Aridity and climate change in the western US

The potential spread of arid conditions within the western US in response to projected climate changes is a threat to water supplies, ecosystems, and many human settlements. In order to evaluate such risks in considerable geographic detail, daily precipitation and temperature projections from three General Circulation Models (GCMs) and emission scenario A2 were downscaled using the method of constructed analogues and then used as input to the Variable Infiltration Capacity (VIC) model to determine climate change impacts in the aridity of the western US. Specifically, the ratio of AET/PET was used to classify the western landscape into arid and non-arid regions under historical and future conditions. Future climates from the GFDL CM2.1 model yielded the largest increases in aridity, with expansion of the desert areas on the order of +11 percent and +34 percent during the first and second halves of the 21st century respectively. The CNRM CM3 model yielded expansions of the order of 0 percent and +16 percent for the same periods, while the NCAR PCM 2.1 yielded expansions of only +1 percent and +3 percent. The increase in the areas where actual evapotranspiration (AET) is "energy limited" in the historical period but "water-limited", on average, in the first half of the 21st century are on the order of +2 percent, +9 percent and 0 percent for the CNRM, GFDL and NCAR models respectively. Similar projections for the second half of the 21st century are +25 percent, +40 percent, and +3 percent. The consensus is that aridity in the western US will increase but the wide spread of responses between models suggests that we need many more realizations to determine the real magnitude and range of the desertification. Projected desertification, along with changes from energy limited AET to water limited AET at high altitudes, would have important impacts on the water resources, ecology and wildfire potential in the western US.