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Could Antarctic ice cliffs collapse and make sea levels rise much faster? A continent-wide, multidecadal remote-sensing survey is being designed to test whether marine ice cliffs have a height-related stability limit. That evidence could narrow one of the largest uncertainties in long-range Antarctic sea-level projections, but it is not itself a forecast and does not yet confirm or rule out marine ice cliff instability (MICI).
What the survey is testing
Marine ice cliff instability in plain language
Antarctica’s ice shelves float on the ocean while remaining connected to grounded ice inland. Their friction and geometry can buttress the inland ice, slowing the flow of glaciers into the sea. If a shelf retreats or disappears, the ocean-facing edge of grounded ice can become a tall marine cliff.
MICI is the hypothesis that a sufficiently high cliff cannot support its own weight. Structural failure would produce repeated calving, move the ice front inland, expose another high cliff and potentially create a self-reinforcing sequence. Faster inland discharge would then add to global mean sea level more rapidly than in projections that omit this process.
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The survey described in a SCAR 2024 conference abstract uses observations collected across Antarctica over multiple decades to test an empirical stability limit: how cliff height, retreat, calving and ice flow have related under real environmental conditions. A continent-wide record matters because a conclusion based on one glacier or one climate regime could be misleading.
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The abstract establishes the survey’s purpose, not a completed result. The final dataset and statistical findings were not available in the canonical record, so no responsible account can say that the survey has already measured a universal collapse threshold.
What recent modeling found
Three-model tests in West Antarctica
A 2024 assessment led by Mathieu Morlighem implemented a new MICI parameterization in three ice-sheet models. In experiments focused on the Amundsen Sea Embayment, researchers removed currently stabilizing ice shelves and also imposed grounding-line retreat that exposed substantially higher cliffs.
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Under the tested parameterization, the cliffs did not enter runaway collapse in either set of experiments. Ice discharge increased, but the models also produced faster ice flow and thinning that stabilized the response rather than sustaining an accelerating cliff-failure sequence.
What that result does—and does not—mean
The modeling result argues against near-term, 21st-century MICI vulnerability for the West Antarctic Ice Sheet in those experiments. It does not show that West Antarctica is safe: ocean-driven melting, grounding-line retreat, shelf weakening and continued thinning can still remove large amounts of ice and raise sea level.
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Nor does a model result settle the observational question. A parameterization represents uncertain fracture and calving physics mathematically. If real fracture networks, stress concentrations or calving styles differ from that representation, the modeled stability could be too strong or too weak.
Why scientists still disagree about cliff failure
Fractures and stress are decisive
The 2024 Annual Review of Earth and Planetary Sciences describes long-term internal glaciological stress as the primary driver of calving, while noting that thinning can make an ice front more vulnerable to environmental forcing. Whether a tall cliff fails depends on pre-existing flaws, the concentration and direction of stress, the geometry of the cliff, and how quickly ice flows in to replace lost mass.
Those controls make a simple height-only rule problematic. Two cliffs of similar height can have very different fracture networks and support conditions. A cliff can also lose height through gradual flow and thinning, changing its stability without a dramatic calving event.
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“Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions.”
— Bassis, Crawford, Kachuck, Benn, Walker, Millstein, Duddu, Åström, Fricker and Luckman, 2024 Annual Review
Survey evidence versus parameterized simulations
| Question | Remote-sensing survey | Recent model experiments |
|---|---|---|
| Evidence type | Observed, multidecadal changes in Antarctic ice fronts and cliffs | Parameterized simulations run in three ice-sheet models |
| Ice conditions | Modern cliffs across varied Antarctic environments | Hypothetical shelf loss and imposed grounding-line retreat in the Amundsen Sea Embayment |
| Fracture and stress | Tests how real retreat, calving and flow correlate with cliff geometry; the final statistical result is not yet available | Represented through the chosen MICI parameterization, including stabilizing flow and thinning feedbacks |
| Time horizon | Multiple decades of historical remote sensing | Experiments relevant to future ice-sheet evolution, including the 21st century |
| What can be concluded now | It is designed to test a stability limit, not to provide a sea-level forecast | No runaway MICI under the tested West Antarctic scenarios; other ice-loss mechanisms remain active |
How much sea-level rise depends on MICI?
The Intergovernmental Panel on Climate Change (IPCC) separates processes that can be projected with at least medium confidence from low-confidence processes such as MICI. Its likely global mean sea-level estimates for 2150 are:
| Emissions pathway | 2150 global mean sea-level rise | Qualification |
|---|---|---|
| SSP1-1.9 | 0.6 m (likely range 0.4–0.9 m) | Processes projected with at least medium confidence |
| SSP5-8.5 | 1.4 m (likely range 1.0–1.9 m) | Processes projected with at least medium confidence |
For 2300, the IPCC gives a global mean rise of 0.3–3.1 m under SSP1-2.6 and 1.7–6.8 m under SSP5-8.5 when MICI is excluded. Including the low-confidence MICI contribution raises the upper SSP5-8.5 outcome to as much as 16 m.
These are scenario outcomes, not a prediction that cliffs will collapse. The difference illustrates why the survey matters: if observations show that the proposed instability limit is unrealistic, the extreme upper tail becomes less plausible; if they support repeated failure at high cliffs, long-horizon high-end estimates may need to retain a larger tail. Either way, ordinary shelf weakening, ocean forcing, grounding-line retreat and ice-sheet thinning remain serious sources of sea-level rise.
What the survey can clarify—and what it cannot
Evidence that would strengthen the MICI case
- Repeated, height-dependent cliff failures across independent Antarctic settings.
- A consistent relationship between cliff height, fracture concentration and rapid retreat after shelf loss or thinning.
- Observed behavior that cannot be reproduced without a structural-failure threshold.
Evidence that would weaken the MICI case
- Stable or gradually retreating high cliffs despite sustained thinning and exposure.
- Retreat rates explained by flow, basal conditions, ocean melting and ordinary calving without repeated structural collapse.
- No continent-wide cliff-height threshold after accounting for stress, geometry and pre-existing fractures.
Limits readers should keep in mind
- A multidecadal record is long for satellite observation but short compared with some ice-sheet transitions.
- Remote sensing can measure geometry and motion far more readily than internal three-dimensional fracture structure.
- Past behavior under observed climate conditions may not fully represent extreme future warming or unusual shelf configurations.
- A survey result must be tested against ice-sheet models before it can change a sea-level projection.
Does this disprove the doomsday sea-level scenario?
No. The latest three-model assessment found no MICI-driven runaway collapse in its severe West Antarctic experiments, and the planned survey has not yet published its final statistical result. Together, those facts make an immediate cliff-instability jump less established than earlier high-end scenarios suggested, but they do not eliminate rapid Antarctic ice loss from more familiar processes. The scientifically defensible conclusion is narrower: the survey may reduce a major uncertainty in the extreme upper tail, while the underlying risk from a warming ocean and weakening ice shelves remains.
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