Human population size is one of the most problematic environmental issues. Population size directly affects other environmental issues, like pollution and resource depletion. Because human interactions with the environment are mediated through society, human environmental impact is a social question. The social dimension becomes especially clear when population issues are approached using terms from ecology. This paper will examine ecological terms like "carrying capacity", "environmental impact" and "sustainability", and survey the problems of applying these concepts to human population. By identifying the variables in the human "carrying capacity" equation, the political character of the population issue becomes clear.
The fundamental terms of the debate over the human population size have not changed substantially in the past 200 years since the Reverend Thomas Malthus wrote his 1798 "Essay on the Principle of Population". In his essay, Malthus asserted that population would grow exponentially, while the capacity to feed people would grow only arithmetically. Population would overshoot resources, resulting in famine and death. Marx (1967), among others, argued that "surplus population" was not an absolute term, but rather was relative to the economy's ability to incorporate people into production; the "industrial reserve army" was a direct result of capitalism. The human population at the time Malthus wrote was about one billion people; today we have passed the six billion mark. There were two fundamental flaws in Malthus' argument. First, technological advances increased food production at a much faster rate than Malthus said; and second, an expanding world economy was able to absorb and support a larger population (Foster, 2002). For the most part, Marx was right.
One hundred and fifty years of industrialization later, though, Malthus's arguments have taken on new life as the environmental crisis has revived the idea of absolute limits on human population. Malthus raised the question of what ecologists today would call "carrying capacity." "Carrying capacity" refers to the maximum size of the population of a given species that can be supported in a given environment on a long-term basis (Cunningham, Cunningham and Saigo, 2005). Since the environment can change over time, the carrying capacity can change as well. The concept of the planet's carrying capacity of humans is a much more complicated matter, since several additional variables must be considered. This led demographer Joel Cohen to assert "on examination, none of the existing concepts of carrying capacity in basic or applied ecology turns out to be adequate for the human population." (Cohen, 1996, p. 237, cited in Carnell, 2000) The problem, as Cohen describes elsewhere, is that
It is a convenient but potentially dangerous fiction to treat population projections as exogenous inputs to economic, environmental, cultural, and political scenarios, as if population processes were autonomous. Belief in this fiction is encouraged by conventional population projections, which ignore food, water, housing, education, health, physical infrastructure, religion, values, institutions, laws, family structure, domestic and international order, and the physical and biological environment. (Cohen, 2003)
That is, population is tightly interconnected with both natural and social processes, and so human carrying capacity varies according to culture and economic development. Daily and Ehrlich (1991) distinguish between two different carrying capacities: biophysical capacity ("the maximum population size that could be sustained biophysically under given technological capabilities") and social capacity ("the maxima that could be sustained under various social systems and, especially, the associated patterns of resource consumption").
Technology, for example, extends the number of people that can be biophysically carried. Even today, food production is increasing faster than population. Some estimates suggest that advances in agriculture, waste management and other technologies allow the planet to support 1,000 times as many people per unit of area than 10,000 years ago (Cunningham et al.). Pessimists like Ehrlich question whether such production is sustainable over the long term using current agricultural methods. That 850 million people today suffer from malnutrition, including one in three preschool children (UNDP, 2005), suggests that we have at least exceeded the social carrying capacity of the current system.
Carrying capacity is closely linked to the concept of "environmental impact." The organization Redefining Progess (cited in Cunningham et al.) uses the concept of "ecological footprint", the amount of "biologically productive area" required to support an individual, to measure impact. Daily and Ehrlich consider impact as the energy "consumed, co-opted or eliminated." Ehrlich and Holdren (1971) developed the formula Impact = Population X Affluence X Technology (I=PAT) to measure impact. "Affluence" is a measure of "material throughput" or per capita consumption. "Technology" is the environmental impact per unit of energy used to produce material throughput (Foster, 1999). That is, the human impact on the environment varies with changes in population, affluence and technology. The biophysical carrying capacity can be understood as the upper limit of environmental impact. In the I=PAT formula, the affluence and technology values vary and are interdependent with population (Daily and Ehrlich, 1991).
Technology changes. James Malin (1948) argued that the concept of "resource exhaustion" was effectively meaningless, because as yet undiscovered technology can bring new resources into the "flow of utilization." New technologies can push biophysical limits upwards. Besides raising the biophysical capacity, new technologies can also shrink human environmental impact. Breakthroughs in electronics, computing, materials and biology have upset previous assumptions about technology and resource consumption. In particular, "knowledge-intensive" technology is potentially cheap, resource conservative and energy efficient (Davis and Stack, 1993, 1996). The technology input has the potential of falling, while producing the same or more material throughput. For example, energy consumption per dollar of GDP has dropped from 19.57 in 1949 to 9.20 in 2003 (U.S. Department of Energy, 2005).
"Affluence" is also a variable term, bound up with concepts like "quality of life." When affluence is measured in material goods consumed, material throughput will increase as affluence increases. If affluence is measured by non-consumable factors -- e.g. health, literacy, physical and emotional security -- material throughput can decrease, or at least increase at a slower rate, as quality of life improves. Conservation and recycling reduce the affluence factor as well.
Environmental impact, as adjusted through technology and affluence, affects "sustainability." "Sustainability" describes a process that can be maintained "without interruption, weakening, or loss of valued qualities" (Daily and Ehrlich), and so is a requirement if a population is to remain below the carrying capacity limit. To put the terms together, a population is sustainable if its environmental impact keeps the biophysical and the social carrying capacity above the population size.
For both biophysical and social limits, technology and affluence lead back to social organization. Social priority questions like research funding, tax policy and investment affect what technology is developed and deployed. As such, the promise of new technologies may fail to be recognized, or even add to environmental impact (e.g., "e-waste"). While one may be optimistic about the potential of new technologies to reduce environmental impact, current economic structures will block that potential. Affluence is also socially determined, to the extent that ideology, including values and priorities, is a product of social and economic relations (Marx, 1859). Even human reproduction decisions are socially determined, as factors like education for girls, gender equality, access to birth control and access to health care strongly correlate to declining fertility rates (Population Reference Bureau, 2006; Cunningham et al.).
Applying the scientific concepts of carrying capacity, environmental impact and sustainability to human-centric environmental issues is an example of using ecology terms to understand the human-environment relationship. Although ecology can help us to analyze issues like population size, the human-environment relationship takes place through historically shaped social factors. Since these factors involve who makes decisions and enforces them, they are political questions. The real population problem then is rooted in the political realm. The solution will be found there as well.
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