IMPENDING LOSS 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 from the Little Ice Age. The largest of these, the Lyell and Maclure glaciers, have been the subject of detailed scientific study for nearly 150 years, allowing full accounting of changes since the end of the Little Ice Age. We repeated historical photographs, field surveys, and velocity measurements to document glacier response to climate change. Field surveys and remote sensing data indicate that glacier surface areas have diminished by 68-79% since 1883, with up to 16% of that loss occurring during the 2012-2015 California drought. Volume losses were even larger. Both glaciers have thinned by 25-35 m since 1933, with thinning up to 4 m/yr during the 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 fall. We reproduced Muir’s measurements over the same time period in 2012, as well as over a longer four-year period (2009-2012), and found the glacier to be moving at the same rate that Muir measured, despite marked reductions in surface area and thickness. In contrast, the adjacent Lyell Glacier displayed virtually no movement over the same 2009-2012 period. Relative decreases in ice thickness and increases in ice surface slope affecting ice deformation likely explain the different flow behavior exhibited by the glaciers. Stream gauge data suggest that at times during the late summer and fall of the 2012-2015 drought glacier meltwater contributed nearly 90% of the baseflows of the upper Tuolumne River. If present trends continue, the Lyell and Maclure glaciers will likely disappear within decades, significantly reducing baseflows that maintain downstream ecosystems, and bringing to a close the rich history of scientific research on Yosemite’s glaciers.