A COMPARISON OF THE CCM1-SIMULATED CLIMATES FOR PREINDUSTRIAL AND PRESENT-DAY CO2 LEVELS

A comparison is made between the climatic equilibria of the NCAR CCM f or a pre-industrial atmospheric CO2 level (265 ppm) and a current (cir ca 1975) level of 330 ppm, including statistical estimates of the leve l of confidence in the implied changes. We also compare the model resu lts to observations compiled over the past 100 years, which correspond s to roughly half of the period of change in CO2 from pre-industrial ( 1800) to current levels. A relatively large model response in surface temperature and a smaller response in the precipitation, surface press ure, and storm track fields is obtained. These results are in accord w ith previous findings of the climate sensitivity to systematic changes in CO2 forcings. A t-statistic of the model results indicates a signi ficant surface temperature response to a relatively small change in CO 2, above the inherent model variability. Observations of global surfac e temperature anomalies for the period 1890-1990 show some similaritie s to the model results, especially a warming in regions of the wintert ime northern hemisphere of 2-3 degrees C. A point-by-point correlation of the 330 ppm minus 265 ppm model temperature differences to the obs erved 1980-1890 differences suggests that some of the variance in the observed trends in the surface temperature anomalies may be explained by the model experiments.