Quantifying atmospheric CO2 concentrations (CO2(atm)) during Earth's ancient greenhouse episodes is essential for accurately predicting the response of future climate to elevated CO2 levels. Empirical estimates of CO2(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 CO2(atm) have been significantly overestimated because previously assumed soil CO2 concentrations during carbonate formation are too high. More accurate CO2(atm), resulting from better constraints on soil CO2, indicate that large ( 1,000s of ppmV) fluctuations in CO2(atm) did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A. D. 2100.
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