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Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved October 22, 2009 (received for review March 5, 2009)
Abstract
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.
Footnotes
- 2To whom correspondence should be addressed. E-mail: breecker{at}jsg.utexas.edu.
Author contributions: D.O.B., Z.D.S., and L.D.M. designed research; D.O.B. performed research; D.O.B. and Z.D.S. analyzed data; and D.O.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
See Commentary on page 517.
Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100
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