RETREAT AND STAGNATION OF LITTLE ICE AGE GLACIERS IN YOSEMITE NATIONAL PARK

The high peaks of Yosemite National Park in the Sierra Nevada, California, retain small (<1 km²) glaciers dating back to the Little Ice Age. The largest of these glaciers, the Lyell and Maclure, form the headwaters of the Tuolumne River and are an important source of late summer runoff. The Lyell and Maclure glaciers have been the subject of scientific study and documentation since the late 19^th century and thus offer opportunities for comparative studies of their response to climate change. Beginning in 2007, we repeated a set of historical photographs, topographical surveys, and velocity measurements of the glaciers. By tracking the movement of stakes over a four year period (2009-2012), we found the average horizontal velocity of the Maclure Glacier to be 7.2 m/yr, or 1.9 cm/day. The noted conservationist John Muir first measured the velocity of the Maclure Glacier between 21 August and 6 October of 1872. In 2012, we reproduced Muir’s measurements over the same time period and found the glacier to be moving at the same rate, about 2.6 cm/day. This consistency in velocity occurs despite an approximate 60% reduction in glacier surface area and about 30 m of thinning. Much of the movement of the Maclure Glacier occurs as sliding at the bed, which may be enhanced by greater amounts of meltwater. In contrast, we found that the adjacent Lyell Glacier displayed virtually no movement over the same four year period, suggesting that it has stagnated. As velocity measurements by the National Park Service in 1935 indicated that the east lobe of the Lyell Glacier was moving at about 3.2 m/yr, stagnation has occurred since that time, and perhaps within the past decade. A time series of surface elevation surveys across the width of the Lyell Glacier shows that the Lyell Glacier surface has lowered by as much as 40 m since 1932, with about 3 m of thinning in the past five years; this thinning is likely why the glacier has stagnated. The surface area of the Lyell Glacier has diminished by about 62% since 1903. Ongoing work involving numerical modeling of glacier mass balance from the Little Ice Age into the next century will help to predict the future fate of these glaciers.