"Trees form a new ring every year," said Valerie Trouet, a researcher at the University of Arizona's Laboratory for Tree-Ring Research. "In a year with a wet winter, these trees will grow a lot, and form a wide ring. In years with dry winters, they will not grow a lot and and they will show a very narrow ring."
The team examined the concentric rings that formed in the trunks of 1,500 blue oak trees from that region, paying particular attention to the width of each ring.
They compiled measurements of these tree rings over the last several centuries and paired the data with an already published reconstruction of seasonal temperatures over the same amount of time.
The results showed that the 2014-15 winter in California brought record low snow levels and record high temperatures—two factors that limited the amount of snowpack.
The team published its results Monday in the journal Nature Climate Change.
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California is in its fourth year of drought, and the state has implemented water-rationing measures in cities and is struggling to keep crops irrigated.
The low snowpack in the Sierras in the winter was an early indicator of how severe shortages would be this summer.
"Snowpack is a natural water storage system," Trouet said. "When you have no precipitation in summer, you can access water from snowmelt."
Trouet also said that the wildfires currently raging across the state can be directly linked to the extremely low snowpack levels, as well.
"I am looking at the uncontrollable wildfires in California, and that is not a coincidence. It has been showed for more than a decade that there is a direct link between the amount of snowpack in the winter and the wildfire risk in the following summer."
And if the climate continues to warm as it has, "chances of this happening again in the future are much higher than they were in the past," she said.