Mass balance of a cirque glacier in the U.S. Rocky Mountains

Glacier National Park, Montana, USA, contains 27 cirque glaciers, most less than 1 km² and together comprising about 17 km². These glaciers lie at relatively low elevation (2000 – 3000 m a.s.l.) and latitude (48^o N) and have undergone dramatic retreat since the mid-nineteenth century, when an estimated 150 glaciers existed. Continuing volume losses and the disappearance of glaciers in recent decades are used as key indicators of regional warming. Here we present initial results from a long-term study initiated in 2005 on Sperry Glacier (48.6^oN, 113.75^oW), a 0.8 km² cirque glacier that has undergone an 80% reduction in size since the mid 1800s. We calculated seasonal and annual balances using the direct glaciological method augmented with data from an automated weather station adjacent to the glacier and data from a nearby automated snow pillow. The net annual balance averaged -1.0 m w. eq. for the 2005 and 2006 balance years. Specific balances showed significant transverse spatial variability due to site-specific processes that augment accumulation or mute ablation. These processes have a significant effect on the mass balance of the glacier. Proxy measures showed that the 2005 and 2006 balance years likely had very low accumulation or very high ablation, respectively, relative to other years in recent decades.