And if you look to Europe, N America, and other "developed" countries, you'll see that after industrialization occurs, population rates even out. I won't argue that there shouldn't be a limit to human population (within reason), but we aren't there yet, at least as far as this solar system goes. Terraforming becomes a distinct possibility, not to mention that Nickel and hydrogen are abundant in our own solar system (and many other solar systems I'm guessing). This is excluding the (very real to my mind) possibility of zero-point energy, ie bringing order from 'chaos' energy, of which I would consider cold fusion to be a derivative. Desalination is ezmode with a nonpolluting energy source, building atmospheres (and ecosystems), several -- to many for ecosystems -- steps removed but not impossible at all. Our atmosphere, and all the others in our solar system, didn't happen accidentally. Assuming a population of 10 billion humans, it is very sustainable in our solar system in the next thousand years would be my conservative guess. Studies on demographics have shown large losses in population gain beyond 'industrialization', the most notable being Europe where immigration is needed to keep the population from dropping more quickly. If you want to drag "population control" theories into it, I couldn't say that they weren't happening, but to me they don't seem necessary for the continuation of humanity in an expression similar to what we have now. That is assuming we switch to something higher energy density, cleaner and more sustainable than what we are currently using (and I would put the 'renewables' into that category for our current total population).