Comment by hrvbr

Yes, the worst is mass extinction because the worst greenhouse-effect gas is water vapor. The risk with human-generated chemicals is that they bootstrap a transfer of ocean water into the atmosphere in a vicious circle: more heat => more evaporation => more heat. The Earth could become as hot as Venus. I don't think scientists have enough informations to tell if the Earth would have ways to stop this circle. Even if it did, the event would be bad for humans.