Heat Waves Making Wildfires Worse in the Western United States

Key points:

• Researchers found that heat waves are a critical enabler and driver of wildfire-burned areas across the western U.S.

• The findings suggest that heat waves contribute not only to increased flammability, but to longer burn periods and increased lightning.

• Burned area in forests during the summer more than doubled between 2001 and 2024. Most of this recent increase - 64 percent - occurred during heat waves.

Heat waves are associated with a disproportionate share of wildfire activity and trends in the western U.S. in recent decades, UC Merced researchers found in a new study.

The study, published Wednesday in Science Advances, analyzed links between heat waves in both wildfire occurrence and growth across the western United States. It was co-written by a team of postdoctoral researchers, including Dimitri Kalashnikov, Emily Williams and Cong Yin, working under management of complex systems Professor John Abatzoglou. Four of the researchers also discuss their findings in a piece for The Conversation. 

While overall impacts of heat waves on human health, infrastructure and ecosystems have been extensively studied, the connection between heat waves and wildfire activity remains relatively underexplored, the researchers said.

They found that periods of intense heat fuel significant increases in how much area burns from wildfires as meteorological and fuel flammability conditions promote new ignitions and exacerbate ongoing fire activity. And in most regions, these effects persist even after the heat wave ends.

"I think we need increased awareness that heat waves aren't simply a nuisance or an opportunity to escape to the mountains to enjoy outdoor recreation," Kalashnikov said. "Our study shows that heat waves are in fact a major driver of wildfire activity across the western United States, and these impacts could potentially increase as summers continue to get hotter."

The area of forest land burned in the western United States increased 2.5-fold since 2001. Roughly 64 percent of that increase coincided with heat waves. There were not similar increases in non-forested areas, the researchers found, as fire activity in those regions is closely linked to availability of fuels rather than hot weather.

While periods consisting of heat waves and the five days immediately after them represent only 12 to 15 percent of warm-season days, they accounted for roughly 42 percent of total burned area between May and October in 2001-2024.

The growing influence of heat waves in shaping burned area in western United States forests has important implications for fire management and public health and can improve predictions of wildfire risk, they wrote.

Their results suggest several processes through which heat waves serve as both catalysts and enablers of fire activity. However, the study found that not all heat waves have the same impact on fire activity. A moist heat wave - accompanied by rain or higher humidity - does not increase forest fire activity as much as drier conditions do.

Dry heat waves desiccate fuels and increase flammability. Those conditions remain for long periods after the heat ends. And dry lightning that often accompanies heat waves can also increase ignitions. At the same time, human-caused wildfires also increase during heat waves, especially in the Northwest, likely because the conditions are ripe for a flame from an accidental or negligent source - such as a towing chain sparking on a road, careless use of fireworks or escaped campfires.

"Climate impacts aren't neatly siloed," Williams said "They are messy, they interact and exacerbate each other. We all know that heat waves mean dangerous heat stress for people. But here we show that heat waves can also be dangerous in another way - by increasing the risk of large fires. And more large fires means a heavier smoke burden, meaning communities aren't just sweltering, but are sweltering in the smoke."

"We know people in the Western United States are suffering from both heat waves and wildfires," Yin said. "These compounding climate threats and their interactions pose escalating risks to our daily life. For example, increasing medical stress from heat- and smoke-related illnesses."

The report outlines several precautions that could go a long way toward mitigating some of these impacts.

Presently, heat waves are not directly linked to fire early warning systems or warnings that are issued when conditions merit, but could be, the researchers said. Explicitly accounting for heat waves could also help people take more precautions during periods of high fire danger, and fire-suppression resources could be placed on alert in areas where a heat wave is imminent.