Rich Pedroncelli/AP photoIf winter does not produce enough rain

Rich Pedroncelli/AP photoIf winter does not produce enough rain

Study: Carbon emissions, climate change likely behind California drought

The worst dry period in California's natural history may not be an entirely natural disaster.

Carbon emissions that have been put into the Earth's atmosphere by human activities since the Industrial Revolution are “very likely” behind California's ongoing epic drought, a study led by a Stanford University climate change scientist has found.

The winter storms that normally bring the rain and snow that fills California reservoirs have stayed away for the past two winters, steered north to Alaska and the Arctic Circle by a wall of high atmospheric pressure.

This wall has periodically popped up consistently throughout the history of the region. But thanks to greenhouse-gas emissions, the likelihood of the wall of pressure appearing — and not moving — has now greatly increased, according to the study published in the journal of the American Meteorological Society.

The drought-causing ridge is now three times as likely to appear as it was in the days before industrialization, the study found.

“This highlights the fact that the climate has already changed as a result of human activities,” said Daniel Swain, a Ph.D. student at Stanford and the study's lead author.

The study was one of five reviews of human impact on changing weather patterns around the world published Monday, three of which studied climate change's impact on the California drought.

One review found that while the overall “warming trend” helps the rain-blocking wall of pressure to appear, warming also creates more humidity that could bring more rain.

Another review found that “substantial warming” in the Pacific region did not contribute heavily to the drought, but noted that extreme weather events like the drought are “very unlikely” to happen without human-caused climate change.Stanford researchers dubbed the wall of pressure the “ridiculously resilient ridge.” The phenomenon appeared suddenly in January 2013, abruptly cutting that winter's rains short.

Instead of disappearing as predicted, the ridge hung around for a year, making the 2013-14 winter bone-dry and delaying any subsequent rainy season “by four months,” according to the Stanford study.

The drought, which is about to enter its fourth year, is one of the driest periods ever recorded in California. The dry spell has cost the state economy $2.2 billion and 17,000 jobs, officials say.

All 58 counties have been declared disaster zones, and Gov. Jerry Brown has called on Californians to cut their water use by 20 percent.

San Francisco has fared better than most — Mayor Ed Lee has called on The City to cut its water use by only 10 percent — but 20 percent cutbacks appear likely next year if this winter is not remarkably wet, the San Francisco Public Utilities Commission said last week.

Will the ridge reappear this winter? It's hard to say, Swain said Monday.

“Hopefully not, but we don't know,” he said. “There aren't right now any strong signals for a wet or dry winter … we don't know what's going to happen this winter.”

Even if the rains begin in the coming weeks, another year of drought appears certain if the ridge pops up again in January.

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