CLIMATE MODEL SENSITIVITY, PALEOCLlM.l~TE J~ND FUTURE CLIMATE CHANGE

Radiatively active trace gases such as CO2, methane, nitrous oxide and chlorofluorocarbons warm the surface-troposphere system by iru~reasing the infrared opacity of the atmosphere. Climate models have been used to estiimate the warming associated with future increases in these gases. An important issue is the sensitivity, or the amount of climate response associated with a given amount of radiative forcing. Models currently used for climate change studies differ substantially iin their sensitivity, and these differences contribute prominently to the uncertainty in estimating the magnitude and rate of future climate change. The observed climate record Ci3.n be used to calibrate the sensitivity of climate models. Calibration based on the irlstrumental record (the last -100 years) has the advantage of using direct observation~; of climatic parameters with relatively dense spatial coverage, but suffers from uncertaintie,s about sources of climate forcing other than greenhouse gases and the possibilit)' that natural climate variability may be comparable in magnitude to the greenhouse gas..induced change. In contrast, calibration using paleoclimate data from the late Quaternary involves larger changes in forcing, most of which are relatively well-documented, tlUt is s;ubject to the inherent uncertainties of reconstructing past climate from a variety o.r sometimes sparsely-distributed proxy data. While better monitoring of ongoing changE~s in climate forcing and response may be the best way to calibrate climate model sensitivity, paleoclimate information can provide much-needed insights into climate model performance, particularly as improvements occur in the quantity and quality of paleoclimate data.

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