Last year, the Golden State was hit by the deadliest wildfires on record, forcing hundreds of thousands of Californians to evacuate and provoking a strange and terrifying-sounding weather phenomenon called a "firenado". 

But this state of affairs could now become a more regular occurrence, according to a study published in the Proceedings of the National Academy of Sciences. A team of scientists has concluded that wet winters may no longer be a sign of a quiet wildfire season.

The researchers used climate model simulations and historical data on temperature, precipitation, and fires as well as "natural archives" (tree rings) that reveal the climate and fire severity at different points in time. From this information, they were able to track how the North Pacific jet stream has affected wildfire season since 1571 CE. 

"The method we used to determine the average winter jet stream conditions is a real advance," Eugene R. Wahl, a paleoclimate scientist at the National Oceanic and Atmospheric Administration and lead author of the paper, said in a statement.

"Coupled with independent precipitation and fire records, this is a state-of-the-art coupling of paleoclimate and paleoecology."

Up until the beginning of the 20th century, wet winters brought in by the strength and position of the jet stream signaled a season of low wildfire activity, whereas a dry winter foreshadowed a season of more severe wildfire activity. Then, in 1904, things changed. Fire suppression policy was introduced for US federal land and the connection between the jet stream (specifically, the winter moisture it brings) and wildfire severity weakened. By the late 70s, there was no correlation whatsoever. 

"When the jet stream is positioned over California, it's like a fire hose – it brings storms and moisture straight over California," Valerie Trouet, an associate professor of dendrochronology at the University of Arizona Laboratory of Tree-Ring Research, explained.

"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."

Now, climate change and the rising temperatures it brings are exacerbating the situation. Combined with decades of fire suppression policy, climate change means any year could see wildfires on the scale of 2018. It doesn't matter how wet or dry the previous winter is. Add in 149 million or so dead trees and you have a major fire hazard on your hands. 

"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 added.

"The last three years may be a harbinger of things to come," co-author Alan Taylor 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."