The snowpack in California’s Sierra Nevada mountain range is the lowest it has been in the last five centuries, according to a study published online in Nature Climate Change. This snowpack decline, which is likely to impact agricultural and urban water supplies, and could affect hydroelectric power and increase the risk of wildfires, is linked to and confirms the severity of the California drought.

Snow accumulates in the Sierra until April, when it starts to melt due to decreased precipitation and higher temperatures. The instrumental record shows the 1 April snowpack in Sierra Nevada in 2015 was 5% of the historical average (from 1951 to 2000), but a longer-term historical record in which to place this recent decline is not yet available.

Valerie Trouet and colleagues reconstructed the 1 April winter snowpack conditions in the Sierra Nevada for the last 500 years by combining a tree-ring series that reflects winter precipitation with another proxy that reflects winter temperature. They find that the record low snowpack in 2015 is exceptional, with a probable return period of once every 1,000 years at lower elevations, where the effect of warm winter temperatures on the snowpack is more pronounced; while at higher elevations the return period is 95 years.

These findings highlight the critical condition of the state’s primary natural water storage system.