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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.

Range of projections of sea level rise, relative to 2015, from the Greenland ice sheet using ice-sheet models (shading area, representing model spread) and satellite observations (dark line). Adapted from Aschwanden et al. (2021).

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”.

Schematic diagram of grounding zones, where ice sat on top of bedrock transitions into a floating ice shelf.
Schematic diagram of grounding zones, where ice sat on top of bedrock transitions into a floating ice shelf. Relatively warm ocean water can enter into grounding zones by either tidal intrusion or porous intrusion. Credit: Bradley and Freer

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.

Map of the relative susceptibility to porous intrusion from modelling.
Map of the relative susceptibility to porous intrusion from modelling. Superimposed are locations of key Antarctic ice shelves (with ovals indicating likely ranges); those located in purple areas appear relatively more susceptible to porous intrusion, while those located in orange areas appear less susceptible. The inset shows the locations of these ice shelves in Antarctica in corresponding colours. Bradley & Hewitt (2024)

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.

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Landmark deal to share Chile’s lithium windfall fractures Indigenous communities

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Rudecindo Espíndola’s family has been growing corn, figs and other crops for generations in the Soncor Valley in northern Chile, an oasis of green orchards in one of the driest places on Earth the Atacama desert.

Perched nearly 2,500 metres above sea level, his village, Toconao, means “lost corner” in the Kunza language of the Indigenous people who have lived and farmed the land in this remote spot for millennia.

“Our deep connection to this place is based on what we have inherited from our ancestors: our culture, our language,” said Espíndola, a member of a local research team that found evidence that people have inhabited the desert for more than 12,000 years.

This distant outpost is at the heart of the global rush for lithium, a silvery-white metal used to make batteries for electric vehicles (EV) and renewable energy storage that are vital to the world’s clean energy transition. The Atacama salt flat is home to about 25% of the world’s known lithium reserves, turning Chile into the world’s second-largest lithium producer after Australia.

For decades, the Atacama’s Indigenous Lickanantay people have protested against the expansion of the lithium industry, warning that the large evaporation ponds used to extract lithium from the brine beneath the salt flats are depleting scarce and sacred water supplies and destroying fragile desert ecosystems.

Espíndola joined the protests, fearing that competition for water could pose an existential threat to his community.

But last year, he was among dozens of Indigenous representatives who sat across the table from executives representing two Chilean mining giants to hammer out a governance model that gives Indigenous communities living close to lithium sites a bigger say over operations, and a greater share of the economic benefits.

A man wearing a black T-shirt and a hat stands in front of a tree
Rudecindo Espíndola stands in a green oasis near the village of Toconao in the Atacama desert (Photo: Francisco Parra)

A pioneering deal

The agreement is part of a landmark deal between state-owned copper miner Codelco and lithium producer the Sociedad Química y Minera de Chile (SQM) to extract lithium from the salt flats until 2060 through a joint venture called NovaAndino Litio.

The governance model that promises people living in Toconao and other villages around the salt flats millions of dollars in benefits and greater environmental oversight is the first of its kind in mineral-rich Chile, and has been hailed by industry experts as the start of a potential model for more responsible mining for energy transition metals.

NovaAndino told Climate Home News the negotiations with local communities represented an “unprecedented process that has allowed us to incorporate the territory’s vision early in the project’s design” and creates “a system of permanent engagement” with local communities.

The company added it will contribute to sustainable development in the area and help “the safeguarding of [the Lickanantay people’s] culture and environmental values”.

    For mining companies, such agreements could help reduce social conflicts and protests, which have delayed and stalled extraction in other parts of South America’s lithium-rich region, known as the lithium triangle.

    “Argentina and Bolivia could learn a lot from what we’re doing [here],” said Rodrigo Guerrero, a researcher at the Santiago-based Espacio Público think-tank, adding that adopting participatory frameworks early on could prevent them from “going through the entire cycle of disputes” that Chile has experienced.

    Justice at last?

