Two years into California's drought and locals are repeating (mantra-like) "we've never seen anything like it." They are right, of course, since this is the worst period of rainlessness since records began... but if Cal Berkeley professor Lynn Ingram is correct, they ain't seen nothing yet. The paleoclimatologist fears, if very long-run history repeats, California should brace itself for a mega drought, as National Geographic reports, a drought that could last for 200 years or more.

Via National Geographic,

California is experiencing its worst drought since record-keeping began in the mid 19th century, and scientists say this may be just the beginning. B. Lynn Ingram, a paleoclimatologist at the University of California at Berkeley, thinks that California needs to brace itself for a megadrought—one that could last for 200 years or more. As a paleoclimatologist, Ingram takes the long view, examining tree rings and microorganisms in ocean sediment to identify temperatures and dry periods of the past millennium. Her work suggests that droughts are nothing new to California. ... "During the medieval period, there was over a century of drought in the Southwest and California. The past repeats itself," says Ingram, who is co-author of The West Without Water: What Past Floods, Droughts, and Other Climate Clues Tell Us About Tomorrow. Indeed, Ingram believes the 20th century may have been a wet anomaly. ... Unfortunately, she notes, most of the state's infrastructure was designed and built during the 20th century, when the climate was unusually wet compared to previous centuries. That hasn't set water management on the right course to deal with long periods of dryness in the future. Given that California is one of the largest agricultural regions in the world, the effects of any drought, never mind one that could last for centuries, are huge. About 80 percent of California's freshwater supply is used for agriculture. The cost of fruits and vegetables could soar, says Cantu. "There will be cataclysmic impacts." ...

So what is causing the current drought?

Ingram and other paleoclimatologists have correlated several historic megadroughts with a shift in the surface temperature of the Pacific Ocean that occurs every 20 to 30 years—something called the Pacific Decadal Oscillation (PDO). The PDO is similar to an El Nino event except it lasts for decades—as its name implies—whereas an El Nino event lasts 6 to 18 months. Cool phases of the PDO result in less precipitation because cooler sea temperatures bump the jet stream north, which in turn pushes off storms that would otherwise provide rain and snow to California. Ingram says entire lakes dried up in California following a cool phase of the PDO several thousand years ago. Warm phases have been linked to numerous storms along the California coast.

"We have been in a fairly cold phase of PDO since the early 2000s," says Brian Fuchs, a climatologist at the National Drought Mitigation Center, "so the drought we are seeing now makes sense."

That said, scientists caution against pinning the current drought on the PDO alone. Certainly ocean temperatures, wind, and the weather pattern in the Pacific have contributed to the drought, says Nate Mantua, a professor in the department of atmospheric sciences at the University of Washington in Seattle, where the PDO pattern was first discovered and named. "But it's more nuanced than saying the PDO did this." After all, as its name suggests, the PDO is decades in the making.