The International Tribunal for the Law of the Sea (ITLOS) today rendered a landmark, unanimous advisory opinion on climate change in response to a request from the Commission of Small Island States on Climate Change and International Law (COSIS). The Tribunal determined that the United Nations Convention on the Law of the Sea (UNCLOS) requires the 169 nations that are party to it to take specific, concrete steps to prevent, reduce, and control pollution of the marine environment by greenhouse gas emissions. This is the first time that an international court has ruled directly on countries’ international legal obligations to mitigate climate change.

In this Q&A, Catherine Amirfar—Chair of the Subcommittee on Litigation Management of COSIS’s Committee of Legal Experts and Co-Chair of the International Dispute Resolution Group at Debevoise & Plimpton LLP—and Duncan Pickard answer key questions about COSIS, ITLOS’s advisory opinion, and what it means for the fight against climate change.

What is COSIS?

COSIS is an intergovernmental organization comprising small island States formed in October 2021 by Antigua and Barbuda and Tuvalu at the start of the COP26 United Nations climate negotiations in Glasgow. Its mandate is to “promote and contribute to the definition, implementation, and progressive development of rules and principles of international law concerning climate change.” 

COSIS now has nine Member States comprising small island States from the Caribbean and the Pacific: Antigua and Barbuda, Tuvalu, Palau, Niue, the Republic of Vanuatu, Saint Lucia, Saint Vincent and the Grenadines, Saint Kitts (Saint Christopher) and Nevis, and The Bahamas. 

It is advised by a Committee of Legal Experts with deep expertise in international law: Payam Akhavan, Catherine Amirfar, the late Alan Boyle, Jutta Brunnée, Eden Charles, David Freestone, Vaughan Lowe QC, Makane Moïse Mbengue, Phoebe Okowa, Nilüfer Oral, Shaista Shameem, Jean-Marc Thouvenin, Philippa Webb, Margaretha Wewerinke-Singh.

Why are small island States fighting for climate justice?

Climate change presents an existential threat to small island States. Tuvalu, for example, with an average elevation of around six feet, is on track to be fully submerged by sea-level rise this century at current rates of warming and without sufficient adaptation. Even islands that remain above water may soon become uninhabitable due to harms such as salinization of aquifers or agricultural lands, declines in life-sustaining fish stocks, or more intense tropical cyclones, such as Hurricane Irma which drove out most of the population of Barbuda in 2017.

Projections for future emissions have actually gotten worse since 2015, not better. The U.N. Framework Convention on Climate Change (UNFCCC) Secretariat’s First Global Stocktake warned last year that “the window to keep warming to 1.5º Celsius within reach is closing rapidly” and “much more is needed now on all fronts.” It further confirmed that even if all current Nationally Determined Contributions under the Paris Agreement are implemented, the world is still on track for global warming between 2.4 ºC and 2.6ºC.

There is simply no room to argue with the certainty of the science at this point, particularly as reflected in the reports of the Intergovernmental Panel on Climate Change (“IPCC”), on the drivers, impacts, and necessary mitigation pathways of climate change. The IPCC has shown that Earth’s carbon budget for 1.5ºC is nearly exhausted, underscoring the need for dramatic and urgent reductions in greenhouse gas emissions.

Why did COSIS turn to ITLOS?

Climate change is largely a marine phenomenon. The ocean is by far one of the world’s largest greenhouse gas and heat sinks. It has absorbed more than 90 percent of the excess heat and nearly one quarter of the excess carbon that has accumulated in the atmosphere since the 19th century. This causes a number of profound harms to marine ecosystems, including sea-level rise, ocean warming, ocean acidification, and ocean deoxygenation, and related impacts on humans and marine flora and fauna.

UNCLOS contains rigorous obligations related to the protection and preservation of the marine environment and the prevention, reduction, and control of marine pollution. And these obligations apply broadly: to the Convention’s 169 States Parties, and in large part through customary international law.

What did COSIS request of ITLOS?

On December 12, 2022, COSIS requested an advisory opinion from ITLOS regarding States Parties’ specific obligations under UNCLOS, including Part XII, regarding climate change. Specifically, the request sought the Tribunal’s opinion on States’ obligations to prevent, reduce, and control pollution of the marine environment by greenhouse gas emissions and to protect and preserve the marine environment from the adverse effects of climate change.

ITLOS held oral hearings over two weeks in September 2023, with unprecedented participation from more than 50 States Parties and international organizations.

What did ITLOS decide?

ITLOS unanimously determined that Part XII of UNCLOS requires States to take specific, concrete steps to prevent, reduce, and control pollution of the marine environment by greenhouse gas emissions and to protect and preserve the marine environment from climate change.

The critical threshold question was whether greenhouse gas emissions meet UNCLOS’s definition of “pollution of the marine environment.” This is important because States have specific and rigorous obligations under Part XII of UNCLOS to prevent, reduce, and control marine pollution. The Tribunal answered this question in the affirmative.

The Tribunal confirmed that the best available science is the starting point in determining what steps are necessary to combat climate change under the Convention. ITLOS underscored the irrefutable scientific evidence, particularly as reflected in the conclusions of the IPCC. In fact, virtually all countries and international organizations that made submissions to ITLOS recognized the irrefutable scientific evidence on these points.

In particular, the Tribunal recognized the critical role of the ocean to the climate system as Earth’s largest heat and carbon sink. It also found that climate change has already caused profound harm to the marine environment—including sea-level rise and warming-related devastation of coral reefs—and that the risk of further catastrophic harm increases exponentially with average global warming of 1.5ºC above pre-industrial levels. 

The Tribunal linked States’ obligations to these scientific facts, concluding in part that, under Article 194(1) of UNCLOS:

States Parties to the Convention have the specific obligations to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions and to endeavour to harmonize their policies in this connection.

Such measures should be determined objectively, taking into account, inter alia, the best available science and relevant international rules and standards contained in climate change treaties such as the UNFCCC and the Paris Agreement, in particular the global temperature goal of limiting the temperature increase to 1.5°C above pre-industrial levels and the timeline for emission pathways to achieve that goal.

The scope and content of necessary measures may vary in accordance with the means available to States Parties and their capabilities. The necessary measures include, in particular, those to reduce GHG emissions.

Why is ITLOS’s opinion important?

The advisory opinion is an authoritative statement on how States must act to fulfill their obligations to prevent, reduce, control greenhouse gas emissions, and to protect and preserve the marine environment from climate change. 

The opinion thus serves as an important reference point for States’ obligations with respect to climate change under UNCLOS. It can also strengthen the legal basis for small island States’ claims for climate justice, including for loss and damage, and help hold high-emitting States accountable for their contribution to climate change. In diplomatic channels, the opinion can inform climate policy makers of existing obligations under international law, thereby setting the “floor” for climate negotiations going forward.

The Tribunal’s opinion brings us from abstract principles to concrete and rigorous legal obligations that can influence national, regional and international courts’ interpretation of State duties and adjudication of pending and future cases, and it can help cut through the political inertia that has plagued climate negotiations and national level climate action.

IMAGE: Sea ice is seen from NASA’s Operation IceBridge research aircraft off the northwest coast on March 30, 2017 above Greenland. Scientists have said the Arctic has been one of the regions hardest hit by climate change. (Photo by Mario Tama/Getty Images)