Solve global warming, overfishing, soil destruction, pollution, and a great deal of poverty in one fell swoop over 30 years:
Cost: $2T. Pay 1 billion people $1,000 to get sterilized, and use the other half to pay for the operations.
Financed over 30 years at 5%, that’s about $60B per year, or 0.6% of GDP (and less than the Iraq war by far). Almost no one in the developed world would take the money, because it’s nothing to them. However, that would feed people in the developing world for about 3 years in some places, and those are the exact areas that are suffering the worst from overpopulation.
But since you’re not forcing it on anyone–they get to choose–it’s not a scary totalitarian thing.
The total world fertility rate is about 2.6. The total world replacement rate is about 2.3. At present that means a growth rate of about 13%. Reducing the fertile population by 1B would increase the replacement rate, by my amateurish calculations, to about 2.7, which would mean, on average, a population decrease of about 600m per generation. I suppose that’s about a billion in the 30 year period, assuming no countervailing increase in fertility elsewhere.
That calculation could be even greater if not spread out over the whole world. Inasmuch as the appeal of $1,000 would correlate strongly with places with higher birthrates, it might skew that number even higher, so the 1B reduction over 30 years is conservative.
Of course, a simpler and even less scary way of doing this would be to provide free of charge birth control of all kinds to everyone in the world, everywhere. It would cost much, much, much less than $1,000 per person, could arguably reach everyone in the world instead of 15%, and could work an even more drastic reduction.
All of this involves no killing.
Of course, the alternative is to wait for the earth to just choke and kill us through famine and disease.