@davidho Eliezer Yudkowsky suggests AI might use climate change to kill us all: it needs more compute power to accomplish whatever its goals are, and that process inevitably creates waste heat which will eventually make the planet uninhabitable. (I think he wouldn't say this is the most likely thing that kills us first, though.)

Normally, climate change as we think of it wouldn't cause human extinction, of course. I mean, worst-case scenario is bad enough, just not that bad.