A new study finds that burning 500,000 acres of California conifer forests each year with prescribed fire could cut deadly pollution from wildfire smoke by roughly 10% over a decade.
That's slightly more than the state's goal for annual prescribed burns and about four times the area currently treated with prescribed burns across all California wildlands.
The study, published June 11 in Science by Stanford Doerr School of Sustainability researchers Iván Higuera-Mendieta and Marshall Burke , finds prescribed burns would initially raise smoke pollution, then deliver a net reduction as early as year four.
Using two decades of satellite data, the authors found that the risk of very severe fires immediately drops 92% in areas burned in low-severity fires, with risks remaining lower for up to a decade and extending as far as 5 kilometers (about 3 miles) beyond directly burned areas.
A double fire paradox
Smoke from wildfires increasingly threatens public health across the American West and beyond, exposing millions of people to toxic particles in plumes that can travel thousands of miles and blanket entire regions.
"Part of our wildfire problem, and the air quality problem it generates, is ironically the fact that we've been putting out fires for a long time," said Burke, a professor of environmental social sciences at Stanford and senior author of the study.
Scientists have long warned about this fire paradox, in which suppressing fires reduces smoke and damage in the short term, while allowing flammable dead wood, brush, grasses, and small trees to accumulate in forests evolved to burn regularly. Climate change multiplies the risk of catastrophic fires by drying out plants and extending the fire season.
Scientists say part of the solution is to bring back beneficial fire, deliberately igniting and managing fires under carefully chosen weather conditions to reduce built-up fuels.
"The key proposal is to reintroduce fires to the landscape – specifically, low-severity fire that replicates the fire regimes that we had 100 years ago before we started suppressing fires," Burke said. "But what scale of fire reintroduction do you need to actually move the needle on the overall wildfire and related air pollution problem?"
A central concern is that these controlled or "prescribed" burns also produce smoke, creating what the researchers call a new fire paradox. Land managers, policymakers, air quality regulators, and communities must decide how much pollution from prescribed burns is worth tolerating today in hopes of avoiding worse smoke from future wildfires.
Previous studies show treated patches often burn less severely in subsequent fires and deliver a net reduction in smoke pollution in the treated area. Burke said that was not enough to guide air quality and fire management decisions because a treated acre only pays off if wildfire returns. "You have to account for the fact that you don't know whether or when a wildfire is going to blow through or not," he said. "Our contribution is to put some numbers on these trade-offs."
Low-severity fire as a proxy for prescribed burns
Prescribed burning has been so limited in the U.S. West that there isn't enough historical data to study directly what happens when it's done at a large scale over many years. As a stand-in, the researchers used data from low-severity wildfires.
"We look across California as a whole over multiple decades and say, part of the problem is we just haven't been doing that much prescribed burning. It's a chicken and egg thing. How do we study prescribed burning at scale? Well, what is it trying to replicate? It's trying to replicate low-severity fire," said Higuera-Mendieta, a PhD student in Earth system science and a member of Burke's lab, and lead author on the study
The team built satellite-derived measures of fire severity for 98.9% of California wildfire events reported to the federal government's Monitoring Trends in Burn Severity program from 2000 to 2021. They paired these data with fire-specific estimates of smoke PM2.5, the fine particles that can penetrate deep into the lungs and enter the bloodstream, from 2006 to 2020.
"Our approach allows us to study decades of fires, burning in different landscapes under very different conditions," said Higuera-Mendieta. Focusing on fire severity, they measured how initial exposure to low-severity fire reduced subsequent extreme fire risk in areas directly burned in those fires and nearby unburned areas.
Where low-severity fire reduces severe wildfire risk
The study revealed that low-severity fires reduced future fire severity for at least a decade, both in places that initially burned and in unburned regions up to 5 kilometers away.
"You get large benefits in the place that you treated," Higuera-Mendieta said. "You get benefits that are maybe half as big but pretty meaningful in the nearby places that you didn't burn. And so those are sort of 'for free' from an air quality perspective, because you didn't have to burn them."
Results varied by vegetation type. In shrublands, the researchers found a reduction in wildfire risk in the first year after low-severity fire, but the effect faded within four years and did not clearly reduce the risk of very severe wildfires.
In conifer forests, by contrast, the effect was dramatic. These forests span much of the Sierra Nevada and parts of Northern California, where many of the state's largest and most damaging fires have burned in recent years – including the 2020 Creek Fire, one of more than 1,000 blazes in the study.
When do prescribed burns reduce wildfire smoke?
In simulations that accounted for reduced fire risk and air pollution beyond burned areas, the researchers estimated how adding more low-severity fire to California landscapes starting in 2010 might have changed the state's fire activity and smoke pollution over the subsequent decade. They simulated scenarios ranging from 125,000 acres to 1 million acres per year, both with and without spillover benefits for surrounding areas.
The results indicate that treating 500,000 acres per year would have increased PM2.5 smoke pollution by about 50% in early years, when wildfire activity happened to be low. The researchers estimate a sustained effort could have reduced cumulative smoke exposure as early as year four and cut it by about 10% over the full decade. The reductions would have been largest during 2020 and 2021, by far the worst smoke years on record in California. Lower treatment levels delivered lower, but still positive, benefits.
Even under high discount rates, which translate future costs and benefits into their present value, the modeling suggests air quality benefits can outweigh the cost of smoke from prescribed burns in conifer forests by a factor of about 5 to 1 over a decade. Benefits and costs rise roughly in line with the amount of area treated.
"On average, we estimate that each individual acre you treat has an impressive benefit-cost ratio, with the largest benefits in the worst wildfire years," said Burke. "The challenge is that you really have to treat a lot of acres to make a dent in the overall problem."
Study limits and next steps
The study does not directly estimate health impacts from the modeled smoke changes, such as how many asthma attacks, heart problems, or premature deaths might be avoided by shifting smoke from catastrophic wildfires to controlled burns. And it does not account for the direct financial costs of fuel treatments, which the authors cite as roughly $170 per acre in California, although costs vary widely depending on terrain and proximity to urban areas.
The study also does not evaluate alternative treatments, like mechanical thinning, which produce fewer emissions. Assessing those strategies will be important as the state considers different ways to meet its treatment goals, Higuera-Mendieta said.
The team is now working to connect the smoke findings with health outcomes using local air quality measurements, and looking to inform decisions about where to target prescribed burns.
"The simulations we run assume that treatments are not that targeted. We're saying, you have 20 million acres of conifers in California, we need to treat them all, we're going to just do it everywhere it's needed, such that by the end of the period, we've treated everywhere," Burke said. Ideally, he said, the places that are most likely to burn in wildfire and where prescribed burns are viable would be prioritized.
Burke is also a senior fellow at the Freeman Spogli Institute for International Studies, Stanford Woods Institute for the Environment, and Stanford Institute for Economic Policy Research; a professor (by courtesy) of Earth system science in the Stanford Doerr School of Sustainability; and a research fellow at the National Bureau of Economic Research.
This research was supported by the Keck Foundation and by a Stanford Data Science fellowship. The Stanford Research Computing Center provided computational resources and support.
