With the International Court of Justice’s landmark advisory opinion on climate change hot off the press this July, hopes were high it could be used as a diplomatic lever for stronger climate action at COP30 in Brazil. But it proved a difficult tool to wield in a tense atmosphere.

The advisory opinion (AO) from the world’s top court – which determined that all states have obligations to protect the climate system from significant harm – has already been woven into new climate litigation and existing legal cases, and judges are starting to reference it in their rulings.

The Mexican community of El Bosque in Tabasco even managed to use it as leverage in recent negotiations with the central government over its latest national climate plan (NDC).

Yet, while some countries wanted the ICJ’s non-binding conclusions to feature in the main political decision approved at November’s climate COP in the Amazon city of Belém, the lack of a coordinated strategic push meant that did not happen, legal experts said.

Monaco, Mexico, the Alliance of Small Island States (AOSIS) and the group of Least Developed Countries (LDCs) all called for the ICJ’s decision – and two other climate advisory opinions from the Inter-American Court of Human Rights and the International Tribunal on the Law of the Sea – to be recognised during various COP30 presidency consultations.

But Jennifer Bansard, the Earth Negotiations Bulletin team leader, told journalists at COP30 that these requests were “at very generic levels” and did not go into the courts’ actionable findings.

“Deep, deep, deep red line”

The closest the ICJ advisory opinion came to being mentioned in a formal text was during a review of the Warsaw International Mechanism for Loss and Damage (WIM). This is key as experts believe the decision has particularly significant implications for the new loss and damage fund.

During these discussions, the Independent Alliance of Latin American and Caribbean Nations (AILAC) said the AO provides “an informed legal foundation” for advancing work on loss and damage. They pointed to “the need for comprehensive assessment and health protection” for vulnerable groups and “forms of reparation” This was supported by Vanuatu, which led the diplomatic work resulting in the ICJ opinion.

But Saudi Arabia, representing the Arab Group, responded that the ICJ’s final outcome is “non-binding” and “does not represent parties’ views” even though it participated in the process. Negotiations, it added, are a “party-driven process based on consensus, and not litigation”.

According to a source in the room, the Arab Group described the inclusion of the ICJ AO anywhere in the WIM document as a “deep, deep, deep red line”. “If you insist on discussing it, we might as well just suspend this session to not waste each other’s time,” said Saudi Arabia’s negotiator. The AO is not mentioned in the final agreed WIM text.

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Harjeet Singh, founding director of the Satat Sampada Climate Foundation and strategic advisor to the Fossil Fuel Non-Proliferation Treaty Initiative, said the group was particularly concerned about the ICJ’s reference to the status of a state as developed or developing as “not static”.

“They feared that formally recognising the opinion would open the door to limitless legal liability for fossil fuel production,” he explained.

Left out of the COP30 cover decision

In addition, the AO’s recognition of a “just and fast transition in line with best available science” was mentioned by Fiji, for the Alliance of Small Island States (AOSIS), at an inaugural meeting on the Just Transition Work Programme. AILAC, Egypt and the UK also raised it during just transition negotiations, while Malawi used it to try to frame transition finance as a legal necessity.

Some states had expected the cover decision to recognise the AO in some form, but text drawn up by Brazil’s COP presidency did not include relevant wording.

The lack of references came despite the fact that the UN asked the ICJ for the advisory opinion unanimously and 96 countries spoke at the hearings.

Data visualisation developed by law professor Margaret Young and designers Dan Parker and Stanislav Roudavski.

Singh said the COP30 battle lines were drawn so sharply on the ICJ opinion because it validates the claims of vulnerable countries for climate justice, while historical and large polluters wanted “to avoid acknowledging any legal framework that implies liability”.

But, he added, while pushing back strongly against it, developed countries “neither championed nor explicitly opposed it in open plenary to avoid negative optics”.

The ICJ’s recognition that COP decisions may have legal effects could also make negotiators more wary of what they agree to.

In the closing COP30 plenary, Palau for AOSIS noted the ICJ’s clear assertion of 1.5C as the legal temperature limit. Yet the final Mutirao decision explicitly reiterates the Paris Agreement’s language of “pursuing efforts” to reach that level, while retaining the original goal of “well below 2°C”.

No coordinated push to champion the AO

Harj Narulla, a barrister specialising in climate litigation and counsel for the Solomon Islands, argued the COP30 decision “undermined” the ICJ’s conclusions. But barring a few nations like Saudi Arabia, he saw the overall outcome as a “failure of capacity and coordination, rather than a principled opposition to using the AO”.

Insiders said government negotiating teams remain too separate from their legal teams, and the former were not properly briefed on how the AO could be used in practice.

The leadership expected from climate-vulnerable countries, particularly the island nations that had advocated for the AO in the first place, also seems to have been absent. A briefing by Ed King and Lindsey Smith, who work on international climate strategy for the Global Strategic Communications Council, described AOSIS’s showing at COP30 in particular as “insipid”.

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Ralph Regenvanu, minister of climate change of Vanuatu and a key architect of the AO campaign, mentioned it several times in public, including at Cambodia’s announcement that it would formally support a fossil fuel non-proliferation treaty. But his focus seemed to be on pursuing a new UN resolution recognising the ICJ’s findings.

Neither AOSIS nor Regenvanu responded to requests for comment.

Influencing the wider narrative

Nonetheless, Mohamed Adow, director of Power Shift Africa who has followed the climate talks for many years, believes the AO is “starting to influence the wider narrative around responsibility and liability”.

“Though it did not make the ‘waves’ in the formal text that many hoped for, it was clearly the ‘undercurrent’ beneath many streams of negotiation,” agreed Singh.

Nikki Reisch, climate and energy programme director at the Center for International Environmental Law, an organisation that supports the youth activists who sparked the AO process, said the opinion also supports “the need to reform the UNFCCC to make it fit for purpose”. That includes preventing fossil fuel industry influence and allowing majority voting so that a handful of countries cannot block climate action.

Eyes on Colombia fossil fuel transition conference

In 2026, the opinion may start to play a stronger role on the global stage, including at an international conference on a just transition away from fossil fuels co-hosted by Colombia and The Netherlands next April.

The Fossil Fuel Treaty initiative says that gathering will align with the AO, “which confirmed that states have a legal obligation to protect the climate, including by addressing fossil fuel production, licensing and subsidies”.

Colombia seeks to speed up a “just” fossil fuel phase-out with first global conference

Experts, meanwhile, expect more domestic lawsuits underpinned by the advisory opinion aimed at pushing countries to raise their ambition on cutting emissions and say inter-state litigation cannot be ruled out.

“COP30 in Belém is by no means the last word on the ICJ AO or the climate duties it confirms,” Reisch said.

A version of this article was originally published in The Wave.

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Why the ICJ’s advisory opinion on climate change took a backseat at COP30