The vast ice sheets that cover Greenland and Antarctica have the potential to trigger catastrophic sea level rise as the climate warms.
But the ice-sheet models that scientists use to project future changes underestimate how fast sea levels are rising now and how much they have risen in the past.
This suggests the models are missing important processes driving ice-sheet retreat.
New research suggests that melting at grounding zones – where the ice transitions from sitting on land to floating on water – could be the missing piece of the puzzle.
And it is potentially a big piece. For example, when one study on the Thwaites glacier in west Antarctica included these processes, its projections of ice-sheet loss more than doubled.
In this guest post, we unpack the latest research on grounding-zone mechanisms and why they suggest that current global projections of sea level rise may be substantially underestimated.
Ice sheets are shrinking faster than models project
Scientists use ice-sheet models to project how fast – and by how much – the Earth’s ice sheets will shrink as global temperatures rise.
These models, developed by scientists at universities and national research institutes, represent physical real-world processes using sets of equations. By solving these equations, scientists gain understanding how the physical system will respond to different scenarios. There are around 20 ice sheet models in use around the world today.
The models capture what we know about how ice sheets flow and deform. Projections using these models suggest that ice sheets will contribute 10s of centimetres to sea level rise by the end of the century.
However, it increasingly looks like these models underestimate the true sensitivity of ice sheets to climate change.
First, we can look at sea level rise from ice sheets over the past 20-25 years. Over this period, we have detailed satellite observations to compare to model predictions.
The figure below shows the spread in projections from models (blue shading) for Greenland’s contribution to sea level rise compared to the satellite data (blue line). This spread is generated by a range of factors, such as how different models describe ice flow, how fine of a resolution they use to represent the real system and the initial state used for model runs. Sea levels are presented relative to 2015, as this was chosen as a benchmark year.
It is clear that the majority of models underestimated the rate of Greenland melt.
Next, we can consider whether models have succeeded in reproducing ice sheet loss from historical warm periods.
For example, proxy records, such as reconstructions of previous shorelines, indicate that during the Pliocene (5.3-2.6m years ago) sea levels were between six and 40 metres higher than today.
This extreme sea level rise would have required a substantial contribution from the world’s ice sheets. However, models are largely unable to reproduce these values, with almost all failing to even attain lower bounds.
Finally, from a different standpoint, we can ask how much climate forcing is required to reproduce current ice sheet retreat rates. Models of the west Antarctic ice sheet typically require ice shelves, which restrain the flow of grounded ice, to be melted very rapidly in order to reproduce current rates. However, recent observations of melting in these regions suggest it is in fact far smaller than models require.
Together, this evidence suggests that ice-sheet models are not entirely capturing what is going on: they should be more sensitive to changes in the climate than they currently are. Two recent papers have suggested that what happens in so-called “grounding zones” could be the missing piece of this puzzle.
Grounding zones
Ice sheets are not static; they spread out like a very thick liquid. The majority of ice in the world’s ice sheets sits on top of land, above sea level. As the ice spreads out and thins, it begins to float, transitioning into a floating ice shelf.
The boundary between ice on land and floating ice shelves – shown in the figure below – is known as the “grounding zone”.
The classical picture of grounding zones is as distinct boundaries between floating and grounded ice. It was previously thought that they migrate on annual-to-decadal timescales as ice sheets retreat and advance, and that little melting took place there.
Recently, however, a different picture has emerged, where grounding zones are actually highly dynamic regions.
In particular, two independent mechanisms – called “tidal intrusion” and “porous intrusion” – are thought to be taking place in grounding zones.
Both of these mechanisms are thought to allow relatively warm ocean water to rush under ice sheets, enabling vigorous melting to take place there.
Tidal intrusion
Antarctica has one of the largest tidal ranges in the world, with sea levels fluctuating by up to seven metres each day.
As the tide rises, water pressure under ice shelves increases, lifting the ice and creating a new cavity in the grounding zone. As a result, this newly formed cavity, which can be up to 15km long, is rapidly filled with ocean water.
