The combination of drought conditions and heat waves, which can make wildfires more likely, is becoming increasingly common in the American West, according to a new study. The results may be predictably disastrous.

It has been well established that both droughts and heat waves have been occurring more frequently in recent decades. And while those conditions can cause damage singly, “their concurrence can be even more devastating,” the authors wrote.

What used to be a rare weather double whammy has been occurring more frequently in recent decades because of climate change, said Mojtaba Sadegh, an assistant professor of civil engineering at Boise State University and an author of the new study, published Wednesday in Science Advances. “These events, dry-hot events, are intensifying,” he said. “This is rising at an alarming rate.”

The new paper extends the historical weather record from the National Oceanographic and Atmospheric Administration back 122 years and examines heat and drought events that occurred across the contiguous United States.

The researchers found that the two phenomena feed each other in an intensifying cycle. They can also spread downwind, expanding drought and heat into broader areas like a storm front spreads rain, so “that self intensification will happen in a new region,” Dr. Sadegh said.

While the researchers expected that the records they examined would show an increase in the combined events, “we didn’t expect to see the increase at this magnitude.”

As a result of the warming that is characteristic of climate change, droughts such as the one California experienced this year can occur even when the amount of rainfall is not terribly low. And levels of drought and extreme heat that, in the absence of the warming brought about by climate change, might have been expected to recur once every 75 years hit the Northwest five or six times between 1993 and 2017.

While people generally look at climate phenomena within the context of what they have known in their lifetimes, “those norms, in terms of climate, might not mean anything from now on — or they might not have meant anything for a while, and we’re just now realizing it,” he said.

The new study also examined the combined drought and heat conditions that happened during the 1930s drought that led to the Dust Bowl. That national tragedy was driven largely by lack of rainfall, which led to the air becoming hotter, and poor land management practices that caused astonishing dust storms, the scientists said. But recent dry-hot disasters are driven more by excess heat than a lack of rainfall.

So the triggering mechanism for heat-drought events is shifting, according to the researchers, from lack of rain to excess heat. The authors conclude that “if meteorological droughts of the length and severity observed in the 1930s occur during the hot years that are increasingly common in recent decades due to global warming, their concurrence can have devastating impacts.”

What’s more, they wrote, no major region of the continental United States is immune to severe droughts. They warn that the increased heat makes megadroughts more likely.

Perhaps most chillingly, they cite research that warns that the heat from climate change will increase demand for water and lead to scarcity. , That could put strains on national infrastructure and society that, the scientists wrote, “ might push them to unprecedented states.”

A climate scientist who was not involved in the study, Daniel Swain of the University of California, Los Angeles, said the new research expands on previous work that focused on the West. The most important finding of the new paper, he suggested, was the increasing importance of heat as the driver of droughts.The warming global temperatures, he said, “are making it easier to achieve historically rare levels of dryness.”

Dr. Swain recently noted in a series of comments posted on Twitter that California, and possibly the Pacific Northwest as a whole, are likely to see another severe heat wave in early October, which could “bring extreme wildfire burning conditions once again.”

The new study, he said, does a good job of describing how drought and wildfire conditions spread and can occur over broad regions at the same time, making fire management much harder. “We are certainly seeing this with respect to wildfire in 2020,” he said.

Dr. Sadegh noted that the nation has done little to address the changing conditions and continues to build new homes in wildfire-prone areas, yet “every year there is a new record,” he said. With 3.6 million acres burned this year in California alone, and five of the six biggest wildfires in the state’s history occurring in 2020, the answer, he said, is to address the issue of climate change squarely.

“This is only getting worse,” he said, “until we act on it and reduce emissions.”





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