Tall ice cliffs can calve faster than previously thought, but new observations suggest they are unlikely to trigger the runaway retreat feared in some projections of Antarctic ice loss. Ice sheets lose ice to the ocean through glaciers and ice streams, ending either in floating ice shelves or steep ice cliffs. It has been proposed that, should floating ice shelves disappear and ice sheets retreat inland, these tall cliffs could become unstable and collapse. This process, known as marine ice cliff instability (MICI), could lead to catastrophic ice-sheet retreat if successive collapses expose increasingly taller cliffs. However, because MICI has never been directly observed, its role in future ice loss and sea level rise remains uncertain. To address these gaps, Caroline Needell and colleagues combined airborne and satellite laser altimetry measurements collected between 2003 and 2023 to map the positions and heights of ice cliffs along the margins of Antarctica and Greenland.

Needell et al. found that very tall cliffs are uncommon – in Antarctica, only 5% of observed cliffs exceed 52.2 meters, while in Greenland, only 5% exceeded 76.6 meters. Very few cliffs in either region approached the roughly 100-meter height previously proposed as the threshold for instability. According to the authors, despite major differences between the two ice sheets, their cliffs had remarkably similar average and maximum heights, with the tallest cliffs concentrated in areas where glaciers are flowing rapidly toward the ocean. The findings show that ice cliff height is most strongly associated with water depth, the thickness of the ice mélange in front of the glacier, and the amount of crevassing near the glacier’s terminus. To test how ice cliffs behave when they approach heights that could trigger MICI, Needell et al. focused on 16 glaciers with cliffs taller than 90 meters, identifying four that had both detailed repeat observations and measurable retreat during the study period. They found that these tall cliffs tend to calve more rapidly than previously assumed. However, the likelihood of runaway retreat depends strongly on the slope of ice thickness inland as gradual changes in ice thickness appear to limit the buildup needed for sustained instability. Applying these findings to the vulnerable Thwaites Glacier of West Antarctica, Needell et al. argue that it would be unlikely to undergo immediate MICI-driven retreat if an ice shelf failure exposed tall cliffs at its current grounding line.