More specifically, I am proposing a limit to the impact of human populations on the rest of Earth's environment -- in practice this is a limit on human populations too, though it could still go far.<p>I don't have any answers I really believe in as yet. One incomplete idea I have been toying with is that much of human population growth is driven by the 'biological clock,' one's reproductive age ends relatively soon, and so if (a woman) wants to have children she needs to do so relatively early in life. Were lives longer and were the reproductive years extended, perhaps there would not be such a drive to reproduce so quickly; in a similar if distinct way to how reducing infant mortality reduces birthrates.<p>Reducing the age of reproduction is nearly as important as reducing the number of children, in terms of limiting the rate of exponential expansion. I don't know any collection of strategies that seem complete, however.