When the tide falls, the reverse happens: the ice settles back down and water flushes out of the cavity. This cyclic process – named “tidal intrusion” – occurs up to twice per day, driving rapid flows of warm ocean water into and out of the grounding zone, where it can melt the ice from below.
The tidal opening and closing of grounding zone cavities can be observed using surface elevation and radar measurements from satellites. However, these observations – particularly in fast-changing regions – have typically been limited by how frequently the satellites pass over the same spot, which varies between weeks and months.
Recent research, published last month, has used a unique set of sub-daily satellite observations from the ICEYE constellation to show that tidal intrusion is widespread beneath Thwaites glacier.
This research indicates that warm water is able to intrude up to 6km beneath the Thwaites ice shelf during each high tide, where it could accelerate ice melt from below.
Thwaites – nicknamed the “doomsday glacier” – holds enough ice to raise global sea levels by up to 65cm and has long concerned scientists because of its rapid present retreat, potential for runaway ice loss and possibly important role as a keystone in the west Antarctic ice sheet.
This new research suggests that Thwaites may be even more vulnerable than previously thought.
Porous intrusion
Beneath ice sheets, there are networks of channels and tunnels, as well as porous sediments, through which meltwater from the bottom of ice sheets flows.
In grounding zones, this cold, fresh meltwater meets relatively warm, salty ocean water. The ocean water is denser, allowing it to intrude beneath the meltwater and into the grounding zone. This process is called “porous intrusion”.
Recent modelling has suggested that ocean water can intrude kilometres beneath ice sheets via this mechanism.
However, previous research has not considered how the geometry of these regions change in response to melting. In our new study, published in Nature Geoscience, we show that, when this effect is considered, the porous intrusion mechanism can be far more powerful.
In particular, we show that these systems display a tipping-point like behaviour, in which a small change in ocean temperatures can lead to a dramatic change in the distance that warm water is able to intrude through grounding zones.
We also show that the porous intrusion mechanism is not only applicable to ice shelves exposed to warm water or with high melt rates at a glacier’s base, such as Thwaites, but can also influence cold water ice shelves.
The susceptibility of a glacier to porous intrusion is linked to the slope of the seabed and how quickly melted ice is replaced. This is illustrated in the figure below; glaciers in the purple-shaded area are the most susceptible.
In fact, Thwaites appears to be relatively unsusceptible to porous intrusion – despite being highly vulnerable to tidal intrusion – because it flows very rapidly and ice is replaced very quickly.
Other ice shelves, such as the Ross and Ronne ice shelves, which sit in relatively cold water may, surprisingly, be amongst the most susceptible ice shelves.
Are grounding zones the missing piece?
The majority of ice-sheet models still represent grounding zones as a distinct transition between grounded and floating ice, with no melting there from either tidal or porous intrusion.
The only models able to attain anything resembling sea level rise values during past warm periods have included a grounding-zone melting mechanism or a similar mechanism that boosts their sensitivity to climate change.
These same models have also received much attention for their pessimistic future sea level rise projections. Capturing the past requires an increased sensitivity to climatic change, meaning that they predict much higher sea level rise in future.
So, grounding zone intrusion and melting might be the missing piece to reconcile observed and modelled sea level rise.
While scientists have yet to run model simulations with grounding-zone melting included for the whole of Antarctica, studies focusing on specific regions of the continent’s ice sheets project up to twice as much sea level rise.
Current global projections of sea level rise also do not include grounding-zone melting. This means that these projections – including those that inform the reports of the Intergovernmental Panel on Climate Change (IPCC) – may be substantial underestimates.
Ice-sheet models have “known unknowns” – things we know that we do not know perfectly, but can account for imperfect knowledge of. However, they also have “unknown unknowns” – things that we do not even know are happening and therefore cannot quantify the full effects of.
Although grounding-zone melting might result in higher sea level than we expected, at least we now know that it is happening and can begin to incorporate it into our models. The devil we know is better than the devil we don’t.
