I realize you were looking end-of-earth cataclysm, however I think we should consider pandemic as a possibility for the end of humankind. This could actually take place in three ways, I believe.
The obvious is a deadly disease with extraordinarily easy transmission.
Less obvious would be a disease or environmental factor that prevents procreation.
A third possibility is a disease that destroys major food sources such as wheat, corn or rice. If for some reason we were unable to produce ample grain we would experience worldwide famine and also experience consequences of famine, such as disease and war.
I would say that pandemic or famine should rank right up with global warming, and they could certainly tag team us.
Sunfyre, welcome to the Straight Dope Message Boards, we’re glad to have you with us. When you start a thread, it’s helpful to other readers to provide a link to the column in question. Yeah, it’s on the front page now, but in a few days/weeks it will disappear into the bowels of the Archives. So, the link saves search time and keeps us all (mostly) on the same page. No biggie, I’ve added the link at the bottom of your post, and you’ll know for next time. And, as I say, welcome!
All of this is possible, and even likely, but as it has not happened that scenario is fiction fantasy. More probable is we grind ourselves into the dust of entiquity by less dynamics.