    As part of the governance deal, NovaAndino has pledged to adopt technologies that will reduce water use and mitigate the environmental impacts of lithium extraction.

    It has also committed to hold more than 100 annual meetings with community representatives to build a “good faith” relationship, and an Indigenous Advisory Council will meet twice a year with the company’s sustainability committee to discuss its environmental strategy, company sources said. The meetings are due to begin next month.

    To oversee the agreement’s implementation, an assembly – composed of representatives from all 25 signatory communities – will track the project’s progress. In addition, NovaAndino will hold one-on-one meetings with each community to address issues such as the hiring of local people and the protection of Indigenous employees.

    A flamingo at the Chaxa Lagoon in the Atacama salt flat (Photo: REUTERS/Cristian Rudolffi)

    Espíndola said the deal, while far from perfect, was an important step forward.

    “Previously, Indigenous participation was ambiguous. Now we talk about participation at [every] hierarchical level of this process, a very strong empowerment for Indigenous communities,” said Espíndola, adding that it did not give local communities everything they had asked for. For instance, they will not hold veto power over NovaAndino’s decisions or have a formal shareholder role.

    But after years of conflict with mining companies, a form of “participatory justice is being done”, he said.

    Not everyone is convinced that the accord, pushed by Chile’s former leftist government, marks progress, however.

    “Not in our name”

    The negotiations have caused deep divisions among the Lickanantay, some of whom say greater engagement with mining companies will not stop irreparable damage to the salt flats on which their traditional way of life depends. Others fear the promise of more money will further erode community bonds.

    In January 2024, Indigenous communities from five villages closest to the mining operations, including Toconao, blocked the main access roads to the lithium extraction sites. They said the Council of Atacameño Peoples, which represents 18 Lickanantay communities and was leading discussions with the company, no longer spoke for them.

    Official transcripts of consultations on the extension of the lithium contracts and how to share the promised benefits reveal deep divisions. Tensions peaked when communities around the mining operations clashed over how to distribute the multimillion-dollar windfall, with villages closest to the mining sites demanding the largest share.

    Eventually, separate deals establishing a new governance framework over mining activities were reached between Codelco and SQM with 25 local communities, including a specific agreement for the five villages closest to the extraction sites.

    Codelco’s chairman Maximo Pacheco (Photo: REUTERS/Rodrigo Garrido)

    The division caused by the separate deal for the five villages “will cause historic damage” to the unity of the Atacama desert’s Indigenous peoples, said Hugo Flores, president of the Council of Atacameño Associations, a separate group representing farmers, herders and local workers who oppose the mining expansion.

    Sonia Ramos, 83, a renowned Lickanantay healer and well-known anti-mining activist, lamented the fracturing of social bonds over money, and for the sake of meeting government objectives.

    “There is fragmentation among the communities themselves. Everything has transformed into disequilibrium,” said the 83-year-old.

    “[NovaAndino] supposedly has economic significance for the country, but for us, it is the opposite,” she said.

    The company told Climate Home News it has “acted consistently” to promote “transparent, voluntary, and good-faith dialogue with the communities in the territory, recognising their diversity and autonomy, and always respecting their timelines and forms of participation”.

    A one-off deal or a model for others?

    The NovaAndino joint venture is a pillar of Chile’s strategy to double lithium production by 2031 and consolidate the copper-producing nation’s role in the clean energy transition as demand for battery minerals accelerates.

    Chile’s new far-right president, José Antonio Kast, who was sworn in last week, promised to respect the lithium contracts signed by his predecessor’s administration – including the governance model.

    Still, some experts say the splits over the new model highlight the need for legislation that mandates direct engagement and minimum community benefits for all large mining projects.

    “In the past, this has lent itself to clientelism, communities who negotiate best or arrive first get the better deal,” said Pedro Zapata, a programme officer in Chile for the Natural Resource Governance Institute.

    “This can be to the detriment of other communities with less strength. We cannot have first- and second-class citizens subject to the same industry,” he added.

