As the Earth’s average temperatures have risen, evaporation rates have increased, making more water available for precipitation in some areas but contributing to drying in others. As the climate continues to change, some areas will likely experience increased precipitation and flooding, but other locations will likely experience less precipitation and increased risk of drought.
For example, climate projections for the southwestern United States indicate that this region may experience chronic future precipitation deficits, particularly in the spring, increasing the risk of meteorological drought. Significant declines in precipitation are not confidently projected in the U.S. outside of the Southwest; however, higher temperatures in the future may increase the frequency and magnitude of agricultural drought across the country as a result of increased evapotranspiration.
The Climate Science Special Report, Part I of the Fourth National Climate Assessment, found that annual trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western United States. These trends are expected to continue. The report also points out that, under higher emission scenarios and assuming that no changes are made to current management of water resources, chronic, long-duration hydrological drought is increasingly possible before the end of this century.