Wildfire risk in California no longer coupled to winter precipitation

Recent large fires may be harbingers of things to come, researchers say

Eiler Fire, Northern California, August 6, 2014. Photo by Bill Gabbert.

By Mari N. Jensen, University of Arizona College of Science

Wet winters no longer predict possible relief from severe wildfires for California, according to a new study from an international team that includes a University of Arizona scientist.

From 1600 to 1903, the position of the North Pacific jet stream over California was linked to the amount of winter precipitation and the severity of the subsequent wildfire season, the team found. Wet winters brought by the jet stream were followed by low wildfire activity, and dry winters were generally followed by higher wildfire activity.

After 1904, the connection between winter moisture brought by the jet stream from December through February and the severity of the wildfire season weakened. The weakened connection between precipitation and wildfires corresponds to the onset of a fire suppression policy on U.S. federal lands, the team reports in the March 4 issue of the Proceedings of the National Academy of Sciences.

The connection disappeared altogether after 1977.

Now, fuel buildup from decades of fire suppression in the 20th century plus rising temperatures from climate change means any year may have large fires, no matter how wet the previous winter, the team writes.

“The moisture availability over California is still strongly linked to the position of the jet stream, but fire no longer is,” said co-author Valerie Trouet, an associate professor of dendrochronology at the UA Laboratory of Tree-Ring Research.

The finding surprised Trouet.

“I didn’t expect there to be no relationship between jet stream dynamics and fire in the 20th century. I expected it to be maybe weaker than before, but not to completely disappear,” Trouet said.

California’s wet winter of 2016-2017 is a good example, she said. That winter was followed by many large fires in 2017, including the Tubbs fire in October and the Thomas fire in December. Twenty-four people died and 6,699 structures burned in those two fires, according to the California Department of Forestry and Fire.

“It’s not either climate change or historical fire management–it’s really a combination of the two that’s creating a perfect storm for catastrophic fires in California,” Trouet said.

To reconstruct California’s fire and moisture patterns and the position of the North Pacific jet stream for the past 400 years, the researchers combined instrumental and historical records of temperature, precipitation and fires with the natural archives of climate and fires stored in tree rings that go back in time for centuries.

Lead author Eugene R. Wahl of the National Oceanic and Atmospheric Administration said, “The method we used to determine the average winter jet stream conditions is a real advance. Coupled with independent precipitation and fire records, this is a state-of-the-art coupling of paleoclimate and paleoecology.”

The study is the first to show the close connection between winter precipitation in California and the position of the jet stream back to the year 1571, Trouet said. The study is also the first to examine the relationship of past winter precipitation, the position of the jet stream and past fire activity stretching back to 1600, she said.

The paper by Wahl, Trouet and two co-authors is, “Jet Stream Dynamics, Hydroclimate, and Fire in California from 1600 CE to Present.”

Initially, Wahl and co-author Eduardo Zorita of the Helmholz-Zentrum Geesthacht in Germany were working independently of Trouet and co-author Alan Taylor.

As part of a larger project to extend global reconstructions of temperature, precipitation and atmospheric circulation further into the past, Wahl and Zorita were figuring out how the North Pacific jet stream affected precipitation in California for centuries. Wahl, a paleoclimatologist at NOAA’s National Centers for Environmental Information in Boulder, Colorado, was a co-leader for the North America part of the larger project.

Trouet and Taylor of Penn State in University Park, Pennsylvania, had already reconstructed California’s fire history back to 1600, and Trouet had reconstructed the behavior of the North Atlantic jet stream back to 1725.

After Wahl heard Trouet give a presentation about her North Atlantic jet stream research, the four scientists joined forces to see whether there were links between the past behavior of the North Pacific jet stream and California’s fire and precipitation history.

“When the jet stream is positioned over California, it’s like a fire hose–it brings storms and moisture straight over California,” Trouet said. “What we see post-1900 is that the position of the jet stream is still an important driver of moisture to California–it brings moisture to California when it’s in the right position–but there’s a disconnect with fire.”

The likelihood that every year may be a high-fire year will be a significant societal challenge, Taylor said.

“Fire not being influenced by moisture anymore? That is surprising. It’s going to be a problem for people, for firefighters, for society,” he said. “The only thing we can control is fuels, so what it suggests is that we take that very seriously.

“The last three years may be a harbinger of things to come,” he said. “Between 1600 and 1903 there was not a single case of a high-precipitation year coupled with a high-fire year as occurred in 2017.”

The research team’s next step is to expand this research to see how the jet stream patterns correspond with fire in other types of forested ecosystems farther north.

The research was funded by the U.S. National Science Foundation, the U.S. Geological Survey’s Southwest Climate Science Center, the U.S. Department of Agriculture Forest Service, the German Science Foundation Cluster of Excellence Clisap, a George H. Deike, Jr. Research Grant, and the Swiss National Science Foundation.


Note from Bill. Even though the research was funded by universities and government agencies, you would still have to pay $10 to a private organization to read the report. It does not comply with Open Access policies.

Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire. Google+

2 thoughts on “Wildfire risk in California no longer coupled to winter precipitation”

  1. Thank you for your posting of the Gabbert-Jensen article on the disconnect between California precipitation and wildfire severity.

    I was a volunteer for the U. of Arizona’s Tree Ring Lab at its Bristlecone Pine dendrochronology camp in the White Mountains of eastern California during the summers of 2009-10. I taught forestry at Feather River Community College here in Quincy with colleague Mike Yost who did extensive fire scar analysis around the Plumas National Forest. It parallels your findings that the interval between fire events rose dramatically resulting from intense fire suppression policies.

    Mike was a part of the Quincy Library Group, which produced federal legislation to test a custom thinning prescription for the Sierra’s mixed conifer type. Its application was intended for a 10-year period from 1998 and was repeatedly delayed by environmentalist lawsuits. Post-treatment research showed that when uncontrolled wildfire reached the boundary of treated lands, the fire slowed its advance, moved to the forest floor and preserved all but the smallest trees on the treated site.

    We still have too little regulation of fuels within the WUI (Wildland-Urban Interface). Second-homers and permanent WUI residents keep far too many live trees on their parcels, especially near their homes for shade, and they tend to pile firewood against outside walls. Fire Safe organizations attempt to promote a fuel-less clearing radius with mixed results. We need better regulation in the form of ordinances that mandate such practices instead of the tree preservation regulations in place for Paradise, CA, destroyed by 2018’s “Camp” fire. Perhaps we can involve insurance companies in helping to enforce better practices through rate discounts or surcharges. Otherwise, we face increased fire insurance rates (if we can find coverage) and higher electric bills. Water and wastewater districts can starve for cash or become financially unstable if most of their ratepayers lose houses to fire when communities are wiped out.

    The value of your article is that controlling fuel density by thinning is our only potential for lowering the risk of catastrophic fire, soil losses, structure damage, and loss of life. I would prefer battalions of woods workers doing thinning on a three season schedule instead of spending billions on air assets and “project fire” contracting. Such a campaign could represent more than an ounce of prevention. The avoidance of catastrophic costs could be far greater.

    Thinning and good slash treatment will minimize the loss of precipitation that gets re-evaporated into the air instead of reaching mineral soil. This increases tree vigor and helps against bark beetle infestations which result in additional fuels. Mother Nature used to prevent 300+ conifer stems per acre by natural fire events. Native Americans copied this method to some extent. We should do the same.

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