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A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 12, NUMBER 5, MAY 1999

Increased Precipitation for Central United States

Item #d99may49

In an April 19, 1999, press release, the National Center for Atmospheric Research (NCAR) released new projections for future climate change that were produced with the latest version of NCAR’s Climate System Model (CSM). The model simulated the climate from 1870 through 2100, with two scenarios being projected for the 21st century: a business-as-usual projection in which atmospheric CO₂ concentrations double from today’s levels to about 710 parts per million by the year 2100 and a stabilization scenario that meets the greenhouse-gas-reduction objectives of the UN Framework Convention on Climate Change. The projections also employed a more realistic scenario for the future emissions of sulfur dioxide than had been considered previously.

Initial analysis of the 21st century model results show substantial global-scale warming during the next century in both simulations: approximately 2° C for the business-as-usual case and 1.5°C for the stabilization case, three to four times larger than the warming that has occurred over the 20th century. The simulations indicate that CO₂ stabilization would have a more marked influence on temperature over Eurasia than over North America. However, no clear separation would occur between the business-as-usual and stabilization cases until around 2060. This multidecadal lag between the changes in emissions pathways and changes in the modeled climate results from the large thermal inertia in the climate system, especially the oceans.

The model results also show appreciable precipitation changes that vary markedly from region to region and season to season as well as between the two cases studied. In the business-as-usual case, winter precipitation increases by more than 40% over the U.S. Southwest and central Great Plains. The slight changes along the eastern coastal areas are generally towards drying in winter and moistening in summer. In the stabilization case, the changes in the Southwest and Great Plains regions are reduced substantially.