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Study: Chance of extreme autumn fire weather up 40%

Image courtesy of Oregon State University A satellite image shows smoke and flames from the November 2018 Camp fire that destroyed Paradise, California, and killed more than 80 people.
Jamie Lusch / Mail Tribune Smoke from a controlled burn is visible from Medford Thursday. Burns help decrease the accumulation of fuels in forests in preparation for fire season.
Climate change making falls hotter, drier

The likelihood of hot, dry autumn weather that can set the stage for severe fires in California and Oregon has increased 40% due to human-caused climate change, according to new computer modeling led by an Oregon State University researcher.

The study looked at four areas in Oregon and California that suffered catastrophic wind-driven wildfires.

Researchers found climate change actually decreased the risk of strong, dry winds in the fall during the two-year study period, but higher temperatures and dried-out fuels caused an overall jump in fire risk.

To measure the impact of human-caused climate change, researchers ran computer models with atmospheric carbon dioxide and aerosol levels set at pre-industrial levels versus modern day amounts.

“We found that when CO₂ and aerosols from human activity were included, the chance of extreme conditions was 40% higher in those areas of California and Oregon where recent autumn fires have occurred. The jump was mainly because of an increase in temperature and fuel aridity and not an increase in wind speeds. In fact, we found anthropogenic climate change slightly decreased the frequency of strong, dry, offshore winds,” said Linnia Hawkins, a postdoctoral researcher in the OSU College of Forestry.

Researchers looked at four study areas that included land burned by the September 2020 Lionshead fire that swept across the Warm Springs Reservation in Central Oregon; the Northern California wine country fires in October 2017; Southern California’s Woolsey fire in November 2018, and the November 2018 Camp fire that destroyed the town of Paradise, California, and claimed more than 80 lives.

In September 2020, a rash of wind-driven fires destroyed more than 4,000 homes in Oregon, with the most destruction in Jackson County from the urban Almeda fire and the rural South Obenchain fire.

The Almeda fire alone destroyed 2,500 homes and 170 businesses. Hot weather, strong winds and dried-out fuels fed the fire that burned from Ashland through Talent and Phoenix to the southern outskirts of Medford. Entire neighborhoods and manufactured home parks were incinerated by the blaze.

Researchers said they can’t say climate change specifically caused any of the individual fires they studied. However, they said the fires provided examples of wind-driven autumn fires to study.

Hawkins emphasized that the 40% increase in the likelihood of hot, dry autumn weather is the average across the western United States. The increase is smaller or larger in specific regions. She also stressed that the study examined extreme fire weather conditions as opposed to average conditions, and only in one season of the year.

Additional factors contributing to escalating fire risk include an accumulation of fuels in forests, and more people living in fire-prone wildland-urban interface areas, Hawkins said.

Other researchers who collaborated on the study were David Rupp of the Oregon Climate Change Research Institute, Sihan Li of the University of Oxford and John Abatzoglou of the University of California, Merced.

The National Science Foundation and the National Oceanic and Atmospheric Administration supported the research. The findings were published this month in the Geophysical Research Letters science journal.

Reach Mail Tribune reporter Vickie Aldous at 541-776-4486 or valdous@rosebudmedia.com. Follow her on Twitter @VickieAldous.