Climate Change: Glaciers

Lyell Glacier: 1883 vs. 2013

Left image
Lyell Glacier in 1883 (Israel Russell, USGS)

Right image
Lyell Glacier in 2013 (NPS Photo)

Of at least four glaciers that John Muir recorded in Yosemite during the 1860s–70s, only one still qualifies as a glacier. These dwindling bodies of ice, cradled in tracks of the vast glaciers that sculpted Yosemite’s canyons and domes over the past 2–3 million years, are shrinking largely due to global climate change. While few visitors to Yosemite ever lay eyes on a glacier, each year hundreds of thousands of sightseers and hikers visit ecosystems fed by their meltwater. Glacial runoff differs from snowmelt in one important way: it continues through late summer and fall after the snow disappears, sustaining life in high-elevation ecosystems that would otherwise be dry. During spring, the trickle of glacial meltwater barely registers in the thundering, snowmelt-fed Lyell Fork of the Tuolumne River. However, by September and October in dry years, that same steady glacial runoff can make up 90% of the river’s smaller flow. This reliable water source shapes ecosystems and human recreation in Lyell Canyon, Tuolumne Meadows, and the Grand Canyon of the Tuolumne River.

 

Two hikers ascend the remnant Lyell Glacier in October, 2016. The Lyell Glacier no longer moves downhill, meaning that it no longer formally meets the definition of "glacier."

Changing Giants

Earth goes through natural cycles of warming and cooling, typically lasting tens of thousands of years. Because glaciers respond to changes in Earth’s climate and their movements carve long-lasting records into the landscape, they are key to allowing geologists to “see” into the past. Around 20,000 years ago, the Tioga Glaciation (the last glacial maximum) filled much of Yosemite with glaciers and ice fields; the largest of these stretched nearly 50 miles from Mount Lyell, down the Grand Canyon of the Tuolumne River, and through Hetch Hetchy Valley. These massive glaciers retreated, then crept forward and retreated again for thousands of years. For several periods around 10,500–5,400 and 4,800–3,200 years ago, the Sierra Nevada may have been entirely free of glaciers. By around 3,200 years ago, as Earth entered a natural cooling period known as neoglaciation, small glaciers began to return to the Sierra Nevada. While Earth is still within this natural cooling period, these glaciers are likely to vanish within 50–250 years due to warming caused by greenhouse gas emissions.

What changes are we seeing?

• Surface area loss: As recently as 100–200 years ago, Yosemite’s glaciers were advancing, forming small moraines sometimes called “Matthes moraines” after geologist ⁠François Matthes. However, by the late 1800s, observers had begun to notice shrinking and loss of glacial features, including crevasses. From 1883–2017, the surface area of Yosemite’s two active glaciers (Lyell Glacier and Maclure Glacier) decreased by 67–78%. Up to 16% of that loss took place during the historic drought of 2012–2015 alone.
• Thinning ice: Since 1933, both glaciers have lost 80–115 feet of dept—around the height of a six-story building. During the 2012–2015 drought, they lost around 13 feet (one story) each year.
• Stalled motion: According to measurements made by park geologists over 2009–2012, the Maclure Glacier is still moving downhill at around one inch per day (the same rate measured by John Muir in 1872). The Lyell Glacier, however, displayed virtually no movement over that four-year period, meaning that it likely no longer meets the definition of glacier.

 

Duration:

42 seconds

This animation shows the retreat of the Maclure and Lyell glaciers from 1883 to 2016. Note: There is no audio.

 

What’s next?

A glacier's health is determined by a combination of winter snowfall and summer and winter temperatures. Although snowfall at high elevations has not declined, both summer and winter temperatures are continuing to rise across the western US. Glaciers may not shrink during the winter, but higher winter temperatures warm the snowpack and require less energy for the ice to reach melting point in summer. Between 1903 and 2004, the volume of water frozen in Sierra Nevada glaciers dropped by over 155 billion gallons—more than the size of the entire Hetch Hetchy Reservoir.

Geologists don’t know for sure how long the Lyell and Maclure Glaciers will be around. Climate change is associated with extreme weather events, which make it more difficult to predict changes from year to year. However, based on current rates of loss, glaciers are likely to disappear from Yosemite’s peaks within decades. When that happens, part of Yosemite’s geologic story will slip into the past, sending ripple effects to plant communities, wildlife, and all those who visit downstream ecosystems.

 

 

References

• Basagic, H. J., and A. G. Fountain. “Quantifying 20th Century Glacier Change in the Sierra Nevada, California.” Arctic, Antarctic, and Alpine Research 43, no. 3 (2011): 317–30. https://doi.org/10.1657/1938-4246-43.3.317.

• Stock, Greg M., Robert S. Anderson, Thomas H. Painter, Brian Henn, and Jessica D. Lundquist. “Impending Loss Of Little Ice Age Glaciers In Yosemite National Park.” Geological Society of America Meeting, 2017. https://doi.org/10.1130/abs/2017am-299617.