Heat Islands...
I think the biggest problems are (ranked in order):
1) Overpopulation
2) which leads to Over Development. New suburbs constantly being created.
3) which means more roads, concrete, etc, ie. heat islands, which exacerbate the amount of heat
4) and also means more fuel burned to get all these people to work, bring them food and keep their Walmarts full.
5) not to mention extracting the resources necessary to keep up with 8 fucking billion people
Unless we can find a way to live in more sustainable ways with less energy use or at least use more renewable energy, reduce the amount of pollution we create, not to mention our massive piles of garbage streams, humans are doomed.