Global dimming and warming.

[downbeatplumb]

There's countless billions of tons of methane hydrate frozen under the oceans. If the planet warms up enough, it'll all be released and if this happens. It'll all be over. After all, all that methane would make all that CO2 look nothing more than a puff of smoke.

I think this is what happened in the Permian period.

http://en.wikipedia.org/wiki/Permian%E2%...tion_event