Photo courtesy of University of Arizona. California’s fire and moisture patterns used to be more predictable, but new research indicates that the connection has gradually declined since the early 1900s and has now totally disappeared. Why? The weakened connection between precipitation and wildfires corresponds to the development of a fire suppression policy on federal lands, an international team reports in the Proceedings of the National Academy of Sciences on March 4. 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. However, the team reported that, after 1904, the connection between winter moisture brought by the jet stream from December through February and the severity of the wildfire season weakened and totally disappeared after 1977. 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 University of Arizona Laboratory of Tree-Ring Research. 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 in a release.

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