Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on life and ecosystems

Assessment of Potential Future Vegetation Changes in the Southwestern United States

Robert S. Thompson
U.S. Geological Survey

Katherine H. Anderson
INSTAAR, University of Colorado


Patrick J. Bartlein
University of Oregon

Warming of global climate may occur over the next century due to increased concentrations of carbon dioxide and other trace gases in the atmosphere resulting from human industrial activities (see "Global Climate Change: The 1995 Report by Intergovernmental Panel on Climate Change"). It is difficult to assess how warmer climatic conditions would be distributed on a regional scale and what the effects would be on the landscape, especially for temperate mountainous regions such as the southwestern United States. Below we present a strategy that can be used to estimate the potential changes in the distributions of plant species that may occur under the future climatic conditions simulated by Atmospheric General Circulation Models (AGCMs) and Regional Climate models. In this example, a 2xCO2 climate (climatic conditions under atmospheric carbon-dioxide concentrations twice the pre-industrial level) simulation by the National Center for Atmospheric Research (NCAR) GENESIS ACGM provided the boundary conditions for a regional climate model simulation on a relatively fine geographic scale (Giorgi and others, 1994a, 1994b). Output from the regional model was then interpolated on to a 15-km equal-area grid of the western United States and then used to initialize models that estimate the probability of occurrence of plant species from climatic data (Thompson and others, in review).

The geographic ranges of plant species are affected by climatic change (see "Past Climate and Vegetation Changes in the Southwestern United States"). The relations between the modern distribution of a plant species and climatic parameters provides the basis for estimating how future climatic changes may influence plant species distributions. Our first step in exploring these relations was to construct a 25-km equal-area grid of modern climatic data for North America (Figs. 1 and 2; Bartlein and others, 1994). Distribution maps of plant species (Little, 1971, 1976) were digitized and the presence or absence of each plant species determined for each point on the climate grid (Thompson and others, in preparation). The probability of occurrence of each species can then be determined from these climatic and distributional data (fig. 3) and response surfaces based on these relations used to estimate changes in distributions (figs. 4 and 5).

Figure 1. Thirty-year climate normals (1951-1980) for January, July, and Mean Annual Temperature and Growing Degree Days on a 5° C base for North America. Click on any graphic to view a larger image.

Figure 2. Thirty-year climate normals (1951-1980) for January, July, and Mean Annual Precipitation and a Moisture Index (based on Thornthwaite and Mather, 1957).Click on any graphic to view a larger image.

Figure 3. The modern relations between the distributions of big sagebrush (Artemisia tridentata) and saguaro (Cereus giganteus) and climatic parameters. Big sagebrush lives in relatively cool and dry environments, whereas saguaro lives in some of the hottest and driest environments in North America.

Figure 4. In this illustration the modern distributions of Douglas Fir (Pseudotsuga menziesii) and California White Oak (Quercus lobata) on the 25-km calibration grid are shown in the far-left panels. The simulated modern distributions of the two species are shown on the 15-km target grid in the left-center panels. Comparisons of the modern and simulated distributions suggests that the response-surface based method does an adequate job of simulating the modern distributions of the species. The right-center panels illustrate the estimated distributions under the 2xCO2 climate simulation, and the far right panels show the changes between the modern and 2xCO2 simulations (here green indicates grid points where the plant species lives under both the modern and potential future simulations; blue indicates new grid points where the species can live under the 2xCO2 simulation; and red indicates grid points where the plant species dies off between the two simulations).

Figure 5 uses the format explained above to illustrate the potential range changes that could occur under the 2xCO2 simulation. Under this scenario, major forest trees and range shrubs (Engelmann-spruce, Douglas-fir, lodgepole pine, and big sagebrush) die off across much of their modern range without new potential replacement habitats becoming available in other areas of the western United States. Ponderosa pine would lose much of its western range and find new potential habitat east of its modern limits. White oaks from California and Oregon could potentially grow under a 2xCO2 winter-wet summer-dry climate in the Southwest. Gambell oak and pinyon pine would die off in the Southwest, but could potentially find new range in the northern interior. Joshua tree and creosote bush, two shrubs adapted to arid conditions, would considerably expand their spatial coverage relative to today under the 2xCO2 simulation. Saguaro, a frost-limited dry-adapted plant of the Sonoran Desert, would largely die off in its modern range, but could potentially find new range farther east and at higher elevations. Index to common and scientific names: Engelmann Spruce (Picea engelmannii), Douglas Fir (Pseudotsuga menziesii), Lodgepole Pine (Pinus contorta), Ponderosa Pine (Pinus ponderosa), Oregon White Oak (Quercus garryana), California White Oak (Quercus lobata), Gambel Oak (Quercus gambelli), Pinyon Pine (Pinus edulis), Big Sagebrush (Artemisia tridentata), Joshua Tree (Yucca brevifolia), Creosote Bush (Larrea divaricata), and Saguaro (Cereus giganteus).


The range changes simulated to occur between today and the 2xCO2 climate would dramatically affect ecosystems across the western United States. Similarly large shifts in plant distributions have been simulated for a potential 2xCO2 climate in Europe (Dahl, 1990; Huntley et al., 1995; Sykes et al., 1996) and in the Yellowstone region (Bartlein and Whitlock, in press). The plant species simulated here exhibit individualistic responses to climatic change, as has been demonstrated in the response of western plants to warming following the end of the last Ice Age (Thompson, 1988; Betancourt et al., 1990; Thompson et al., 1993). In the latter case, the warming was relatively gradual and plants had thousands of years to disperse from their old ranges to the new ones. The anticipated onset of "greenhouse" warming may occur in only a few decades, and it is unknown whether or not displaced plants and animals could migrate to new habitats this rapidly.

The strategy presented here permit us to interpret global climate simulations at the scale of landscape processes and impacts. In this particular example, plant distributions are strongly affected by the simulated global climate change. However, not all possible future climates would necessarily have such strong environmental impacts. This strategy provides a means of exploring the range of possible future climates to identify processes and regions at risk.


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Bartlein, P.J. and Whitlock, C., in press. Potential Future Environmental Changes in the Yellowstone National Park Region. Conservation Biology.

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Thompson, R.S., Anderson, K.H., and Bartlein, P.J., in preparation: Atlas of Vegetation-Climate Relationships in North America. U.S. Geological Survey Professional Paper.

Thompson, R.S., Hostetler, S.W., Anderson, K.H., and Bartlein, P.J., in review: A Strategy For Assessing Potential Future Vegetation Changes In The Western United States. U.S. Geological Survey Circular.

Thompson, R.S., Whitlock, C., Bartlein, P.J., Harrison, S.P., and Spaulding, W.G., 1993, Climatic Changes in the Western United States since 18,000 yr B.P. In Wright, H.E., Jr., Kutzbach, J.E., Webb, T. III, Ruddiman, W.F., Street-Perrott, F.A., and Bartlein, P.J. (eds.), "Global Climates since the last Glacial Maximum". University of Minnesota Press. pp. 468-513.

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