Wildfire response to changing daily temperature extremes in California’s Sierra Nevada

Study key findings & significance

• A rise of 1.8°F (1°C)  in mean summer temperature over the past 20 years increased the risk of a fire starting on a given day — either by human activity or a lightning strike — by 19 to 22 percent, and increased the burned area by 22 to 25 percent.
• The research examined daily temperatures and data from nearly 450 Sierra Nevada fires from 2001 to 2020 and projected the analysis into the future.
• The number of fires could increase by about 20 percent or more by the 2040s, and the total burned area could increase by about 25 percent or more.
• The work adds to the growing scientific literature of climate-driven fire potential in forests of the West.

Author quotes

“[The findings] show how short events like heat waves impact fires. We were able to quantify that.”

Aurora Gutierrez, lead author and researcher at the University of California Irvine

“What makes this novel is that we were trying to identify the role of individual temperature extremes on individual dates."

Jim Randerson, senior author on the paper and a UC Irvine professor of earth systems science

“If it’s a normal day, say 80 degrees Fahrenheit, and you accidentally create a spark and there’s an ignition, you can probably stomp on it, or local fire agencies can come and put it out. The vegetation still contains a significant amount of moisture that the heat from the fire must evaporate first. That slows the spread of flames. But on a 100-degree day, the vegetation is so dry, with so little moisture to evaporate, that a fire spreads quickly, and grows. You get rapid expansion, and eventually a fire so big it can last for weeks and weeks.”

Jim Randerson, senior author on the paper and a UC Irvine professor of earth systems science

Abstract

Burned area has increased across California, especially in the Sierra Nevada range. Recent fires there have had devastating social, economic, and ecosystem impacts. To understand the consequences of new extremes in fire weather, here we quantify the sensitivity of wildfire occurrence and burned area in the Sierra Nevada to daily meteorological variables during 2001–2020. We find that the likelihood of fire occurrence increases nonlinearly with daily temperature during summer, with a 1°C increase yielding a 19 to 22% increase in risk. Area burned has a similar, nonlinear sensitivity, with 1°C of warming yielding a 22 to 25% increase in risk. Solely considering changes in summer daily temperatures from climate model projections, we estimate that by the 2040s, fire number will increase by 51 ± 32%, and burned area will increase by 59 ± 33%. These trends highlight the threat posed to fire management by hotter and drier summers.