One year on: Impacts of a landmark legal opinion on climate change

06 06 2025 | 16:10Isabella Kaminski

An international court’s opinion that greenhouse gases are an ocean pollutant is already being used in the fight against climate change.

The city of Hamburg lies on the banks of the River Elbe in Germany, its busy port one of Europe’s most important gateways to the North Sea and beyond.

It is also home to the wave-shaped headquarters of the International Tribunal for the Law of the Sea (Itlos), a little-known court that is responsible for interpreting and upholding the 1982 United Nations Convention on the Law of the Sea (Unclos).

The court usually deals with fairly technical matters and maritime disputes between states. But in 2022, it looked at perhaps the biggest international question there is: climate change.

Specifically, the court was asked what legal duties Unclos imposes on signatory states to protect the marine environment from the impacts of climate change. The request came from the Commission of Small Island States on Climate Change and International Law (Cosis), a group led by Tuvalu, and Antigua and Barbuda. These states are among those that will be hit hardest by rising sea levels in a warmer world.

A year ago, the court issued its advisory opinion that greenhouse gases are pollutants, which are severely damaging the marine environment. It says states have a legal responsibility to control them that goes beyond the Paris Agreement. Itlos became the first court in the world to produce such a document on climate change.

“We could not have expected a better outcome,” says Payam Akhavan, professor of international law at the University of Toronto and chair of the Cosis committee of legal experts. “We succeeded on all the essential points that we put to the tribunal … and the tribunal went further in some respects.”

Akhavan says the power of the request was in its simplicity. It relied heavily on the established principle in international law that states must avoid transboundary harm. “We didn’t try to be incredibly creative or to push the envelope.”

Experts say Itlos firmly established that courts and governments cannot ignore the scientific reality of climate change.

Dialogue Earth spoke to Kate McKenzie, CEO and director of the Climate Change Legal Initiative, which works to establish legal means for forcing climate action.

She notes that the Itlos opinion brought together the 2015 Paris Agreement on climate change and Unclos as “two complementary legal documents” – it stresses that ocean protection measures must incorporate the Paris Agreement’s aim to limit the average global temperature rise to 1.5°C. “Scientifically, it’s all been connected forever; legally, it hasn’t,” says McKenzie. “We’re finally … starting to connect the dots.”

Filling in the gaps

The tangible impacts of this legal opinion are still difficult to determine one year on.

“The full implications of saying that greenhouse gas emissions are pollution haven’t necessarily been worked through yet,” says Kate Cook, a barrister at Matrix Chambers. Cook acted for Mauritius during the Itlos proceedings, but spoke to Dialogue Earth in a personal capacity.

“It wasn’t a contentious issue in the proceedings – most states supported it. But when you actually think about the breadth of obligation under the convention, and then you look at what states are doing at the national level, I would imagine there’s quite a lot of gap-filling to do.”

McKenzie says the opinion is also “really clear” that states have due diligence obligations, which require them to put in place national legislation and regulations to protect and preserve the marine environment – and to enforce them. However, she says this has not yet led to any satisfactory new or updated laws.

Another concrete statement in the Itlos advisory opinion is that developed states should provide financial assistance to developing states, particularly those vulnerable to climate change. Such help should support global and regional cooperation, technical assistance and monitoring.

Dialogue Earth also consulted Shobha Maharaj, a professor at the University of Fiji’s Ecological and Climate Crises Legal Institute, and a climate scientist who gave evidence to Itlos. Maharaj had hoped the legal opinion would bear tangible results, particularly for small island developing states. “So far, I haven’t seen anything like that happening,” she admits.

There were always going to be limits to the impact of the tribunal’s opinion. Although 169 countries have ratified Unclos, there are some notable exceptions, such as the United States.

Moreover, advisory opinions are not legally binding.

“States that are laggards [on climate action] are not going to be doing anything that they’re not obliged to do,” notes Carly Hicks, chief strategy and impact officer for the London-based NGO Opportunity Green, which uses legal, economic and policy knowledge to tackle climate impacts.

Hicks adds that the court has nevertheless provided an “additional string” to the bow of people working on climate action.

