Nighttime fires have become more intense and more frequent in recent decades, as hot, dry nights are more commonplace, according to a new study. A key factor—changes in Vapor Pressure Deficit (VPD), which essentially indicates how fast the atmosphere is sucking moisture out of vegetation and fuels. A lower deficit signifies cool and moist air, while a high deficit means the air is hot and dry and conducive to burning. The study team found the daily minimum vapor pressure deficit increased by 25 percent from 1970 to 2020. Forty years ago, cool, moist nights regularly provided relief to firefighters -- and "flammable nights" were rare. Now, due to a changing climate, nights are warming faster than days are, and there are 11 more flammable nights every year in the U.S. West -- a 45 percent spike, the team found. The team also used a novel remote sensing and modeling technique to evaluate fire progression hourly for tens of thousands of fire events. And they found globally, night fires have become 7.2 percent more intense from 2003-2020. In the U.S. West, that number was much higher at 28 percent.
Wildfires are becoming more intense at night and lasting longer, study finds
Nighttime-fire intensity in the U.S. West has increased by 28 percent over the past two decades.
When the Cameron Peak Fire ignited in northern Colorado in August 2020, few could foresee its longevity. As it burned, summer turned into winter. Nearly a semester of school passed. By the time the fire was fully contained in December, it had become the state’s largest on record.

In recent decades, wildfires have become more intense and longer lasting amid rising temperatures linked to human-caused climate change. A key influence on their growing duration? Their increasing ability to survive the night, when temperatures typically dip and humidity rises.

A study published Wednesday in Nature shows that a trend toward warmer and drier conditions after sundown is helping blazes withstand what should be unfavorable conditions — making fire containment more difficult for responders. Crews are less able to rely on relief in fire intensity previously offered by nighttime cooling.

“The fact that the [Cameron Peak] fire was burning for months, to me is an indication that we were essentially able to pass through the night,” said Jennifer Balch, the director of Earth Lab at the University of Colorado at Boulder. “We were able to burn from day to night, from day to night, and that fire burned over 200,000 acres.”

Cameron Peak isn’t the only notable example of nighttime fire growth. During Australia’s 2019-2020 bush fire season, fires seemed to spread more rapidly at night than during the day for the Snowy Complex Fire in the southeast of the country. In fact, about three-quarters of satellite fire detections occurred at night.

In 2017, the Tubbs Fire in Northern California burned across 36,000 acres and became the most destructive fire in the state at that time. Satellite data showed more than half of the fire detections occurred at night.
Across burnable lands globally, the annual number of flammable nighttime hours increased by 110 hours over the past four decades — allowing five additional nights when flammability does not cease, the study stated. It also found that nighttime fires globally increased in intensity by 7 percent from 2003 to 2020.

“Our nights have been warming more than our days have been warming as a function of human-caused climate change, and that’s having a direct impact on fires,” said Balch, the lead author of the study. “We’re losing the brakes on fires in terms of the cooling and moisture accumulation that happens at night.”
Balch and her colleagues analyzed satellite and hourly climate data for more than 81,000 fires globally to pinpoint when conditions become hot and dry enough for fires to burn at night.

Specifically, the team measured vapor pressure deficit (VPD), which essentially indicates how fast the atmosphere is sucking moisture out of vegetation and fuels. A lower deficit signifies cool and moist air, while a high deficit means the air is hot and dry and conducive to burning. The team found the daily minimum vapor pressure deficit increased by 25 percent from 1970 to 2020.

“The two things that change that ability to hold moisture are temperature, or how hot it is, and how much moisture is already in the air,” Balch said.
Some ecosystems were hit harder by nighttime activity than others. For instance, nighttime fire detections were dominant in temperate evergreen forests, where 38 percent of fire detections occurred at night. Cropland fires, however, mainly occurred during the daytime.

“In really hot, dry places like desert systems, you have really high vapor pressure deficit," said Balch. “Then in places that are hot and moist compared to a desert … vapor pressure deficit goes down a little bit just as a function of the moisture that’s already in the air.”

The increase in nighttime fire activity was not necessarily surprising, said co-author John Abatzoglou at University of California Merced, but “the magnitude of the change was noteworthy” in certain regions.

For example, the U.S. West stood out against the global average. Nighttime fire intensity from 2003 to 2020 increased by 28 percent in the region. The West also experiences around 11 more flammable nights compared to four decades ago.

Grasslands and savannas in South America, Africa and Asia and open shrub lands in Australia also saw an increase in the number of flammable nighttime hours.

“People tend to pay more attention to conditions during the daytime, when fires are most active. But there’s not enough attention put on nighttime, when cooler conditions tend to slow fires down or even extinguish them completely,” said Adam Mahood, postdoctoral researcher at Earth Lab and co-author on the paper, in a statement.

The effects of nighttime warming on fire activity adds to a growing number of factors, linked to human-caused climate change, that are intensifying fires, increasing their duration and extending their season. In California, 18 of the top 20 largest wildfires in state history have occurred in the past two decades and many now consider the fire season, previously mostly restricted to the summer and fall, as year-round. All 20 of Colorado’s largest wildfires on record have occurred in the past 20 years.

The study states that continued nighttime warming from human-caused climate change “will promote more intense, longer-lasting and larger fires” in the future.

Abatzoglou said solutions to reduce wildfire impacts during the day or night include more preparation by communities in fire prone locations, scaling up intentional and beneficial offseason fires to better manage forests and fuels and taking steps to mitigate our effect on the climate.

“Climate mitigation measures can bend the curve on increasing fire weather," said Abatzoglou.
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