In 1986, Stanford biologist Peter Vitousek calculated how much of the earth's primary productivity went to support human beings. He added together the plants eaten as food, the plants eaten by the animals then eaten as food, and the forests cut for timber and paper; he added the losses in food due to overgrazing grassland and other types of land degradation. The total was 38.8 percent. In other words, directly or indirectly, humans consume 38.8 percent of what it is possible to eat.
The demographer Joel Cohen collected and examined every estimate of carrying capacity made in recent decades. Despite their differences in assumptions and conclusions, he uncovered that the median low value for the planet's carrying capacity was 7.7 billion people, and the median high value was 12 billion. That is just the range that the UN predicts we will inhabit by the middle of the next century.
2. We don't need worst-case scenarios: best-case scenarios make the point. The population of the earth is going to nearly double one more time. We simply don't know what that means. The arguments put forth by cornucopians is that human intelligence will get us out of any scrape, that human beings are "the ultimate resource," that Malthusian models "simply do not comprehend key elements of people" -- all rest on the same premise: that human beings change the world mainly for the better. But that is simplistic and insupportable. We've already done deep and systemic damage. We won't be able to grow back the glaciers again in the near future; the oceans won't suddenly drop. To use a human analogy, we've already said the angry and unforgivable words that will haunt our marriage till its end. Nature may still meet us halfway, but halfway is a long way from where we are now, when the The UN's Intergovernmental Panel on Climate Change projects that an immediate 60 percent reduction in fossil-fuel use is necessary just to stabilize climate at the current level of disruption.
3. Human beings, unlike deer, can eat almost anything and live at almost any level they choose. Hunter-gatherers used 2,500 calories of energy a day, whereas modern Americans use seventy-five times that. Human beings can import what they need from thousands of miles away. And human beings, unlike deer, can figure out new ways to do old things. If, like deer, we needed to browse on conifers to survive, we could crossbreed lush new strains, chop down competing trees, irrigate forests, spray a thousand chemicals, freeze or dry the tender buds at the peak of harvest, genetically engineer new strains -- and advertise the merits of maple buds until everyone was ready to switch. The variables are so great that professional demographers rarely even bother trying to figure out carrying capacity. The demographer Joel Cohen reports that at two recent meetings of the Population Association of America exactly none of the more than 200 symposia dealt with carrying capacity.
4. Many people who are sure that controlling population is the answer overseas are equally sure that the answer is different here. If those people are politicians and engineers, they're probably in favor of our living more efficiently -- of designing new cars that go much farther on a gallon of gas, or that don't use gas at all. If they're vegetarians, they probably support living more simply -- riding bikes or buses instead of driving cars. Both groups are utterly correct. Environmental damage can be expressed as the product of Population x Affluence x Technology. Surely the easiest solution would be to live more simply and more efficiently, and not worry too much about the number of people.