[fallingfrog]

This is indeed worrisome, but the evidence I've heard suggests that it won't be as bad as +12C. Also the Earth was at +14C with respect to today at it's hottest point during the PETM, which was very bad for the earth, but life survived.

The earth will one day reach a true runaway greenhouse when the sun brightens by 10% in about 1.1 billion years. But according to J. Hansen in 2013 a complete greenhouse runaway can't happen unless the high temperatures are maintained for several million years, long enough for all the oceans to evaporate, and our own forcing will only(!) last a few tens of thousands. So my understanding is that we won't cause all life to go extinct, if that makes you feel any better.

[m_mueller]

Didn’t Hansen state that burning all coal and oil sand would do it? I‘m always wondering how much methane is assumed to be mixed in, as the amount of methane to be released is poorly understood as pointed out above.

[fallingfrog]

Sorry, I should clarify: Hansen's paper indicated that burning all fossil fuels would create a 16 degree C warming, which would definitely be the end of the line for us, for reasons of heatstroke if nothing else, but in the same paper he does say that the temperature can't go up to Venus levels. So nature would eventually recover.

[m_mueller]

Yes Venus isn‘t possible afaik, but I think around 125C is an equilibrium.