Regional Climate Dynamics

Observed changes in the Sierra Nevada snowpack: potential causes and concerns

A study of the California Sierra Nevada snowpack has been conducted using snow station observations and observed surface temperature data. Monthly snow water equivalent ("SWE") measurements were combined from two data sets to provide sufficient data for statistical analysis from 1930 to 2008. The monthly snapshots are used to calculate peak snow mass timing for each snow season. Since 1930, there has been an overall California trend towards earlier snow mass peak timing by 0.6 days per decade. The trend towards earlier timing also occurs at nearly all individual stations. Even stations showing an increase in late season snowpack values (April 1st SWE) exhibit the trend toward earlier timing, indicating that enhanced melting is occurring at nearly all stations even when more snow is accumulating during the season. Analysis of individual years and stations reveals that warm daily maximum temperatures averaged over March and April are associated with earlier snow mass peak timing for all spatial and temporal scales included in the data set (see accompanying figure). The influence is particularly pronounced for low accumulation years indicating the potential importance of albedo feedback for the melting of shallow snow. The robustness of the early spring temperature influence on peak timing suggests the trend towards earlier peak timing is attributable to the simultaneous warming trend (0.1 degrees Celsius per decade since 1930, with an acceleration in warming in later time periods). Given future scenarios of warming in California, we can expect acceleration in the trend towards earlier peak timing; this will reduce the warm season storage capacity of the California snowpack.



Scatterplot of peak snowpass timing ("SCD") versus April 1st SWE value for 70 stations with at least 75% of years available from 1950 to 2003; colored by the local averaged March and April daily maximum temperature. Temperature data is from the Hamlet and Lettenmaier (2005) data set (available from 1915 to 2003) and has been adjusted for station elevation assuming a constant lapse rate of 6.5 degrees Celsius per kilometer. If the graph is confined to stations with at least 75% of years available from 1930 to 2003, a similar distribution is found. The average SCD for the data set is Julian day 73, and is given by the dashed black line.

Download the paper (Kapnick et. al. 2009) describing these results in more detail.