The Sierra Nevada’s snowpack is at the lowest level ever seen (Image: AP Photo/Rich Pedroncelli)

The snow cover on the iconic US mountain range of Sierra Nevada has hit a 500-year low, with the snowpack in April this year just 5 per cent of the average volumes recorded for that month between 1951 and 2000.

It’s bad news for California – which is in the midst of its worst drought on record – since melting snow from the Sierra Nevada fills 30 per cent of its reservoirs.


“Snow is a natural way to store water, and now with rain-less summers, the only water available in California comes from melting of snow that fell the previous winter,” says Valerie Trouet of the University of Arizona in Tucson. “When snowpack is low, as in 2015, California summer droughts can therefore be very severe, as we are experiencing now.”

Her team compared the snowpack levels in April 2015 with those seen over the past 500 years. To estimate past snowpack volumes, they analysed tree rings from 1500 blue oak trees that have been growing in central California for centuries. The tree rings are wider in wet winters, and narrower in dry winters, allowing them to build up a historical record of how much precipitation there was.

Snowpack cover has reduced since 2010 (Image: NASA/MODIS)

Trouet says the vanishing snowpack this year was triggered by a freak “double whammy”: unusually low winter precipitation combined with exceptionally high winter temperatures – the highest ever recorded were seen between January and March this year.

Based on past data alone, the chance of a repeat event in the next 500 years is less than 5 per cent, but projected global warming means the chance is actually far higher, says Trouet.

“Temperatures in California and worldwide will continue to rise, and the chance of low precipitation events co-occurring with high temperatures will increase,” she says.

The only long-term option to protect the snowpack is to cut back on the carbon emissions, says Trouet.

More immediately, water providers in California need to revise water management and supply strategies to take account of the reducing snowpack, says Trouet.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate2809