Climate Model Projections of the 21st Century for the U.S. National Assessment

The U.S. National Assessment of the Potential Consequences of Climate Variability and Change has incorporated a number of methods of generating climate scenarios for the future, including projecting historical trends into the future, applying 'what-if' scenarios, and using results from several general circulation models (GCMs). These GCM scenarios are coupled atmosphere-ocean models that use time-varying greenhouse gases and sulfate aerosols for both the 20th and 21st centuries. The two primary GCMs used by most regions and sectors are the Canadian (CGCM1) and UK Hadley (HadCM2) models; recently, additional results from several U.S. models have also become available.

Most regions and sectors within the National Assessment have utilized projections of changes in temperature and precipitation. While both CGCM1 and HadCM2 show warming across the U.S., which is generally largest in the West, the precipitation anomalies from the two models disagree in many areas. Both models show large increases in precipitation in the Southwest, while the CGCM1 shows less precipitation and the HadCM2 shows more precipitation in the Southeast. The precipitation increase in the southwest is an extension of a larger region of increased precipitation in the Pacific Ocean off the west coast of North America that is associated with a southward-shifted jet stream, a deepened and southward-shifted Aleutian Low, a weaker subtropical high, and warmer sea surface temperatures (SSTs). While both models show decreased frequency of Atlantic storms, consistent with decreased meridional and land/sea temperature gradients, the more coastal position of the storm track in CGCM1 results in less precipitation than modern along the eastern seaboard of the U.S. During summer, differences in land surface models within the two GCMs sometimes leads to differences in soil moisture that feedback to the precipitation over land due to available moisture. More recent results from newer generation GCMs will also be discussed.