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Observations of atmospheric carbon dioxide (CO2) concentrations in recent decades have shown an accelerating upward trend due mainly to burning of fossil fuels. Physical considerations indicate that observed and projected increases in the concentrations of CO2 and other greenhouse gases in the atmosphere will lead to significant global warming. Previous authors have followed both empirical and theoretical approaches to develop scenarios for changed climate conditions. The most common empirical approach is the use of analogs of global warming drawn from historical records and paleoclimates. The principal theoretical approach involves the interpretation of output for general circulation models of the atmosphere.

This report compares previously published results in the light of basic physical and meteorological considerations to derive an initial "first-guess" climate scenario for the western United States under doubled-CO2 conditions. The scenario features a northward displacement of existing storm tracks across western North America and changes in lengths of seasons and airmass characteristics. This scenario is then applied to two sets of watersheds chosen for study in a joint Bureau of Reclamation and Geological Survey study, namely, the Gunnison in Colorado, and the American, Carson, and Truckee basins in California and Nevada, to yield estimates of changes in temperature, cloudiness and precipitation that an altered climate might bring. Attachment A is a summary of selected publications on global climate change.


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