The frequency of combined droughts and heatwaves - which are more devastating when they occur in unison - has substantially increased across the western U.S. and in parts of the Northeast and Southeast over the past 50 years, according to a new study. The findings also suggest areas that experience compound dry-hot extremes are growing less scattered and more connected, resulting in larger impacted regions that place enormous strain on regional and national relief efforts. "Episodes of extreme dryness and heat are the recipe for large forest fires," said Mojtaba Sadegh, the senior author of the study. "These extremes are intensifying and extending at unprecedented spatial scales, allowing current wildfires to burn across the entire U.S. west coast." Climate risk analyses have typically focused on shifts in one climate parameter at a time, such as changes in heatwave magnitude or trends in aridity. But while multiple extreme events rarely occurred at the same time in the past, they have begun to coincide more often as climate change progresses. To better understand how the frequency of concurrent droughts and heatwaves has changed over time in the contiguous U.S., Mohammad Reza Alizadeh and colleagues analyzed combined dry-hot extremes using 122 years of climate data based on ground observations. While most prior analyses of these concurrent events rely on post-1950s data, the researchers extended their analysis to cover the years 1896-2017, incorporating the 1930s megadrought that, combined with inappropriate farming practices, led to the Dust Bowl phenomenon. The findings indicate that the climate factors driving concurrent droughts and heatwaves has shifted from lack of precipitation in the 1930s to excess heat in recent decades. The authors suggest their findings may be used to bolster risk assessment frameworks and inform climate adaptation and mitigation efforts.
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