From Skeptical Science:
https://www.skepticalscience.com/print.php?r=77
Climate Myth: CO2 was higher in the past
"The killer proof that CO2 does not drive climate is to be found during the Ordovician- Silurian and the Jurassic-Cretaceous periods when CO2 levels were greater than 4000 ppmv (parts per million by volume) and about 2000 ppmv respectively. If the IPCC theory is correct there should have been runaway greenhouse induced global warming during these periods but instead there was glaciation."
(The Lavoisier Group)
http://www.lavoisier.com.au/articles/greenhouse-science/climate-change/mcclintock-proofnotco2-2009.pdf
Over the Earth's history, there are times where atmospheric CO2 is higher than current levels. Intriguingly, the planet experienced widespread regions of glaciation during some of those periods. Does this contradict the warming effect of CO2? No, for one simple reason. CO2 is not the only driver of climate. To understand past climate, we need to include other forcings that drive climate. To do this, one study pieced together 490 proxy records to reconstruct CO2 levels over the last 540 million years (Royer 2006). This period is known as the Phanerozoic eon.