WASHINGTON — The number of wildfires burning in the Western United States each year dropped roughly 28% over the past three decades, even as annual burned area and damage from wildfires have soared. A decline in fires accidentally sparked by humans accounts for over 40% of the overall trend, according to a new study.
In areas with few people, fires get more frequent as population density rises. Push past a certain density, however, and fire frequency starts to fall. Understanding how shifting human demographics influence wildfire can help scientists more accurately predict how fire patterns will change going forward.
"It would be premature to talk about informing fire management [based on] these results, but the main implication is that we can incorporate these results into projections of future fire activity," said Gavin Madakumbura, an atmospheric and oceanic scientist at the University of California, Los Angeles, and lead author of the study.
The study will appear Thursday, April 30 in Earth's Future, AGU's journal for research on the state of the planet and its inhabitants and their future resilience in the Anthropocene.
More burning, fewer blazes
Wildfire impacts are mounting across much of the Western U.S. The rise in annual burned area from 1992 to 2020 amounts to an increase of about 4% each year. Nine million acres burned in 2020, more than the entire land area of the state of Maryland.
Because of all of this, people often assume the number of fires is increasing, too. But after examining a comprehensive dataset of fire occurrence in the Western U.S. including fire dates, locations, sizes and causes, Madakumbura and his coauthors uncovered a decline from over 25,000 fires per year in 1992 to about 18,000 per year in 2020 — equivalent to 305 fewer fires each year across the 11 continental western states.
Madakumbura said the rise in burned area is due largely to human-driven climate warming, which amplifies the hot, dry conditions that stoke fires to huge sizes. But how often fires start in the first place depends on more than just climate: everyday human activities and fire prevention measures play a role, too.
"We hear people saying the burned area has been increasing, fire damages have been increasing, fire frequency has been increasing. But…fire frequency is more complicated than that," Madakumbura said. "We wanted to take a stab at that with the best tools we have right now, to see if fire frequency is, in fact, increasing."
In many parts of the West, the team noted, fires started by humans declined more steeply than those from natural causes like lightning. The trend varied by region, however: human ignitions dropped significantly in California and Arizona, for instance, but rose in Wyoming.
To unravel the discrepancy, the team delved into population density data over the same period, as well as financial data from the U.S. Census Bureau showing fire protection and management expenditures at the state level. States with higher population densities, they noticed, spend disproportionately more on fire protection: California's average annual fire spending, for instance, outstrips Wyoming's by over seven billion dollars.
The findings align with something scientists call the "pyric transition." In sparsely populated regions, the idea goes, adding more people leads to more fires accidentally sparked by human activity. But past a certain threshold of population density, more people also means more coordinated fire prevention and public awareness efforts to tackle the rising risk. This, coupled with the fragmented, patchwork nature of more populated landscapes, can end up making fires less frequent.
While the correlation between population density and fire-prevention spending is strong, the team's findings don't absolutely prove that the former directly causes the latter. Fire prevention efforts may also arise in response to rising fire risks as climate change makes western landscapes more arid, Madakumbura said. In this sense, climate change could be triggering both a rise in fire frequency and, in places with sufficient resources, prevention efforts to push it back down. But Madakumbura suspects population density plays an active part as well.
"We know that with increasing human activity, we get more accidental human ignitions. But at the same time, a lot of regions are spending so much money on fire prevention and fire awareness, so we should see this in the data," he said. That made it extra satisfying, he added, to see the regional differences in fire frequency trends his team uncovered. "The data definitely seem to indicate that this is a possible causative relationship."
Not all good news
Exceptions do exist: urban hotspots including Los Angeles, Phoenix, and Denver appear to be seeing more frequent fires despite their high population densities. Madakumbura said this may owe partly to inconsistent reporting practices among local and regional fire agencies, with city- and county-level agencies recording more small, urban fires as their populations rise, while state and federal agencies focus more on wildland fires. The "pyric transition" may also not apply to every city or county individually, although it appears once you zoom out to the state and multi-state levels.
Unraveling the connection between human demographics and fire frequency, the team wrote, will help scientists gauge future fire patterns more accurately than if they considered only the impact of climate change. Previous research, they also noted, has shown that most fire damage to human communities comes from fires sparked by humans themselves, and that aggressive fire suppression efforts can cause wildfire fuels to build up over time, making fires more extreme when they do occur.
"Even though we have been able to reduce the number of fires through fire preparedness, fire awareness, and spending a lot of money on protective measures, there is a disconnect," Madakumbura said. "We haven't been able to reduce the damages."
Notes for journalists:
