Ken and I watched an interesting video on the great extinction event that brought the Permian (and the entire paleozoic, life's first run on Earth) to an end. Virtually everything on Earth died, and the next era that passed was so barren that the vicious mindless dinosaurs were able to take over.
But what was interesting was that they've worked out the two-phase cause of the whole thing. It started in Siberia, with a titanic basaltic eruption that lasted tens of thousands of years. This massive outpouring of heat and carbon dioxide gradually warmed the Earth by about five degrees. Species died out from climate change (and possibly alteration of the acidity of the planet, I'd imagine).
The second phase was the real killer: the five degree difference introduced in the first phase caused the sea-bed's methane deposits to thaw. This released vast amounts of methane into the atmosphere, and a greenhouse warming episode began—lasting a further span of tens of thousands of years. Before this was over, temperatures had risen a full ten degrees, and 95% of all species were dead.
Here's the video. It's a bit low-budget on the attempted graphics, but it's definitely worth a look. Not least because a) we're currently doing everything we can to raise the temperature of the planet and b) we're also looking at extracting those same methane deposits, which have during the past 250 million years built up again.