The Sierra Nevada snowpack—the source of more than one-third of California's water supply—is the lowest it has been in 500 years.
The finding comes from a new historical analysis of tree rings published Monday in the peer-reviewed journal Nature Climate Change. It demonstrates how severe California's four-year-old drought has become. The state began rationing water for the first time in its 165-year history earlier this year.
The Sierra Nevada snowpack is a critical source of water for California during the spring and summer months, when rain is rare. As the snow melts, the water replenishes soil moisture and reservoirs below. With less snow this year, the region's already strained agriculture sector, drinking water supplies and hydroelectric power face serious challenges. It also means California's forests will remain dry, fueling already rampant wildfires.
But the research also gives a clue as to how global warming could change the nature and severity of the region's droughts in the decades to come.
"We know that the drought in California is a hot drought, its high temperatures differentiate it from droughts in the past," said Soumaya Belmecheri, a geochemist who works on climate reconstructions at the Laboratory of Tree-Ring Research at the University of Arizona and lead author of the study. "As temperatures continue to rise from global warming, this research shows the snowpack may no longer be a reliable source of water. That has huge implications on everything from wildfires to biodiversity to human civilization."
Scientists had previously put the current California drought into historical context, looking back thousands of years. But they hadn't yet done the same for the Sierra Nevada snowpack levels.
Belmecheri and her colleagues at the National Oceanic and Atmospheric Administration (NOAA) and the University of Arkansas studied precipitation records dating back five centuries by examining blue oak trees, a water-sensitive species that can live for hundreds of years. Although the trees are in the foothills of the Sierra Nevada, blue oaks can serve as a proxy for precipitation at higher elevations, Belmecheri said.
"The water a storm tract forming in the Pacific Ocean and coming across California dumps over the blue oaks is the same that would fall in the mountains," she said.
The scientists found that a few years in the 16th and 17th centuries—periods that also experienced severe droughts—came close to the 2015 low, but didn't quite match it.
The low snowpack follows a record-warm winter in California. Temperatures from January to March averaged 53 degrees Fahrenheit, 7.5 degrees above the 20th century average and 1.8 degrees hotter than the previous record, in 2014. The warmer weather means that more precipitation in the mountains falls as rain, not snow. Instead of being released slowly over the spring and summer months, the water rushes downhill as soon as it falls. Water managers in California are scrambling to figure out a way to best store the winter rain so it lasts during the dry season.
Ultimately, however, the scientists found that low precipitation in the winter months was more responsible for the 2015 snowpack shortfall than temperature. Park Williams, a bioclimatologist at the Lamont-Doherty Earth Observatory at Columbia University agrees with that general conclusion, but also believes this study ultimately underestimates the effect of temperature on Sierra Nevada snowpack.
"The scientists used the best temperature record available, but unfortunately it is still an extremely coarse reconstruction," Williams said. "Their estimation of temperature's influence on the snowpack is an underestimation."
Williams and colleagues published a paper in August that found up to 27 percent of California's drought is attributable to climate change—a situation that will continue to get worse in the decades to come.
"We know that natural variability is capable of producing drastic, severe droughts," Williams said. "But we also know the warming is going to continue. This means the dice are being loaded more and more toward a record-low snowpack year."