Human population is estimated to increase from 6 to 9 billion by 2050, but humans already use over half of accessible runoff and about 40% of plant growth for the energy stored in plants by photosynthesis . We have created large holes in the ozone layer and increased the concentration of CO2 in the atmosphere by more than a third. The rate of species extinctions is growing exponentially (we are actually in the midst of earth's 5th mass extinction event, primarily caused by humans), and the percentage of fisheries fully exploited is nearly 80%. How many humans can Earth support? In this chapter, we look at sustainability as a balance between ecological resource supply and demand.
The ecological footprint is a measure of your resource demand. It is an attempt to estimate how much of earth’s resources you consume, and how much of an impact you have on the environment. There are many ways of calculating the size of your footprint, as you will find if you search the Internet for ecological footprint calculators. One way to express the size of a footprint is as the number of Earths that would be required to support the world’s population if all humans consumed resources at the same rate, i.e., the global ecological footprint. One problem with this approach is that the number of Earths then depends on the global population, which is exponentially increasing. It is thus a moving target. In spite of this problem, it is still a useful way to compare qualitatively the environmental impact of different lifestyles. Another more accurate method is to estimate the area of earth and sea that are required to support an individual’s lifestyle by regenerating renewable resources and absorbing wastes. If measured in productive land area, the global ecological footprint in acres per capita is 6.8, while citizens of the U.S. require 23.5 . I like to think that I could sustain my family of four on our relatively large suburban yard of 1 acre, but to live like average U.S. citizens we would need 4*23.5 = 94 acres!
The concept of the ecological footprint is clearly Malthusian. It assumes that there is a fixed amount of resources available. It raises an important question: What is the carrying capacity of the Earth, the number of humans that Earth can support sustainably? Estimates range between 4 and 10 billion, depending on the average environmental impact of humans . Remember that I = P*C*T. Assuming T is equal to one (*elsewhere we will debate whether T is smaller or greater than one, i.e., whether technology increases or decreases our environmental impact), then one Earth can sustainably withstand a maximum level of human impact Imax = P*C. If we reduce consumption C, we can increase population P and still maintain the equality. If all humans minimized C by becoming vegetarians and we farmed all arable land, then the maximum population that Earth could support Pmax = Imax/Cmin = 10 billion people. However, currently our consumption rate is closer to Cmax, and in this case Pmin = Imax/Cmax is about 4 billion. The current global population is 6 billion. According to the Living Planet Report 2008 , the current global ecological footprint of that 6 billion people is 1.3 planet Earths (See Fig. Global ecological footprint from ). This means that humanity uses ecological services 1.3 times faster than Earth can renew them. We are in “ecological overshoot”, i.e., our population and impact have grown so much that the earth can no longer support us sustainably. In order to live sustainably, humanity must reduce its total ecological footprint to one earth, either by reducing consumption or population. We can choose now to reduce consumption, but if we don’t act then eventually nature will make the choice for us and without mercy, and global population will decrease until it reaches a sustainable level.
The maximum amount of ecological services and resources that Earth can provide is termed the biocapacity. It is a measure of supply, expressed as the amount of land available for production per capita. The global biocapacity is -0.6 hectares per capita, again indicating that we have a global ecological overshoot (see Fig. Footprint and Biocapacity factors that influence overshoot from ). For the U.S. it is -4.4 hectares per capita , which explains why the U.S. has to import so many goods. Our use of ecosystem services and resources is sustainable when we demand less than the Earth can supply, i.e., when the global ecological footprint equals or is less than the global biocapacity. Unfortunately, the footprint now exceeds the biocapacity, and the gap between the two is increasing. The Figure Ecological footprint Biocapacity Overshoot  illustrates an optimistic scenario in which we act quickly to close the gap between supply and demand. In the unsustainable situation when demand is greater than supply, as it is now, we build up an ecological debt. If we decrease the demand until it is less than the supply, then we can build up a reserve.
Cornucopianists argue that advances in technology could make the term T in I = P*C*T vanishingly small, so that both population and consumption are relatively unconstrained. To understand their reasoning I give the following quote from Edward O. Wilson’s Consilience, in which Wilson uses the term “Exemptionalist” synonymously with Cornucopianist: “Of course the exemptionalists will say that new technology and the rising tide of the free-market economy can solve the problem. The solution, they explain, is straightforward: Just use more land, fertilizer, and high-yield crops, and work harder to improve distribution. And, of course, encourage more education, technology transfer, and free trade. Oh, and discourage ethnic strife and political corruption. All that will certainly help, and should have high priority, but it cannot solve the main problem, which is the finite resources of planet Earth. It is true that only 11 percent of the world’s land surface is under cultivation. But that already includes the most arable part. The bulk of the remaining 89 percent has limited use, or none at all.” 
1. Speth, J.G., The Bridge at the Edge of the World: Capitalism, the Environment, and Crossing from Crisis to Sustainability. 2008, New Haven, CT: Yale University Press. 295.
2. Hails, C., ed. Living Planet Report. 2008, WWF, ZSL, and the Global Footprint Network. 48. http://www.footprintnetwork.org/download.php?id=505
3. Wilson, E.O., Consilience: The Unity of Knowledge. 1998, New York, NY: Vintage Books. 367.