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The only scenario developed to date in the GCCRP is a doubled-C02 scenario. It is recognized that doubled-C02 scenarios, being steady state, are not entirely realistic, but there are not enough available GCM simulations of continually increasing concentrations of greenhouse gases to provide much guidance on their probable impacts on climate. The concentration of CO2 in the atmosphere is likely to reach a value double that of the pre- industrial era somewhere around 2050 (Nordhaus and Yohe, 1983), and the enhanced greenhouse effect will be hastened by increases in trace gases such as methane. However, the oceans will impose a lag of several decades on any resultant climate change. Considering all known factors, the scenario can be viewed as a crude, fIrst guess at the climate of western North America around the year 2100. Although the scenario team has paid close attention to the available analog studies, the first- guess scenario is based mainly on GCM results. The team has not simply averaged the available GCM results, but has placed greater credence in the recent output from second- generation GCMs coupled to ocean models. Results from those models are less startling than some of those published 10 to 15 years ago. For example, according to Boer et al. (1990), runs with the second-generation version of the Canadian Climate Center GCM suggest only a 4 percent increase in world-wide precipitation in a doubled-C02 world, compared to predictions of 10 percent or more based on runs with early versions of well known American GCMs. However, the greater attention paid to the recent results is not due to their greater palatability, but to the improvements that have been made to the various models. 3. A DOUBLED CO2 SCENARIO Subjective combination of the available modeling results and analog information leads to the following scenario for a doubled-C02 world. The equatorial regions would be about 20C wanner than at present, with the location of the Intertropical Convergence Zone (ITCZ) varying with the seasons much as it does now. Global warming would tend to shift the other weather belts around the world poleward by 2 or 30 of latitude. However, weather systems anchored to major geographic features, such as the polar front which often forms in winter off eastern North America along the northwestern edge of the Gulf Stream, would tend to hold their positions. The ainnasses on both sides of the polar front would be warmer than at present, the sub-tropical ainnasses on the warm side by about 20C and the polar ainnasses by larger amounts, ranging up to 50C for yearly averages in the Arctic and Antarctic regions. Mid-latitude frontal cyclones would be weaker than those of today (in terms of winds) because of the reduced temperature contrasts, but would produce abundant precipitation because of the increased water vapor content of the atmosphere. Printed January 18, 1991