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More Frequent Combined Droughts, Heatwaves Are Recipe for U.S. Megafires

Drought and heatwave combinations are becoming more frequent throughout the U.S. | Mojtaba Sadegh

The frequency of combined droughts and heatwaves — which together form a devastating climatic brew — 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 published in the September 25 issue of Science Advances.

While most parts of the contiguous U.S. experienced only one once-in-a-quarter-century compound dry-hot extreme event between 1943 and 1967 (as would be expected), most regions experienced between one and three of these combined events between 1968 and 1992. Between 1993 and 2017, however, several regions have experienced more than five of these events, with the Pacific Northwest, southern parts of the Southwest, Florida, and parts of the Northeast seeing the most dramatic increases.

The findings also suggest that areas experiencing compound dry-hot extremes are growing less scattered and more interconnected, 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, an assistant professor in the department of civil engineering at Boise State University in Idaho and the corresponding 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."

While the ongoing COVID-19 pandemic has made 2020 a hellish year for many people worldwide, the American West has been transformed into a literal inferno. Wildfires have ravaged more than 3.7 million acres in 11 states this year — a situation so out of control that fire officials are calling in the military and seeking international help to extinguish the blazes. California has experienced five of its 20 largest fires in history this season, and smoke from burning trees and other materials recently made Portland, Oregon home to the worst air quality in the world. On September 15, the city's air quality index score of 455 far surpassed notoriously smoggy Delhi, India. Anything over 300 is "actively hazardous."

On September 11, Portland resident Chelsea Dascher barricaded herself in her home to escape the noxious air.

"We're just trying not to let [the smoke] in the apartment," she said. "Our tiny AC unit seems to not be filtering the air well, and we realized that we had taped down the mail slot in our door, but it wasn't fully shut. We stuffed a rag in it and now it seems to be better in here."

"[My boyfriend] asked me how I protected my house from getting smoke inside when I was growing up in California," she added. "But this didn't happen when I was growing up."

Multiple extreme events such as 2020's combined Western U.S. drought and record-breaking heatwaves — which aligned with wind and ignition sources to spark hundreds of fires — rarely occurred at the same time in the past. But as the climate has warmed, this norm has changed.

"Traditionally, probability of multiple extremes co-occurring was quite low, so many of the traditional risk assessment frameworks were focused on separate hazards," said Sadegh. "However, since climate change has escalated the probability of multiple extremes happening simultaneously, a paradigm shift is required to address these emerging threats."

Most prior analyses that did look at concurrent events relied on post-1950s data, which tend to provide better spatial and temporal resolution but come with other limitations, Sadegh said.

"The last 50 years in the U.S. have been relatively wet in the context of long-term history, and focusing on recent data can underestimate potential for megadroughts," he said. "We wanted to provide a much broader perspective of how compound dry and hot events are evolving."

To achieve this broader perspective, Mohammad Reza Alizadeh, a Ph.D. candidate in the department of bioresource engineering at McGill University and the lead author of the study, and his colleagues analyzed combined dry-hot extremes in the U.S. from 1896 to 2017. These 122 years of ground observation-based climate data incorporated the 1930s megadrought that, along with inappropriate farming practices, led to the Dust Bowl phenomenon.

The researchers performed hazard scenario analyses to estimate the frequencies of compound dry-hot extremes over this time period, finding that these combined droughts and heatwaves have become increasingly common, progressively more intense, and ever more interconnected.

The findings also indicate that the climate factors driving concurrent droughts and heatwaves have shifted. Lack of precipitation was the dominant driver in the 1930s, while both low precipitation and excess heat contributed to compound events in the mid-2000s. Excess heat has primarily driven such events since 2010.

"We expected the background warming trend should play a role in driving concurrent dry and hot events," said Sadegh. "But its rates of intensifying and spatially expanding were certainly a surprise."

The authors suggest their findings may be used to bolster risk assessment frameworks and inform climate adaptation and mitigation efforts. Sadegh believes extensive measures must be taken to prepare for climate disasters on the horizon, including aggressive greenhouse gas emission cuts, improving irrigation systems, rethinking housing development projects in fire territory, instituting wildlife refuges to prevent ecological collapse, and bolstering watershed capacity by perhaps reintroducing beavers to North American river valleys.

"This study, and the 2020 fires that have burned and are burning the entire West Coast, show us that our infrastructure design frameworks may not be appropriate anymore in a warming climate," said Sadegh. "Our agriculture system is too vulnerable to dry-hot events, our housing codes are not proper for an era of megafires, and our wildlife is too fragmented and too stressed to tolerate a future of increasing extremes."

"We need to recognize the power of Mother Nature, and we need to respect it," he added. "We need to listen! The more we play deaf, the more Mother Nature will raise her voice."

[Credit for related image: Project LM/ Flickr]