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Snow levels in the Sierra Nevada mountain range are at their lowest point in 500 years, depriving drought-plagued California of a much-needed source of water, and leaving the state vulnerable to the wildfires currently burning across the landscape.
An analysis of data by researchers from two universities and the National Oceanic and Atmospheric Administration used information from tree rings to reconstruct winter precipitation levels over the last five centuries.
Snowpack refers to the snow that accumulates on the ground after it falls. It is important for the health of ecosystems, and is a crucial source of water in the drier seasons.
Because much of California has a Mediterranean climate, summers tend to be dry, with little rainfall. Most of the rain that trees will receive falls during the other three seasons. The Sierra Nevada snowpack supplies provides California with 30 percent of its water supply, according to the study.
To reconstruct the winter snowpack history over the last several centuries, the researchers focused on tree ring samples taken from a kind of tree commonly known as the blue oak, a tree found in parts of California, especially in the Sierra Nevada.
Blue oaks are especially sensitive to winter precipitation; the amount of rain or snow they receive during the winter months is one of the main factors determining how much they grow in a year. The amount that a tree grows in a single year can be measured in the width of the rings that form in the trunk.
"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.
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.