It’s been at least half a millennium since California has been this dry.
The snow in the Sierra Nevada mountains — which provides nearly a third of the state’s water supply — is the lowest it has been in 500 years, according to a new study published in Nature Climate Change on Monday.
The researchers compared blue oak tree rings during known time periods of precipitation, snowpack, and temperatures — beginning in 1930 — and found that the data accurately reflected snowpack levels. They then looked at rings going back 500 years to chart California’s historic snowpack supply. The findings revealed the “exceptional character” of California’s ongoing, four-year drought. As of April 1, 2015, the Sierra Nevada snowpack was only 5 percent its historical average, the researchers found.
It’s not that California has never had this little rain, explained Soumaya Belmecheri, lead author of the study and a post-doctoral research associate at the Laboratory of Tree-Ring Research. It’s that the high temperatures have combined with the drought to reduce snowpack.
“What is different is the record high temperatures that exacerbated or made this drought more severe,” Belmecheri told ThinkProgress. High temperatures affect the quality of precipitation — whether water falls as snow or rain — and whether the snowpack has a chance to stick around.
“The snowpack is like a reservoir. It’s a water bank,” Belmecheri said. “If this kind of drought in California is expected to become more common in the future, you can imagine all the impacts it will have for water in California.”
California is in the midst of a staggering drought, which has contributed to widespread wildfires, agricultural strain, and, in some cases, drinking water shortages. In April, Gov. Jerry Brown (D) issued water restrictions for the state for the first time ever, mandating that urban water suppliers cut their use by a quarter. The drought has been linked to climate change.
“This is probably the biggest water supply concern our state is facing,” Mark Gold, associate vice chancellor for environment and sustainability at UCLA told the Los Angeles Times. “On a scale of 1 to 10, it’s 11.”
The researchers found that for low-elevation sites, winter temperatures have a greater influence on snowpack. “The 2015 record low snowpack coincides with record high California January–March temperatures and highlights the modulating role of temperature extremes in Californian drought severity,” they write. This year’s snowpack is “strongly exceptional” the report says, exceeding the 95 percent confidence interval for 1,000-year return period. In other words, over 1,000 years, there is only a 5 percent chance of the snowpack being this low. For contrast, the second-driest year, in 1977, exceeded the 95 percent confidence interval for only a 60-year return period.
“The 2015 snowpack in the Sierra Nevada is unprecedented,” Valerie Trouet, one of the authors of the study and a paleoclimatologist at the University of Arizona, told the New York Times. “We expected it to be bad, but we certainly didn’t expect it to be the worst in the past 500 years.”