Climate projections for western North America suggest temperature increases that are relatively robust across climate models. However, projections for precipitation are less consistent, especially in the Southwest, promoting uncertainty about the future of soil moisture and drought. In particular, the impact of climate change on soil temperature and moisture regimes, which are increasingly utilized as indicators of ecosystem vulnerability, has not been assessed. We utilized a daily time-step ecosystem water balance model to characterize soil temperature and moisture patterns on a 10-km resolution across western North America for three time periods: historical (1980-2010), mid-century (2020-2050), and end of the century (2070-2100). We incorporated site-specific soil conditions and simulated temperature and moisture in multiple soil layers based on gridded daily weather observations and downscaled climate projections for two RCPs and eleven global climate models (GCMs; selected strategically to represent variation among the full set of GCMs in the latest CMIP5 climate projections). We identified areas with robust projections as those where 10 of 11 GCMs agree in the direction of increase or decrease in specific conditions or regimes.
Rising temperatures will increase average soil temperatures reasonably uniformly across western North America and expand the area of mesic and thermic soil temperature regimes while decreasing the area of cryic and frigid regimes. Despite variability in precipitation projections among climate models, future soil moisture conditions are relatively robust across climate models for much of the region. Robust projections for drier soils are expected in most of Arizona and New Mexico, consistent with previous studies. Other regions with robust projections for declining soil moisture include the central and southern U.S. Great Plains and large parts of southern British Columbia. By contrast, areas with robust projections for increasing soil moisture include northeastern Montana, southern Alberta and Saskatchewan, and many areas dominated by big sagebrush (particularly the Central and Northern Basin and Range and the Wyoming Basin ecoregions). In addition, many areas dominated by big sagebrush are expected to experience pronounced shifts toward cool season moisture, which will create more area with xeric moisture conditions and less area with ustic conditions. These results provide a framework for assessing climate change impacts on the resistance and resilience of dryland ecosystems while recognizing uncertainty in climate projections.