A large fraction of California's water supply arrives not as rain, but as snow. In the current severe Califonian drought, levels of snowpack in the Sierra Nevada are at just 5% of the historical average, however records of this significant water resource are short. Soumaya Belmecheri and Valerie Trouet from the Laboratory of Tree-Ring Research are the main authors on a study showing that the present lack of snow is not only unusual since records began, but probably the lowest level for the past 500 years.
The study used tree-ring data to reconstruct the past levels of snowpack, but not from the direct affects of snow accumulating around trees. Some oaks growing at lower elevations are sensitive to the amount of moisture at the times of year when most snow falls in California, and a large collection of other tree-ring data provided a temperature reconstruction for the same months. The researchers could reliably match the historical low snowpack records by a careful combination of the two sets of tree-ring data, and could then extend this back in time for several centuries, demonstrating how rarely the current levels had occurred in the past. Dry, warm winters are particularly important for droughts, and these become more frequent in projections of future human-caused climate change.
Multi-century evaluation of Sierra Nevada snowpack
Soumaya Belmecheri, Flurin Babst, Eugene R. Wahl, David W. Stahle & Valerie Trouet
Nature Climate Change (2015) doi:10.1038/nclimate2809
Published online 14 September 2015