People are starting almost all the wildfires that threaten U.S. homes, according to an innovative new analysis combining housing and wildfire data. Through activities like debris burning, equipment use and arson, humans were responsible for igniting 97 percent of home-threatening wildfires, a CU Boulder-led team reported this week in the journal Fire. 

Moreover, one million homes sat within the boundaries of wildfires in the last 24 years, the team found. That’s five times previous estimates, which did not consider the damage done and threatened by small fires. Nearly 59 million more homes in the wildland-urban interface lay within a kilometer of fires.

“We have vastly underestimated the wildfire risk to our homes,” said lead author Nathan Mietkiewicz, who led the research as a postdoc in Earth Lab, part of CIRES at the University of Colorado Boulder. “We’ve been living with wildfire risk that we haven’t fully understood.” 

To better understand wildfire trends in the United States, Mietkiewicz, now an analyst at the National Ecological Observatory Network, and his colleagues dug into 1.6 million government spatial records of wildfire ignition between 1992 and 2015; Earth Lab’s own compilation of 120,000 incident reports; and 200 million housing records from a real estate database from Zillow. 

Among their findings: 

• Humans caused 97 percent of all wildfires in the wildland-urban interface, 85 percent of all wildfires in “very-low-density housing” areas, and 59 percent of all wildfires in wildlands between 1992 and 2015.
• Human-started wildfires are expensive, eating up about one-third of all fire fighting costs.
• Overall, about half of fire suppression costs were related to protecting houses in all locations: the wildland-urban interface, low-density housing areas, and elsewhere.
• Most human-caused wildfires were relatively small (<4 km2) but were responsible for most homes threatened (92 percent). 
• The wildland-urban interface or “WUI,” represented only 10 percent of U.S. land in 2010, but was the site of 32 percent of all wildfire ignitions. 
• The WUI is also expanding our vulnerability: between 1992 and 2015, we built 32 million new homes in the WUI. 

“Our fire problem is not going away anytime soon,” said co-author Jennifer Balch, director of Earth Lab, CIRES Fellow and associate professor of geography. It’s not just that we’re building more homes in the line of fire, she said, but climate change is creating warmer, drier conditions that make communities more vulnerable to wildfire. 

The new study, she said, does provide guidance for policy makers. “This provides greater justification that prescribed burns, where safe, can mitigate the risk and threat of future wildfires,” Balch said. And we need to construct more fireproof homes in these beautiful, but flammable landscapes, she added. “We essentially need to build better and burn better.”

“Smokey Bear needs to move to the suburbs,” Mietkiewicz concluded. “If we can reduce the number of human-caused ignitions, we will also reduce the amount of homes threatened by wildfires.”

Authors of “In the Line of Fire: Consequences of Human-Ignited Wildfires to Homes in the U.S. (1992–2015),” published in Fire this week are Nathan Mietkiewicz (Earth Lab/CIRES/CU Boulder and NEON), Jennifer K. Balch (Earth Lab/CIRES/CU Boulder), Tania Schoennagel (CU Boulder), Stefan Leyk (Earth Lab/CIRES/CU Boulder), Lise A. St. Denis (Earth Lab/CIRES/CU Boulder) and Bethany A. Bradley (University of Massachusetts-Amherst) 

This work was funded by the University of Colorado Boulder’s Grand Challenge Initiative and also received support from the Humans, Disasters, and the Built Environment program of the NSF, Award Number 1924670.