Anthropogenic CO2.
R.L., do you still have that link, Scott has forgotten again....
Mankind did not create the greenhouse gasses that are warming out planet.
Forest fires, Volcanism, and other natural sources release thousands of times more CO2 than mankind.
Well, there's this.
From: http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/icecore.html
And that only goes back a thousand years....