I have never found a useful overview of how paleo people look at CO2 and not for lack of trying.
So I'm happy to see that Richard Alley is giving a plenary talk on this very subject at AGU Tuesday afternoon 1:40 PM Pacific time..
Title is "The Biggest Control Knob: Carbon Dioxide in Earth's Climate History".
It will be webcast. Google AGU webcast and you should find it.