Nighttime wildfires growing in frequency, intensity: study

Nocturnal blazes have grown more intense and frequent in recent decades as hot, arid nighttime weather becomes the norm across the western U.S., a new study has found.

While such “flammable nights” were a rarity just 40 years ago — with cool, moist nights offering firefighters some respite — the changing climate has caused nights to warm faster than days, according to the study, published on Wednesday in Nature. There are now 11 more flammable nights each year in the West than there were in 1979, equivalent to a 45 percent surge, the authors found.

“Night is the critical time for slowing a speeding fire — and wildfire’s night brakes are failing,” Jennifer Balch, lead author and professor of geography at University of Colorado Boulder, said in a statement. 

Balch’s team at the Cooperative Institute for Research in Environmental Sciences’s Earth Lab conducted their assessment using a key measurement of the atmosphere’s thirst called the vapor pressure deficit.

When that deficit is relatively low, cool and moist air conditions prevent fires from thriving and enable firefighters to extinguish the flames. But if the deficit is high, the resultant hot and dry conditions create a parched environment that is ideal for burning, according to the study.

The researchers analyzed satellite observations and hourly climate data for 81,000 global fires to identify the precise vapor pressure deficit at which point conditions are hot and dry enough to create a flammable night.

Over the past 40 years, they found that burn nights had grown by a full week — a 36 percent surge — in a fifth of ignitable lands around the world. In comparison, the planet’s daytime flammable hours grew by only 27 percent during these same four decades.

The increase in burnable nights was much more dramatic in U.S. West, which saw an additional 11 nights, or a 45 percent rise, according to the study.

In the 2003-2020 range alone, night fires have become 7.2 percent more intense across the globe, while in the western U.S., these blazes have become 28 percent more intense, the authors found.

They also observed that certain ecosystems were particularly vulnerable, including evergreen and broadleaf forests, as well as scrublands and grasslands.

Human-induced climate change has caused even more heating at night than during the day over the past 70 years, the scientists noted, warning that this trend only going to continue to accelerate. The persistence of such climate conditions, according to the study, will “promote more intense, longer-lasting and larger fires.”

Given the recently destructive winter fires in region — including ones in midwestern Kansas and Boulder County, Colo. — the research team stressed the importance of elucidating the climate-related drivers of wildfire.

As scientists continue to track nighttime fire behavior over the next decade and beyond, the authors also emphasized a need for openly accessible drone-, airborne- and space-based fire observations and fine enough resolutions to allow for the reconstruction of fire spread.

“With continued nighttime warming, we expect to see more runaway wildfires that are more intense, faster, and larger,” Balch said. “That means firefighters don’t get the breaks at night they used to get — they have to battle flames 24/7.”