Atmospheric CO 2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100

Quantifying atmospheric CO 2 concentrations ([CO 2 ] atm ) during Earth’s ancient greenhouse episodes is essential for accurately predicting the response of future climate to elevated CO 2 levels. Empirical estimates of [CO 2 ] atm during Paleozoic and Mesozoic greenhouse climates are based primarily on the carbon isotope composition of calcium carbonate in fossil soils. We report that greenhouse [CO 2 ] atm have been significantly overestimated because previously assumed soil CO 2 concentrations during carbonate formation are too high. More accurate [CO 2 ] atm , resulting from better constraints on soil CO 2 , indicate that large (1,000s of ppmV) fluctuations in [CO 2 ] atm did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A.D. 2100.