By Lucas Carlos Lima

Rendered in July 2025, the International Court of Justice (ICJ) Advisory Opinion on Obligations of States in respect of Climate Change is among the most significant pronouncements of the UN’s principal judicial organ. It is not merely a reply to the UN General Assembly’s (UNGA) April 2023 request. It offers an up-to-date snapshot of the international climate law regime with its evolving dynamics – and an eye into the future.

The opinion reads in two parts. The first part recaps States’ duties under the climate regime and links them to general international law and the interpretative techniques that drive normative development. The second part, in practical terms, delves into the main rules on the consequences of internationally wrongful acts in the climate context – setting the stage for potential future contentious cases and bilateral claims.

Beyond those two dimensions, the Court’s pronouncement is likely to shape not only law but climate politics, with the UN Climate Change Conference (UNFCCC COP 30) in Belém, Brazil, expected to provide a sense of the Advisory Opinion’s initial impact. This article reflects on that prospect and advances three points:

• The Court’s confirmations mark a point of no return, becoming part of the applicable body of international law.
• Its identification of customary rules that are binding on all States is a powerful lever that operates beyond any single negotiating track.
• The Court’s reflections on the role of Conferences of the Parties (COPs) themselves may prove especially consequential for the negotiations.

A point of no return: The force of law

It is well known that ICJ advisory opinions are not binding per se, but the rights and duties they identify – when grounded in treaty or customary international law – are. The opinion creates no new obligations; it interprets existing ones. And, despite some criticism, its unanimous adoption is hard to dismiss in assessing its legal value. The Advisory Opinion turns ambition into a legal floor.

Nor should one ignore that States advanced divergent views in written and oral pleadings during the proceedings. Fundamental disagreements revealed a divided international community on key interpretations. For example, the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC) was a fault line between the Global North and the Global South, particularly over its legal status and operational consequences – an issue the Court confronted and, in part, clarified in its reasoning.

By spelling out stringent due diligence duties and pairing obligations of conduct (such as taking domestic measures, assessing risk, running environmental impact assessments (EIAs), and cooperating) with obligations of result (including preparing, communicating, and maintaining nationally determined contributions (NDCs) that collectively align with Paris Agreement goals), it narrowed the political room for excuse making.

Put plainly, the Court’s reading now sets the legal frame for climate politics. Negotiations and policies will have to operate within it rather than around it. These pronouncements mark a point of no return: they cannot be wished away, and shifting them would require States, collectively, to change the law.

The force of customary law: The rules for the international community as a whole

The Court also roots key duties in customary international law – prevention of significant harm and cooperation – making them binding on all States, not only parties to climate treaties.

Prevention entails stringent due diligence: using all available means to ensure activities under a State’s jurisdiction or control do not cause significant harm to the climate system or other parts of the environment, with the standard shaped by common but differentiated responsibilities and respective capabilities (CBDR-RC). This point is reinforced also by the advisory opinions rendered by the International Tribunal for the Law of the Sea (ITLOS) and the Inter-American Court of Human Rights (IACtHR). Cooperation requires good-faith, sustained, and continuous joint action to avert such harm. More can be expected in the future about the clear content of this obligation of conduct.

These are not rhetorical flourishes. They furnish arguments deployable in any negotiation, while future practice – COP decisions, domestic measures, and case law – will flesh out specific conduct and may ratchet up the applicable standard. That reframes climate diplomacy: the question is no longer whether to act, but how to meet pre-existing legal baselines, with the best science available and precaution setting the yardsticks. In short, the opinion fixes the coordinates; negotiators (party to treaties or not) must now work within law, not around it.

The force of COPs: A tool in the toolbox

Several States used the ICJ proceedings to argue over what, if anything, COP decisions mean in law. The Court was clear: when interpreting climate treaty obligations, States should consider the decisions of the treaties’ governing bodies – and, in some circumstances, those decisions have legal effect. It identified three channels. First, where a treaty expressly empowers the COP, a decision can carry binding weight for specified obligations. Second, even without such powers, COP outcomes may constitute “subsequent practice” under Article 31(3)(a) of the Vienna Convention on the Law of Treaties if they evidence a substantial agreement among parties on how to read the treaty. Third, COP decisions may be relevant to identifying customary international law when they reflect State practice and opinio juris (meaning the practice is legally required) – bringing us back to the customary law point.

The Court stopped short of cataloguing which past COP texts fall where, inviting lawyerly argument – though its reasoning drops hints. The practical upshot is twofold. Negotiators, governments, and civil society now have a clearer map of how COP language can harden into law. Tighter drafting and more explicit signals of consent can be expected going forward. Language agreed in previous COPs will be quoted again, this time with claims of legal effect. At the same time, new fault lines may open – especially on technology transfer and finance – as States contest whether particular phrases are politics or subsequent practice with bite. In Belém, the ICJ’s guidance will not be a footnote: it will shape how texts are framed, read and, if need be, litigated.

Concluding reflections: Out of the lamp

This article has sketched three channels through which the ICJ Advisory Opinion could shape climate politics: it fixes legal baselines that are hard to row back; it grounds key duties in custom that bind all States; and it clarifies how COP decisions can acquire legal effect. To borrow a familiar metaphor, the genie is out of the lamp – and it will not go back in.

For COP 30, that legal anchoring shifts leverage. Small island, vulnerable States and developing countries can press for NDC updates and implementation guidance that meet the Court’s yardsticks: science-based due diligence; transparent planning; and credible pathways consistent with 1.5°C. Finance, technology, and capacity building can be framed less as generosity and more as modalities of the duty to cooperate, raising the political cost of backsliding. We can also expect a litigation-diplomacy loop: courts will cite the opinion to test weak targets or rollbacks, nudging negotiators to codify higher standards in COP decisions. There will be complaints of “judicial overreach,” but the center of gravity has shifted. The ICJ has made climate obligations harder to ignore and easier to enforce. What happens next is up to States – and the coalitions that move them.

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Lucas Carlos Lima is a professor of International Environmental Law at Universidade Federal de Minas Gerais (UFMG), Brazil, and Coordinator of the Research Group on International Courts and Tribunals at UFMG.

This Guest Article is part of a project that seeks to raise awareness and build momentum and knowledge around the ICJ advisory opinion on obligations of States in respect of climate change and to promote a better understanding of the implications of the advisory opinion among sustainable development decision makers.