The Earth's Carrying Capacity

It turns out Warren Buffett is interested in the earth's capacity to sustain us. On page 642 of the new biography, The Snowball, he holds forth on the subject to author Alice Schroeder:

There is a carrying capacity to the earth. It's far, far, far, far greater than [Thomas] Malthus ever dreamed. On the other hand, there is some carrying capacity, and the thing about carrying capacity is that you want to err on the low side. If you were provisioning a huge rocket ship to the moon and had enough for two hundred people and didn't know how long the journey would take, you probably wouldn't put more than a hundred fifty people on the ship. And we have a spaceship of sorts, and we don't know how much the provisions are good for. It's very hard to argue that the earth would be better off in terms of average happiness or livelihood with twelve billion people instead of six.

Buffett is right that Malthus' original estimates of the maximum human population were way off. But that doesn't mean the limits are meaningless, and Buffett argues that we should plan for a margin of safety in calculating the effect of population growth, just as he does when making investments.
The earth's population is expected to reach 9 billion by the middle of this century, and gradually slow its growth. A country's population tends to grow more slowly and stabilize as education and living standards improve. Earth watchers like Lester Brown expect the earth's population to stabilize eventually, either through social restraints or, as he wrote in Beyond Malthus, "because nature ruthlessly imposes its own restraints."
The earth's capacity to sustain human life depends as much on technology as it does on sheer numbers. It is also worth pointing out that markets and populations start to behave erratically when their limits are reached, whether those limits are fuel, food, land or water.
The fabled Green Revolution greatly improved crop yields. Equally dramatic changes in energy technology will be needed to reduce carbon emissions and maintain our ability to feed our growing numbers. It's hard to cultivate land that's under water. And Brown points out that reliable water sources from mountain ranges like the Rockies or the Himalayas will be affected as snow accumulation is reduced by global warming.