On April 1 each year, researchers ski and snowshoe out into the high mountains of the western United States to jab stakes into the bright, crystalline snow, checking the thickness of the blanket. But in 2012, many researchers could barely travel on snow to their test sites—and when they got there, there was almost no snow to measure.

2013 was almost as bad. 2014, the same. And in 2015, on the April 1 assessment, many sites across the Sierra Nevada mountains were bald and snowless—the worst snow drought, scientists found, in at least the last 500 years.