Climate and CO2 levels have always varied together. During ice ages CO2 levels were low, and during warm periods CO2 was higher. In the Eocene (56-34 million years ago) there were no polar ice caps, temperatures were about
10ºC hotter than the 20th Century, and CO2 was about
1,500ppm (Westerhold et al. 2020, Rae et al. 2021). During the last Ice Age, CO2 varied between about
180 and 300ppm as ice sheets waxed and waned with orbital wobbles (Rae et al. 2021). CO2 was also
about that level during the Paleozoic Ice Age, 340-290 million years ago (Foster et al. 2017).
Early attempts to estimate CO2 for that long ago in Earth’s past were broad-brush and very uncertain (eg Royer 2006), leading to the high CO2 estimates referred to in the myth. New data and refined techniques have since clarified the picture considerably. The 2006 estimates, for example, averaged data across 10-million-year timesteps, the 2017 data in the figure below used 0.5-million-year timesteps, and newer compilations don’t average across timesteps. At the same time, CO2 and temperature uncertainties have reduced considerably so that climates from the geological past are now
a useful reality check for climate models (Tierney et al. 2020, IPCC 2021, see the intermediate version for more detail).