Saw a documentary a few weeks back where they were studying ice samples taken from the Antarctic, something like 3 km down.
Apparently by thawing the ice, and analising the gasses released, they can get an accurate picture of the makeup of the Earth's atmospheric gasses from thousands of years ago.
To cut a long story short, all the evidence seems to point to the fact that global warming is a naturally occuring phenomenom and that we are actually in a period of unusual stability.
It has happened before, many times, and a lot of those times were far more extreme than the current situation.
Yeah, I know, my spelling sux.