It's no secret that destructive wildfires have proliferated across the West in recent years.
But new research shows the number of structures destroyed by wildfires has grown significantly more than fires themselves have increased — and humans are to blame.
In the Western U.S., 30% more land burned in wildfires in the 11 years from 2010 to 2020 than burned in the previous 11 years, 1999–2009. But between those same two periods, the number of structures burned by wildfires increased nearly 250%. So, while wildfires burned somewhat more land, they burned a whole lot more homes and outbuildings. That meant that the structure-loss rate, or the average of how many structures were lost per area burned, increased from 1.3 structures per 4 square miles burned to 3.4 structures per 4 square miles burned. That outpaced not just the increase in wildfires in the West, but also the 40% increase in homes.
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Human-caused fires drove the accelerated destruction of property: Three-quarters of the fires that destroyed structures were started by humans.
The revelation was the product of a study published Wednesday in the Proceedings of the National Academy of Science-Nexus. University of Montana Professor of Fire Ecology Philip Higuera was the lead author. The study was the result of Higuera's visiting fellowship at the University of Colorado Boulder. CU Professor Jennifer Balch, doctoral student Maxwell Cook and Director of Earth Lab Analytics Hub Natasha Stavros co-authored the study.
In a phone call Friday, Higuera said the team zeroed in on the changing rate of property destruction because structure loss is one of the "most acute" ways in which wildfires hurt humans: "We are seeing increasingly negative impacts on humans. Wildfires are increasingly causing human suffering."
He said the team knew from previous work that most structures burned in wildfires are lost to fires started by humans. But they wanted to understand how the phenomenon varied regionally, such as by state. And they wanted to see if there was a detectable trend from one decade to the next.
The study looked at 15,001 western U.S. wildfires from 1999 through 2020. In that time, the team found, 62% of all structures lost were burned in three exceptionally brutal years for destructive wildfires in the West: 2017, 2018 and 2020. And one state – California – accounted for 77% of the 85,014 structures burned in those 22 years.
"It's dominated by California, but it's not just California," Higuera said. "The direction that every western state was going in all points the same way," of increased structure loss. "Those are some sobering findings that I think should be alarming to us and they should help motivate us to be proactive and acknowledge that, with growth and expansion, we need to be very conscious of the flammability of the environment that we live in and the fact that that is increasing as climate is changing."
Only New Mexico and Arizona saw decreases in the number of structures lost per area burned. States with high percentages of undeveloped or public lands and relatively small populations — like Montana, Wyoming and Idaho – accounted for significantly smaller portions of structure losses.
Although structure losses in Montana increased alongside other Western states, the growth was not as dramatic. In Montana, the number of structures lost to wildfire increased 36% between the two spans of time in the study. The structure-loss rate – the number of structures lost per area burned – in Montana increased 44%. While less than some other states' increases, Higuera said it was still significant.
"We do live in a state still where only 11% of the fires resulted in home and structure loss," he said. "The majority of fires do not destroy homes and structures. That's even more true in Montana."
Human causes, solutions
Across the West, only 12% of fires burned structures, the study found. The team determined that was at least partly because lightning-caused fires, which often occur in remote areas with few or no structures, accounted for 65% of the area burned.
But 76% of structures burned were destroyed in human-caused fires. The increase in structure losses corresponded with a 51% increase in human caused wildfires between 1999–2009 and 2010–2020. Human-caused fires often occur around developed areas, and on average destroy more than 10 times the number of structures per square mile burned than lightning-caused fires.
Because most structures are burned in human-caused fires, Higuera said, "it really misguides us" to think of wildfire only in terms of overall acres burned. In terms of impacts to structures, a more telling measurement is the acres burned by human-caused fires, because those are the fires most likely to destroy structures.
Climate change plays a role, too, the team found, by extending fire season in the West well beyond early fall. As the West warms and dries, arid and windy conditions ripe for wildfire can now last through December and begin earlier in spring. That means that more times of the year without lightning now offer conditions primed for human-caused wildfire.
But because most structures are destroyed in an increasing number of human-caused fires, Higuera said, humans have the ability to affect the trend the team discovered.
"The flip-side of that is that gives us a big lever we can change pretty immediately, or at least quicker," he said. "We need to mitigate climate change, but it's not like that's going to happen in five years."
Solutions could include more careful planning around development in fire-prone landscape, the team advised, and improvements to structures' fire resistance. Improving landscape resilience to severe wildfire could also help, particularly around development. And people need to be more conscious throughout the year of activities that can start wildfires.
"It is amplifying Smokey Bear's message," Higuera said. "The key message of this age, is we need to not be thinking about Smokey Bear just in the summer."