WildfiresWildfire risk in California no longer linked to winter precipitation

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.

Arizona says that 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.