High heat driving Colorado droughts even when it rains and snows, study finds

Climate change has made Western air so hot that drought will threaten the region even in years of decent snow and rainfall, according to a new study by UCLA and NOAA scientists detailing the scary new normal of warmer global temperatures. 

From 2020 to 2022, a searing drought in Colorado and Western states was caused more by hot air robbing water from the landscape through evaporation than by the lack of precipitation, the study concludes. As average summer temperatures climb higher, that means the West will suffer even when the water falling from the sky approaches historic averages. 

“It is becoming the reality of the world that we’re living in,” said Joel Lisonbee, regional drought information coordinator with NOAA’s National Integrated Drought Information System and coauthor of the study published in “Science Advances.”

“Across the western United States, we are prone to drought. We’re also prone to high temperatures, and we depend a lot on the water that we have,” Lisonbee said. “We need to prepare for more frequent and more severe drought.”

Any patch of ground needs normal levels of moisture to promote healthy growth, and that moisture is present from two causes: precipitation, or lack of evaporation or “evaporative demand.” The scientists also refer to evaporative demand — what is taken from the ground and air by evaporation — as the “thirst of the atmosphere.” 

Hotter air can also hold more evaporated moisture than cool air before it falls to ground as precipitation, exacerbating the drought cycle. 

“Even if precipitation looks normal, we can still have drought because moisture demand has increased so much, and there simply isn’t enough water to keep up with that increased demand,” said Rong Fu, a UCLA professor of atmospheric and oceanic sciences and a study coauthor. “This is not something you could build bigger reservoirs or something to prevent because when the atmosphere warms, it will just suck up more moisture everywhere. The only way to prevent this is to stop temperature increase, which means we have to stop emitting greenhouse gases.” 

The study logged a reversal of the usual ratio of drought causes from 2000 to 2022, a Western dry spell now considered a historic “megadrought” occurring only every 1,000 years. During that long drought, evaporation from higher temperatures accounted for more of the severity than did lack of precipitation. 

That ratio tilted further in the West’s extreme drought recorded from 2020 to 2022. In that three years, “evaporation accounted for 61% of the drought’s severity, while reduced precipitation only accounted for 39%,” according to the study. 

The researchers studied the recent drought against a historic period from 1948 to 1999. During the megadrought, the average drought area increased 17% in the western United States because of the higher evaporative demand from warmer temperatures. 

Since 2000, “thirsty” atmosphere alone could have caused drought on 66% of historically vulnerable Western land, even if the precipitation was average. That was up sharply from the 1948 to 1999 period, when only 26% of that land was vulnerable to evaporation-caused drought. 

The stark findings have Lisonbee and his colleagues steering their research focus to the very definition of modern drought, in order to make the discussion more relevant to the realities of global warming. 

“Do we define drought using the whole period of record, all the way back to the late 1800s? Or should we be defining drought based on a shorter period of record that is more representative of the last 30 years?” Lisonbee said. 

“It’s almost more philosophical.”