A comparison of the CCM1-simulated climates for pre-industrial and present-day CO₂ levels

A comparison is made between the climatic equilibria of the NCAR CCM for a pre-industrial atmospheric CO₂ level (265 ppm) and a current (circa 1975) level of 330 ppm, including statistical estimates of the level of confidence in the implied changes. We also compare the model results to observations compiled over the past 100 years, which corresponds 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 pressure, and storm track fields is obtained. These results are in accord with previous findings of the climate sensitivity to systematic changes in CO₂ forcings. A t-statistic of the model results indicates a significant surface temperature response to a relatively small change in CO₂, above the inherent model variability. Observations of global surface temperature anomalies for the period 1890-1990 show some similarities to the model results, especially a warming in regions of the wintertime northern hemisphere of 2-3° C. A point-by-point correlation of the 330 ppm minus 265 ppm model temperature differences to the observed 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.