The easy answer is that no one knows how many people can live in a sustainable way on this amazing world. We can make some guesses about the number and we can review what the limiting factors will be. Let’s do that.
There a lot of factors that could pose an eventual limit on population.
Global food supply is one of the potential limiting factors. This one seems like it might come into play because so much of the planet’s food supply is now produced through petro-chemical industrial agriculture and this form of agriculture is doomed by peak oil. This industrial food supply is also subject to the issues that plague mono-crop agriculture.
This is an important problem and we should think hard about how we create a sustainable planet agriculture, but I don’t think this is likely to be the primary limiting factor on human population.
The atmosphere is primarily nitrogen with a healthy splash of oxygen and a sprinkle of carbon dioxide and then a whole bunch of trace gases. Lots of the green beings on the planet really dig the nitrogen, these green beings grow, other beings eat them. The green beings breathe in the nitrogen and carbon dioxide and breathe out oxygen. The beings that are grazing the green beings breathe in the oxygen and breathe out carbon dioxide. It’s a pretty good arrangement. As we foul the atmosphere we end up with all sorts of population constraints – like lung cancer, asthma, heart disease, etc. But I don’t think air supply is going to be the primary constraint on human population.
Here’s the one that I think is going to hit the hardest and soonest: water. Even with the bottled water industry working overtime it seems likely that a day is coming when water is going to be scarce.
The water factor is likely to be exacerbated by global warming. We can anticipate increased water shortages in areas that rely on glacial melt, like India and Pakistan, for a good portion of their fresh water. But the simple fact is that nothing we can do is going to change the fresh water supply on this small blue planet. The fresh water supply is the same now as it was 2000 years ago when the world population was 3% of what it is today.
Can we calculate how many people this small blue planet can support? I don’t think we can. I think we are probably already exploring and living in the realm of unsustainable human population at 6 billion plus. My guess is that the planet can sustain a population of 2 or 3 billion humans. It’s an educated guess, but still a guess. The troubling aspect of our situation is that no one is coming up with a good way to reduce the human population. Famine and conflict are ugly population controls. We can do better. I hope we do. And sooner rather than later.