The world population is currently more than two and a half times what it was when my parents were born. It can’t keep on growing forever.
One point of view is that as the population continues to grow, technology will grow apace — so that, no matter how high the figures climb, human ingenuity will always find a way to make sure there is enough food, water, oxygen, and living space to go around.
Obviously that can’t be true. Maybe the earth can be made to support a very large number of people — trillions, say — but it can’t support an arbitrarily large number. There must be a limit somewhere. Sooner or later, if the population continues to grow, we will reach that limit.
And long before we reach the point where it is physically impossible for the earth to support us, we will reach the point — many countries have reached it already — where population density begins to have a negative effect on the quality of life. No one wants to live on a planet — or in a country — which is filled to capacity, where there is no extra space, no countryside, no wilderness, not even any backyards or roomy living rooms. Living in Taiwan (with 19 times the population density of the U.S.), I’ve had a little foretaste of that, and it isn’t very nice.
Nor is space colonization a viable solution. Even assuming it were technically feasible, uprooting a few billion people every generation or two and shipping them off to other planets isn’t going to do much for our quality of life either.
Another “optimistic” point of view is that the problem will solve — is solving — itself, as more and more peoples voluntarily choose sub-replacement fertility. In most of the developed world, population growth rates are low or even negative, and what growth there is comes more from immigration than from the reproduction of the indigenous population. And the developing world is, as the euphemism implies, assumed to be developing. That is, given time, it will become “developed” and stop reproducing.
But such a situation is evolutionarily unstable and cannot possibly be permanent. So long as there is variation in fertility rates, and so long as that variation is correlated with heritable (genetic) or quasi-heritable (religious) features, those who reproduce at above-replacement levels will multiply at the expense of those who do not, and overall rates of population growth will go back up.
Since voluntary population control cannot by its nature be a long-term solution, that leaves Chinese-style coercive population control — which is obviously morally unacceptable. To be effective, it would have to be applied to the entire world, and it would have to be strictly enforced — meaning, in practice, forced abortion or infanticide. Without strict, universal enforcement, it would be no more stable than voluntary control. Some people would inevitably go on reproducing, regardless of social pressure or sanctions, and whatever it was that made them do so, they would be likely to pass it on to the next generation — leading in the long term to rising fertility.
So there’s no solution at all.