A Comparison of Simulations of Current Climate from
Two Coupled Atmosphere-Ocean Global Climate Models
Against Observations and Evaluation of Their Future Climates

Report to the National Institute for Global
Environmental Change (NIGEC)

In support of the US National Assessment

Ruth Doherty and Linda O. Mearns

Revised 27 April 1999

Revision of the Report

We found a small error in the processing of the HADCM2 current climate segment (1921-1980), referred to as the control in the report. The control serves as the baseline for comparison with the observations and for determining changes in the future climate segments. The greenhouse gas only control (i.e., without the effect of sulphates) mistakenly was used instead of the greenhouse gas plus aerosols control when creating the baseline. The latter run is considered the most accurate representation of the current climate since it considers both major forcings (greenhouse gases and sulphate aerosols). In most regions and in most seasons, however, the differences in the two controls are quite small, considering the regional and temporal scale of our investigation. The largest difference occurs in the mean temperature field in summer when the greenhouse gas plus sulphate control is between 1 and 1.5 ° C cooler than the greenhouse gas only control in the central portion of the U.S. Differences in precipitation are minor in all seasons. However, the region that is cooler in the summer is also slightly wetter.

The error had little or no effect on the comparison of the control with observed data sets (given the qualitative comparison considered in this report). The main effect shows up in the calculation of changes in future temperature for the central US. For the sake of accuracy, we have replaced all figures in the report that used the HADCM2 control. We also made some minor changes in the text and corrected Tables 2 and 3. However, it is recommended that those researchers who obtained hard copies of the February 4 version of the report or downloaded it from the web, replace (downloaded from the web) only the HADCM2 future temperature change figures (Figures 18, 20, and 22), along with Tables 2 and 3. Textual changes are on pages 11-13 of the report.

Hard copies of the revised report may be obtained by contacting Ms. Tanya Beck, ESIG Administrative Assistant: tbeck@ucar.edu; 303-497-8117.

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