Global warming may cause Sierra Nevada glaciers to vanish this century

For generations, the snow and ice tucked into high Sierra cirques, bowl-shaped hollows carved by ancient glaciers, looked small but stubborn.

New research reveals that the region may soon see something no person has witnessed.

The study concludes that several of California’s longest-lived glaciers persisted through the Holocene, the roughly 11,700-year-long warm period since the last Ice Age, and are now on track to vanish within this century.

The finding reframes both the past and the future of ice glaciers in the Sierra Nevada.

Sierra glaciers withstood the test of time

Andrew G. Jones of the University of Wisconsin-Madison Department of Geoscience (UWMDG) led the team that tested whether Sierra glaciers ever fully disappeared during the current interglacial period, a time between major ice ages when global temperatures rise.

The answer points to continuous ice cover across key sites for thousands of years. “Mountain glaciers are sensitive climate indicators,” wrote Jones.

Evidence from paired isotopes, slightly different forms of the same element, in rock shows long burial by ice rather than long intervals of bare exposure.

The theory of iceless glaciers

The researchers sampled recently uncovered proglacial bedrock, the solid rock lying just in front of a glacier. They also dated boulders on moraine ridges, piles of rocky debris left behind as a glacier retreats.

Cosmogenic nuclide concentrations, rare atoms created when cosmic rays strike rock surfaces, were measured in the bedrock.

The team focused specifically on carbon-14 and beryllium-10, a radioactive isotope that reveals how long a rock has been exposed to the sky.

These rare isotopes build up when rock is uncovered and fade when it’s buried under thick ice.

What the isotopes revealed

Numbers from Yosemite’s East Lyell forefield tell a clear story. One sample near the remnant ice shows 1.7 ± 0.1 thousand years of apparent exposure.

A down-valley sample recorded 7.9 ± 0.4 thousand years, and a bedrock site just outside the Matthes moraine.

A glacial ridge formed during the Little Ice Age recorded 9.3 ± 0.5 thousand years, all consistent with repeated burial by advancing ice rather than a fully ice-free valley for most of the Holocene.

At nearby Maclure and Conness forefields, nuclide concentrations, the measured amounts of these isotopes in the rock, are near detection limits, which fits long cover by ice.

Moraine ages at Palisade, the largest Sierra glacier, cluster in the late Holocene, which indicates glaciers hovered near preindustrial extents for much of that period rather than rebuilding from scratch.

Sierra glacier debris and lake sediment

Debris that falls from headwalls and rides on glacier surfaces can change melt rates.

Thick supraglacial debris, the rock and sediment resting on top of a glacier, insulates ice and can make it respond in complex ways to warming. This matters for reading lake sediment records.

Slow-moving, debris-muffled ice grinds less rock flour, the fine gray powder produced when glaciers scrape bedrock, leaving a muted downstream signal that might be mistaken for glacier absence when ice still lingers in cirques.

Paired carbon-14 and beryllium-10 in bedrock offers a check on interpretations based only on lake sediments.

One isotope decays quickly on geologic timescales while the other persists, so their ratio helps separate periods of exposure from burial.

Combining those bedrock records with moraine ages gives both ends of the glacier story. Bedrock signals reveal when ice pulled back from sampled sites, and boulder ages show when the margin last reached a ridge.

Sierra glaciers linked to Little Ice Age

Western North America shows a broader pattern of glacier minimum size in the early Holocene and growth during the late Holocene.

A lake sediment record from Grand Teton points to glacier activation around 6.3 thousand years ago, which aligns with the Sierra evidence for mid-Holocene expansion.

The Sierra also carries signatures of the Little Ice Age, the cool centuries from about 1300 to 1850.

The Matthes moraines mark those late Holocene highs, yet the new ages imply many ridges were already sizable before the Little Ice Age peak.

Unprecedented glacier-free conditions

The isotope pairs favor long ice cover over explanations like the deep scouring of all sampled rock or thick, unbroken blankets of slope debris across entire forefields, the areas directly in front of glaciers, for millennia.

The pattern of ages down East Lyell valley is hard to square with a glacier-free Sierra for most of the Holocene.

“These findings imply that a glacier-free Sierra Nevada is unprecedented since before the Holocene,” wrote Jones.

The phrasing is careful, and it is backed by multiple lines of dated evidence.

What a no-analog future could mean

The global picture is sobering. A comprehensive analysis projects that 49 to 83 percent of the world’s glaciers could disappear by 2100, depending on how much the planet warms.

Scientists call this a no-analog scenario, meaning a situation with no past equivalent in Earth’s climate history.

California summer temperatures have risen by about 3 degrees Fahrenheit since the late 1800s. That increase rivals or exceeds early Holocene warmth that trimmed Sierra glaciers but did not erase the most resilient ones.

The Sierra’s ice plays a significant role beyond scenery. Even small, it cools headwaters in dry years and steadies late-season flows that support alpine life.

Loss of that buffer will change stream timing, sediment flux, the pace at which fine material moves downstream, and habitat conditions high in the watershed.

Uncertain future for Sierra glaciers

Debris thickness varies over time and space. That complexity can mask or mimic glacier behavior in forward models, computer simulations used to test how glacier data evolve, and in field observations.

Storm tracks and snowfall matter too. The Holocene saw shifting winter precipitation and summer sun, and those patterns shaped ice even without human-caused warming.

In the deep past, Sierra glaciers shrank and grew with natural pushes and pulls, yet the largest patches of ice endured. Today, the planet’s temperature is rising fast and steadily.

An ice-free Sierra Nevada within decades would be new in the span of the Holocene. It would also be a clear sign that recent warming has pushed beyond the range that alpine glaciers, small mountain glaciers formed high in steep valleys, can absorb.

The study is published in the journal Science Advances.

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