    The government is already negotiating two more public-private partnerships to extract lithium with mining giant Rio Tinto, which it said would include a framework to engage with Indigenous communities and share some of the revenues. The details will need to be negotiated between local people, the government and the company.

    Sharing the benefits of mining

    Under the deal in the Atacama, NovaAndino will run SQM’s current lithium concessions until they expire in 2030 before seeking new permits to expand mining in the region under a vast project known as “Salar Futuro” – a process which will require further mandatory consultations with communities.

    Besides the participatory mechanism, the new agreement promises more money than ever before for salt flat communities.

    A stone arch welcomes visitors to the village of Peine, one of the closest settlements to lithium mining sites in the Atacama salt flat (Photo: REUTERS/Cristian Rudolffi)

    Depending on the global price of lithium and their proximity to the mining operations, Indigenous communities could collectively receive roughly $30 million annually in funding – about double what SQM currently disburses under existing contracts.

    When taking into account the company’s payments to local and regional authorities, contributions could reach $150 million annually, according to the government.

    To access these resources, each community will need to submit a pipeline of projects they would like funding for under a complex arrangement that includes five separate financial streams:

    • A general investment fund will distribute funding based on each village’s size and proximity to the mining sites
    • A development fund will support projects specifically in the five communities closest to the extraction sites
    • Contributions to farmers and livestock associations
    • Contributions to local governments
    • A groundbreaking “intergenerational fund” held in trust for the Lickanantay until 2060

    For many isolated communities in the Atacama desert, financial contributions from mining firms have funded essential public services, such as healthcare and facilities like football pitches and swimming pools.

    In the past, communities have used some of the benefits they received from mining to build their own environmental monitoring units, hiring teams of hydrogeologists and lawyers to scrutinise miners’ activities.

    Espíndola said the new model could pave the way for more ambitious development projects such as water treatment plants and community solar energy projects.

    A man in a white shirt and glasses stands in front of a stone wall
    Sergio Cubillos, president of the Peine community, was one of the Indigenous representatives in the negotiations with Codelco and SQM (Photo credit: Formando Rutas/ Daniela Carvajal)

    Competition for water

    The depletion of water resources is one of local people’s biggest environmental concerns.

    To extract lithium from the salt flats, miners pump lithium-rich brine accumulated over millions of years in underground reservoirs into gigantic pools, where the water is left to evaporate under the sun and leaves behind lithium carbonate.

    One study has shown that the practice is causing the salt flat to sink by up to two centimetres a year. SQM recently said its current operations consume approximately 11,500 to 12,500 litres of industrial freshwater for every metric ton of lithium produced.

    NovaAndino has committed to significantly reduce the company’s water use by returning at least 30% of the water it extracts from the brine and eliminating the use of all freshwater in its operations within five years of obtaining an environmental permit.

      Cristina Dorador, a microbiologist at the University of Antofagasta, told Climate Home News that reinjecting the water underground is untested at a large scale and could impact the chemical composition of the salt flats.

      Continuing to extract lithium from the flats until 2060 could be the “final blow” for this fragile ecosystem, she said.

      Asked to comment on such concerns, NovaAndino said any new technology will be “subject to the highest regulatory standards”, and pledged to ensure transparency through “an updated monitoring system with the participation of Indigenous communities”.

      High price for hard-won gains

      For the five communities living on the doorstep of the lithium pools, one of the biggest gains is being granted physical access to the mining sites to monitor the lithium extraction and its impact on the salt flats.

      That is a first and will strengthen communities’ ability to call out environmental harms, said Sergio Cubillos, the community president of Peine, the village closest to the evaporation ponds. It could also give them the means to seek remediation through the courts if necessary, Espíndola said.

      Gaining such rights represents long-overdue progress, Cubillos said, but it has come at a high price for the Lickanantay people.

      “Communities receiving money today is what has ultimately led to this division, because we haven’t been able to figure out what we want, how we want it, and how we envision our future as a people,” he said.

