The problem is that would take a drastic reduction of human population. By one estimation earth cannot sustain a human population of more than 200,000,000 using organic farming techniques, not to mention returning to hunter gatherer life. Who knows what the real number is? But it seems obvious that it's much, much less than the present population.
So, the problem is how to effect such a reduction ourselves when the whole question seems to be taboo to most people. Nature may do it for us, or we may do it to ourselves in some unthinkable way, but neither of those are alternatives that many people would wish for. — Janus
I once did a back-of-the-napkin analysis and came up with 250-500k. I figured that at our current rate of consumption, as an apex predator, and based upon the richest, most consumptive person on the planet (I don't know who that is) we would need what we had as hunter-gatherers. I figure 10k square miles of temperate zone land per 35 people would be about right. (Fewer as we go closer to the poles and the equator.) That's a block 100 X 100 miles. The center of that is about 50 miles to any border of the next group's area. That is a reasonable distance to travel once a year to party, diversify the gene pool, trade lithic knowledge, etc. and yet still be able to slaughter mammoths, bison, fish, etc. without really harming biodiversity. Of course they could go different directions, seasonally, to link up with other groups, maybe four times per year.
As modern homo sapiens, we would not want all 500k living in and around the same city because that would present a danger from a localized catastrophe, not to mention disease. So we'd spread out but maintain travel and communications. Ideally those 500k would be made up of as much genetic diversity as we could cram into 500k. In this way, we could maintain the current highest standard of living of the richest, most consumptive person on Earth, without jeopardizing maximum complexity and biodiversity of the planet.
Not that anyone would want more people, but if we decided that we wanted more people, we could easily do that with lowered consumption. Anyway, if human beings are as smart as we think we are, we should be able to automate extraction of resources and the production of all the food and luxuries we want, speed around and off the planet, reserve all knowledge gained thus far, continue to gain it, and generally leave the planet whole.
How do we get there? Well, if we're so fucking smart, then it shouldn't be a problem. And we should be able to do it without killing billions of individuals before their normal time would be up. But we are not as smart as we think we are, and we are no different than any other species on this planet. We certainly are not humble. Rather, we are desperate, insecure, over-compensating, fearful little creatures scurrying around the pizza, grabbing slices because we don't think there will be enough. And we are right, because we are scurrying around the pizza grabbing slices because we don't think there will be enough. We are who we are, and no better. And we could never agree on who should stop the grabbing. So we'll just let the pizza run out. Earth can only make so much pizza.