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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Observations support faster calving from very tall ice cliffs, but they do not show a self-sustaining marine ice-cliff instability (MICI). A 2026 dataset covering 15,330 cliffs around Greenland and Antarctica finds that cliffs over 100 metres exist and can calve up to three times faster than older model parameterizations predict. The same observations show why retreat usually loses momentum: dynamic thinning and movement into regions with smaller ice-thickness gradients limit further cliff growth.
What the large observational dataset measured
Needell, Walker and Bassis’ 2026 dataset catalogued 15,330 ice cliffs along the margins of Greenland and Antarctica. The inventory combines laser altimetry, optical imagery, airborne observations and historical records, allowing cliff geometry to be compared across many environments rather than inferred from one glacier.
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Cliff height is not an isolated measurement in the dataset. Taller or more exposed faces are associated with the surrounding mélange thickness, water depth and crevassing. Those relationships matter because the waterline, floating debris and fracture network can either remove support from a cliff or help hold an ice front together.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallCliffs above 100 metres are unusual, not imaginary
Faces higher than 100 metres are rare, but the observations find them in more than one setting. Their existence establishes that the geometry assumed in marine ice-cliff instability scenarios can occur naturally. It does not establish that every tall cliff will collapse, or that a collapse will continue without an external forcing change.
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What the measurements say about calving speed
The tallest observed cliffs calve faster than the existing parameterizations used in some ice-sheet models, by as much as a factor of three. That result is a constraint on the equations used to convert cliff geometry and environmental conditions into a calving rate.
It is not evidence that calving speed rises in a simple, universal line with height. Height changes the stress state, but the measured rate also depends on water depth, mélange, crevasse density, ice thickness gradients, viscous deformation, brittle tensile failure, shear-band formation and pre-existing flaws. Two cliffs with similar freeboard can therefore retreat at different speeds.
How to interpret the older 0–3 km-per-year range
The 2025 Needell, Walker and Bassis preprint compares the new observations with older parameterizations that ramp calving rates from approximately 0 to 3 kilometres per year. That range describes model behaviour in its original context; it is not a universal observed rate for ice cliffs. The newer measurements indicate that some of the tallest cliffs calve faster than those formulas allow, while still not demonstrating runaway retreat.
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Why the observations do not show runaway MICI
MICI describes a positive-feedback scenario: a retreating ice front exposes a higher cliff, the higher cliff calves faster, and the cycle continues inland without needing a new trigger. The observed cliff population does not display that self-sustaining outcome.
Dynamic thinning reduces the height available to fail
As an ice stream accelerates and thins, the exposed face can become lower even while the front retreats. Thinning removes the geometric amplification that a runaway model requires. Retreat can therefore continue for a time without producing ever-taller, ever-faster cliffs.
Retreat reaches gentler thickness gradients
Ice thickness commonly changes sharply near a margin and more gradually inland. When a front retreats into an area with a smaller ice-thickness gradient, each additional kilometre of retreat produces less increase in freeboard. The mechanism that would amplify calving consequently weakens.
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Buttressing and mélange can change the boundary condition
Floating ice, mélange and contact with the bed provide resistance to flow and calving. Their thickness and extent vary seasonally and geographically. A loss of mélange or buttressing can accelerate a front, but recovery or persistence of that resistance can prevent a temporary acceleration from becoming a self-sustaining instability.
What controls whether a cliff fails
Cliff height is one input to stability, not a stand-alone threshold. The main controls identified by the observations and accompanying analyses are:
- Geometry: freeboard, total thickness, thickness gradients and any bed overdeepening beneath a floating or grounded margin.
- Fracture state: crevasse density, the size and orientation of pre-existing flaws, tensile failure and the formation of shear bands.
- Forcing and resistance: water depth, mélange thickness, ocean and atmospheric forcing, viscous deformation and ice-shelf buttressing.
- Time-dependent response: whether the ice deforms viscously before a crack can run, or fails abruptly in a brittle event.
The most consequential unresolved variable is the size and evolution of pre-existing flaws. They are difficult to measure over an entire ice margin and can change as an ice stream stretches, bends and interacts with its bed.
How competing calving explanations should be tested
A useful comparison is broader than asking which formula gives the fastest retreat. A model should be tested against the following dimensions:
| Test axis | Questions to ask |
|---|---|
| Geometry | Does it use cliff height, freeboard, thickness gradients and bed overdeepening rather than height alone? |
| Fracture state | Can it represent crevasses, flaw size, tensile failure and shear-band formation? |
| Forcing and resistance | Does it include water depth, mélange, ocean and atmospheric forcing, viscous deformation and buttressing? |
| Validation | Does it reproduce observed cliff populations and calving rates across both Greenland and Antarctica, not just one site? |
| Outcome | Does it distinguish a temporary acceleration from self-sustaining runaway retreat? |
This framework prevents a model from being judged solely by whether it can generate a dramatic collapse. A parameterization that matches one high cliff but fails across the 15,330-cliff population is not a general explanation.
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What this means for sea-level projections
The observations constrain how models should represent calving, especially at unusually tall faces. They do not, by themselves, provide a future sea-level contribution. A measured calving rate is a local or episodic response; a sea-level projection also requires the ice flux, grounding-line geometry, shelf buttressing, ocean conditions and the persistence of retreat.
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The 2024 review of ice-cliff instability states that MICI has not yet been directly observed and calls for tests in both modern and paleo environments. Its conclusion is a demand for wider validation, not a dismissal of the mechanism:
“Theories of iceberg calving, ice shelf demise, and ice cliff instability must now be fully tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions.”
What future observations need to resolve
The next decisive tests must link repeated cliff geometry to the same front’s fracture evolution and calving history. In practice, that means combining high-resolution topography with measurements of crevasses, mélange and water depth, then following how those variables change before and after individual calving events.
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- Track whether a cliff becomes taller or thins before its next major failure.
- Measure how mélange thickness and buttressing change at the time of acceleration.
- Map flaw sizes and shear bands, not only visible surface height.
- Test parameterizations across Greenland and Antarctica and across modern and paleo records.
- Separate a short-lived burst of calving from retreat that continues after the original forcing is removed.
On current evidence, the defensible conclusion is narrow but important: very tall cliffs can calve faster than older models predict, yet the observed system contains geometric and mechanical feedbacks that suppress the runaway instability those models sometimes allow.
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