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The 19th-century anthem, America the Beautiful [RM / ZIP / MP3], celebrated the wonder of the Rocky Mountain/Great Basin (RMGB) region. The purple mountain majesties have long conjured the image of the American frontier west, while providing world-class recreational opportunities for residents and visitors to this dramatic region. Exceptionally diverse natural resources and ecosystems characterize the RMGB, including alpine tundra and permanent glaciers, coniferous forests, high mountain grasslands, dry steppe, grassland and rangeland, stunning canyon formations, basin and range, expansive salt flats, and even an inland salt sea -- Utah's Great Salt Lake.

For purposes of this paper, this region covers the mountainous zone extending from northern New Mexico northwestward to western Montana and the Bitterroot Range of the eastern Idaho panhandle. The Wasatch Mountains, backbone of Utah, also are considered part of the Rockies. The Great Basin encompasses the area between the Wasatch and the Sierra Nevada of California and covers the western half of Utah and all of Nevada from about 65 miles north of Las Vegas northward. Also included in this region is southern Idaho, south from the Snake River plains, the Columbia Plateau of eastern Oregon, part of southwestern Wyoming, and a small part of southeastern Washington.

Much (e.g., 80%) of the RMGB is public land (national forests, national parks, wildlife refuges, public domain, state land), and about 80 % of its population lives in urban areas. These urban areas are becoming increasingly congested, because the RMGB has one of the fastest growing populations in the country. As this region's population is growing, income from traditional activities -- mining, farming, ranching, and timber harvesting -- is making-up a declining fraction of the region's economy. Analyses indicate that resulting changes in land-use patterns, coupled with potential impacts of climate change, could affect the ecological and economic health of this region, both positively and negatively.

The climate varies across this extensive and topographically complex region. Its topography is bounded on the east and west by major mountain chains rising to between 12,000 and 14,000 feet at their highest points. In between is the Great Basin, dotted with a washboard pattern of lesser ranges and intervening dry valleys, making what the geologists call the Basin-and-Range Province.

In the West, precipitation increases and temperature decreases with increasing elevation. These variations are quite large; for example, precipitation in the RMGB region varies from as low as 2 inches per year up to 100 inches per year. The region's mountains force much of the atmosphere's moisture out as precipitation, primarily as snow on the mountains thereby drying the air and creating an arid climate in the valleys. The mountain snow supports a lucrative skiing and tourism economy in the winter. Spring runoff provides the needed water for irrigation, industry needs, and power generation, and a rapidly growing urban population cloistered in the lower elevations. The importance of the snowfall in these mountains is not confined to this region because the headwaters of the Colorado, Columbia, Missouri, Rio Grande, Platte, and Arkansas Rivers are all in the region and carry this valuable water to distant down-stream users.

Seasonally, most of the precipitation in the northwestern two-thirds of the region falls from autumn to spring. But in the southern third of the region, the pattern is a monsoonal one, with most precipitation occurring as rain in late summer (July-September). Precipitation in the region is subject to natural (i.e. non-human) sources of variation. Atmospheric circulation patterns, particularly those connected with the Pacific Ocean, influence the climate of the region. The two most prominent patterns of Pacific climate variability are: the Pacific Decadel Oscillation (PDO) and the El Niño Southern Oscillation (ENSO) (see boxes for more detailed descriptions). The PDO alternates over a 20-30-year timescale between cool-wet and warm-dry conditions. The ENSO recurs on a 2-7 year timescale, and alternates causing wet and dry periods such that wet periods in the south tend to be dry in the north, and vice versa. To be detectable and influential, any long-term trends in climate during the 20^th century must therefore be strong enough to be seen above these natural climate variations.

The concentrations of greenhouse gases in the atmosphere have been increasing since the beginning of the Industrial Revolution in the mid-1800s, so it is reasonable to ask whether the predicted climate effects of that increase have begun to appear during the last 150 years. Over much of the US, dependable weather records do not go back much before the turn of the 20^th century. There are enough data, however, for scientists to conduct analyses of climate trends over the past 100 years.

In the RMGB region average annual temperatures have risen about 1°F during the 20^th century in the northwestern two-thirds of the region, but have not increased significantly in the southeastern third (i.e. Colorado and New Mexico). The lowest nighttime temperatures have increased more than the highest daytime temperatures, consistent with projections by climate models. Average yearly precipitation has increased from about 5 to 20% during the 20^th century, again mostly in the northwestern two-thirds of the region. There has been no significant increase in precipitation in the southeastern one-third of the region. These changes are in the general direction of those projected by the computer models.

Because the atmospheric concentrations of greenhouse gases are increasing, and are expected to increase substantially by the year 2100 over the pre-industrial levels, the patterns of climate change in the 21^st century are likely to be different from those of the 20^th century. Therefore the 20^th century trends cannot be used reliably to predict the changes in the 21^st century.

The primary means for making such projections is with the use of computer models that simulate the most important aspects of the behavior of the Earth's climate system. These models can be used to project future climatic conditions when given projections of future greenhouse-gas emissions. Known as global-circulation models (GCMs), these models are mathematical representations of important physical processes -- atmospheric physics, oceanographic and terrestrial influences -- that determine the Earth's climate. Characterizing the extremely complex interactions of factors that drive the Earth's climate system, and representing these in mathematical statements are on the cutting edge of science. Therefore it is inevitable that there are uncertainties about how accurately the models are likely to project future climatic conditions. It is generally agreed that the models provide credible estimates of changes at the continental to global scales. Such models are being developed in a number of nations. The scientists are all in contact with each other and compare notes on their efforts.

Projecting climate changes on a regional scale is more difficult than projections on a continental or global scale because model representation of these finer scales is not as well developed and tested as at the global scale. Particularly in the RMGB region, the complex topography creates a mosaic of localized climates for which the present generation of GCMs have only a very limited ability to represent. Thus, while the model results suggest plausible types of changes that could occur, their results should not be considered firm predictions of what will occur at a localized site; rather the results are best thought of as typical "what-if" projections for the region as a whole. While it is important to be careful in interpreting the projections, some confidence arises because analyses of historical climate changes in this region for the 20^th century point in similar directions as the regional GCM projections for the 21^st century.

Recognizing these limitations of the models, the more conservative of the two GCMs used by the National Assessment process project average, annual temperature increases of 2 to5°F by the end of the 21^st century. As noted earlier, temperatures over much of the RMGB rose 1°F during the 20^th century when the CO₂ increase was just getting underway. The models also project that total precipitation for the region will increase by 50-100%.

Once again it needs to be emphasized that these are projections by computer models that simulate the behavior of the extremely complex global climate system. Thus they need to be considered "what-if" possibilities. However, the measured trends during the 20^th century also point in these directions.