Droughts are recurrent, disruptive weather events whose impacts are often compounded by extreme and prolonged heat waves. Now a new NOAA study in the Journal of Climate warns that in the already warm and frequently dry southern Great Plains and Southwest, climate change will make these “hot droughts” significantly hotter—and longer—than they used to be. 

The research, led by CIRES scientist Linyin Cheng, now at the University of Arkansas, assessed how, and by how much, human-caused climate change affects compound drought and heat wave events over the contiguous United States. The researchers ran multiple simulations using a sophisticated climate model, comparing the response of heatwaves to an underlying drought in our current atmosphere, and in the atmosphere of the 19th century, before appreciable global warming.

They found that in the southern Plains and Southwest especially, as soil moisture vanishes during severe droughts, cooling by evapotranspiration is more severely curtailed during droughts in today’s climate than in the climate of the 19th century. This results in amplification of the surface warming and hotter heat waves. The paper notes that this supercharging of drought-related heat waves is a factor separate from, and additive to, the overall rise in global temperature as a whole, which acts to further increase temperature in virtually all weather regimes, drought or not. 

“When we modeled conditions of moderate or severe drought, we found that heat waves in the climate of today were appreciably worse than they were in the climate of 100 years ago,” said co-author Martin Hoerling, NOAA research meteorologist. “What this tells us is that droughts occuring in our warmer world will produce hotter temperatures than the same droughts in our forebears’ cooler world.” 

The authors point out that not every region of the U.S. responded the same way. In the cooler and typically wetter northeastern and northwestern regions of the country, heat waves did get more intense, but not because of this positive feedback loop between soil moisture and temperature. 

“In our models, the drought-heatwave relationship undergoes little change over northern U.S regions in the warmed climate,” said Cheng. “Instead, the long-term warming trend plays the primary role in intensified heat waves during droughts in those regions.”

In the Ohio Valley and Great Lakes regions, the model paradoxically projects decreased heat waves during droughts, for reasons not yet understood. In the southern Plains, however, the amplification of high temperatures during compound drought and heat events was significant.  

How significant? The team looked at Texas-Gulf Basin, where a heat wave during a moderately severe summer drought in the 1800s would typically be amplified by 2.7 to 3.6 degrees Fahrenheit (F) by the absence of cooling evaporation. In the 21st century, model runs showed heat waves during drought were amplified by 5.4 to 6.3 degrees F. 

“It’s important to understand that these stronger heat waves that the model generates are not from warming due to climate change, but an enhanced feedback between low soil moisture and high temperatures,” Hoerling said.  

If the background trend of increasing temperatures due to climate change is factored in, a compound heat wave-drought in the 21st century could end up being between 7.2 and 9.9 degrees F hotter than a similar drought in 1850. 

The authors caution that their results are based on a single climate model, and will need to be compared to results from other models running comparable scenarios, though one other recent study did find that compound drought-heat waves are becoming more frequent in parts of the U.S., including the semi-arid Southwest. They would also like to determine whether the sensitivity of compound drought-heat waves to climate change can be identified in historical weather records. 

The authors believe that this information will be important for officials seeking to better understand how climate change may expose vulnerabilities of human health, water supply, and agricultural productivity to more frequent and intense hot droughts.

“We need to better understand drought-heat wave interactions because of the risk that these events may become especially severe in regions that are already water-short,” said Hoerling.

This story was written by NOAA Communications.