I recently sat next to someone on an airplane who was pretty knowledgeable on the subject of global warming (due to her work and educational background) and she had two interesting points.

1) The oceans act as a huge buffer for CO2 and they have been getting close to their limit on how much they can absorb after the past several decades. Another bad byproduct of this is the acidification of the sea which destroys coral and is happening much faster than wildlife can adapt.

2) There are so many variables... and the climate data available for scientists obviously represents a tiny fraction in time compared to geological timescales that in is very difficult to accurately predict what is going to happen.

I'm not particularly any sort of environmentalist but I think Carl Sagan's "Pale Blue Dot" quote is relevant on the subject. We can't afford to be wrong on this one as we've got no where else to go.