Take a period of limited rainfall. Add heat. And you have what scientists call a 'hot drought' – dry conditions made more intense by the evaporative power of hotter temperatures.
"The frequency of compound warm and dry summers particularly in the last 20 years is unprecedented," said Karen King, lead author of the study and an assistant professor at the University of Tennessee in Knoxville.
For much of the last 20 years, western North America has been in the grips of a megadrought that's strained crop producers and ecosystems, city planners and water managers.
Cross-sections or cores of trees, both living and dead, can offer scientists windows into climate conditions of the past. Dark scars can denote wildfires. Pale rings can indicate insect outbreaks. "Narrow rings [mean] less water," said King, a dendrochronologist, who specialized in tree ring dating. "Fatter rings, more water."
Scientists have looked at tree ring widths to understand how much water was in the soil at a given time. King and fellow researchers did something different. They wanted to investigate the density of individual rings to get a picture of historical temperatures. In hotter years, trees build denser cell walls to protect their water.
King collected samples of tree species from mountain ranges around the West, road-tripping from the Sierra Nevada to British Columbia to the southern Rockies. She and her co-authors used those samples and others to reconstruct a history of summer temperatures in the West over the last 500 years.
It also suggests that future droughts will be exacerbated by higher temperatures, particularly in the Great Plains, home to one of the world's largest aquifers, and the Colorado River Basin, the source of water for some 40 million people.
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