In my grad program, we've spent a lot of time talking about Paul Ehrlich and John Holdren's IPAT equation. It's pretty simple:
Impact = Population x Environment
English translation: A society's environmental impact is proportional to its population, its wealth, and its technological capacity to mitigate the impacts of its population and its wealth.
So how do you reduce impact? Well, it's too ethically and politically dicey to do a whole lot about population -- at least beyond educating women. Affluence? Let's put it this way: How would you like to be the one to tell El Salvador or Namibia to stay poor because the world has all the rich countries it can take?