ICJ Climate Ruling Leans on Law of the Sea, and Legal Scholars Say the Reasoning Is Thin

When the International Court of Justice delivered its advisory opinion on the obligations of States in respect of climate change on 23 July 2025, much of the world celebrated. The Court affirmed that the United Nations Framework Convention on Climate Change constitutes the primary legal framework governing the international response to climate change, and it clarified that States’ climate obligations derive not only from climate treaties but also from the law of the sea, international human rights law, other environmental treaties, and the customary international law duties of prevention and cooperation. Now a critical assessment published in Frontiers in Marine Science argues that the Court’s treatment of the law of the sea, while consequential, rests on reasoning that is thinner than its landmark status suggests. The analysis, by Ying Yao and Xintong Tong of Jilin University, examines how the ICJ selected the United Nations Convention on the Law of the Sea as applicable law, how it applied the principle of systemic integration, and whether its evolutionary interpretation of treaty terms crossed the line from interpretation into judicial law-making.

The convergence of the climate and ocean agendas has become a defining feature of contemporary global governance, and the ICJ’s opinion advanced that convergence in international adjudication. When interpreting legal questions within one regime, international courts and tribunals increasingly draw on the norms and principles of the other, and this interaction operates in both directions. The law of the sea has incorporated norms from international climate law, while the climate law regime has drawn upon the law of the sea. The first step in this judicial convergence was taken not by the ICJ but by the International Tribunal for the Law of the Sea, which delivered its own advisory opinion on climate change on 21 May 2024. The ICJ then endorsed the ITLOS opinion and treated the law of the sea as part of the most directly relevant applicable law, placing considerable reliance on the Tribunal’s conclusions without undertaking an extensive separate examination of the relationship between UNCLOS and climate change.

The technical foundation of this convergence lies in the ITLOS’s interpretive methodology. From a normative perspective, the Tribunal characterized the provisions of UNCLOS that refer to external rules as rules of reference. Part XII of UNCLOS, which governs the protection and preservation of the marine environment, contains numerous such provisions, and together with Article 237 of the Convention these rules demonstrate that Part XII adopts an open approach toward other conventions. From the perspective of treaty interpretation, the principle of systemic integration embodied in Article 31(3)(c) of the Vienna Convention on the Law of Treaties requires that, in interpreting a treaty, account be taken of any relevant rules of international law applicable in the relations between the parties. The ITLOS concluded that the UNFCCC-centered climate treaty regime should be taken into account in answering the questions before it, and it went on to characterize anthropogenic greenhouse gas emissions as pollution of the marine environment within the meaning of Article 1(1)(4) of UNCLOS, thereby bringing such emissions within the scope of the relevant provisions of Part XII.

The ICJ then consolidated this convergence in several significant respects. A key contribution lies in clarifying the relationship between climate-related obligations and the secondary rules of State responsibility. The Court went beyond treating the law of the sea as an interpretative aid for climate-related obligations; it also relied on the law of the sea to define the content of those obligations, whose breach may give rise to State responsibility. More specifically, a breach of climate-related obligations may constitute an internationally wrongful act, giving rise to the legal consequences of cessation, assurances and guarantees of non-repetition, and reparation for injured States. The ICJ also expanded the scope of the duty to cooperate under Article 197 of UNCLOS, endorsed the characterization of the due diligence obligation as particularly stringent given the seriousness of the climate threat, and addressed sea-level rise by holding that UNCLOS does not require States to update charts or lists of geographical coordinates to reflect physical changes, and that the loss of territory does not necessarily result in the loss of statehood.

One study identified 21 references to the ITLOS Advisory Opinion in the ICJ Advisory Opinion, and extending the analysis to the separate opinions and declarations of ICJ judges reveals a further six. This reflects a form of judicial dialogue between international courts and tribunals: the ITLOS first established the relationship between climate change and UNCLOS within the law of the sea framework, and the ICJ subsequently incorporated and endorsed that interpretation within the broader framework of international law. UNCLOS is no longer merely a legal framework governing navigation, maritime delimitation, resource exploitation, and traditional marine pollution; it has also become an important legal framework for addressing climate change. Ocean warming, ocean acidification, sea-level rise, and the ocean’s role as a carbon sink have all become integral to the interpretation and application of the Convention. Yet it is precisely this far-reaching influence that makes the ICJ’s methodology warrant critical examination, the authors argue, because the quality of a court’s legal reasoning is what lends its pronouncements persuasive authority.

The first controversy concerns the ICJ’s selection of UNCLOS as applicable law. The Court relied solely on the ITLOS Advisory Opinion as the basis for treating UNCLOS as part of the applicable law, and whether that reasoning is sufficiently justified is open to question. Although the ITLOS correctly concluded that Part XII of UNCLOS is open to other conventions, its reasoning does not fully support the incorporation of climate treaties. The rules of reference in Part XII carry qualifying terms such as generally accepted, applicable, and global, indicating that only rules enjoying broad acceptance among States are capable of being incorporated as external rules. Moreover, the Convention neither specifies whether external rules may be taken into account in its interpretation nor identifies which external rules qualify. Article 237(1) of UNCLOS refers to conventions previously concluded relating to the protection and preservation of the marine environment and to agreements concluded in furtherance of the general principles outlined in the Convention. The UNFCCC, adopted in May 1992, cannot be classified as a previously concluded special convention, and its objective does not appear to be directly related to furthering the general principles of UNCLOS. The preamble to the UNFCCC makes no reference to UNCLOS, and to infer a legal connection solely from the word marine is unpersuasive.

