That contrast has occurred this decade in the state, where years of drought were followed last winter by very wet weather that led to a bumper crop of grasses and other vegetation.
That season was followed this year by more dryness: a hot, desiccating summer and fall that turned all the vegetation into tinder. Coupled with strong, warm winds, the fire risk was extreme. The resulting blazes destroyed parts of Santa Rosa and other communities in the north and now threaten greater Los Angeles.
“For fires, sequencing is really important,” said Alex Hall, a climate scientist at the University of California, Los Angeles. “The sequence we’ve seen over the past five or six years is certainly very similar to the changes that we project as climate change continues to unfold.”
It is too early to know if climate change is directly responsible for all of these conditions in California over the past several years. But studies, including one led by Dr. Williams, have shown that human-induced global warming contributed to the drought that gripped the state beginning in 2012.
Wildfires in coastal California are not uncommon because the strong winds — known as Diablo winds in the north and Santa Anas in the south — descend from the high desert of Utah and Nevada and blow from October into the winter. Fire season usually ends around October, when autumn rains eliminate the threat.
But this year in Southern California, those rains have not arrived yet. “It’s as if it is still summer in Southern California when it comes to fire risk,” Dr. Hall said.
Climate change may not be to blame for this. Meteorologists suggest a ridge of air over the Pacific Northwest, perhaps related to the cooling of Pacific waters under current La Niña conditions, is the likely culprit. But more generally, many climate change forecasts suggest that there will be less rain in Southern California in the fall in the future, and more rain in December and January. That means fires could continue later into the fall, greatly extending the fire risk season.
The gradual warming caused by emissions of carbon dioxide and other greenhouse gases makes fires more likely across the planet, as warmer air dries the soil and vegetation more, allowing it to ignite more readily. California is no exception: average annual temperatures in the state have increased by about 1.5 degrees Fahrenheit since 1895, and the Central Valley and Southern California have warmed even more.
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