Can Controlled Burns Reduce California’s Air Pollution?

His research, published in the June 11 issue of Science, found that a sustained campaign of yearly prescribed burning — in line with pre-existing state goals — could reduce smoke severity during bad wildfire years by 25%. Averaged over a decade of good, normal and bad fire years, the net reduction in smoke pollution is about 10%.

The study, conducted with co-author Marshall Burke, examined two decades of fire and smoke data and provides the first large-scale estimate of how prescribed-fire-like burns influence future smoke exposure.

“We know that smoke is killing a lot of people. Any reduction in that is meaningful,” said Burke, professor in the Doerr School of Sustainability at Stanford and Higuera-Mendieta’s advisor.

Prescribed fire, or the intentional and cautious burning of land at low severity, is a smoke solution that comes with trade-offs. A prescribed burn produces its own smoke, so the net benefits to air pollution are felt during the next wildfire, which may be soon or may be years away.

“You’re not eliminating the problem,” Burke said. “But you’re making a meaningful dent in the problem, particularly in the worst years, and to me, that’s important.”

In past research, Burke’s group at Stanford has estimated that wildfire smoke will be the key way most Californians will feel the harmful effects of climate change. A drying, heating atmosphere, caused primarily by the burning of fossil fuel, is leading us to more severe wildfires, which are reversing decades of air quality improvements in California and across the West.

Wildfire smoke will kill an estimated 70,000 Americans each year by 2050, if the planet continues to warm at its current rate, according to research Burke and colleagues published last September.

California has set a goal of treating half a million acres annually with low-severity fire. The state is about 20% of the way towards that goal, depending on how you count the progress. But such fires don’t have unalloyed support from the public. Some people complain about the smoke they inevitably produce.

One of the implications of Burke and Higuera-Mendieta’s research is that policymakers who support prescribed fires should emphasize educating the public about the delayed benefits.

“A message to all of us living in California that we are probably going to have to tolerate some low-severity smoke during parts of the year when we’re not used to seeing it,” Burke said.

“We’ve been putting out fires for a century. This has caused a big problem. We’ve got a warming climate, and these put us in a world of hard trade-offs. We just have to be clear-eyed about those trade-offs.”

For the study, researchers mined decades of satellite readings, from data sources like the U.S. Forest Service, U.S. national parks and the National Center for Atmospheric Research. Working in such a data-rich environment stood out to Higuera-Mendieta as one of the best things about studying a scientific problem in the U.S.

“I wish people knew that part of America’s greatness is this investment in all the open data,” he said.

Building on earlier work done by Burke’s group, they used models to trace smoke detected by satellites to the originating fire. They could then simulate how much less smoke those fires would have produced if they’d been less severe because there was a history of prescribed fire in the area. They used natural low-severity fire as a proxy for prescribed fire.

A parallel, but not novel, finding was that low-severity fire reduced the chance of very severe fires in the area by more than 90%, with reductions trailing off over a decade. This finding is backed by a large body of pre-existing research.

One interesting wrinkle was that the greatest benefits were found in conifer forests, the site of many of the most severe fires, but not all.

“But if you think of the L.A. fires,” Higuera-Mendieta said, “that’s a completely different animal.”

In Southern California’s brushy, chaparral landscapes, low-severity fires were not very protective from future fires and future smoke.

“We need to find solutions for that second half [of the state],” Higuera-Mendieta said. “I think that opens new research avenues for me, asking what interventions work for Southern California.”