Wildfires have often been a significant environmental problem, particularly in areas with drier and hotter climates such as the western United States. However, four decades ago, cool, moist nights regularly provided relief for firefighters, and so-called “flammable nights” were an extremely rare event.
Now, due to climate change, the nights are warming even faster than days. According to a new study led by the University of Colorado Boulder (CU Boulder), there are 11 more flammable nights in the U.S. West every year – a 45 percent increase since 1979.
“People tend to pay more attention to conditions during the daytime: when fires are most active,” said study co-author Adam Mahood, a postdoctoral associate in CU Boulder’s Earth Lab. “But there’s not enough attention put on nighttime: when cooler conditions tend to slow fires down or even extinguish them completely.”
“Night is the critical time for slowing a speeding fire – and wildfire’s night brakes are failing,” added study lead author Jennifer Balch, the director of the Earth Lab.
The scientists analyzed satellite observations and hourly climate data for 81,000 global wildfires, and identified variations in the Vapor Pressure Deficit (VPD) – a key measurement of the “thirst of the atmosphere,” signaling the level of atmospheric heat and dryness – in order to determine the VPD tipping points when the air became hot and dry enough to allow wildfires to burn unabated during nighttime too. The results revealed a seven-days increase in flammable nights per year in a fifth of burnable lands around the globe, and an 11-days rise in the U.S. West over the past four decades (1979-2020).
By using a novel remote sensing and modeling technique to evaluate fire progression hourly for a large number of wildfires, the researchers also found that, globally, the night fires have become 7.2 percent more intense from 2003 to 2020, with a staggering 28 percent increase in intensity in the U.S. West. Moreover, the results shown a 36 percent increase of flammable nighttime hours during this period, compared to a 27 percent increase of daytime flammable hours. The most affected ecosystems were evergreen and broadleaf forests, shrublands, and grasslands.
“With continued nighttime warming, we expect to see more runaway wildfires that are more intense, faster, and larger. That means firefighters don’t get the breaks at night they used to get – they have to battle flames 24/7,” concluded Professor Balch.
The study is published in the journal Nature.