      Main image: A truck loads concentrated brine at SQM’s lithium mine at the Atacama salt flat in Chile (Photo: REUTERS/Ivan Alvarado)

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      Roadmap launched to restart deadlocked UN plastics treaty talks

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      Diplomats will hold a series of informal meetings this year in a bid to revive stalled talks over a global treaty to curb plastic pollution, before aiming to reconvene for the next round of official negotiations at the end of 2026 or early 2027.

      Hoping to find a long-awaited breakthrough in the deeply divided UN process, the chair of the talks, Chilean ambassador Julio Cordano, released a roadmap on Monday to inject momentum into the discussions after negotiations collapsed at a chaotic session in Geneva last August.

      Cordano wrote in a letter that countries would meet in Nairobi from June 30 to July 3 for informal discussions to review all the components of the negotiations, including thorny issues such as efforts to limit soaring plastic production.

        The gathering should result in the drafting of a new document laying the foundations of a future treaty text with options on elements with divergent views, but “no surprises” such as new ideas or compromise proposals. This plan aims to address the fact that countries left Geneva without a draft text to work on – something Cordano called a “significant limitation” in his letter.

        “Predictable pathway”

        The meeting in the Kenyan capital will follow a series of virtual consultations every four to six weeks, where heads of country delegations will exchange views on specific topics. A second in-person meeting aimed at finding solutions might take place in early October, depending on the availability of funding.

        Cordano said the roadmap should offer “a predictable pathway” in the lead-up to the next formal negotiating session, which is expected to take place over 10 days at the end of 2026 or early 2027. A host country has yet to be selected, but Climate Home News understands that Brazil, Azerbaijan or Kenya – the home of the UN Environment Programme – have been put forward as options.

        Countries have twice failed to agree on a global plastics treaty at what were meant to be final rounds of negotiations in December 2024 and August 2025.

        Divisions on plastic production

        One of the most divisive elements of the discussions remains what the pact should do about plastic production, which, according to the UN, is set to triple by 2060 without intervention.

        A majority, which includes most European, Latin American, African and Pacific island nations, wants to limit the manufacturing of plastic to “sustainable levels”. But large fossil fuel and petrochemical producers, led by Saudi Arabia, the United States, Russia and India, say the treaty should only focus on managing plastic waste.

        As nearly all plastic is made from planet-heating oil, gas and coal, the sector’s trajectory will have a significant impact on global efforts to reduce greenhouse gas emissions.

        Countries still far apart

        After an eight-month hiatus, informal discussions restarted in early March at an informal meeting of about 20 countries hosted by Japan.

        A participant told Climate Home News that, while the gathering had been helpful to test ideas, progress remained “challenging”, with national stances largely unchanged.

        The source added that countries would need to achieve a significant shift in positions in the coming months to make reconvening formal negotiations worthwhile.

        Deep divisions persist as plastics treaty talks restart at informal meeting

        Jacob Kean-Hammerson, global plastics policy lead at Greenpeace USA, said the new roadmap offers an opportunity for countries to “defend and protect the most critical provisions on the table”.

        He said that the document expected after the Nairobi meeting “must include and revisit proposals backed by a large number of countries, especially on plastic production, that have previously been disregarded”.

        “These measures are essential to addressing the crisis at its source and must be reinstated as a key part of the negotiations,” he added.

        The post Roadmap launched to restart deadlocked UN plastics treaty talks appeared first on Climate Home News.

        Roadmap launched to restart deadlocked UN plastics treaty talks

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        Iran War Shows That Doubling Down on Fossil Fuels Is ‘Delusional,’ UN Climate Chief Says

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        Price spikes from the war highlight the necessity of the renewable energy transition for stability and national security, the U.N. official says.

        The Iran war’s disruption to the global energy market should be a wake-up call for countries that continue to rely on fossil fuels, said United Nations climate chief Simon Stiell in a speech on Monday.

        Iran War Shows That Doubling Down on Fossil Fuels Is ‘Delusional,’ UN Climate Chief Says

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