Swapping a grounding line for a grounding zone doubles Thwaites' 2100 sea-level estimate
UC Irvine and JPL glaciologists project up to 30 mm of Thwaites sea-level rise by 2100, twice earlier estimates, even with no added warming. Replacing the grounding line with a grounding zone made the model's reconstruction of past change four times more pronounced.
The Scientist · Science desk
Key figures: Poinelli says simulations with a 4-6 km grounding zone give a 60% probability that the Mouginot Ridge unpins by the end of the century. The model projects up to 30 mm of sea-level rise from Thwaites by 2100 and was calibrated against more than 20 years of observations.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Chance Mouginot Ridge unpins by century end, per Poinelli | 60% | with a 4–6 km grounding zone | 5 |
| Sea-level rise by 2100 | 30 mm, up to | 1 | |
| Calibration record | 20 years, over | 8 |
What happened
- Radar from the ICEYE satellites showed the grounding zone on the Thwaites main trunk fluctuating over kilometers, with seawater lifting grounded ice during tidal cycles.
- According to lead author Mattia Poinelli, simulations with a 4 to 6 kilometer grounding zone give a 60% probability that the Mouginot Ridge unpins by the end of the century.
- Thwaites sits in the Amundsen Sea Embayment, which accounts for roughly one-third of mass loss from the Antarctic Ice Sheet.
- Models showing slower movement had been hard to reconcile with satellite observations of substantial ice flow acceleration at Thwaites.
Why it matters
- decision Projection teams that keep a fixed grounding line now have to decide whether to add grounding-zone physics, since that is the assumption this model drops.
- exposure Co-author Eric Rignot says the results imply greater coastal flooding risk and exposure in the coming decades than earlier projections suggested.
- constraint The model was calibrated against more than 20 years of observations, so its match to the 1990s-to-present record is a fit. Only data it has not seen can test the 2100 number.
The change is one assumption in the model. Glaciologists have long treated the grounding line as a discrete, narrow and mostly stable boundary where ice leaves the land and begins to float, according to lead author Mattia Poinelli [12]. A grounding zone lets the model put warm, pressurized seawater several kilometers under ice that is grounded, with the water pushed toward the coast by currents the source says climate change has altered [14]. Poinelli said the resulting melt is detaching Thwaites from the ridges that hold it back, and that flow then speeds up [9].
Co-author Eric Rignot, a UC Irvine professor of Earth system science, put the claim in one sentence. "Most projections to date have struggled to match the rapidity of the mass loss because of missing physics; our new model with more complete physics is able to match the record and then produces projections that are twice as large as before," he said [11]. The report quotes only the paper's authors.
The 30 mm is an upper figure. If it is twice the earlier estimate, that estimate was about 15 mm [15]. The report does not give the lower end of the new range, the number of model runs, or the forcing assumptions beyond "no additional warming". The ridge probability has a flip side: under the same assumed zone width, there is a 40% chance the Mouginot Ridge is still in place at the end of the century [16].
The thing this doesn't tell you is how far a differently built ice-sheet model, given the same grounding-zone physics, would land. The result comes from one model, described in a Nature Communications paper [3].
The ridge matters because of what lies behind it. "The Thwaites main trunk is being temporarily stabilized by the Mouginot Ridge," Poinelli said. "Once that ridge is unpinned, the glacier will continually retreat across a predominantly retrograde seafloor toward its next topographic high, the South Ridge" [10].
What to watch
- Whether other ice-sheet modeling groups add grounding-zone physics and reproduce the doubling.
- Further ICEYE radar passes over the Thwaites main trunk, to see whether the grounding zone keeps fluctuating at kilometer scale and the Mouginot Ridge loosens.
- The full paper's range, ensemble size and forcing assumptions behind the 'up to 30 mm' figure.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence58
- Adoption
- Insufficient
- Hype gap+15
- Incentives
- Insufficient
- Confidence50
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
A new ice sheet model developed by glaciologists at UC Irvine and NASA's Jet Propulsion Laboratory projects that melting of Thwaites Glacier will contribute up to 30 millimeters of sea level rise by 2100, twice previous estimates, even without additional climate warming.
- [2]
By adjusting a computer model of Thwaites to include a grounding zone instead of a grounding line, the researchers reconstructed historical glacier change from the 1990s to the present that is four times more pronounced than traditional models.
- [3]
The paper, Sea-level contribution from Thwaites Glacier doubles in an observation-constrained ice-sheet model, was published in Nature Communications in 2026.
- [4]
Synthetic aperture radar readings from the Finnish ICEYE satellite constellation let the researchers detect kilometer-scale fluctuations in the grounding zone along the main trunk of Thwaites; the data indicate seawater penetrates several kilometers beneath grounded ice and lifts it during tidal cycles.
- [5]
Poinelli said computer simulations that factor in a 4-6 kilometer grounding zone predict a 60% probability of unpinning the Mouginot Ridge by the end of the century.
- [6]
Thwaites Glacier is in the Amundsen Sea Embayment sector of West Antarctica, which accounts for roughly one-third of the mass loss from the Antarctic Ice Sheet.
- [7]
Scientists have been struggling to reconcile ice sheet models that show slower movement with satellite observations that show substantial ice flow acceleration at Thwaites.
- [8]
The UC Irvine-led team calibrated ice sheet models against observations spanning more than 20 years.
- [9]
According to Poinelli, the study showed that grounding zone processes are causing accelerated detachment of Thwaites Glacier from stabilizing ridges, leading to more rapid ice flow and deterioration.
- [10]
"The Thwaites main trunk is being temporarily stabilized by the Mouginot Ridge," Poinelli said. "Once that ridge is unpinned, the glacier will continually retreat across a predominantly retrograde seafloor toward its next topographic high, the South Ridge."
- [11]
Rignot said: "Most projections to date have struggled to match the rapidity of the mass loss because of missing physics; our new model with more complete physics is able to match the record and then produces projections that are twice as large as before."
- [12]
Poinelli said glaciologists have long based their assumptions of ice sheet dynamics on the grounding line, a discrete, narrow and mostly stable demarcation where ice leaves the land and begins floating on water.
- [13]
Rignot said the results predict greater risk of and exposure to coastal flooding than anticipated in the coming decades.
- [14]
The seawater beneath the ice is warm and pressurized, pushed to the Antarctic coastline by currents that have been altered by climate change.
- [15]
If 30 mm is twice the earlier estimate, the earlier estimate was about 15 mm.
- [16]
Under the 4-6 km grounding zone assumption, there is a 40% chance the Mouginot Ridge is not unpinned by the end of the century.
Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgThwaites Glacier could contribute twice as much sea-level rise as previously projected
1 article · October 13, 2026
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Topics
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Entities
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