As if California doesn't have enough water problems. Now scientists say the Sierra Nevada snowpack—which gives the state nearly a third of its surface water—is at a shocking 500-year low, the Guardian reports. That means California, now in its fourth year of drought, can't rely as much on winter snow accumulation in the mountains to fill streams and reservoirs during the hot summers. "It was unprecedented," says Valerie Trouet, who led the study published in Nature Climate Change. "When you are a climate scientist, first you get excited by the result. Then you realise the extreme level of the snowpack is not something to get excited about."
Trouet's team reached their conclusion by analyzing rings in the trunks of 1,500 blue oak trees in the region, CNBC reports. Winter precipitation plays a big role in the trees' growth, she says, which can be measured by the width of each year's ring—and last year's was pretty darn thin. So how did it get this bad? Seems that last winter's record-high temperatures and lack of rain led to the dismally low snow accumulation. Trouet also warns that California's "uncontrollable wildfires" can be linked to the lack of Sierra Nevada snowmelt, which scientists had already figured was at a 100-year low. If the climate keeps warning, she adds, "chances of this happening again in the future are much higher than they were in the past." (See how LA is fighting drought with 96 million black plastic balls.)