Sierra Nevada snowpack hit 500-year low in 2015

Gov. Jerry Brown announces mandatory drought restrictions while standing in a snowless Sierra Nevada meadow on April 1. In an average year, the grass would have been buried under 5 1/2 feet of snow. (Rich Pedroncelli / Associated Press)When California Gov. Jerry Brown stood in a snowless Sierra Nevada meadow on April 1 and ordered unprecedented water restrictions because of the drought, it was the first spring in 75 years of observation that the area lacked snow.

Now, six months later, researchers say this year’s record-low snowpack may be far more historic -- and ominous -- than previously realized.

In a paper published Monday in the journal Nature Climate Change, scientists estimate that…