Atmospheric CO2 Concentrations During Ancient Greenhouse Climates were Similar to those Predicted for A.D. 2100

That's the title of a paper published last year by Breecker et al. in the Proceedings of the National Academy of Sciences (Also see the commentary on the article). The key figure is this:The yellow band are previous estimates of atmospheric CO2 concentrations based on calcium carbonate in ancient soils. The revised estimates from this paper are marked in red and are more in agreement with