Regardless of our actions at this point, climate change will make this planet uninhabitable for civilization in a couple of thousand years. Climate scientests have predicted another ice age will eventually occur, which would effectively end any civilization that still existed at that time.

I can however think of another possibility for our "extinction" A lot of times extinction of a species occurs when it is displaced by a superior form. I think personally what will doom humanity is the inevitable merger of man and machine. I think that somewhere down the road our successors will be some form of nano-tech based human-machine hybrid being.

This type of being will displace us, in the same way our Cro-Magnon ancestors displaced the neandretal man. The world, will belong to the 'metal' so to speak.