Increased temperatures due to global warming and more effective efforts to contain fires mean there's now more dry wood to burn. This means that large wildfires of the kind seen in 2018 can now happen in any year, regardless of how wet the previous winter was.

The researchers say huge blazes may be a sign of things to come. Their study is published in the Proceedings of the National Academy of Sciences. Last year was California's most destructive and deadly wildfire season on record. Pictures beamed around the world revealed the havoc wrought as fires devastated whole communities. During winter, moisture in the form of precipitation is delivered to California by a fast moving band of air called the jet stream. A US-German team of scientists reconstructed fire and moisture patterns, along with the position of the North Pacific jet stream, over the past 400 years.

They found that from 1600 to 1903, the position of the North Pacific jet stream over California was linked to the amount of winter rainfall and the severity of the subsequent wildfire season.

Wet winters brought on by the jet stream were followed by a less intense wildfire season, while dry winters were followed by more intense fires.

But after 1904, the connection between winter moisture and wildfires was seen to weaken. This coincides with the beginning of a fire suppression policy on the US federal lands. 

The connection disappears completely after 1977. 

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