While Boyle and Lavkulich (1997) show that the rate of carbon
W.E. Rees / Ecological Economics 29 (1999) 23–27
26
Table 2 Ecological footprints of Vancouver and the Lower Fraser Basin Geographic unit
Population
Land area (ha)
Ecological footprint (ha)
Overshoot factor
Vancouver City L. Fraser Basin
472 000 2 000 000
11 400 830 000
2 360 600 10 000 000
207 12
release from the Lower Fraser has been declining since the 1930s, the regional population and economy are among the most rapidly growing in North America. These opposing trends are possible only because consumption in the region is now sustained largely by production outside its boundaries. A new tool I have developed with my students, ecological footprint analysis, shows that the average resident of the Lower Fraser currently appropriates through trade and natural flows the biophysical goods and services of about 5 ha of productive land and water (adjusted from Rees and Wackernagel, 1994, Rees, 1996, Wackernagel and Rees, 1996). Thus, the 472000 residents of the city of Vancouver, sitting on just 11400 ha, actually use the ecological output of 2.3 million ha. Similarly, the 2 million inhabitants (1997) of the basin as a whole require about 10 million ha of productive ecosystem to support their consumer lifestyles. In short, the true but invisible ‘ecological footprint’ of the region’s population is 12 times the geographic area of its home territory (Table 2). Having overused or drawn down their own natural capital stocks, the people of the Lower Fraser are now nearly wholly dependent on temporary surpluses elsewhere.
6. Conclusions: the economy as parasite Our technological prowess feeds the common belief that humans and their economies have become increasingly independent of nature. Indeed, the dollar contribution of the primary sector to many service- and knowledge-based economies is so small that economists now routinely omit land/ resources from their production functions in accounting for economic growth. This presumed economy – environment decoupling is pure illusion sustained by abstract mone-
tary models of economic process. The foregoing biophysical analysis shows that modern high income economies are actually becoming increasingly indebted to nature. Most are running massive unsustainable ecological deficits with the rest of the world (Rees, 1996; Wackernagel and Rees, 1996; Wackernagel et al., 1997). Technology and trade have merely obscured this relational truth by displacing the negative consequences of growth to distant ecosystems and the future. Rising productivity and incomes produce overweening confidence in human ingenuity while imposing ever-larger ecological footprints on the earth. The latter goes unnoticed by policy advisors and ordinary citizens alike. Meanwhile, the trophic relationship of industrial economies to their host environments remains that of parasite to host—the former gains vitality at the expense of the vitality and regenerative capacity of the latter (see Peacock, 1995; Rees, 1998). Temporarily shielded from the negative consequences of material profligacy, high-income countries continue blindly to consume the earth while actively encouraging the developing world to adopt the same driving values and lifestyles. This virtually guarantees that the entropic degradation of the environment will be raised to the level of the ecosphere, eventually affecting everyone more or less simultaneously (as is happening with ozone depletion). From this perspective, the prognosis for sustainability may seem bleak indeed. While the mainstream approach to sustainability requires a 5- to10-fold expansion of world industrial activity, the present analysis shows that throughput growth on anything like this scale is physically impossible. One can hardly imagine more diametrically opposed positions on humanity’s and the ecosphere’s future prospects. However, a clear understanding of the sustainability problem is a
W.E. Rees / Ecological Economics 29 (1999) 23–27
prerequisite for any effective solution. To the extent that the biophysically-based ecological economics advocated here approximates reality better than prevailing expansionist models, it may help to break the stalemate. In any event, it shows that economists can never become true human ecologists until they embrace the need for integrated biophysical analyses.
References Boyle, C., Lavkulich, L., 1997. Carbon pool dynamics in the Lower Fraser Basin from 1827 to 1990. Environ. Manag. 21, 443 – 445. Houghton, R.A., Skole, D.L., 1990. Carbon. In: Turner, L.B. II (Ed.), The Earth as Transformed by Human Action. Cambridge: Cambridge University Press, pp. 393–408.
.
27
Peacock, K.A., 1995. Sustainability as symbiosis. Alternatives 21 (4), 16 – 22. Rees, W.E., 1996. Revisiting carrying capacity: area-based indicators of sustainability. Popul. Environ. 17 (3), 195 – 215. Rees, W.E., 1998. How should a parasite value its host? Ecol. Econ. 25, 49 – 52. Rees, W.E., Wackernagel, M., 1994. Ecological footprints and appropriated carrying capacity: measuring the natural capital requirements of the human economy. In: Jansson, A.-M., Hammer, M., Folke, C., Costanza, R. (Eds.), Investing in Natural Capital: The Ecological Economics Approach to Sustainability. Island Press, Washington, DC., pp. 362 – 390. Sterrer, W., 1993. Human economics: a non-human perspective. Ecol. Econom. 7, 183 – 202. Wackernagel, M., Rees, W.E., 1996. Our Ecological Footprint: Reducing Human Impact on Earth. New Society Publishers, Gabriola Island, BC/Stony Creek, CT. Wackernagel, M. et al., 1997. Ecological Footprints of Nations. Report to the Earth Council, Costa Rica.