Wildfires burning in California — pushing two northern counties into a state of emergency — are one symptom of one of the worst droughts in the state’s history. Another measure of the drought's severity puts another point on the problem: The snowpack in the Sierra Nevada mountains hasn’t been this low in the last 500 years, reports Francie Diep for Pacific Standard.

The 400-mile long mountain chain’s name translates to "snowy mountain range," but the drought has made that moniker less apt. Governor Jerry Brown announced mandatory water restrictions in April while standing on bare grass in a mountain meadow that typically would have five feet of snow at that time of year. The snowfall that coats the peaks refills the state’s reservoirs and provides about 30 percent of its drinking water. As of April 1, the levels were at 5 percent of the historical average.

“We knew it was an all-time low over a historical period, but to see this as a low for the last 500 years, we didn’t expect that," says Valerie Trouet, of the University of Arizona, in a Washington Post story by Darryl Fears. Trouet and her colleagues analyzed data that included tree ring measurements from more than 1,500 blue oaks at 33 sites in the Central Valley to arrive at their conclusion. (Blue oaks grow larger bands in years with good winter precipitation.)

Fears points out that the last time California was this dry was in the 1500s. In more historic terms, European explorers had yet to land in what is now San Diego and King Henry VIII was alive. "There’s very little doubt about it," Trouet tells Fears. The team published their findings in Nature Climate Change.

For The New York Times Nicholas St. Fleur reports that this year’s low snowpack levels were a once-in-1,000-years event, based on historical trends. However, climate change could make an extreme event like this much more frequent. Thomas Painter, a hydrologist with NASA who was not involved in the new work tells St. Fleur that studies like this can help build a picture of what they future may look like. “This has been a very bad drought, and being able to understand the context of it is extraordinarily important," he says.

Diep notes that previous studies show that the West can expect more precipitation to fall in the form of rain rather than snow, and for what snow does accumulate to melt sooner. And the Sierra Nevada mountains aren’t the only ones seeing this trend. Tree ring data in the upper Colorado, Northern Rockies and greater Yellowstone areas also show low snowpack levels in recent years.

To make up for the lack of precipitation, California has leaned heavily on its groundwater supply. But as Dennis Dimick reports for National Geographic, aquifers are essentially "fossil water," and if that resource is used faster than it can recharge, water supply is in real trouble.