Malthus was wrong
When I was growing up, the problem at the heart of every environmental question was human population growth. If there aren't many people around, what they do matters little. If there are lots, even careful living is likely to have bad environmental consequences. At that time, the Earth's population was about three billion. It has now doubled to six. Not, on the face of things, great grounds for optimism.
The population curves in the newspapers and television programmes of my youth went relentlessly upwards. That was because they had only one, exponential, term. A real population curve, however, is logistic, not exponential. It does not rise indefinitely. Eventually, it reaches an inflection point and starts to level off. That is because a second term in the form of lack of space, lack of resources, disease or direct conflict between individuals stabilises it by matching the birth and death rates. And that was the fate the environmentalists of the 1970s predicted for humanity.
Such pessimism, however, failed to take account of the demographic shift that all populations (so far) have undergone as they have enriched themselves. For the negative exponent is starting to show up, and its cause is not lack of space or resources, nor yet is it conflict or disease (even AIDS, malaria and tuberculosis make only a small difference in global terms). Instead, it is the thing that the doomsters feared most after population growth — economic growth.
As a quondam evolutionary biologist, I find the demographic transition in response to higher living standards hard to explain. On the face of things, better conditions should lead to larger families, not smaller ones. However, it is impossible to argue with the facts, and the facts are that the rate of population increase is dropping, and that the drop is correlated with increases in personal economic well-being.
Perhaps the answer lies in the old idea of r and K selection. Indeed, the terms r and K come from variables in two-term logistic equation that describes real population dynamics. K-selected species, people may remember from their college ecology classes, have few offspring but nurture them lovingly. Those that are r-selected have lots, but display a Devil-take-the-hindmost attitude to their issue's survival. The crucial point is that K-selected species live in safe, predictable environments, while r-selected ones live in unsafe, unpredictable ones. If the individuals of a species were able to shift opportunistically between r and K strategies in response to shifts in the environment, then something like the demographic transition in response to wealth might be the result.
None of this means that the eventual human population of, say, ten billion will be easy for the planet to support. But such support will not be impossible, particularly as it is also the case that economic growth in rich countries is less demanding of natural resources for each additional unit of output than is the case for growth in poor countries.
Malthus was wrong to observe that population increases geometrically while the resources available to support it increase arithmetically. It was an understandable mistake. It flies in the face of common sense that population growth will actually slow down in the face of better resources. But that is what happens, and it might yet save humanity from the fate predicted for it by the Club of Rome.