This contribution describes and critiques the Advisory Opinion of the International Tribunal for the Law of the Sea (‘ITLOS’ or ‘Tribunal’) on the obligations of States in relation to the sea and its marine environment that was delivered on 21 May 2024. Although the Advisory Opinion is not explicitly about sea level rise, it considers the relationship between climate change, the marine environment, and the consequence of sea level rise. The Opinion clarifies the legal obligations of States for preserving the marine environment and thereby makes a significant contribution to addressing the consequences of sea rise caused by climate change. The ITLOS Advisory Opinion was a precursor for the Advisory Opinions on climate change given by the Inter-American Court of Human Rights and the International Court of Justice in 2025. Much of this contribution relies on the ITLOS Advisory Opinion itself as well as the submissions by United Nations Member States and non-States actors. The background to the Request for an Advisory Opinion is discussed together with the questions that were submitted to the Tribunal. The submissions of States and non-State actors are described. And the unanimous Advisory Opinion of the Tribunal is described and evaluated together with the separate opinions of individual judges that accompanied it. Finally, some conclusions are drawn about the future impact of the Advisory Opinion.