Assessing the Impacts of Global Warming on Snowpack in the Washington Cascades

The decrease in mountain snowpack attributable to global warming is difficult to estimate in the presence of the large year-to-year natural variability in observations of snow water equivalent (SWE). A more robust approach for inferring the impacts of global warming is to estimate temperature sensitivity of spring snowpack and multiply it by putative past and future temperature rises attributable to global warming. Estimates of sensitivity can be obtained from (a) geometric considerations regarding the change in the climatological snow line resulting from warming, (b) regression of historical April 1 SWE measurements upon mean winter temperatures, (c) a hydrological model forced by daily temperature and precipitation observations, and (d) the distribution of precipitation versus temperature at SNOTEL stations. All four methods yield an estimated 20% loss of spring snowpack for 1deg C warming; considering warming-induced precipitation increases, the sensitivity would decrease to 16%. Using various rates of temperature rise over the Northern Hemisphere, it is estimated that spring snow water equivalent in the Cascades portion of the Puget Sound drainage basin should have declined by 8-16% over the past 30 years due to global warming and it can be expected to decline by another 11-21% by 2050.