[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.

[anthonyb]

Water vapour has a very short life cycle in the atmosphere, so while it's theoretically the worst, it's not actually particularly bad in the long term, and it's not likely to change by much. There's also some question as to the cooling effect of clouds.

CO2 on the other hand, stays in the atmosphere for quite a few years (30-95 according to wikipedia), so it's far worse in the long term.