By Nicholas St. Fleur / Source: nytimes

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Half Dome in the eastern part of Yosemite Valley in July. The gradual melting of snow in California’s mountains usually fills the state’s reservoirs with water, but researchers said this past year’s snowpack was unusually low. François B. Lanoë

The snow that blanketed the Sierra Nevada in California last winter, and that was supposed to serve as an essential source of fresh water for the drought-stricken state, was at its lowest levels in the last 500 years, according to a new study.

The paper, published in the journal Nature Climate Change, used tree-ring data from centuries-old blue oaks to provide historical context for the mountain range’s diminished snowfall. As of April 1, the snowpack levels were just 5 percent of their 50-year historical average.

The paper is the first to create a model that describes temperature and precipitation levels on the Sierra Nevada that extend centuries before researchers started measuring snow levels each year.

“The 2015 snowpack in the Sierra Nevada is unprecedented,” said Valerie Trouet, one of the authors of the study and a paleoclimatologist at the University of Arizona. “We expected it to be bad, but we certainly didn’t expect it to be the worst in the past 500 years.”

Snowmelt from the Sierra Nevada fills reservoirs that provide a third of all of the drinking water for the state of California, as well as water to fight wildfires and to generate electricity.

“The scope of this is profound,” said Thomas Painter, a snow hydrologist with NASA’s Airborne Snow Observatory, adding that models like the one developed in the study suggested a dry future for California in years beyond the current drought. “This has been a very bad drought, and being able to understand the context of it is extraordinarily important.”

To determine snowpack levels from 500 years ago, the research team combined two data sets of blue oak tree rings. The first set provided historical precipitation levels from more than 1,500 blue oaks from 33 sites in California’s Central Valley. The team compared part of that data from the years 1930 to 1980 with actual snowpack measurements and found that both findings matched.

Using this correlation, the team combined the precipitation data with a second data set of tree rings that looked at winter temperatures from 1500 to 1980.

After analyzing the data, the team determined with its model that snowpack levels as low as this year’s were a once-in-1,000-years event. But because of rising temperatures caused by human activities, the researchers said they thought that snow droughts would become much more frequent.

California has a Mediterranean like climate, which means that it receives most of its precipitation in the winter and is dry during the summer. The blue oaks that encircle the Central Valley and cover the rolling foothills of the Sierra Nevada serve as a good indicator of snowfall on the mountains because they are very sensitive to winter precipitation, according to David W. Stahle, a geoscientist from the University of Arkansas and an author of the paper.

Many of the winter storms that pile snow on the Sierra Nevada also fall as rain on the blue oaks. The trees use the moisture stored in the soil to grow during the spring and summer, and the width of their tree rings reflects the amount of precipitation from the preceding winter. Wide rings indicate wet winters, while narrow rings denote dry ones.

“Having an ultrasensitive record of wet-season precipitation in ancient blue oak trees is a gift of nature to the modern water-dependent world,” Dr. Stahle said in an email.

Some researchers said the results were valuable to understanding the current drought. Others found the results to be less surprising.

“I don’t think anything they say is alarmingly shocking,” said David Rizzardo, the chief snow surveyor at the California Department of Water Resources. “From a department perspective, you can go back 500 years or 10,000 years, it doesn’t really change the context of the here and now. We’re stuck in this situation.”

Noah Diffenbaugh, a climate scientist at Stanford, said the study provided valuable information about the historical context of the drought, which would help in understanding its causes. He said that, when combined with previous studies, the new findings helped “provide strong evidence that global warming has substantially increased the probability of getting these extremely low snow conditions.”

A. Park Williams, a bioclimatologist at Columbia University, said the study added to evidence that rising temperatures had exacerbated the lack of snow in California.

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