Lifespan solves for this. If people knew they would be living long enough to experience what future generations would have to experience in terms of environmental impact, many would take on a different viewpoint toward climate issues as opposed to the "let them deal with it, I ain't gonna be alive anyway" approach.

In terms of population control, it's all about lifespan combined with more efficient discovery as fewer ideas would die with those moving them forward. Space travel will solve for moving the human race to further inhabitable planets, but not without a solution for the more important lifespan issue which is a requirement for extended space travel. We've got 4 billion years before our sun runs out and before the Andromeda galaxy will cause us a bit of trouble.

4 billion years from now, lets at least be known as the generation that began to make an impact in this area.