Hotter, drier nights mean more runaway fires
Warmer, drier nights can no longer dampen wildfires' flames

Date:
February 16, 2022
Source:
University of Colorado at Boulder
Summary:
A new study shows that an increase in hot, dry nights in recent
decades has resulted in nighttime wildfires becoming more intense
and more frequent. Researchers found there are 11 more flammable
nights every year in the U.S. West compared to 1979 -- a 45%
increase over the past four decades. Nighttime warming is only
expected to increase with climate change, intensifying wildfires'
size and speed, causing more firefighters to work around the clock.



FULL STORY ========================================================================== Nighttime fires have become more intense and more frequent in recent
decades, as hot, dry nights are more commonplace, according to a new study
led by the Cooperative Institute for Research in Environmental Sciences' (CIRES) Earth Lab at the University of Colorado Boulder.


========================================================================== 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% spike,
the team found.

"Night is the critical time for slowing a speeding fire -- and wildfire's
night brakes are failing," said Jennifer Balch, CIRES Fellow, Director
of CIRES' Earth Lab and lead author on the study out Feb. 16 in Nature.

The new analysis relied on a key measurement of the thirst of the
atmosphere - - the Vapor Pressure Deficit, or VPD. When VPD is relatively
low, the air is cool and moist, and fires cannot thrive. Fire suppression operations take advantage of these nighttime conditions to squelch
flames. But when the VPD is high, the air is hot and dry, parched and
primed for burning.

And in a first: the team analyzed satellite observations and hourly
climate data for 81,000 global fires to pinpoint the exact VPD tipping
point when it becomes hot and dry enough to burn at night. The researchers found one full week of additional flammable nights per year in a fifth
of burnable lands across the globe in the last 40 years. And in the
western U.S., it increased by more: 11 nights -- a 45% increase over
the four decades (1979-2020).

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% more
intense from 2003-2020. In the U.S.

West, that number was much higher: 28%.

Balch and her team also reported:
* a 36% increase of flammable nighttime hours between 1979 and
2020. By
comparison, daytime flammable hours "only" increased by 27%.

* certain ecosystems were hit harder than others: evergreen and
broadleaf
forests, shrublands and grasslands.

* and they expect to see changing fire emissions resulting from lower
intensity or smoldering burning that often occurs at night.

"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.

And the nighttime really matters: human-caused climate change has
comparatively warmed the night more than the day over the past seven
decades -- and it's only going to get worse from here, the team noted.

Recent destructive fires in December 2021 -- in midwestern Kansas and
the Marshall Fire in Boulder County, Colorado -- further highlight
the importance of better understanding the climate-related drivers
of wildfire.

"With continued nighttime warming, we expect to see more runaway wildfires
that are more intense, faster, and larger," said Balch, associate
professor of geography at CU Boulder. "That means firefighters don't get
the breaks at night they used to get -- they have to battle flames 24/7." special promotion Explore the latest scientific research on sleep and
dreams in this free online course from New Scientist -- Sign_up_now_>>> ========================================================================== Story Source: Materials provided by
University_of_Colorado_at_Boulder. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Jennifer K. Balch, John T. Abatzoglou, Maxwell B. Joseph, Michael J.

Koontz, Adam L. Mahood, Joseph McGlinchy, Megan E. Cattau,
A. Park Williams. Warming weakens the night-time barrier to global
fire. Nature, 2022; 602 (7897): 442 DOI: 10.1038/s41586-021-04325-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220216112204.htm

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