Fast-growing fires were responsible for nearly 90% of fire-related damages despite being relatively rare in the United States between 2001-2020, according to a new study.
"Fast fires," which thrust embers into the air ahead of rapidly advancing flames, can ignite homes before emergency responders can intervene. The study, published recently in Science, shows these fires are getting faster in the Western U.S., increasing the risk for millions of people.
"In California, we've been transfixed by so-called megafires because of their massive size, but it turns out that the most destructive fires are ones that grow so fast they can't be stopped," said Professor Crystal Kolden, director of the UC Merced Fire Resilience Center and a co-author of the study. "Fast fires are the ones that destroy homes and lives."
Kolden said the deadliest wildfire in California history - the Camp Fire, which killed 85 and destroyed the Sierra foothill town of Paradise, displacing 50,000 - was a fast fire.
The research, led by the University of Colorado, Boulder, highlights a critical gap in hazard preparedness across the United States: National-level fire risk assessments do not account for fire speed or describe how people and communities can better protect against rapid fire growth events.
The researchers used satellite data to analyze the growth rates of more than 60,000 fires in the contiguous U.S. from 2001-2020. Using a cutting-edge algorithm that applies a set of calculations to each satellite pixel, they identified and recorded the perimeter of each fire for each day it was active.
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% increase in the average maximum growth rate of the fastest fires over the past two decades in the Western U.S.
In California, the trend was even higher, with a 400% increase in the growth rate of fast fires.
"We already knew that more and more fires were demonstrating explosive, unstoppable growth, both during the fall when we get offshore winds and during the summer as climate change is increasing the frequency of extreme heat events," Kolden said.
"This research highlights that these fast fires are causing the majority of home losses and fatalities and highlights how important it is to increase mitigation activities that we know are effective, such as defensible space and home hardening."
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% of the homes destroyed between 2001 and 2020 despite only representing 2.7% of recorded fires. 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.
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, butdo 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.
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