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

The decrease in mountain snowpack associated with 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 the temperature sensitivity (lambda) of spring snowpack and multiply it by putative past and future temperature rises observed across the Northern Hemisphere. Estimates of lambda can be obtained from (i) simple geometric considerations based on the notion that as the seasonal-mean temperature rises by the amount delta T, the freezing level and the entire snowpack should rise by the increment delta T/Gamma, where Gamma is the mean lapse rate; (ii) the regression of 1 April SWE measurements upon mean winter temperatures; (iii) a hydrological model forced by daily temperature and precipitation observations; and (iv) the use of inferred accumulated snowfall derived from daily temperature and precipitation data as a proxy for SWE. All four methods yield an estimated sensitivity of 20% of spring snowpack lost per degree Celsius temperature rise. The increase of precipitation accompanying a 1 degrees C warming can be expected to decrease the sensitivity to 16%. Considering observations 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 yr resulting from global warming, and it can be expected to decline by another 11%-21% by 2050. These losses would be statistically undetectable from a trend analysis of the region's snowpack over the past 30 yr.