The post Guest post: The critical role of ‘grounding zones’ in the retreat of Earth’s ice sheets appeared first on Carbon Brief.
Guest post: The critical role of ‘grounding zones’ in the retreat of Earth’s ice sheets
Climate Change
Australia’s climate credibility tested at Pacific Pre-COP talks, as High Court fossil fuel ruling puts government on notice
NADI, FIJI Thursday 8 October 2026 — As the Pacific Pre-COP talks wrap up and Australia prepares to take the reins of COP31 Negotiations in Türkiye next month, Greenpeace Australia Pacific says the government is on notice over fossil fuel expansion and exports, and must accelerate action to align with a 1.5°C pathway.
Following yesterday’s landmark High Court ruling that the climate impacts of coal and gas exports must be considered by New South Wales planning authorities, Greenpeace Australia Pacific is calling on the Albanese government to find the “courage, leadership and grit” to chart a new course away from fossil fuels.
High res images and video from yesterday’s ‘Keep 1.5C Alive’ flotilla in Nadi can be found here
Speaking from Nadi, Shiva Gounden, Head of Pacific at Greenpeace Australia Pacific, said:
“The outcomes of this week’s talks are a drop in the ocean given the scale of need, and urgency of the crisis our communities are facing. It is like taking a glass of water to a burning house if we do not urgently act to address the root cause of the existential threat facing Tuvalu, Fiji and all Pacific countries: fossil fuel expansion.
“The Electrification Pledge must end fossil fuel dependence, not be an end in itself — its ultimate success depends on ensuring electricity comes from renewable sources that displace fossil fuels and align with a 1.5°C pathway. It must be underpinned by justice and backed by finance flowing from polluters to communities.
“Limiting global warming to 1.5°C is a non-negotiable survival line for humanity and Australia must act. The landmark climate advisory ruling from the ICJ is clear — 1.5°C is the moral, the scientific and the legal limit. Continuing down the fossil fuel path, and failing to align efforts with limiting warming to 1.5°C, is a breach of our international legal obligations, and risks making Australia liable for future reparations from climate-vulnerable nations.”
Also in Nadi, Dr Simon Bradshaw, COP31 Lead and climate expert at Greenpeace Australia Pacific, said: “The Pacific was never going to be a mere backdrop for Australia in its role as incoming chair of the COP31 climate talks, but where its credibility and commitment to climate leadership would be tested.
“Here we see communities fighting for their survival and doing everything possible to hold the line on returning warming to 1.5°C. When governments profess to take their concerns seriously, only to then throw more fuel on the fire, the pain and sadness is visceral.
“This week the High Court of Australia recognised what the Federal Government refuses to — that Australia is responsible for the climate damage of our fossil fuel exports and if governments don’t act, the courts will intervene. The message is simple: this is not someone else’s problem, it is ours.
“We must now follow other countries in developing a national roadmap away from fossil fuels that ensures a managed wind-down of fossil fuel production, including exports, in line with our legal obligation to help return warming to 1.5°C.”
ENDS
Media contact: Kate O’Callaghan in Nadi on +61 406 231 892 (Whatsapp/Signal)
A Dutch NGO has filed the first climate complaint under the European Union’s trade rules, arguing that New Zealand violated the environmental provisions of its free trade agreement with the bloc by weakening its climate regulations.
The case will test whether binding climate provisions in the EU’s free trade deals can be enforced to hold governments accountable to their climate obligations, experts told Climate Home News.
The EU-New Zealand free trade agreement, which came into force in 2024, was the first in the world to include legally-binding climate provisions and possible sanctions for violating them, as the EU seeks to use its trade partnerships to advance greater environmental protection.
Under the deal, both parties committed not to weaken their environmental law to promote trade or investment and to “refrain from any action or omission that materially defeats the object and purpose of the Paris Agreement”.
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At the time the agreement was signed, EU Commission President Ursula von der Leyen said the agreement included “unprecedented social and climate commitments”. But experts warned it was unclear how the Paris Agreement provisions would be enforced.
