Ice Age glaciers played an essential role in shaping Yosemite’s landscape. Most of this ice had melted away due to natural warming by about 10,000 years ago. During a more recent cold period called the Little Ice Age, small glaciers formed below the highest peaks. Currently, two remain: the Lyell and Maclure glaciers. These ice bodies are important to local ecosystems because they provide a year-round supply of cold water to the Lyell Fork of the Tuolumne River, especially during the late summer and fall after the winter snowpack has melted.
Glacier Research in Yosemite
Integrating this history of research allows park scientists to compile maps of the ice extent through time. Glaciers are sensitive indicators of climatic changes because their health depends on winter precipitation and summer temperatures. Both the Lyell and Maclure glaciers have been steadily retreating since they were first mapped and photographed in 1883, but melting has accelerated rapidly in the past decade, especially during the drought that began in 2012. Repeat photography shows the glaciers melting back as well as thinning through time. Although glaciers have come and gone from Yosemite many times in the past, scientists agree that the current melting is primarily due to warmer temperatures caused by human activities.
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This animation shows the retreat of the Maclure and Lyell glaciers from 1883 to 2016.
Glaciers move when they are thick enough to flow under their own weight, and when water underneath them reduces friction allowing them to slide. In 2012, with a grant from Yosemite Conservancy, scientists repeated an experiment that John Muir performed in 1872, which measured the movement of the Maclure Glacier using stakes placed in the ice. They discovered that Maclure Glacier was flowing at a rate of about one inch per day—the exact same rate that Muir measured 140 years earlier. This was surprising because the glacier is now less than half the size that it was in Muir's time. Scientists concluded that the Maclure Glacier continues to move mainly because its own meltwater is helping it slide. In contrast, the scientists found that the Lyell Glacier had not moved at all, probably because it is too thin. Glaciers by definition move, so the term "glacier" may no longer properly apply to the Lyell ice body.
Depending on whether drought conditions continue in the coming years, the Lyell Glacier could disappear as early as 2020. The Maclure Glacier is expected to remain longer, but will ultimately follow suit. When that happens, Yosemite's glaciers will become part of its past, marking the end of a rich history of glacier research.