@SteveRichter: Population is NOT growing unsustainably, anywhere in the world, actually. http://www.bbc.co.uk/news/magazine-24303537. In fact, current rates of reproduction worldwide are less than what's required to replace the current population. There are a few countries that are growing, but most are not. And currently, we're not even close to carrying capacity, and that's not even considering improvements in agriculture and all other technologies required to improve living conditions. Right now we produce, apparently, enough to feed 10 billion, and we have about 7 billion. We have 5 acres per person, and 1/2 acre of arable land per person, based on current technology. Not to mention, in another 100 years we should be mining asteroids for resources and have as efficient solar power as any plant has, and just doing that should provide enough resources for 100's of billions. If we actually figure out terraforming with nanotechnology, the solar system can sustain a crazy estimate of 10^6 quadrillion people, according to some estimates.