Global warming/ice core data
Ice core data is one of the primary methods of reconstructing environmental conditions far enough back in the earth's history to see how various factors affect (and are affected by) the climate. The oldest data currently available, going back 740 kyr and covering 8 glacial cycles, comes from the EPICA [W] core.
The core data shows that CO2 levels seem to lag global temperature rises at the end of global glaciation. This might seem to indicate that CO2 rises are caused by, rather than being a cause of, global warming -- but additional data seems to indicate that the two actually work together, such that a small "push" towards warming ends up causing a much larger temperature rise over the long run.
What seems to be happening is that when the temperature goes up due to orbit-change, this causes CO2 outgassing from the ocean (because warmer water has less CO2-absorption capacity). This in turn causes a greenhouse effect [W] which amplifies the warming trend in a positive feedback loop.
This does not directly defend the claim that CO2 causes global warming; what it does is refute the claim that CO2 levels lagging global warming refutes the claim that CO2 causes global warming.
It also suggests that if the warming could be accounted for entirely by factors other than CO2 greenhouse warming, then CO2's role would be shown to be negligible.
- The Vostok core is often mentioned as a source of such data (going back 420 kyr, 4 glacial cycles), but it was far surpassed in 2004 by the EPICA [W] core, which provided nearly twice as much history (740kyr, 8 glacial cycles).