July 6, 2022

New study shows how drought led to deadly temperatures in the southwest in 2021

Laura Paskus

The Rio Grande near Mesilla in 2014.

With the changing climate, incidents like droughts and heatwaves can be interlinked. 

This was the case in 2021 when the drought conditions in the southwest United States triggered record-breaking heat, according to a study published in the journal Geophysical Research Letters on June 18.

Johns Hopkins University researchers Benjamin Zaitchek and Mahmoud Osman along with their colleague Nathaniel Winstead conducted the analysis. Osman and Zaitech typically study flash droughts, or droughts that come on very quickly.

“We usually think about drought as being kind of more of a creeping disaster that comes over time,” Zaitech said.

He said flash droughts come fast and can have really damaging effects, like destroying soy crops in the midwest.

During that research, Zaitech and Osman became interested in the 2021 heat wave in the southwest that killed hundreds of people.

While the heat came during extreme drought conditions, the researchers wanted to see if the drought led to the heatwave or if the two events were unconnected, meaning they had no influence on each other despite occurring at the same time.

The heatwave brought some of the most extreme temperatures the region has ever experienced, which came on in June rather than later in the summer.

“Drought, through its influence on surface fluxes, might have triggered or intensified the heat wave, which in turn could exert a positive feedback on drought through increased evaporative demand or decreased precipitation,” the researchers wrote.

What that means is that when drought impacts the surface of the earth, such as by killing off vegetation and leading to more light being reflected from the ground, it could trigger a worse heat wave and that heat wave could then worsen the drought conditions by increasing the amount of water that evaporates or decreasing precipitation.

Zaitech said record-shattering heat and events like wildfires are drawing attention and people are asking if they are becoming more frequent, which, he said, they are. The answer to why they are becoming more frequent, Zaitech said, is often climate change. Within those big areas are questions about what are known as cascading events. For example, when a drought occurs, is a more intense heat wave likely to occur?

“We know that there’s a connection, but this connection has always been a hypothetical,” Osman said.

Osman and Zaitech set out to see if the hypothesis that the drought and heatwave were interconnected was supported by evidence. 

The researchers used satellite imagery and computer modeling. Zaitech said they asked the question, “what if this happened back in 2019” when there wasn’t as severe of drought in the southwest.

“In other words, we removed the drought,” Osman said.

The researchers input soil moisture, soil temperature, canopy water, snow, the fraction of sunlight reflected and vegetation fields data from 2019 into the model to determine how the weather patterns, especially heat, would likely have looked without the drought conditions.

“Our simulations are not designed to simulate the full impact that the heat anomaly had on drought, but they can be used to examine simulated drought feedbacks on drought, including any cascading drought-heat-drought feedbacks,” the researchers reported.

Zaitech said areas of the southwest were several degrees hotter in 2021 because of the drought than they would have been if the drought hadn’t existed.

And, because the temperature increased, the drought became amplified, Osman said. He described it as a drought-heat-drought feedback.

Osman said anthropogenic climate change is causing warming, which makes drought more likely and, when drought does occur, there’s an increased risk of heatwaves.

Zaitech said people in the southwest should be prepared for more heat waves amid drought, especially earlier in the year when residents may not be as prepared for high temperatures.

“Early season heat for humans, that’s pretty deadly,” he said, adding that early season heat waves “catch you before you’re ready.”