Hotter Summer Days Mean More Sierra Nevada Wildfires, Study Finds

The hottest summer days in California’s Sierra Nevada dramatically increase the risk of wildfires igniting or spreading, and as the planet continues to warm, the risks will grow even more, scientists said Wednesday.

The research, which examined daily temperatures and data from nearly 450 fires in the Sierra Nevada from 2001 to 2020 and predicted future analysis, found that the number of fires could increase by about 20 percent or more by 2040, and that total fires burning could increase area at about 25 percent or more.

The findings “show how short events such as heat waves affect fires,” said Aurora Gutierrez, a researcher at the University of California Irvine and lead author of a paper describing the work in Science Advances. “We were able to determine that.”

As for the outlook for the next two decades, she said, “Our days are getting hotter and that’s why the risk of fires in the future increases.”

Wildfires are increasing in size and intensity in the western United States, and wildfire seasons are getting longer. California in particular has suffered in recent years, including last summer, when the Sierra Nevada experienced several major fires. One, Dixie Fire, burned nearly a million acres and was the largest single fire in the state’s history.

Recent research has indicated that the heat and drought associated with global warming are among the main reasons for the increase in larger and stronger fires.

The results of the new study are generally in line with that of previous research, but there is an important difference. Most previous studies looked at temperature and other data collected at monthly to annual time intervals. The new research looked at the daily data.

“What makes this novel is that we were trying to determine the role of individual temperature extremes on individual dates,” said Jim Randerson, senior author of the paper and professor of Earth systems science at the University of California, Irvine.

Over the past 20 years, researchers have found that a 1°C (1.8°F) increase in average summer temperature increased the risk of a fire on a given day — either by human activity or lightning — by 19 to 22 percent, and the area increased burnt by 22 to 25 percent.

Dr. Randerson gave an example of why very hot weather could lead to more fires and spread more easily.

“If it was a normal day, say 80 degrees Fahrenheit, and you accidentally made a spark and there was an ignition, you would probably step on it, or the local fire agencies could come in and put it out,” he said. Vegetation still contains a large amount of moisture, which must first be evaporated by the heat of the fire. This slows down the spread of the flame.

But on a day at 100 degrees, Dr. Randerson said, the plants are so dry, with so little moisture to evaporate, that the fire spreads quickly and grows.

“You get a rapid expansion, and eventually a fire so big that it can last for weeks and weeks,” he said.

John Apatzoglu, who studies the impact of climate change on wildfires at the University of California, Merced, said the work “adds to the growing scientific literature on the potential for climate-driven fires in the forests of the West.”

“The observed and predicted rise in temperatures is exacerbating pre-existing conditions, namely the accumulation of fuel in our forests, to escalate fire risks,” said Dr. Apatzoglu, who was not involved in the study.

The researchers used meteorological data, averaged over an area, and fire data from two sources: the California Department of Forestry and Fire Protection, which accurately records when fires broke out, and sensors on two NASA satellites that can measure the spread of fires on a daily basis.

For Ms. Gutierrez, who worked in Dr. Randerson’s lab during her undergraduate studies in Irvine and full-time there after earning her undergraduate degree in 2018, it meant wrangling over a deluge of data over the course of several months.

But, she said, researching the link between daily extremes and wildfires is worthwhile.

“We’ve decided that’s a question we need to ask,” Ms. Gutierrez said. “And yes, it’s a bit boring with the amount of data we have to process, but it’s an important question.”

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