I think the world population growth will start to slow a bit over the next few decades. Birth rates i the west are around or below replacement, and in the developing world, as people move out of poverty, infant mortality lowers, contraception becomes widely available and accepted, and people simply start seeing having multiple kids as a cost rather than an asset, their rate will slow too.
Or maybe i'm kidding myself. Even if population rates do slow, the burgeoning middle class will want stuff, they will want cars for the first time, they will want to travel, and this will have a negative impact on the environment, at least for a while.