In a Warming California, a Future of More Fire

In a Warming California, a Future of More Fire

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.

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The fires around Ventura, Calif., on Thursday morning. Credit Kyle Grillot/Agence France-Presse — Getty Images

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.

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Where Wildfires Are Raging in Southern California

Wildfires burned near and in Los Angeles and other parts of Southern California, forcing thousands to evacuate.

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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.

While climate change models suggest that the state’s climate will remain variable — some even suggest that the northern Sierra Nevada will see more precipitation in the future — “whatever happens it’s all superimposed on a warmer world,” Dr. Williams said. California is going to continue to need more precipitation, he added, as warming leads to more water loss through evaporation.

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Climate change may affect fires in the state in other ways. While there is conflicting evidence as to whether Santa Ana and Diablo winds are becoming more frequent, Dr. Hall said that they should become drier as the planet warms, because warmer air over the high desert of Utah and Nevada has lower relative humidity and will become drier still as it descends into California. Drier air leads to more desiccation and greater fire risk.

And a paper published this week suggests that even Arctic sea ice may play a role in California wildfires by contributing to droughts. The analysis, by scientists from Lawrence Livermore National Laboratory and other institutions, argues that loss of sea ice at the pole may be affecting atmospheric circulation and blocking winter storms from reaching the state.

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