Study finds snowpack in California’s Sierra Nevada to be lowest in 500 years – ‘The 2015 snowpack in the Sierra Nevada is unprecedented’

By Nicholas St. Fleur
14 September 2015 (The New York Times) – The snow that blanketed the Sierra Nevada in California last winter, and that was supposed to serve as an essential source of fresh water for the drought-stricken state, was at its lowest levels in the last 500 years, according to a new study. The paper, published on Monday in the journal Nature Climate Change, used tree-ring data from centuries-old blue oaks to provide historical context for the mountain range’s diminished snowfall. As of April 1, the snowpack levels were just 5 percent of their 50-year historical average. The paper is the first to create a model that describes temperature and precipitation levels on the Sierra Nevada that extend centuries before researchers started measuring snow levels each year. “The 2015 snowpack in the Sierra Nevada is unprecedented,” said Valerie Trouet, one of the authors of the study and a paleoclimatologist at the University of Arizona. “We expected it to be bad, but we certainly didn’t expect it to be the worst in the past 500 years.” Snowmelt from the Sierra Nevada fills reservoirs that provide a third of all of the drinking water for the state of California, as well as water to fight wildfires and to generate electricity. “The scope of this is profound,” said Thomas Painter, a snow hydrologist with NASA’s Airborne Snow Observatory, adding that models like the one developed in the study suggested a dry future for California in years beyond the current drought. “This has been a very bad drought, and being able to understand the context of it is extraordinarily important.” To determine snowpack levels from 500 years ago, the research team combined two data sets of blue oak tree rings. The first set provided historical precipitation levels from more than 1,500 blue oaks from 33 sites in California’s Central Valley. The team compared part of that data from the years 1930 to 1980 with actual snowpack measurements and found that both findings matched. Using this correlation, the team combined the precipitation data with a second data set of tree rings that looked at winter temperatures from 1500 to 1980. After analyzing the data, the team determined with its model that snowpack levels as low as this year’s were a once-in-1,000-years event. But because of rising temperatures caused by human activities, the researchers said they thought that snow droughts would become much more frequent. [more]

Study Finds Snowpack in California’s Sierra Nevada to Be Lowest in 500 Years

To the editor – California is currently experiencing a record-setting drought that started in 2012 and recently culminated in the first ever mandatory state-wide water restriction. The snowpack conditions in the Sierra Nevada mountains present an ominous sign of the severity of this drought: the 1April2015 snow water equivalent (SWE) was at only 5% of its historical average. In the Mediterranean climate of California, with 80% of the precipitation occurring during winter months, Sierra Nevada snowpack plays a critical role in replenishing the state’s water reservoirs and provides 30% of its water supply. As a result, a multi-year and severe snowpack decline can acutely impact  human and natural systems, including urban and agricultural water supplies, hydroelectric power and wildfire risk. The exceptional character of the 2012–2015 drought has been revealed in millennium-length palaeoclimate records, but no long-term historical context is available for the recent snowpack decline. Here, we present an annually resolved reconstruction of 1April SWE conditions over the whole Sierra Nevada range for the past 500years (Fig.1). We combined an extensive compilation of blue oak tree-ring series that reflects large-scale California winter precipitation anomalies (Supplementary Information and Supplementary Fig.1) with a tree-ring-based California February–March temperature record in a reconstruction that explains 63% of the Sierra Nevada SWE variance over the instrumental period (Supplementary Table1). Our reconstruction shows strong statistical skill (Supplementary Table2), but underestimates anomalously high SWE values over the instrumental period (for example, in 1952 and 1969). However, SWE lows (for example, in 1934 and 1977) are reliably captured and our reconstruction reveals that the 2015 low is unprecedented in the context of the past 500years (Fig.1). Our error estimation indicates that there is a possibility that a few (primarily sixteenth century) years exceeded the 2015low, but the estimated return interval for the 2015SWE value — as calculated based on a generalized extreme value (GEV) distribution (Supplementary Information) — is 3,100years and confirms its exceptional character.

Multi-century evaluation of Sierra Nevada snowpack