ITLOS Climate Advisory Opinion

The ITLOS Climate Advisory Opinion is the advisory opinion delivered unanimously on 21 May 2024 by the International Tribunal for the Law of the Sea (ITLOS), sitting in Hamburg, in Case No. 31. Its jurisdictional basis lies in the request submitted on 12 December 2022 by the Commission of Small Island States on Climate Change and International Law (COSIS), an intergovernmental body created by Antigua and Barbuda and Tuvalu under an agreement of 31 October 2021. ITLOS grounded its competence in Article 21 of its Statute (Annex VI to the UN Convention on the Law of the Sea, UNCLOS) and Article 138 of its Rules, which permit the full Tribunal to render advisory opinions where an international agreement related to the Convention's purposes so provides. The COSIS agreement supplied that authorising instrument, and the Tribunal rejected objections — advanced by states including China, India and Australia — that it lacked jurisdiction or should decline to exercise it.

The procedural sequence began with the formal transmission of two questions: what specific obligations UNCLOS states parties bear to prevent, reduce and control pollution of the marine environment arising from anthropogenic greenhouse gas (GHG) emissions, and what obligations they bear to protect and preserve the marine environment from climate-change impacts including ocean warming, sea-level rise and acidification. ITLOS opened written proceedings, receiving thirty-one written statements from states and nine from international organisations, followed by oral hearings held from 11 to 25 September 2023 in which more than fifty delegations participated — the largest turnout in the Tribunal's history. The Tribunal then deliberated and issued a single unanimous opinion of the full bench of twenty-one judges, rather than referring the matter to its Seabed Disputes Chamber.

The opinion's central analytical move was to interpret marine pollution under UNCLOS Article 1(1)(4) so as to capture anthropogenic GHG emissions: the carbon dioxide absorbed by the ocean constitutes a "substance" introduced "directly or indirectly" by humans that produces deleterious effects, satisfying the definitional elements. From this the Tribunal derived that Article 192's duty to protect and preserve the marine environment, and the more specific Article 194 duty to prevent, reduce and control pollution, apply directly to climate-change harm. Crucially, the Tribunal characterised these as obligations of conduct subject to stringent due diligence, not obligations of result, and held that the standard is heightened given the high risks of serious and irreversible harm. It read UNCLOS as informed by external rules including the Paris Agreement, but stressed that compliance with Paris does not automatically discharge UNCLOS obligations, which are autonomous and may demand more.

The opinion has reshaped the contemporary climate-litigation landscape. It was the first of three coordinated international advisory proceedings: the Inter-American Court of Human Rights followed with its opinion in 2025, and the International Court of Justice delivered its own climate advisory opinion on 23 July 2025, which expressly drew on the ITLOS reasoning regarding the marine environment. Vanuatu's foreign ministry, the Pacific island coalition, and COSIS members Antigua and Barbuda and Tuvalu treated the Hamburg ruling as a foundational precedent. Major emitters that had contested jurisdiction — Beijing's and New Delhi's submissions in particular — found their argument that the climate regime should be the exclusive forum rejected by the bench.

The opinion must be distinguished from adjacent instruments. Unlike a contentious judgment of ITLOS or the ICJ, an advisory opinion is non-binding and creates no res judicata; its authority is persuasive, flowing from the Tribunal's interpretive standing. It differs from the ICJ climate advisory opinion of 2025 in scope: ITLOS spoke specifically to obligations concerning the marine environment under UNCLOS Part XII, whereas the ICJ addressed the broader corpus of international law including customary obligations and human-rights treaties. It is also distinct from the Paris Agreement's nationally determined contributions (NDCs), which are procedural pledges; ITLOS held that the UNCLOS due-diligence standard is a separate, potentially more exacting legal benchmark that exists independently of any state's NDC.

Controversy centres on the legitimacy of advisory jurisdiction triggered by a small intergovernmental commission rather than a UN organ, and on the consequences of folding climate obligations into the law of the sea. Critics warn of fragmentation and of subjecting compliance to ITLOS's interpretation of evolving scientific standards, including the Tribunal's reliance on Intergovernmental Panel on Climate Change findings to define the required level of diligence. Proponents counter that the unanimity of the bench and the breadth of state participation lend the opinion exceptional weight. Open questions remain over how the due-diligence standard translates into specific reduction targets, and whether breach could ground state-responsibility claims in future contentious cases under UNCLOS dispute-settlement provisions in Part XV.

For the working practitioner, the opinion is now an operative reference point. Government legal advisers must assess whether national mitigation measures meet a UNCLOS due-diligence threshold that may exceed Paris commitments, and litigators in domestic and regional courts cite it to argue that states owe enforceable obligations to protect the ocean. Foreign ministries advising on maritime delimitation, deep-seabed activity and emissions policy should treat the marine-pollution characterisation of GHGs as the prevailing interpretive baseline, anticipating that it will be invoked in negotiations, compliance review and any subsequent Part XV proceedings.