Only 5% of Antarctic ice cliffs top 52 meters, making a runaway Thwaites collapse less likely

Only 5% of the ice cliffs mapped around Antarctica stand taller than 52.2 meters, according to a study in Science that combined two decades of laser measurements of the ice sheet edge. The finding bears directly on one of the most feared scenarios in climate science, a self-feeding collapse of Thwaites Glacier, and it makes that outcome look less likely in the near term.

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Cliffs of roughly 100 meters have long been proposed as the height beyond which exposed ice would fail under its own weight, shedding blocks faster than snowfall could replace them. Few cliffs in either Antarctica or Greenland approach that mark, and the researchers report that the tallest ones concentrate where glaciers flow rapidly toward the ocean. That pattern matters because the places with the fastest ice flow are also the places researchers watch most closely for signs of trouble, yet even there the measured heights fall short of the proposed instability threshold for the large majority of cliffs.

Twenty years of laser altimetry along the ice edge

The paper, “Empirical constraints on ice cliff calving,” appeared in Science on October 15, 2026, with a summary distributed through a AAAS press release on EurekAlert. Lead author Caroline Needell of the MIT-WHOI Joint Program in Oceanography worked with Catherine Walker of the Woods Hole Oceanographic Institution and Jeremy Bassis of the University of Michigan. They combined airborne and satellite laser altimetry collected from 2003 to 2023 to map the positions and heights of ice cliffs around Antarctica and Greenland.

The central result is a distribution rather than a single measurement. In Antarctica, 5% of observed cliffs exceed 52.2 meters, and in Greenland 5% exceed 76.6 meters. The two ice sheets showed similar average and maximum cliff heights. The tallest cliffs were concentrated where glaciers flow rapidly toward the ocean, and cliff height tracked most strongly with water depth, the thickness of ice mélange packed in front of the glacier, and crevassing near the terminus.

Marine ice cliff instability and the 100-meter threshold

The concern behind the work is marine ice cliff instability, a theory in which a tall cliff, once exposed by the loss of a buttressing ice shelf, fractures and retreats in a runaway chain. The release notes that because the process has never been directly observed, its role in future ice loss and sea level rise remains uncertain. Observations are therefore the only way to test how often cliffs reach heights where the theory would apply.

The team examined 16 glaciers with cliffs taller than 90 meters, but only four of them had both detailed repeat observations and measurable retreat during the study period. Where tall cliffs did exist, they calved more rapidly than previously assumed. Runaway retreat, however, also depends strongly on the slope of the inland ice thickness, and gradual thickness changes appear to limit the buildup needed for sustained instability.

Thwaites Glacier and the 52-meter threshold

Applying the findings to West Antarctica, the authors argue that Thwaites would be unlikely to undergo immediate retreat driven by marine ice cliff instability if an ice shelf failure exposed tall cliffs at its current grounding line. That is a statement about the near-term cascade, not about the glacier’s overall fate. The International Thwaites Glacier Collaboration reports that complete collapse would raise global sea level by about 65 centimeters, that the Thwaites Eastern Ice Shelf covers roughly half of the glacier’s 120-kilometer-wide front and is likely to break up within the coming decade, and that retreat has accelerated over the past 40 years.

Earlier modeling pointed in a similar direction. A 2021 University of Michigan study led by Bassis found that stretching and thinning of ice, together with buttressing from trapped ice chunks, may moderate fracture-driven collapse. Bassis said then that over long timescales ice behaves like a viscous fluid, and that rapid collapse becomes possible only if the ice cannot thin fast enough. He also stated plainly, “There’s no doubt that sea levels are rising.”

The new result does not make Thwaites safe. Ice flow from Thwaites and its neighbors more than doubled between the 1990s and 2010s, according to the collaboration, and models project further, faster retreat through the 21st and 22nd centuries. What the cliff survey removes is a particular worst case in which very tall cliffs appear everywhere at once and drive retreat that feeds itself. The authors’ evidence says such cliffs are uncommon, which leaves slower, shelf-dependent processes as the main sources of uncertainty for sea level projections.

The International Thwaites Glacier Collaboration, a $25 million effort funded jointly by the United Kingdom’s Natural Environment Research Council and the U.S. National Science Foundation, was announced in 2018 because researchers did not know whether the next century would bring roughly 50 centimeters or a full meter of sea level rise from the glacier. For readers tracking Thwaites, the practical takeaway is narrow but useful: cliff height data now exist as a measured distribution, so future models of cliff failure can be tuned against what ice sheet edges look like rather than against assumed heights.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.