The ITLOS Advisory Opinion on Climate Change and International Law : A Deep Dive into External Rules and Materials

On 21 May 2024, the International Tribunal for the Law of the Sea (ITLOS) delivered its Advisory Opinion on Climate Change and International Law (Advisory Opinion) regarding the obligations of States Parties to the United Nations Convention on the Law of the Sea (UNCLOS) in respect of climate change. ITLOS made several methodological choices to answer the request for the Advisory Opinion, but none were more decisive than its extensive use of external rules and scientific materials to interpret both the questions presented and the relevant UNCLOS provisions. This article provides an account of the ways in which ITLOS engaged with legal and scientific external resources throughout its decision. The final section discusses some concerns raised by this interpretative approach, including unresolved questions related to the criteria for using external rules under the principle of systemic integration, and the justification for the role that scientific information plays in shaping the content of certain UNCLOS obligations as applied to climate change. These issues are important given the influence that the Advisory Opinion will have on future debates and negotiations, and because they are ultimately linked to the need for a sound theoretical foundation for using legal and non-legal external materials in treaty interpretation.