Sierra Nevada Snowpack Reached Five-Century Low This Year

9-14-15 Snowpack blue oaks
The annual tree rings on these blue oaks in California's San Joaquin Valley show how much winter rain has fallen each year. By using tree-ring chronologies going back to 1405, a University of Arizona-led team of researchers figured out that the snowpack in the Sierra Nevada mountains in 2015 was at the lowest level in 500 years. Kevin Anchukaitis

In April this year, the annual measurement of the Sierra Nevada mountains' snowpack in California brought startling results: There was no snow at all. For nearly three-quarters of a century, the average depth of snow in the measurement spot at Phillips Station, about 90 miles east of Sacramento, had been 66.5 inches. A new report published online Monday in Nature Climate Change finds that the snowpack level in 2015 was the lowest in five centuries.

"Our study really points to the extreme character of the 2014-15 winter. This is not just unprecedented over 80 years—it's unprecedented over 500 years," Valerie Trouet, an associate professor of dendrochronology at the University of Arizona's Laboratory of Tree-Ring Research, says in the university's press release.

The snowpack, which is usually at its peak at the start of April, is an accumulation of winter precipitation that slowly melts during warmer months to replenish streams, lakes, groundwater and reservoirs. According to the University of Arizona report, California gets 80 percent of its precipitation during the winter, and the snowpack accounts for 30 percent of its water supply.

"Snow is a natural storage system," Trouet said. "In a summer-dry climate such as California, it's important that you can store water and access it in the summer when there's no precipitation." This year's low was the result of scant winter precipitation and high temperatures between January and March. The drought that began in California in 2012 has been called the worst in over a millennium, and researchers have predicted that the future holds even more severe "megadroughts."

In the recent study, titled "Multi-century evaluation of Sierra Nevada snowpack"—funded by the National Science Foundation, the U.S. Geological Survey and the Swiss National Science Foundation—Trouet and her colleagues looked at previously published data on tree rings of blue oaks in California's Central Valley, whose width reflects annual winter precipitation. They also used previous work by co-author Eugene R. Wahl of the National Oceanic and Atmospheric Administration/National Centers for Environmental Information, who had reconstructed January to March temperatures in the area between 1500 and 1980. Finally, the researchers turned to actual snowpack measurements, which have been conducted at Phillips Station since 1941.

Combining data from the tree rings, temperature reconstruction and snowpack measurements, they found that this year's meager snowpack was an anomaly not only in the context of decades but also in the past several centuries.

This year's currently unprecedented lack of snow may become more commonplace. "We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures," Trouet said. "Anthropogenic warming is making the drought more severe."