Three procedures were used to develop a set of plausible scenarios of anthropogenic climate change by the year 2100 that could be posed to the sectors selected for assessment (Fig. 2.2). First, a workshop of climatologists with expertise in western North American climates was convened from September 10-12, 1998 at the National Center for Ecological Analysis and Synthesis in Santa Barbara, CA to discuss and propose a set of scenarios for the Rocky Mountain/Great Basin (RMGB) region.

Secondly, the 20^th-century climate record was analyzed to determine what trends might have occurred during the period. Since CO2 and other greenhouse gases increased during the century, it was reasonable to examine whether the changes projected for the 21st century had begun to appear during the 20th, at least qualitatively though not quantitatively.

Third, on the assumption of a two-fold increase in atmospheric CO₂ by 2100, climate-change scenarios for the 21^st century were projected with two, state-of-the-art computer models that simulate the complex interactions between earth, atmosphere, and ocean to produce the earth’s climate system. Each of the last two procedures has its strengths and weaknesses, and each can function to some degree as a check on the other. The historical analysis has the advantage of using empirical measurements of actual climate change taken over an extensive network of measuring stations. These make it possible to subdivide a large region like the RMGB into subreqions to assess the uniformity of climate and climate change over the region. And the historical measurements can to some degree serve as a check on the GCM simulations when the two are compared over the same time period.