"The fact is that the last time we had high levels of carbon dioxide in the atmosphere was 100 million years ago and the Sun was a little bit cooler at that time. Now if we push it up...this is not something that most climatologists will talk about but I think that there is a small chance, maybe a 1% chance, that if we really hit the planet too hard we may push it into a runaway system in which the temperature simply goes up and up until the oceans boil into the atmosphere, and that would extinguish all life on Earth."