Debevoise & Plimpton LLP has helped secure a landmark ruling from the International Tribunal for the Law of the Sea (ITLOS) in response to a formal request from the Commission of Small Island States on Climate Change and International Law (COSIS) for an advisory opinion on the legal obligations of countries under international law related to climate change. ITLOS’ opinion sets out the specific steps that the 169 nations party to the United Nations Convention on the Law of the Sea (UNCLOS) must take to prevent, reduce, and control pollution of the marine environment by greenhouse gas emissions. This opinion is historic as it marks the first time an international court has directly addressed countries’ legal obligations to address climate change.

COSIS is an intergovernmental organization comprising nine small island states that is chaired by Antigua and Barbuda and Tuvalu and includes Palau, Niue, the Republic of Vanuatu, Saint Lucia, Saint Vincent and the Grenadines, Saint Kitts and Nevis, and The Bahamas. COSIS is advised by a 14-member Committee of Legal Experts with deep expertise in international law. Catherine Amirfar serves on the Committee, chairs its litigation subcommittee, and served as the Co-Representative of COSIS to ITLOS in this case.

Following the historic hearings held in Hamburg in September 2023 from over 50 state parties and international organizations, ITLOS unanimously concluded that UNCLOS requires states to take the specific measures necessary to prevent, reduce, and control marine pollution from greenhouse gas emissions. The tribunal affirmed that greenhouse gas emissions constitute "pollution of the marine environment" under UNCLOS and specified that states must implement concrete measures to reduce greenhouse gas emissions and protect the marine environment. Importantly, the Tribunal concluded that the best available science plays a “crucial role” in determining what steps are objectively necessary to combat climate change under the Convention. The Tribunal also underscored developed States’ obligations to take the lead in mitigating greenhouse gas emissions and assist developing States with adaptation to climate change.

ITLOS’ advisory opinion is significant as it signifies an inflection point for nations’ obligations under UNCLOS concerning climate change and strengthens the legal foundation for small island states' climate justice claims while holding high-emitting states accountable. This opinion also provides a concrete legal framework that can influence future climate negotiations, including in overcoming political inertia and driving global climate action.

The Debevoise team was led by litigation partners Catherine Amirfar and Conway Blake and included associates Romain Zamour, Duncan Pickard, Janine Godbehere, Beatrice Walton, Perpétua Chéry, Nicole Marton, Sara Kaufhardt, Alix Meardon, Evelin Caro Gutierrez, and Mary Grace McEvoy.