However welcome the change in weather, the bit of rain and snow we received last week does little to affect the drought status and state of emergency in effect for California.

“This could potentially be the driest water year in 500 years,” says Lynn Ingram, a professor at UC Berkeley. Ingram, an Academy fellow and co-author of The West Without Water: What Past Floods, Droughts, and Other Climatic Clues Tell Us About Tomorrow, was featured last fall in our video about Atmospheric Rivers.

And she knows her droughts. As a paleoclimatologist—a scientist who studies changes in climate by teasing data out of rocks, sediments, shells, microfossils, trees, and other sources—she’s accustomed to looking deeply into Earth’s past. According to the width of old tree rings (which can record the coming and going of wet or waterless stretches), California hasn’t been this parched since 1580.

California’s weather isn’t always shaped by local events. As Ingram described in our interview last fall,  “Our climate or our water really is controlled by what goes on over the Pacific Ocean. And so we’re learning we’re very closely tied to what goes on in the tropics, mainly—and that’s the ENSO—El Niño-Southern Oscillation.”

Warmer than usual waters in the Pacific signal El Niño conditions. When this happens, California usually sees more rain throughout the winter, especially in the northern parts. If ocean waters are cooler, then the state is in for a La Niña year, typically associated with drier weather for the state.

A paper published two weeks ago in Nature Climate Change proposes that these El Niño events could occur twice as often in the next few decades. “Potential future changes in such extreme El Niño occurrences could have profound socio-economic consequences,” according to the study. However, we are currently in an ENSO “neutral” period, without indications in the Pacific of either El Niño or La Niña.  So why is California not receiving any precipitation?

The San Francisco Chronicle describes the agreed-upon explanation:

The prolonged dry spell, meteorologists say, is caused by a dense air mass that has parked itself off the West Coast for more than a year and prevented wet weather fronts from passing.

As Daniel Swain, a PhD candidate in the Department of Environmental Earth System Science at Stanford University, explains on his California weather blog, this unusually persistent ridge of dense air has been the main cause of California’s extraordinary dry spell this past year. It has directed the primary storm track north into Alaska and British Columbia, diverting storm systems away from California, making 2013 the driest year on record for the state.

Another Chronicle article addresses the question on everybody’s mind:

California’s drought will be one of the extreme weather events that the American Meteorological Society will examine later this year to determine whether the cause is natural variability or human-caused climate change.

Swain also notes that the incredibly dry conditions brought about by this ridge mean that much of the San Francisco Bay Area has been drier than Death Valley over the past six months. At a recent press conference about the measures currently being undertaken in response to California’s exceptional drought, a Department of Water Resources official claimed that California would need to receive heavy precipitation every other day between now and the beginning of May to eliminate the existing precipitation deficit.

And that gets to the reason Governor Jerry Brown declared the state of emergency two weeks ago. UC Berkeley’s Michael Hanneman explained it on NPR very well:

First of all, it gives sanction to water supply agencies to invoke special rules that they’ve developed banning lawn watering, washing cars. Secondly, it orders the Water Rights Agency in California to move some water from agriculture to urban uses and also to ecosystem protect. But most of the water in California, maybe 75, 80 percent of the water, is used in agriculture.

On Friday, officials made several cuts to the availability of the water supply. (Read more in the New York Times.) The Los Angeles Times reports that fishing bans are already in effect due to the drought. And Peter Fimrite describes potential impacts and how we can learn from the past in an excellent recent Chronicle article, “California drought: Water officials look to rules of ’70s.”

Conservation will play a critical role, and we can all do our part. The California Department of Water Resources has a great website devoted to water conservation—start shortening those showers, people!

“We can’t make it rain, but we can be much better prepared for the terrible consequences that California’s drought now threatens, including dramatically less water for our farms and communities and increased fires in both urban and rural areas,” said Governor Brown in his declaration. “I’ve declared this emergency and I’m calling all Californians to conserve water in every way possible.”

Image: Folsom Lake, ewoerlen/Flickr

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