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Fires raging as nights trend warmer, drier; research finds

wildfire rages
The Rim Fire in the Stanislaus National Forest in California began on Aug. 17, 2013. U.S. Forest Service photo. Photo provided by Wikimedia Commons.

When a wildfire rages during the day, the cool, moist air of night promises a chance to slow the fire’s spread, and for firefighters to rest. But there’s a problem: globally, nights are getting warmer and drier.

New research published in Nature by Boise State Assistant Professor Megan Cattau and colleagues in the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder (CIRES) Earth Lab reveals three important takeaways.

“Nights are getting drier, the time during which night time conditions are conducive to burning is increasing, and fires at night are becoming more intense,” Cattau said.

The research team, led by institute director Jennifer Balch, discovered multiple concerning trends. For example, globally, the daily minimum vapor pressure deficit (which indicates how dry the air is) increased by 25% from 1979 to 2020.

“Night is the critical time for slowing a speeding fire—and wildfire’s night brakes are failing,” said Balch.

Across burnable lands, the annual number of flammable night-time hours increased by 110 hours, allowing five additional nights when flammability never ceases. Across nearly a fifth of burnable lands, flammable nights increased by at least one week across this period. Additionally, night fires across the globe have become 7.2% more intense from 2003 to 2020.

To conduct this nearly three-year-long study, the team collected and analyzed global satellite observations of daytime and night-time fire detections and corresponding hourly climate data. The team also used a novel remote sensing and modeling technique to evaluate fire progression hourly for tens of thousands of fire events.

Another important facet of this research is that firefighters who counted on the cooler, damper temperatures of the night to be able to rest may not be able to do so in the future. This means more hours spent fighting longer, and more intense fires than before.

In addition to this research, Cattau published an earlier study with the Earth Lab on the rising trend of human-started (or anthropogenic) fires, revealing that increasing anthropogenic ignitions, such as debris burning or fireworks, are causing increasingly frequent fires over increasingly long fire seasons. However, limiting these ignition sources is one way in which humans can do their part to stop fires from ever starting.

Megan Cattau. Photo provided by Patrick Campbell, University of Colorado.

“At this point in time, we should expect fire as part of life in the West,” Cattau said. “In addition to limiting ignitions, it’s critical that we address the other factors influencing fire: fuels and climate. These factors influence how large and severe a fire becomes once it starts. We can also take actions to protect ourselves from fire when possible, like clearing defensible space around our homes and using firewise construction.”