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A Test of Climate Model C02 Sensitivity by Comparison to Past Warm Climates

A Test of Climate Model CO2 Sensitivity by Comparison to Past Warm Climates

Karen L. Bice
Associate Scientist
Department of Geology and Geophysics
Woods Hole Oceanographic Institution
phone: 508-289-3320
email: kbice@whoi.edu

David Pollard
EMS Earth and Environmental Systems Institute
Pennsylvania State University
email: pollard@essc.psu.edu

The IPCC 4th Assessment Report (AR4) will include the results of quadrupled CO2 scenarios from coupled atmosphere-ocean models. Whether the CO2 sensitivity of these models is correct is the subject of important debate. Does a realistic amount of warming occur in the models when CO2 is increased?

Paleoclimate model-data comparisons represent the best test of model CO2 sensitivity. In one approach, past surface temperatures and pCO2 are estimated from well-preserved geochemical proxies. The inferred CO2 concentration is specified to the model, along with other boundary conditions appropriate for the study interval, and the model is run to equilibrium. If the model CO2 sensitivity is adequate and all other important climate forcings have been properly incorporated in the model, then model-predicted temperatures should match the temperatures inferred from data to within the uncertainty in the paleotemperature proxy technique.

We have compared the AR4 models? predictions of warming to the warming (relative to the modern) inferred for geologic intervals when atmospheric CO2 concentrations are believed to have been about 4 times the modern concentration. Arctic Ocean, tropical north Atlantic and Southern Atlantic upper ocean temperature estimates are available from well-preserved foraminifera and organic carbon compounds preserved in early Eocene (~55 Ma) and late Cretaceous (~90 Ma) marine sediments. The geochemistry of organic carbon in sediments of these ages suggest CO2 concentrations of 1500-2500 ppm. The range of surface warming achieved with quadrupled CO2 in seven of the IPCC AR4 models is 7-18�C in the Arctic Ocean, 3-5�C in the tropics, and 2-8�C in the far South Atlantic. However, the past warm climate data suggest mean annual surface temperatures at least 28�C higher than modern in the Arctic, 6-12�C higher in the tropical Atlantic, and 14-24�C higher at 65�S latitude in the Atlantic. In order to adequately match the temperatures inferred from geochemical data for the Eocene and Cretaceous, the models require CO2 concentrations in excess of 4000 ppm and an imposed increase in poleward heat transport (to reduce the equator-pole temperature gradient). A match can also be achieved with the models if the atmospheric methane concentration is increased by at least an order of magnitude and CO2 concentrations of >2500 ppm are specified. If the data-based estimates of past temperatures and CO2 concentrations are correct, the models being used to predict future warming largely underestimate the temperatures that will result with a quadrupling of CO2, and the underestimate is greatest in high latitude regions.