The speed of the fire, not the size, determines the danger to people and infrastructure

Fast-growing fires accounted for nearly 90 percent of fire-related damage, despite being relatively rare in the United States between 2001 and 2020, according to a new study conducted by CU Boulder. “Fast fires,” in which embers are thrown into the air before rapidly advancing flames, can set homes ablaze before emergency responders can respond. The work was published today in Scienceshows that these fires are growing faster in the western United States, increasing the risk to millions of people.

The study highlights a critical gap in hazard preparedness across the U.S.: National-level fire risk assessments do not consider fire speed or provide insights 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 recognize how quickly fires spread and prepare for them,” said Jennifer Balch, a CIRES fellow and associate professor of geography at the Lead author of the study: “Speed ​​is more important for people’s safety.”

Balch and her colleagues were inspired to study fire speed in more detail 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 strong winds. Less than an hour after the fire was reported, it had spread to a town 3 miles (4.8 kilometers) away and ultimately resulted in the evacuation of tens of thousands of people. Balch’s team was then keen to understand how fire growth rates affected fire risk across the country.

Researchers used satellite data to analyze the growth rates of over 60,000 fires in the contiguous United States from 2001 to 2020. Using a state-of-the-art algorithm that applies a series of calculations to each satellite pixel, they identified and recorded the extent of each fire for each day it was active.

“Until now, we have had scattered information about the rate of fire,” said Virginia Iglesias, interim director of the Earth Lab and co-author of the study. “We used Earth observations and remote sensing data to systematically learn more about fire growth across the country.”

The team used the fire edge maps to calculate the growth rate of each individual fire as it progressed. They then examined the fastest-growing fires, which grew over 4,003 acres (16.2 square kilometers) in a single day, and examined how the highest growth rates changed over time. The analysis found a staggering 250 percent increase in the average maximum growth rate of the fastest fires over the past two decades in the western United States

“In the western United States, the number of fires has increased more rapidly in just a few decades,” Balch said. “We need to focus on what we can do to prepare communities: securing homes and creating robust evacuation plans.”

To assess the impact of fast fires on people and infrastructure, researchers compared the growth rates of the fastest fires with information from incident reports about the number of buildings damaged or destroyed per fire event. They found that rapid fires were responsible for 88 percent of destroyed homes between 2001 and 2020, although they only accounted for 2.7 percent of recorded fires. Fires that damaged or destroyed more than 100 structures experienced a maximum fire growth rate of more than 21,000 acres (85 square kilometers) in a single day.

“These results change the way we think about wildfire risk because they position growth rate as a key factor in a fire’s destructive potential,” Iglesias said.

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

“When it comes to protecting infrastructure and organizing efficient evacuations, the speed at which a fire spreads is arguably more important than its sheer size,” Iglesias said.