The thing is that CO2 levels interact with a multitude of other factors, many of which are not understood and often not known for some time periods. For instance, we know that during the late Ordovician, CO2 levels exceeded 3000ppm but due to 4% less solar output, glaciation occurred at that level of CO2. There is no general theory of climate and many hypothesis being presented oversimplify. However, there is no doubt that CO2 is a greenhouse gas as is methane, water, etc. Currently our 400 ppm CO2 seems close to the lowest levels however, just 3 million years ago, the level was as low as 365 ppm but Artic temperatures were 16C higher and global temperatures were 4C higher than now.
A little more detailed graphic - note the Carboniferous with massive CO2 adsorption.