Hicks says the advisory opinion was referenced during recent International Maritime Organization negotiations by nations vulnerable to climate change, such as Tuvalu. Those talks resulted in the first global agreement to drive down greenhouse gas emissions from shipping. “It’s been a boost to these nations to be able to bring something concrete into the discussion,” says Hicks.

The opinion is also being used to try to push countries to submit more ambitious nationally determined contributions (NDCs) to the UN Framework Convention on Climate Change (UNFCCC), says Hicks. The latest iteration of these documents was due earlier this year, but most states missed the deadline, which has now been extended to September.

Sea you in court?

Another use of advisory opinions is to support litigation, and the Itlos opinion is starting to be referenced in domestic lawsuits.

In March, the High Court of England and Wales heard a challenge to the United Kingdom government’s decision to issue 31 North Sea oil and gas exploration licences. The case against the decision is its perceived failure to take into account the full consequences for the marine environment or climate change.

The case was brought by Oceana UK, a marine conservation group. Among its arguments, it cites the Itlos advisory opinion to support two of its arguments: “there can be no real dispute as to the impacts of global warming and ocean acidification on the sea, and therefore the protected marine sites in question here”; and “insufficient information or uncertainty as to the extent to which the climate impacts of the proposed plans will impact marine sites [means that adverse impacts cannot be ruled out]”.

McKenzie expects the opinion to be used in future to “strengthen” legal arguments being made using national climate laws.

It is also possible that a climate-vulnerable state could start a case against a high-emitting state under Unclos.

“Obviously, there are political issues that would have to be negotiated,” says Hicks. “A lot of those countries would also be dependent on money from countries that they were wanting to take action against. But I don’t think it’s impossible.”

McKenzie once thought such a lawsuit unlikely, but her view has changed: “We’re in a moment of a real shift in the global order and some countries are in an existential crisis; if you’re a small island state, your country is going away, your territory is going under sea – you’re going to start throwing every legal argument [at it] that you can.”

Similar requests for advisory opinions are currently ongoing at the Inter-American Court on Human Rights and the International Court of Justice (ICJ), where different states, NGOs, and academics are offering their views. Itlos is expected to be referenced in the decisions in those cases.

Cosis deliberately sought to get Itlos to issue its opinion first, says Akhavan, expecting a progressive result that it hoped would “lock” the ICJ into its legal reasoning.

The Itlos advisory opinion was cited around 250 times during the ICJ’s climate hearings, which took place in December. But Akhavan says some states involved in the hearings put forward a “creative reinterpretation” of the opinion’s contents, to deny they had any legal obligations on climate change beyond the Paris Agreement. Other states ignored it altogether.

McKenzie believes people are waiting for the opinion of the ICJ, whose role is to harmonise and integrate international law, before they make full use of these documents. “Courts don’t tend to contradict each other dramatically at that level,” she says.

Assuming the various forthcoming court decisions support each other, the combination “could be used as a heavier tool”, says McKenzie. “I think that that’s why we’re not seeing a tonne of activity around it yet.”

Hicks says the Itlos opinion has forged a “solid anchor” in international law. She also believes it has bolstered the hopes of people working to mitigate climate change.

“Particularly with what’s happening in the US at the moment, where institutions are being ransacked, it’s quite reassuring to know that there are solid international legal principles that are being considered in a quiet, reasonable way that will be able to be relied on by people who are advocating for more progressive action in the years to come.”

The Itlos opinion has also helped small-island states make a bigger mark on the international legal stage, despite very limited resources. “It’s very difficult to get a group of small-island states to cooperate,” says Akhavan. “They have different priorities, different agendas. They’re from different regions of the world and they don’t all see eye to eye. But I think the glue that has kept this together so far has been the advisory opinions.”

“I’ve always seen this as a long-term investment,” says Akhavan. “What today may be an obscure opinion that only excites a few international law nerds may, in the long run, be a kind of Nuremberg moment.”

Cover photo:  Small island states are especially susceptible to climate impacts such as flooding. They are also driving legal battles to force action against climate change. Image: , CC BY-SA 3.0, via Flickr.

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