Droughts and Climate Change (AAAS)

 

Climate Change and Droughts

 

Many parts of Colorado, California, and other western states are back in drought conditions. You may wonder if this and other droughts world-wide are exacerbated by a warming planet. If you thought so, you are correct. Here is the explanation from  contributing scientists at the American Association for the Advancement of Scienc (AAAS) Sci-line.

 

“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.

 

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.

 

As temperatures go up, drought risk goes up, irrespective of precipitation, 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 decreases in precipitation alone.

 

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. 

 

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 Colorado ⁶ (which provides water to nearly 30 million people and irrigates nearly 4 million acres of agricultural land ⁷) and the Rio Grande ⁸ (the nation’s fifth largest river and a key source of agricultural and urban water in the Southwest ⁹). Colorado River flow decreased by about 16% from 2000 to 2017, with rising temperatures accounting for approximately half of this decline.

 

Human-caused global warming is responsible for more than half the increase in forest aridity (dryness) across the U.S. West ⁱⁱ since the 1970s, doubling the cumulative area burned in forest fires since 1984. 

 

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. 

 

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 there, ¹⁴ 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. 

 

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.”

Have questions? E-mail me at climatelynx@earthlink.net     Warm regards, Cool climate, grey

 

The American Association of Science (AAAS) was founded in 1848 and is the largest general science organization in the world. It promotes science and, in the recent anti-science years, has focused on defending scientists. Its world-renowned “Science” magazine is published by the AAAS and distributed to its 120,000 members, of which, I am a member.