Observed and Projected Climate Changes in the Western and Southwestern United States
Gregg Garfin1*
1Southwest Climate Adaptation Science Center, Tucson, Arizona, USA; gmgarfin@arizona.edu
Human caused changes to the earth’s climate system are already affecting the climate of the western United States. The most obvious direct observed changes include increased average and extreme temperatures, with increases in the number of extremely hot days and decreases in the number of very cold nights. Increased temperatures have led to a variety of indirect effects on the climate of the region, including decreases in spring snowpack, reduced snow water content, earlier snowmelt runoff in many parts of the region, and an increase in the fraction of winter and early spring precipitation falling as rain rather than snow, in some parts of the region. These hydrological changes, along with reduced soil moisture, at least partly attributed to regional warming trends, affect surface and groundwater hydrology. Recent studies have summarized some of these changes with memorable phrases, such as “warm snow drought,” and “reduced runoff efficiency.” Drought has been a signature impact across the region for millennia, due to the region’s geography and topography. While the most severe and sustained droughts in the region are recorded in proxy records, such as tree-rings, in the period before instrument observations, in recent decades human-caused climate changes have exacerbated hydroclimatic impacts of drought—intensifying the severity of the most recent drought period in southwestern North America. The region is also characterized by high year-to-year and multi-decade precipitation variability. Other regional natural resources and ecosystem impacts related to these factors include decreased surface water reliability, forest mortality, expanded wildfire seasons, and wildlife population stress and mortality episodes.
Given assumptions of continued high rates of greenhouse gas emissions, climate models confidently project further increases in temperature, which will amplify changes to snow hydrology, including an increase in the average lowest elevation at which snow falls (which will reduce water storage in the snowpack, particularly at elevations which are now on the margins of reliable snowpack accumulation), earlier snowmelt and timing of runoff, and less snow-covered area. Temperature and snow-related changes have implications for streamflow and stream temperatures—warmer streams, less snowfall, and changes to snowmelt timing all have important knock-on effects in riparian areas. Models project increased annual average precipitation for the northern part of the western U.S. and decreased annual average precipitation in the southern part of the western U.S., due to projected alterations of the atmospheric circulation patterns responsible for guiding winter and spring storm tracks. Models project increases in extreme precipitation events, due to increases in the atmospheric water vapor generated over warmer ocean source regions.
Although projections of future precipitation totals are accompanied by much uncertainty, increased temperatures will increase the risks of more severe drought, greater regional aridity (especially in the Southwest) and increased wildfire occurrence and severity. Environmental consequences of these changes will depend on management actions and adaptations a variable and more extreme future climate.