The Sierra Nevada snowpack that is a critical water source for California fell to a 500-year low last winter – far worse than scientists had estimated and underlining the severity of the current drought, according to new research.

The snow accumulation in the mountains was just 5% of what is normal, inflating the risk of wildfires, drying up wells and orchards, and pushing communities into water rationing.

Scientists had earlier thought that the snow pack was the lowest in 100 years, after a winter that was the warmest on record. Now it turns out it was actually the lowest in five centuries, according to research published in Nature Climate Change on Monday.

“It was definitely a 500-year low. In the 500-year reconstruction, it was unprecedented,” said Valerie Trouet of the University of Arizona, who led the study. “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.”

Snow surveyors have been trekking into the Sierra Nevada since the 1930s using gauges to measure the accumulation of snow pack, which provides almost a third of California’s surface water, at 108 observation stations.

Trouet’s team studied two independent sets of records of tree rings, whose patterns reflect annual rainfall and temperatures.

They found in the study of the 3,500 samples that 2015 was indeed the worst snowpack in 500 years – a situation that is unlikely to return for 1,000 years at lower elevations.

California gets almost all of its precipitation in the winter months, and the Sierra Nevadas are the state’s most important natural water storage system. Come April, when the snow melts, the water flows into streams and reservoirs to be captured for the hot, dry months ahead.

When the snowpack does not materialise, California faces shortfalls in water supply.

Two factors aligned for the extreme low in the snow pack last year: winter temperatures too warm to allow formation of snow in the Sierras, especially at lower elevations, and a phenomenon known as the “Ridiculously Resilient Ridge”, the high pressure atmospheric formation over the north Pacific that blew storm tracks off course, preventing rains from reaching California.

Last year was the hottest year on record. The warm temperatures, which prevented winter rains from turning to snow, were the bigger factor in the failure of the snow pack, Trouet said.

The blasting temperatures also intensified the impact of the drought.

But both high temperatures and a lack of rain would need to come into play for a recurrence of 2015’s dismal snow pack, and this year’s. “You need to have the coincidence of high temperature and low precipitation,” she said.

At the moment, there is only a tiny chance of a recurrence of such extreme conditions in the coming centuries.

But with winters continuing to warm under climate change, the likelihood of future low snow packs – and future punishing droughts – increase, she said.

This article titled “California drought: Sierra Nevada snowpack falls to 500-year low” was written by Suzanne Goldenberg US environment correspondent, for on Monday 14th September 2015 19.57 UTC

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