PALEOCLIMATE DATA CONSTRAINTS ON CLIMATE SENSITIVITY - THE PALEOCALIBRATION METHOD

The relationship between paleoclimates and the future climate, while n ot as simple as implied in the 'paleoanalog' studies of Budyko and oth ers, nevertheless provides sufficient constraints to broadly confirm t he climate sensitivity range of theoretical models and perhaps eventua lly narrow the model-derived uncertainties. We use a new technique cal led 'paleocalibration' to calculate the ratio of temperature response to forcing on a global mean scale for three key intervals of Earth his tory. By examining surface conditions reconstructed from geologic data for the Last Glacial Maximum, the middle Cretaceous and the early Eoc ene, we can estimate the equilibrium climate sensitivity to radiative forcing changes for different extreme climates. We find that the ratio s for these three periods, within error bounds, all lie in the range o btained from general circulation models: 2-5 K global warming for doub led atmospheric carbon dioxide. Paleocalibration thus provides a data- based confirmation of theoretically calculated climate sensitivity. Ho wever, when compared with paleodata on regional scales, the models sho w less agreeement with data. For example, our GCM simulation of the ea rly Eocene fails to obtain the temperature contrasts between the Equat or and the Poles (and between land and ocean areas) indicated by the d ata, even though it agrees with the temperature data in the global ave rage. Similar results have been reported by others for the Cretaceous and for the Last Glacial Maximum.