California is facing one of its most severe droughts in the last 100 years. The state's Governor, Jerry Brown, issued the first-ever mandatory reduction of water use. And yesterday, because of raging wildfires fueled by the four-year dry spell, California declared a state of emergency. Yet the most drastically drought-hammered part of the Golden State has been its mountains.

According to new research conducted by a team of environmental scientists led by Valerie Trouet at the University of Arizona, the total snowpack in California's Sierra Nevada mountain range—the state's 400-mile-long rocky spine—is at its lowest level in at least the last 500 years. Since this April, scientists says, the mountains have gotten only 5 percent as much snow as their historical average from 1951 to 2000. The research was published today in Nature Climate Change.

"The snow drought we're seeing in 2015 is an ominous sign," Trouet says. "About 30 percent of the state's water reservoirs originate from such snowpack. You can think of the [Sierra] snowpack as California's natural water storage system."

According to Trouet, California's bone-dry mountains are a confluence of two major factors. The first is, as you might expect, a general lack of precipitation. For the most part, the Sierra Nevada snowpack comes during the state's winter snows. But each of the last four years has seen only around 75 percent of an average year's rain and snowfall.

"You can think of the Sierra snowpack as California's natural water storage system."

The second problem is that, thanks to record-breaking winter temperatures over the last four years, the precipitation that has fallen across the mountain range has largely been rain, not snow. Instead of sticking as snowpack and slowly refilling reservoirs over the spring and summer, she explains, winter rains sweat off the mountainsides almost immediately, "complicating water management and, [to make full use of the water,] requiring new reservoirs where you didn't need them before," she says.

The snows of the past

Blue oak. K.J. Anchukaitis

How exactly can scientists compare today's level of snowpack against snowy seasons from centuries ago, even before the state's recorded history? Weirdly enough, by looking at tree growth rings. Trouet and her colleagues can model the mountains' snowy history by comparing the yearly size of tree rings in thousands of blue oaks, which live at the base of the Sierra Mountain range and grow at dramatically different rates depending upon changes in water—with trusted reconstructions of historical temperature.

In other words: Because they can tell how wet and how cold it was each winter, the scientists can say how much snowpack accumulated in a given year. While it's not a perfect method, this odd tree-ring reconstruction faithfully matches the last 80 years of snowpack measurements with astonishing accuracy.

Now, Trouet is careful to point out a tricky caveat. Yes, she says, the Golden State is currently seeing a 500-year record low in snowpack. But that doesn't mean the state is also experiencing a 500-year record drought. The lack of snow reflects years of increasingly hot winters (unanimously agreed in the scientific community to be driven by climate change) in addition to California's current lack of water.

Jan Null, a meteorologist at San Jose State University in California and 20-year lead forecaster at the National Weather Service. says that while California's current drought is no doubt severe, by many measurements it's still not as bad as an even worse drought in the mid 1970s. "The whole issue of drought is very, very complicated," Null says, "snowpack is only one of many measurements." For example, California's reservoirs are still at a much higher level than they were during the 1975-77 drought.

"It would be very misleading to say this [Sierra Nevada] snowpack issue indicates a historically severe drought," he says.

El Niño is coming

Sierra Nevada snowpack comparison. NASA/MODIS

Whether or not the Sierra Nevada snowpack will stay at such historic low levels through this upcoming winter is still very much up in the air. Aggressively warm waters in the southwest Pacific Ocean are leading many, including experts at the U.S. National Ocean and Atmospheric Administration, to expect an upcoming El Niño event. El Niño is a periodic weather change that often dumps massive amounts of rain and even snow on California.

"But... while El Niño increases the odds of above average rain, it's no sure bet," says Null, who explains that there are many instances in the last century where strong El Niño weather events were actually coupled with below-average amounts of rain and snow in California. El Niño weather events are so complex that nobody is really sure why this is so.

Still, Null says, it is nonetheless possible that the upcoming El Niño event could not only refill the Sierras with average levels of snowpack but also halt California's drought entirely. For that to happen, California would have to see two things. The first is a seasonably cold winter, which would allow precipitation to cling as snow. The second is about twice as much rain and snowfall as average (which would erase California's roughly one-year deficit of water.) According to Null, if this year's El Nino hits like it did in 1997 and 1998, "that's certainly a possibility."

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