Michelle Koutnik and Al Rasmussen
Thursday, May 24, 2007
Fifty years at Blue Glacier and South Cascade Glacier
Blue Glacier and South Cascade Glacier in Washington State have been monitored over the past fifty years and both glaciers have experienced significant changes. These two glaciers have very different geometries and this leads to very different responses in glacier length to changes in temperature and precipitation. We discuss the history of research and also the current state of these two glaciers.
At Blue Glacier, we look at changes in temperature, precipitation, and the response of glacier mass balance, thickness, and length. A detailed temperature and precipitation record exists at Forks, WA from 1914-2006. Summer and winter temperatures have increased, especially from 1977-2006. Total winter precipitation has not changed significantly but the percentage of precipitation falling as snow at the glacier has decreased. In response to these changes the ice thickness in both the accumulation and ablation zones has decreased, and the glacier terminus has retreated.
Seasonal components of South Cascade Glacier mass balance are modeled from daily temperature and precipitation at Diablo Dam, 41 km to the north. The model is fit to published glacier-total values over 1959-2005 that have been adjusted to remove the effect of shrinking glacier topography and then is used to reconstruct the record back to 1935. After 1964-1976, when the glacier was nearly in equilibrium with the climate, winter balance became less positive and summer balance more negative. During the period 1977-2005, summer balance became even more strongly negative after 1994 but was slightly overcome by winter balance becoming more positive. The increase in winter balance after 1994 resulted from precipitation that was not only heavier but was also colder, so that a greater fraction of it fell as snow than did over 1977-1994.
Michelle Koutnik is a graduate student working towards her Ph.D. in glaciology at the University of Washington's Department of Earth and Space Sciences. Al Rasmussen is a professor in the UW Department Earth and Space Sciences.