title: Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100.
abstract: 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.
authors.affiliation: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
authors.forename: D O
authors.initials: DO
authors.lastname: Breecker
authors.forename: Z D
authors.initials: ZD
authors.lastname: Sharp
authors.forename: L D
authors.initials: LD
authors.lastname: McFadden
mesh_terms: D002119:Calcium Carbonate / Q000032:analysis; D002245:Carbon Dioxide / Q000032:analysis*; D003889:Desert Climate; D005544:Forecasting; D005580:Fossils; D017752:Greenhouse Effect; D009516:New Mexico; D010163:Paleontology; D006109:Poaceae; D012621:Seasons; D012987:Soil / Q000032:analysis; D013781:Texas
chemical_list: D012987:Soil; D002245:Carbon Dioxide; D002119:Calcium Carbonate