The EU included a similar “trade and sustainable development” clause in 14 other bilateral trade deals in recent years, with several others, including with China, awaiting ratification or being negotiated.
Climate activists at the Dutch NGO Both ENDS argue that the New Zealand government breached these terms by reopening its waters to offshore oil and gas exploration, releasing a climate plan that barely requires any emissions reductions, and passing a law that prevents corporations from getting sued over climate damages.
“Here, we have a so-called gold standard for free trade agreements with sustainability provisions but we have a trading partner that is doing exactly the opposite: regressing, as all the evidence points to, away from the Paris Agreement,” said Marius Troost, a senior policy advisor at Both ENDS.
Can the EU enforce its climate trading rules?
The Dutch environmental group filed the complaint under the EU Commission’s Single Entry Point, a mechanism that allows civil society to request enforcement of the bloc’s trade commitments.
The EU-New Zealand trade deal includes the possibility of suspending beneficial trading arrangements between the two parties in response to serious violations of its climate provisions. This, Troost said, is a “unique” tool to enforce both parties’ obligations under the deal.
“This is an opportunity for the EU and New Zealand to show that they are actually serious about these commitments,” he told Climate Home News.
A spokesperson for New Zealand’s Ministry of Foreign Affairs and Trade denied any violations of the agreement and said the government hadn’t received formal notice of the complaint. The country, they said, “takes its commitments under the NZ-EU Free Trade Agreement seriously, including the agreement’s environment and climate-related provisions”.
An EU Commission spokesperson said it will start a preliminary assessment of the complaint and engage with NGO Both ENDS. “Sustainability is a central pillar of the EU-New Zealand relationship,” they added.
The EU is New Zealand’s second-largest trading partner after China, with about 14% of the country’s exports going to the European market. Agricultural products like meat, diary, fruit and vegetables are the country’s biggest exports to Europe.
New Zealand is ‘having its cake and eating it’
Eliza Prestidge-Oldfield, a senior legal researcher at the New Zealand-based Environmental Law Initiative, which is supporting Both ENDS’s claim, told Climate Home News that if the EU upholds the complaint, both parties would begin a negotiation process.
“The idea is to try and resolve this positively with the New Zealand government acknowledging areas where it needs to change its actions in order to comply with the agreement, and get that change in place as soon as possible,” she said.
But New Zealand’s right-wing coalition government said it won’t take any directives from foreign actors regarding its policies. Trade minister Todd McClay told local media that it was “not for overseas countries, organisations or lobby groups to tell New Zealand how to meet its obligations”.
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Prestidge-Oldfield argued the complaint isn’t about “Europe telling anyone what to do at all”, but rather stressing the conditions under which they are willing to import goods from New Zealand. “The New Zealand government is trying to have its cake and eat it too,” she said.
If New Zealand refuses to adjust its policies in line with the agreement, the complaint will be assessed by an independent panel, which can require the country to make changes. If those changes are not implemented, the panel could decide that New Zealand should lose its preferential EU market access.
A negotiated resolution is more likely, however, with no prior labour-related complaints to the EU having ever reached the panel stage.
Alexander Gillespie, a law professor at the University of Waikato in New Zealand, said sanctions would be a “last resort”. “This is a test case, which will generate considerable attention – as it is not just about climate change, but how free trade and environmental sustainability have been woven together,” he said in a statement.
Trade deals as tool for climate accountability
Experts said the case could set a precedent for how trade deals can be used to hold governments accountable on climate action. The EU has enacted similar “trade and sustainable development” clauses in its trade agreements with Canada, Japan and South Korea.
While still pending ratification, the EU’s 2024 trade agreement with Mercosur – which encompasses Argentina, Brazil, Paraguay and Uruguay – also includes climate provisions, including a commitment to “effectively implement” the Paris Agreement and promote low-carbon trade.
In addition, legal researcher Prestidge-Oldfield noted that last year’s