Prescribed Burns Can Help Prevent Wildfires, but the Climate Crisis Is Making It Harder to Light Them Safely

Controlled burns or “prescribed fires” are used by trained firefighters to prevent wildfires in the Western United States by clearing away dried plant material that could potentially feed a natural, healthy fire.

Certain weather conditions are necessary for firefighters to execute controlled burns safely. They must be dry enough that fires can ignite, but not so windy or dry that more vegetation would catch fire than intended, a press release from University of California, Los Angeles (UCLA), said.

Daniel Swain, a UCLA climate scientist, led a new study that discovered that the times of year and overall number of days when firefighters can safely implement prescribed fires is being reduced by climate change.

The study, “Climate change is narrowing and shifting prescribed fire windows in western United States,” was published in the journal Communications Earth & Environment.

“Escalating wildfire activity in the western United States has accelerated adverse societal impacts… However, the intentional use of fire as a vegetation management tool, known as ‘prescribed fire,’ can reduce the risk of destructive fires and restore ecosystem resilience,” the study said. “Prescribed fire implementation is subject to multiple constraints, including the number of days characterized by weather and vegetation conditions conducive to achieving desired outcomes.”

Scientists’ current projection for planetary warming is two degrees Celsius by 2060, which Swain called “optimistic” considering the current trajectory, according to the press release. However, if the projections turned out to be correct, the number of days that would have favorable weather and vegetation conditions for prescribed fires would be reduced by an average of 17 percent across the Western U.S.

Most of the reductions in favorable conditions would be in the spring and summer, which is when most controlled burns are being done currently. The study also found that there would be a four percent increase in days that were favorable for prescribed fires in winter.

Kristen Shive, co-author of the findings and an expert on forests, prescribed fire and wildfires at University of California, Berkeley (UC Berkeley), said the prescriptions for specific burns vary by type of vegetation and location, but the target temperatures usually range from 50 to 70 degrees Fahrenheit with a relative humidity of 10 to 20 percent.

Swain said the agencies that carry out prescribed fires will have to adjust to climate change, since most of the seasonal workers who prepare for and manage controlled burns only work until mid-autumn.

“Global warming will reduce the number of favorable days for prescribed fires throughout the American West, but winter in particular may emerge as an increasingly favorable time for prescribed burns if the relevant policy and staffing changes can be made,” Swain said in the press release.

The study found regional variation as well. The changes were most noticeable in the southern and coastal regions of California, with many locations losing a month’s worth of days that were favorable for controlled burns annually. In the northern Rocky Mountains, however, there could be a small increase in days that are safe for prescribed burns.

“We’re just not doing a lot of prescribed fire compared to what is needed, at scale, to really address the escalating wildfire crisis,” Swain said.

The study compared climate conditions as well as vegetation dryness from 1980 to 2020 and 2020 to 2060.

Wildfires in the Western U.S. have increased markedly due to factors like human development in areas susceptible to fire and climate change, making controlled burns even more important.

“This paper is giving us advanced warning,” Shive said. “Hopefully we can change policies to either extend those folks or create winter-specific crews.”