No. 041 October 31, 2026 9 min read

The Ridge Is the Prediction

Thwaites Glacier's projected sea-level contribution for 2100 just doubled, to thirty millimetres — while the mechanism behind the old metre-by-2100 headline has quietly been taken out of the models. Both landed as alarm.

Thirty millimetres. That is the number at the centre of a paper published in Nature Communications on 12 October, and the reason it travelled — CNN, ScienceDaily, a long tail of aggregators — is that thirty is twice something. Thwaites Glacier, the West Antarctic outlet the press calls the Doomsday Glacier, will contribute up to 30 mm to global sea level by 2100 in the new model. About double earlier projections, and double without any additional warming beyond what is already committed. The coverage ran with worse than thought.

It is worse than thought. It is also thirty millimetres, a bit over an inch, from a glacier holding somewhere between 0.6 and 0.65 m of sea level depending on who is counting, in a century during which the ocean is already rising about 4.4 mm a year on its own. I have spent today trying to work out why those two sentences feel like they belong to different conversations, and I think the answer is that the date on the headline is doing almost none of the work. The prediction in the paper is not really about 2100. It is about a ridge.

What the paper actually changes

The model is from Mattia Poinelli and Eric Rignot with Hélène Seroussi, John Cuzzone, Charles Pelle and Eric Larour. Its central move is conceptual rather than numerical: it stops treating the place where grounded ice becomes floating ice as a line and treats it as a zone, kilometres wide, in which seawater pushes in and out beneath the ice twice a day with the tide. Models have for decades put a boundary there — grounded on one side, floating on the other, migrating slowly, melting modestly. Replace it with a 4-to-6 km band where warm water intermittently reaches under ice that is nominally sitting on rock, and the glacier thins and ungrounds faster.

Calibrated against more than twenty years of observations, the new model reproduces the glacier’s 1990s-to-present change about four times more strongly than conventional models do. That is the real selling point, because the standing embarrassment in this field is that models have persistently failed to match the mass loss satellites actually recorded. Then the forecast: with a 4–6 km grounding zone, a 60% probability that Thwaites detaches from the Mouginot Ridge, the topographic high currently holding its main trunk, before the end of the century — after which, in Poinelli’s words, it “will continually retreat across a predominantly retrograde seafloor” toward the next pinning point, the South Ridge.

The observational footing under the grounding-zone idea is firmer than the modelling. In 2024 Rignot and colleagues used daily-repeat radar interferometry from the ICEYE satellite constellation, March to June 2023, to map Thwaites’ grounding zone directly: about 6 km wide across the central trunk where the bed slopes gently, about 2 km along the steeper flanks, with seawater films 5 to 10 cm thick reaching roughly 6 km upstream at high tide. That is a measurement, not a parameterisation. Alex Bradley, who models the related process of seawater seeping through subglacial sediment, adds a detail I did not expect: the shelves most susceptible to that porous form may be cold-water ones — Ross, Ronne — that nobody currently worries about. And nobody has yet run a whole-Antarctica simulation with grounding-zone melting in it at all.

The mechanism that went the other way

Here is what makes the headline arithmetic strange. Twenty-six months before this paper, the same small community published a result pointing hard the other way, and Seroussi is an author on both.

In August 2024 Mathieu Morlighem, with Dan Goldberg, Jeremy Bassis, Doug Benn, Hilmar Gudmundsson, Seroussi and others, ran three independent ice-sheet models — ISSM, Úa and STREAMICE — against marine ice cliff instability, the mechanism behind the most alarming Antarctic projections of the last decade. MICI is easy to state: disintegrate an ice shelf, expose a tall cliff of grounded ice, and if stresses at its face exceed the strength of ice it fails structurally, calves fast, exposes a taller cliff behind it, and runs away.

The threshold matters enormously, and it moved. The parameterisation under the old projections had failure starting above 80 m of exposed cliff and calving up to 10 m per day above 100 m. A revision built from coupled viscous and brittle fracture modelling puts cliffs stable to roughly 135 m — 55 m higher — calving around 1 m per day at that threshold and rising as a power law. Morlighem’s team took the most aggressive parameters available and then did the most aggressive thing available to the glacier: deleted every square metre of floating ice instantaneously and ran a century.

Thwaites did not run away. Cliffs crossed 135 m in a handful of places. In a harsher second experiment they drove the grounding line back a kilometre a year for fifty years into a trench 1,400 m below sea level before removing the shelf; cliffs then exceeded the threshold over wide areas and retreat still stalled within two decades, because the glacier’s own acceleration thinned it from upstream and dropped the faces back under the threshold. For unstoppable retreat this century, calving would have to run about twenty-five times faster than the worst case they could justify.

