For centuries, a wet winter in California would significantly have tamped down the chances that a large wildfire would break out the following summer and fall.
However, heavy rain and snowfall no longer protects the state from massive conflagrations in the same way, according to research published Monday in the Proceedings of the National Academy of Sciences.
The authors -- a U.S.-German team, which included scientists at the National Oceanic and Atmospheric Administration -- found that an abundance of fuel in overgrown forests and rising temperatures from global warming are now cancelling out those fire-suppressing benefits.
"The extreme warm temperatures in the summer are really drying things out more than they've had in the past, and there's a lot of fuel," said Alan Taylor, co-author and researcher at Penn State University. "The fire environment is changing, and the last couple of years have been a wake-up call."
As examples, the researchers pointed to the massive Tubbs and Thomas fires in late 2017, which followed one of the wettest winters in the state's history.
Researchers said that a century ago, the massive rain and snow storms that started in 2016 would have likely prevented those fires.
The researchers compared yearly precipitation totals, winter temperatures and soil moisture to wildfire events dating back to the 1600s. The conditions were reconstructed using tree rings and scars.
"We reconstructed total precipitation levels for each year based on tree ring data, which correlates stream flows to the width of the rings," said Eugene Wahl, a coauthor and scientist with the National Oceanic and Atmospheric Administration. "We then compared those yearly totals to the fire events in following dry seasons, documented in scars on separate trees."
For hundreds of years, heavy winters, driven by the North Pacific jet stream, almost completely eliminated the threat of intense wildfire in following months, according to the study.
However, the relationship between wildfire and the moisture-packed jet stream started to break down in the early 20th century. And by 1977 the correlation was completely gone.