Wildfires in the western US aren’t only growing bigger. They are also growing faster, putting millions of people and properties at greater risk.
“In the context of home destruction and lives lost, we really need to think more about fire speed than fire size,” says at the University of Colorado, Boulder.
Rapidly spreading wildfires, such as the Camp Fire in California in 2021 and the Lahaina fire in Hawaii last year, are especially destructive, essentially becoming unstoppable because they move faster than firefighters can combat them. However, while we can tell US wildfires are getting bigger by measuring the increased area they burn, it is harder to assess how their speed has changed.
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To address this, Balch and her colleagues used satellite data to estimate the speed of more than 60,000 fires in the US between 2001 and 2020, using daily growth rate as a proxy for this. They also aggregated millions of government documents and property records to estimate the damage caused by these fires and how much property is at risk of exposure to future ones.
They found fires in most of the western half of the country are growing faster: here, the average maximum daily growth rate more than doubled over the 20 years studied. Balch says this change is linked to climate change making hot, dry, windy “fire weather” more common, as well as a buildup in combustible material in forests.
In the northeastern US, however, fire growth rate has declined, which could have to do with fragmentation of forests by roads and other infrastructure, says Balch.
Linking fires with records of damage also allowed the researchers to determine a growth threshold at which fires were the most destructive. They dubbed those that grew more than 1620 hectares in a day “fast fires”. Though these accounted for just 2.7 per cent of all blazes, they were responsible for 89 per cent of the damage.
The findings suggest more resources should be spent on efforts to minimise damage from unstoppable fires, such as by fire-proofing homes or developing evacuation plans, says Balch. Over the period studied, the researchers found that fast fires came within 4 kilometres of 3 million buildings or other structures.
“They’ve assembled a very large and useful dataset, and they do highlight the role of extreme days,” says at the University of Alberta, Canada. However, he cautions that the growth rate metric they use doesn’t necessarily measure fire speed, because that depends on the size of a fire. “If you’ve got a million-hectare fire, growing a thousand hectares is nothing,” he says.
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