As the Sun set in eastern Australia in late December 2019, a wildfire swept through the region’s eucalyptus forests like nothing scientists had seen before. Blazes typically weaken at night. But high winds, hot temperatures, and extremely dry air helped trigger a firestorm that sent plumes of ash 15 kilometers into the atmosphere. Over the course of 2 days, the fires generated energy equal to 2000 Hiroshima-scale nuclear explosions.
In the devastation, scientists saw our future: a world increasingly scorched by infernos that rage without pause through the night. Warmer, drier nights mean fires that might have once sputtered in the cool of the evening are now more likely to keep burning in the dark.
“Not only is the gas pedal on,” says University of Colorado, Boulder, wildfire scientist Jennifer Balch, “but the brakes are also off.”
Firefighters often rely on milder night weather to make wildfires “lie down.” Winds slow, temperatures fall, and humidity rises. But Balch had been hearing that nights weren’t as restful as they once were. She experienced it firsthand while managing controlled burns of large plots in the Brazilian Amazon to see how jungles responded to repeated fires. During a drought there in 2007, she noticed fires burned through the night, rather than going out like they did in other years.
To see whether these trends were true elsewhere, Balch turned to the European Union’s Copernicus Climate Change Service, which has released estimates of hour-by-hour weather conditions around the world dating back to 1950. She and her U.S.-based collaborators combined the weather data with satellite tracking of wildfires to see how nighttime fire behavior has changed over the years. They zeroed in on a key variable: how humid the air is, compared with how much moisture it could possibly hold at a given temperature. This gap, known as the vapor pressure deficit, is a good gauge of how dry and flammable grasses and pine needles are–much as a wet towel dries faster in hot, dry weather.
The researchers looked at weather conditions during nearly 82,000 nighttime wildfires in landscapes ranging from savannas to tropical jungles to evergreen forests. They identified the combination of humidity and temperature that—in 19 out of 20 blazes—coincided with fires not surviving the night. The researchers then measured how often the weather in these ecosystems went above the limit, creating conditions in which a fire is more likely to keep burning.
In many parts of the world, “burn nights” are increasing. The researchers found that fire-prone ecosystems had five fewer nights per year, on average, of mild, fire-squelching conditions from 1979 to 2020, a 15% decrease. A statistically significant decline in such nights happened in 59% of burnable lands worldwide.
Some of the biggest decreases in mild night weather came in grasslands and savannas in equatorial South America and Africa, grasslands across Asia, and Australia shrublands. Western U.S. forests lost an average of 11 mild days, a 45% decline, the scientists report today in Nature.
At the same time, night fires on a global scale became more intense. The amount of energy released from an average fire burning during the evening in a given year—calculated from satellite imagery—increased by 7% between 2003 and 2020, the researchers found.
The nightly fluctuations in weather are an important factor in fire behavior, but they aren’t “the only story line” for what’s influencing nighttime fires, says Mike Flannigan, a wildfire scientist at Thompson Rivers University. His research into nighttime fire behavior matches the new findings about more frequent and intense fires. But changes in wind behavior play a crucial role, he adds. So does the dryness of fuel, such as downed trees, which is increasing alongside sustained droughts, such as the record-setting one now afflicting parts of the western United States. “The intensity we’re seeing at nighttime is more likely due to a prolonged drying period,” Flannigan says.
The new findings aren’t a surprise, given the experiences of firefighters and unprecedented events like the Australian fires of 2 years ago, says David Bowman, a leader in pyrogeography—the study of how fires move over landscapes—at the University of Tasmania. But the new work does an “elegant” job of quantifying the magnitude of the change at a global scale, he says. It also highlights some of the challenges firefighters will face as nightfall brings less respite. Nighttime can be a time to rest and recover, as the fire slows. Now, “We’re going to have to be thinking about a 24/7 model of firefighting,” Bowman says.