Wildfires are becoming faster and more dangerous in Western U.S.

The research highlights a critical gap in hazard preparedness across the U.S. -- National-level fire risk assessments do not account for fire speed or provide insight into how people and communities can better prepare for rapid fire growth events.

"We hear a lot about megafires because of their size, but if we want to protect our homes and communities, we really need to appreciate and prepare for how fast fires move," said Jennifer Balch, CIRES fellow, associate professor of Geography, and the lead author of the study. "Speed matters more for keeping people safe."

Balch and her colleagues were inspired to look closer at fire speed after the Marshall Fire, which destroyed more than 1,000 homes in Boulder County, Colorado, in December 2021. The fire burned less than 6,100 acres (24.7 square kilometers) but grew quickly due to a combination of dry conditions and high winds. Less than an hour after the fire was reported, it had spread to a town 3 miles (4.8 kilometers) away, eventually prompting the evacuation of tens of thousands of people. In the aftermath, Balch's team was eager to understand how fire growth rates impact fire risk across the country.

The researchers used satellite data to analyze the growth rates of over 60,000 fires in the contiguous U.S. from 2001-2020. Using a cutting-edge algorithm, which involves applying a set of calculations to each satellite pixel, they identified and recorded the perimeter of each fire for each day it was active.

"Until now, we had scattered information about fire speed," said Virginia Iglesias, interim director of Earth Lab and co-author of the study. "We harnessed Earth observations and remote sensing data to learn about fire growth across the nation in a systematic manner."

The team used the fire perimeter maps to calculate the growth rate of each fire as it progressed. They then zoomed in on the fastest fires, which grew more than 4,003 acres (16.2 square kilometers) in a single day, and probed how the highest growth rates changed over time. The analysis revealed a staggering 250 percent increase in the average maximum growth rate of the fastest fires over the last two decades in the Western U.S.

"Fires have gotten faster in the western U.S. in just a couple of decades," Balch said. "We need to focus on what we can do to prepare communities: hardening homes and making robust evacuation plans."

To evaluate the impacts of fast fires on people and infrastructure, the researchers compared the growth rates of the fastest fires to information recorded in incident reports about the number of structures damaged or destroyed per fire event. They found that fast fires accounted for 88 percent of the homes destroyed between 2001 and 2020 despite only representing 2.7 percent of fires in the record. Fires that damaged or destroyed more than 100 structures exhibited peak fire growth rates of more than 21,000 acres (85 square kilometers) in a single day.

"These results change how we think about wildfire risk because they position growth rate as a key determinant of a fire's destructive potential," Iglesias said.

The work also highlights a critical risk assessment gap. At the national level, wildfire risk models include parameters for area burned, intensity, severity, and probability of occurrence, but they do not incorporate growth rate or other measures of fire speed. Government agencies and insurance companies that use these models are therefore missing vital information about how fires spread, which homeowners could use to better protect themselves and their communities. The authors believe this needs to change.

"When it comes to safeguarding infrastructure and orchestrating efficient evacuations, the speed of a fire's growth is arguably more critical than its sheer size," Iglesias said.