This April, we learned that the snowpack in the Sierra Nevada was the lowest it's been since records dating back to 1951. The fact that the mountains had just five percent as much snow as they normally would at that time of year is a serious problem: That snow replenishes California's reservoirs and normally provides about 30 percent of the state's water supply. A new study does nothing to ease these anxieties: It seems the snowpack in the spring of 2015 was likely the lowest it's ever been in 500 years.
This new number doesn't change California's plans. After all, state officials already knew the scarcity of the current snowpack, and in response developed appropriate policies, including mandatory water-use cuts. But the new research confirms that this is, indeed, a historic drought.
The tree-ring team is more than 95 percent sure that this spring's snowpack exceeds the low point of any time in the last 500 years.
To estimate the Sierra Nevada snowpack before the advent of weather stations, researchers from several universities and the National Oceanic and Atmospheric Administration in the United States analyzed the widths of tree rings from more than 1,500 blue oaks, the oldest of which had been alive in 1379—more than a decade before Chaucer wrote his Canterbury Tales. (At least one live blue oak has been found that was more than 400 years old, but scientists can get even older data by looking at dead logs.)
The blue-oak data show it's possible there have been a few years, mostly in the 1500s, when the Sierra Nevada held less snow than in spring 2015, the researchers note in a paper published today in the journal Nature Climate Change. However, the tree-ring team is more than 95 percent sure that this spring's snowpack exceeds the low point of any time in the last 500 years.
What can Californians expect in the future? This current study doesn't say anything about whether we can anticipate such low snowpack again, but other studies have found that global warming is expected to reduce the build-up of snow in the American West in general, by making more winter precipitation fall as rain instead of snow, and by making winter snow melt earlier. Having less and earlier-melting snow—even if some of that is made up by rain—is a problem because snow provides California with a steady supply of water in the summer, which tends to be otherwise dry. An abbreviated snow season also means a longer hot, dry season, which increases the risk of wildfires. With the higher winter temperatures of the future, Californians can expect more historically low snowpack, whether or not it hits the nadir of 2015.
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