Thursday, December 31, 2009

D.O. Breecker et al., PNAS 2009, Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100

Proceedings of the National Academy of Sciences, published online before print December 28, 2009; doi: 10.1073/pnas.0902323106

Atmospheric CO₂ concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100

D. O. Breecker**, Z. D. Sharp and L. D. McFadden (MSC03-2040, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, U.S.A.)

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.

**Correspondence e-mail: breecker@jsg.utexas.edu.

* , where R equals ¹³C/¹²C and the subscripts “sam” and “std” refer to the unknown sample and a standard (Pee Dee Belemnite, PDB), respectively.
† Soil temperature (needed to calculate ) is typically assumed; is either held constant (10) or is estimated from the δ¹³C value of contemporaneous marine carbonates (e.g., 12) or well-preserved organic material (e.g., 8); is either calculated from (e.g., 12) or taken to equal the δ¹³C value of well-preserved organic material (e.g., 8).