The second controversy concerns the legal characterization of greenhouse gas emissions as marine pollution. Article 1(1)(4) of UNCLOS provides a broad definition encompassing the introduction of substances or energy into the marine environment that results, or is likely to result, in deleterious effects, and the characterization of GHG emissions as marine pollution can be reasonably supported on that basis. However, the diffuse, cumulative, and globally mixed nature of GHG emissions distinguishes them from conventional forms of marine pollution in terms of source identification. The UNCLOS framework is structured according to different sources of pollution, and conventional forms typically have relatively identifiable sources and pathways of transmission, whereas the diffuse nature of GHG emissions makes their sources difficult to trace. From the perspective of internal consistency, Article 194(3)(a) defines pollution from land-based sources and from or through the atmosphere as involving the release of toxic, harmful or noxious substances, but existing scientific evidence primarily demonstrates that GHGs produce long-term and cumulative environmental effects by altering the climate system, rather than characterizing GHGs themselves as pollutants with direct toxic effects in the conventional sense. From the perspective of external consistency, the UNFCCC defines emissions as the release of GHGs and/or their precursors into the atmosphere over a specified area and period of time, and it does not characterize GHG emissions as pollution, so the ITLOS interpretation does not achieve the consistency with the climate treaty regime that it purported to maintain.

The third controversy concerns evolutionary interpretation and its tendency toward judicial law-making. Evolutionary interpretation generally involves applying general and open-ended terms in a treaty to new factual circumstances, and it is indispensable to maintaining a treaty as a living instrument. Yet the boundary between evolutionary interpretation and judicial law-making is often a fine one, depending largely on the extent to which the original intent of the States parties is respected. As Judge Sir Gerald Fitzmaurice once observed, the objects and purposes of a treaty follow from and are closely bound up with the intentions of the parties as expressed in the text, and may not legitimately be sought elsewhere save in special circumstances. Anthropogenic GHG emissions existed long before the conclusion of UNCLOS, yet neither the negotiating history nor the successive draft texts of the Convention suggests that the drafters intended to bring activities involving such emissions within the scope of Article 194(2). Both the ICJ and ITLOS relied to a considerable extent on the findings of the IPCC, effectively relying on scientific evidence in place of a more fully developed legal analysis, without sufficiently examining the preparatory work of the Convention, its context, or the subsequent agreements and practice of the States parties. Because advisory opinions produce legal effects and far-reaching practical consequences even though they are not legally binding, as demonstrated when the ITLOS relied on the ICJ’s Chagos advisory opinion in a binding judgment, this approach further blurs the line between interpretation and law-making.

Objectively assessed, the ICJ’s advisory opinion strengthens the coherence of international climate law and may encourage substantive climate action. The Court’s systemic approach, evident in the opinion itself and in the separate opinions of individual judges, promotes the coherent interpretation and application of international law and mitigates fragmentation. Judge Cleveland observed in her declaration that armed conflict is a significant yet often overlooked source of climate harm, directly generating GHG emissions, destroying carbon sinks, and damaging infrastructure, and that States’ climate-related obligations should extend to climate-related harm arising from military activities. The opinion also has the potential to transform the international community’s approach to climate change by raising the costs of failing to adopt meaningful emissions reduction measures, prompting States to reassess the costs and benefits of implementing and strengthening their mitigation efforts. According to the opinion, States may incur responsibility not only for failing to fulfill their commitments but also for failing to make commitments capable of achieving the required level of GHG emissions reductions.

Yet like two sides of the same coin, the opinion may advance climate governance while also posing challenges. Bringing proceedings on the basis of the opinion remains subject to evidentiary difficulties and considerable uncertainty in establishing causation, because GHGs become thoroughly mixed in the atmosphere, making it impossible to attribute pollution affecting a particular marine area directly to activities under the jurisdiction of any specific emitting State, and because of the temporal gap between emissions and resulting injury. The judicialization of climate governance may also indirectly impede the political process of international climate negotiations, a concern that found resonance within the ICJ itself, where Judge Nolte observed that climate litigation may produce only symbolic legal effects and that treating litigation as a substitute for climate policymaking could prove counterproductive. Moreover, the expansion of climate obligations through treaty interpretation may discourage States from assuming responsibility where the boundaries of such responsibility are increasingly blurred, since ambiguity and indeterminacy in treaty provisions are among the important factors contributing to non-compliance. In the long term, the authors conclude, international judicial bodies and Small Island Developing States should reflect on the potential and limitations of advisory opinions, and reconsider the future integration of climate and ocean law regimes, because only by translating abstract legal obligations into concrete and enforceable mitigation measures, as well as binding arrangements on climate finance and technology transfer, can the survival and fundamental interests of small island States be effectively safeguarded.

Subject of Research: International law of the sea and climate governance

Article Title: The ICJ’s application of the law of the sea in its climate change advisory opinion: a critical assessment

Article References: Yao, Y., & Tong, X. (2026). The ICJ’s application of the law of the sea in its climate change advisory opinion: a critical assessment. Frontiers in Marine Science, 13, Article 1968699. https://doi.org/10.3389/fmars.2026.1968699

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1968699

Keywords: ICJ, ITLOS, UNCLOS, climate change advisory opinion, law of the sea, systemic integration, evolutionary interpretation, Small Island Developing States, marine pollution, international climate law, application, climate