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.

A new study, published in the journal Science Advances, Wednesday, finds that hot droughts have become more prevalent and severe across the western U.S. as a result of human-caused climate change.

"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. Scientists believe it to be the driest period in the region in at least 1,200 years. They reached that determination, in part, by studying the rings of trees collected from thousands of sites across the Western U.S.

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.

The tree rings showed that the first two decades of this century were the hottest the southwestern U.S., the Pacific Northwest and parts of Texas and Mexico had experienced during that time. Last year was the hottest year on record globally.

By combining that temperature data with another tree-ring-sourced dataset looking at soil moisture, the researchers showed that today's hotter temperatures – sent soaring by the burning of fossil fuels and other human activities – have made the current western megadrought different from its predecessors.

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.

"As model simulations show that climate change is projected to substantially increase the severity and occurrence of compound drought and heatwaves across many regions of the world by the end of the 21st century," the authors wrote. "It is clear that anthropogenic drying has only just begun."