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 several small (<1 km2) glaciers formed during the Little Ice Age. The largest of these, the Lyell and Maclure glaciers, occupy the headwaters of the Tuolumne River and have been the subject of detailed scientific study since the late 19th century. We repeated historical photographs, field surveys, and velocity measurements on these glaciers to document their response to climate change. Field surveys and remote sensing data indicate that glacier surface areas have diminished by 67-78% since 1883, with 10% of that loss coinciding with the 2012-2015 California drought. The naturalist John Muir first measured the velocity of the Maclure Glacier in 1872, finding that the glacier moved about 2.6 cm/day during the late summer and early autumn. We reproduced Muir's measurements over the same seasonal period and found the glacier to be moving at the same rate, despite the marked reduction in surface area. Time-averaged velocities measured over a four-year period show strong seasonality, with rates near zero in winter. Much of the present movement of the Maclure Glacier must therefore occur as sliding at the bed, which is apparently enhanced by greater melt. The adjacent Lyell Glacier displayed virtually no movement over the same four-year time period, likely because it has thinned below a critical threshold; both glaciers have thinned by more than 40 m since 1932, with thinning up to 3 m/yr during the 2012-2015 drought. New remote sensing data collected as part of NASA's Airborne Snow Observatory project offer opportunities to measure glacier volume and mass balance changes from 2012 onward. Numerical modeling of glacier mass balance will help to predict the timing of complete glacier loss and to assess the associated hydrological impacts on downstream ecosystems.