Falling birth rates in some developed and developing countries (a significant portion of which are due to government-imposed limits on the number of children a couple can have) have begun to reduce or reverse the population explosion.
It remains unclear how many people the planet can comfortably sustain, but it is clear that per capita energy consumption must go down if climate change is to be controlled. Ultimately, a one child per couple rule is not sustainable either and there is no perfect number for human population. But it is clear that more humans means more greenhouse gas emissions.
Scientific American, 26 Nov 2011