Sea-level equivalent, millimetres

Thwaites by 2100, Poinelli et al. 2026

"up to"; no range published

Thwaites by 2100, the figure that doubles

never stated in the release

Antarctica by 2100, DeConto et al. 2021, RCP8.5 median

34 cm

Thwaites, the entire glacier

if all of it went

Antarctica by 2100, DeConto & Pollard 2016, RCP8.5

reported as 64–105 cm

Where the metre came from

The 2016 paper by Robert DeConto and David Pollard is the one everybody remembers: Antarctica alone contributing more than a metre by 2100 under unmitigated emissions, more than fifteen metres by 2500. MICI was its engine, and the case for it was palaeoclimate — without cliff failure the model could not reproduce the metres of extra sea level in the Last Interglacial, or the far higher Pliocene. That argument still stands.

Less remembered is that the same authors revised it downward themselves. The 2021 follow-up ran a 196-member ensemble, history-matched against satellite-era mass loss and against Last Interglacial and Pliocene sea level, and landed on a 2100 median of 34 cm under RCP8.5 — against the 64 to 105 cm they report their own earlier paper as having given, a drop they attribute to improved physics and to forcing that delays surface melt by about twenty-five years. In the same paper they call ice-cliff calving “a key wild card.” The long-term numbers did not soften: a metre by 2125, rates above 6 cm a year by 2150, 9.6 m by 2300.

The IPCC’s Sixth Assessment handled the problem by fencing it off. MICI sits outside the likely ranges entirely, under deep uncertainty, and it is what underwrites the report’s line that a rise approaching 2 m by 2100 and 5 m by 2150 cannot be ruled out. That has not been revisited since Morlighem, and I don’t think it should be yet. Three models agreeing is not a field agreeing, and the 2024 paper omits tabular calving, buttressing by iceberg mélange, shelf regrowth, and coupled viscous–brittle interactions that could raise calving rates by an unknown amount. MICI has still never been observed anywhere.

What nobody has measured

Put the two results side by side and the honest summary is that Thwaites’ century number has come down by an order of magnitude while its post-2100 trajectory has barely moved. Which is roughly the opposite of how either paper was reported.

The weak joints are not hard to find. The 60% unpinning probability is conditional on an assumed 4–6 km grounding-zone width, extrapolated from four months of radar over one glacier. The 30 mm has no published range and no stated scenario, and the baseline it doubles appears nowhere in the press material; if I take the ITGC’s figure that Thwaites supplies about 4% of global sea-level rise today and run it forward at 4.4 mm a year, I get something near 13 mm by 2100, which makes “doubling” look a lot like “the glacier stops being in steady state.” But that is my arithmetic, not theirs, and I would rather have theirs.

And one measurement sits awkwardly against the whole intrusion story. In February 2023 two Nature papers reported what instruments under the eastern Thwaites shelf actually found. Peter Davis and colleagues drilled 600 m through the ice about 2 km from the grounding line and measured basal melt averaging 2 to 5 metres a year — less than models assume — while the water there grew warmer and saltier over nine months without melting speeding up to match. Britney Schmidt’s team, with the Icefin robot, found why that can be true and still alarming: the melt is not spread evenly. It concentrates in staircase terraces and in crevasses, where sloped faces melt far faster than flat ones, and water driven into cracks widens them. “It’s not just how much melting is happening,” as Schmidt put it, “but how and where it is happening that matters.”

So the field’s best direct observation says flat-base melt is weaker than modelled, and its best new satellite observation says water gets further under the grounded ice than modelled, and both are probably right about different surfaces. That is not a contradiction being papered over. It is what an unresolved mechanism looks like from the inside.

The ridge is the prediction

What I keep returning to is that the useful content of the 2026 paper is not a sea-level figure at all. It is a claim about geometry and timing: that one ridge under one glacier is more likely than not to let go within about seventy years, after which the bed slopes the wrong way all the way inland and there is nowhere obvious for the grounding zone to stop short of the South Ridge. The sea-level consequence of that is mostly a twenty-second and twenty-third century quantity. Pinning 30 mm and the year 2100 to it turns a claim about an irreversible threshold into a number small enough to be unimpressive inside a frame alarming enough to mislead, which is a hard combination to reach by accident.

I don’t think it was on purpose. The 2100 horizon is institutional: it is what assessment reports ask for, what adaptation planning budgets against, what proposals are scored on. The object being measured has a characteristic timescale of centuries, and the century window happens to cross it where the number is small. Morlighem’s own summary is the one to keep: “We’re not saying the Antarctic is safe. But we’re confident that the most extreme predictions for this century are less likely.” Both halves, including the first.