A 3.5 degrees C increase is considered to be the extinction point, because in such a world the food chain collapses, oceanic plankton dies off and these temperatures severely limit terrestrial vegetation... Temperatures higher than the extinction point are being predicted, not by crackpots, ideologues of sci-fi writers, but by serious scientists. An increasing number of climate change scientists now fear that our situation is already so serious, and so many self-reinforcing feedback loops are already in play, that we are in the process of causing our own extinction.

http://www.flassbeck-economics.com/how-climate-change-is-rapidly-taking-the-planet-apart/