So here's a scenario.
CO2 emissions cause global warming. Global warming causes antarctic glaciers to melt. As the glaciers melt, they splinter, dropping icebergs into the ocean. The icebergs are rich in minerals and prove to be floating islands of life -- wherever they go, life follows and grows. The life that grows is plankton, which is highly effective at doing what?
Removing CO2 from the atmosphere.
There's some indication that this is what's happening. It's like a natural version of an idea Stephen wrote about a while back. Will iceberg-induced plankton CO2 sequestration be enough to offset the massive human CO2 emissions? Probably not. But whatever it's doing, it needs to be factored in.
UPDATE FROM STEPHEN:
There is a counter-tendency that some climatologists bring up - [drawing in a deep breath] that if the world heats up the polar ice caps melt which means that they are reflecting less solar energy away which means that that Earth absorbs more heat and the process accelerates.
Some of the same scientists that talk about that process will also admit that they don't know how the Earth comes back from an ice age. Huge Ice Age ice caps should reflect the heat away trapping the Earth in a permanent deep freeze.
My guess is that the Earth thaws from an ice age because of the flip-side of Phil's point - there is less life to sequester carbon which means more carbon in the atmosphere. The resulting green house effect warms the planet.
There's also cloud cover to consider. A cold Earth evaporates little water for clouds. Fewer clouds means less solar is reflected and the Earth warms. Fewer clouds also mean a drier planet – the flip-side of Phil's point again: a dry planet will have less life to sequester carbon, which puts more carbon in the atmosphere for the green house effect.
The opposite is true. A hot Earth evaporates more water for clouds. Clouds reflect heat away cooling the planet. They also bring the rain that sustains more life to sequester carbon.
An anthropic proof: this self-correcting system is absolutely necessary for us to be here at all. The planet had to be self-correcting to survive the punishment it has taken since life arose 5 billion years ago - huge asteroid impacts and super-volcanoes would have destroyed any fragile system long ago.
The thing that always stays in the back of my mind in this debate is the fact that this challenge to the Earth - human civilization - is different from any challenge the Earth has dealt with in the past. We need to be careful that we don't overtax a struggling Earth.
The very fact that we can think to be careful is another point – human civilization can also be self-correcting.