I think we are probably hitting a maximum level (at least in terms of still being able to maintain certain standards we've grown accustomed to).
The problem is a mixture of technology and population. Is it possible for 10,000 people to live in the rain forest of Africa or the extreme deserts of Arizona at the hunter/gatherer level indefinitely? Sure! No problem!
Can we increase that number to, oh, 35 million and increase the water/resource demands exponentially? No.
I may be wrong, but it just doesn't seem like the rain forest or the deserts cannot accommodate MILLIONS of people all driving cars, consuming billions of gallons of fresh water and, of course, supporting a vast network of McDonald's and KFC's.
At some point we're going to hit the physical limits of those particular geographical areas ability to sustain millions of people trying to live a typical American lifestyle which is what everyone on earth is striving for.
You simply can't recreate "America" in the middle of the freaking rain forest or the middle of a desert! Well, you could -- sort of -- but only if you bulldoze the entire rain forest down or reroute the entire Colorado river or engage in other extreme (and unsustainable) engineering.