You are reading Part 6 of 9 in this series.What are Quick Facts?
Human-caused global warming is increasing drought risk across much of the United States as rising temperatures accelerate evaporation, increase water uptake by heat-parched plants, and reduce the amount of winter snowpack available to refresh regions during dry summer months.
Facts for Any Story
There have always been droughts—temporary periods when the supply of moisture fails to meet human and environmental demands. But human-caused global warming has brought something different to much of the U.S. West: a long-term drying, or “aridification,” driven primarily by rising temperatures.1Das, T., et al., (2011), The importance of warm season warming to western U.S. streamflow changes, Geophys. Res. Lett., 38, 23. View Source2Overpeck, J.T., and Udall, B., (2020), Climate change and the aridification of North America, Proceedings of the National Academy of Sciences, 117 (22) 11856-11858. View Source3Bonfils, C.J.W., et al., (2020), Human influences on joint changes in temperature, rainfall, and continental aridity, Nature Climate Change. View Source
As temperatures go up, drought risk goes up, even in areas where rainfall has remained close to normal, because of increased evaporation and water uptake by plants. From 2000 to 2010, for example, high temperatures in the upper Missouri River Basin—the United States’ largest river basin—significantly contributed to a decade-long drought that by some measures (e.g., river-flow decline) was more severe than the Dust Bowl and could not be accounted for by the modest decreases in precipitation alone.4Martin J.T., et al., (2020), Increased drought severity tracked warming in the United States’ largest river basin. Proceedings of the National Academy of Sciences, 117 (21) 11328-11336. View Source
Human-caused global warming has approximately doubled the severity of the dry spell that has parched the U.S. Southwest since 1999 in terms of temperature, relative humidity, and precipitation, changing what would have been a moderately arid 20-year stretch into a megadrought worse than any in the region since 1500 and the second worst in 1,200 years.5Williams, P.A, et al., (2020), Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368, 6488: 314-318. View Source
Temperature-related acceleration of evaporation and plant water use is increasingly contributing to declines in the flow rates of the two most important rivers in the U.S. Southwest, the Colorado6Woodhouse, C.A., et al., (2016), Increasing influence of air temperature on upper Colorado River streamflow, Geophys. Res. Lett. 43, 2174–2181. View Source (which provides water to nearly 30 million people and irrigates nearly 4 million acres of agricultural land7Colorado River Basin Focus Area Study, USGS View Source) and the Rio Grande8Lehner, F., et al., (2017), Assessing recent declines in Upper Rio Grande runoff efficiency from a paleoclimate perspective, Geophys. Res. Lett. 44, 4124–4133. View Source (the nation’s fifth largest river and a key source of agricultural and urban water in the Southwest9Rio Grande, Britannica View Source). Colorado River flow decreased by about 16% from 2000 to 2017, with rising temperatures accounting for approximately half of this decline.10Milly, P.C.D., and Dunne, K.A., (2020), Colorado River flow dwindles as warming-driven loss of reflective snow energizes evaporation, Science, 367, 6483, 1252-1255. View Source
California’s climate, which swings from a wet winter season to a dry summer season, is naturally vulnerable to drought. But human-caused warming has greatly increased drought risk there11Swain, D., et al., 2018, Increasing precipitation volatility in 21st century California, Nature Climate Change, 8: 427–433. View Source, in part because winter precipitation increasingly falls as rain rather than snow, reducing the storage of water as snowpack for gradual release during the summer dry season. California dry spells (such as the 2012-2016 drought) are getting drier by multiple measures, including precipitation and river runoff, making the Golden State increasingly brown and prone to severe wildfires.12Mote, P.W., et al., (2018) Dramatic declines in snowpack in the western US, Clim Atmos Sci, 1, 2. View Source13Diffenbaugh, N.S., et al., (2015), Anthropogenic warming has increased drought risk in California, Proceedings of the National Academy of Sciences, 112, 13, 3931-3936. View Source14Mann, M.E., and Gleick, P., (2015), Climate change and California Drought in the 21st Century, Proceedings of the National Academy of Sciences, 112, 13, 3858-3859. View Source
Human-caused global warming is responsible for more than half the increase in forest aridity (dryness) across the U.S. West since the 1970s, doubling the cumulative area burned in forest fires since 1984.15Abatzoglou, J.T., and Williams, A.P., (2016), Impact of anthropogenic climate change on wildfire across western U.S. forests, Proceedings of the National Academy of Sciences, 113, (42), 11770-11775. View Source16Westerling, A.L., et al., (2016), Warming and earlier spring increase western U.S. forest wildfire activity, Science, 313, 5789, 940-943. View Source
Drought doesn’t just weaken and kill plants; it prevents new ones from growing. A decades-long period of hot and dry weather that affected many parts of the Rocky Mountains from the 1980s through about 2010 resulted in significant decreases in tree regeneration and forest resilience.17Davis, K.T., et al., (2019), Wildfires and climate-change push low-elevation forests across a critical climate threshold for tree regeneration, Proceedings of the National Academy of Sciences, 116, (13), 6193-6198. View Source
Pitfalls to Avoid
Don’t assume that drought is just about the amount of precipitation. Though it may seem counterintuitive, even places that see increases in precipitation can face a heightened drought risk because the temperature-related increase in evaporation and water demand by plants can outpace the increase in precipitation.
Drought.gov is a federal site featuring data, maps, tools, and other resources, including the U.S. Drought Monitor, a map that shows the location and intensity of drought across the country, updated weekly.