Even if the world population does stabilise soon and starts to glide downwards, that won't solve the world's environmental problems. The real issue is not overpopulation but overconsumption - mostly in rich countries that have long since given up adding substantial numbers to their population.
Every time those of us in the rich world talk about too many babies in Africa or India, we are denying our own culpability. It is the world's consumption patterns we need to fix, not its reproductive habits.