Fire speed, not size, drives threat to people, infrastructure

"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.