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22 May 2024

The ITLOS Advisory Opinion on Climate Change

An introduction into the joint blog symposium

On May 21, 2024, the International Tribunal for the Law of the Sea (ITLOS) delivered a long-awaited Advisory Opinion on climate change and international law. This marks the first time that an international tribunal has issued an advisory opinion on State obligations regarding climate change mitigation. The Advisory Opinion addresses several key questions regarding application of the United Nations Convention on the Law of the Sea (UNCLOS) in the context of climate change, including the interaction between UNCLOS and the global climate change regime, and the specific obligations of States to reduce climate-damaging greenhouse gas (GHG) emissions.  Among other things, ITLOS concluded that “States Parties to the Convention have specific obligations under article 194 of UNCLOS to take all necessary measures to prevent, reduce, and control marine pollution from anthropogenic GHG emissions and to endeavor to harmonize their policies in this connection.” The tribunal also emphasized that article 194, paragraph 1, of UNCLOS imposes a due diligence standard on States that is “stringent”. 

The ITLOS opinion is the first of three advisory opinions on climate change expected to be issued by international courts and tribunals in coming months. The other two advisory opinions are being considered by the Inter-American Court of Human Rights (see here and here) and the International Court of Justice (see here and here). The ITLOS advisory opinion will likely inform those opinions and have a major impact on international legal understanding of climate-related obligations. 

There is much to explore in terms of both the content of the ITLOS advisory opinion and its potential implications for global, regional, and local efforts to combat climate change. To facilitate discussion and the exchange of ideas, the Sabin Center’s Climate Law Blog and Verfassungsblog are partnering on a blog symposium on the ITLOS opinion. Over the coming days, we will post a series of perspectives on the ITLOS opinion, authored by legal experts from across the globe. In this first, introductory blog, we outline the background to the advisory opinion and highlight some of the key takeaways from it. 


The ocean and climate are inextricably linked. On one hand, numerous adverse effects of climate change manifest in the ocean, such as ocean acidification, temperature changes, and rising sea levels. On the other hand, the ocean plays an important role in combating climate change. As noted in the ITLOS opinion, the ocean is a major carbon sink, and has taken up over 90% of the excess heat in the climate system. However, despite the central importance of the ocean to the climate system, international and national courts and tribunals have paid relatively little attention to the interactions between international climate law and international law of the sea.

Attention to the link between climate change and the law of the sea increased significantly with the formation of the Commission of Small Island States on Climate Change and International Law (COSIS) by Antigua and Barbuda and Tuvalu in 2021. The mandate of COSIS includes promoting the implementation and progressive development of rules and principles of international law concerning climate change. Crucially, article 2(2) of the COSIS Agreement authorizes the commission to request advisory opinions from ITLOS. COSIS quickly made use of this authorization and submitted a request on December 12, 2022, with the following legal questions.

What are the specific obligations of State Parties to the UNCLOS, including under Part XII:

(a) to prevent, reduce and control pollution of the marine environment in relation to the deleterious effects that result or are likely to result from climate change, including through ocean warming and sea level rise, and ocean acidification, which are caused by anthropogenic greenhouse gas emissions into the atmosphere?

(b) to protect and preserve the marine environment in relation to climate change impacts, including ocean warming and sea level rise, and ocean acidification?


The ITLOS Advisory Opinion

The advisory opinion developed a number of important findings, a few of which are discussed in this section

Anthropogenic GHG emissions as a form of pollution of the marine environment 

In line with the overwhelming majority of the submissions, ITLOS held that anthropogenic GHG emissions into the atmosphere constitute “pollution of the marine environment” within the meaning of article 1, paragraph 1, subparagraph 4, of UNCLOS. In reaching this decision, the tribunal noted that the definition of “marine pollution” in UNCLOS contains three element: (1) there must be a substance or energy; (2) this substance or energy must be introduced by humans, directly or indirectly, into the marine environment; and (3) such introduction must result or be likely to result in deleterious effects (para. 161). The tribunal found that anthropogenic GHG emissions met each of these criteria because, “through the introduction of carbon dioxide and heat (energy) into the marine environment, anthropogenic GHG emissions cause downstream climate change and ocean acidification, which results in the deleterious effects illustrated in the definition of pollution of the marine environment.” (para. 178).

Obligations to prevent, reduce and control pollution of the marine environment

The tribunal found that under article 194, paragraph 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” (para. 243). 

The obligation to take all necessary measures is a “due diligence” obligation, which requires States to put in place a national system to regulate polluting activities and to exercise vigilance to ensure the effectiveness of that system (para. 235). The exact content of the due diligence obligation is influenced by several factors, including “scientific and technological information, relevant international rules and standards, the risk of harm and the urgency involved” (para. 239). The tribunal found that, in the context of GHG emissions, the due diligence standard “is stringent, given the high risks of serious and irreversible harm to the marine environment from such emissions” (para. 243). 

The tribunal noted that the due diligence standard may also vary in accordance with State capabilities and available resources. As a result, and consistent with the principle of “common but differentiated responsibilities” in the United Nations Framework Convention on Climate Change, States with greater capabilities and resources may be required to do more than States with lesser means, like the least developed States. However, the tribunal notes that even a State with fewer capabilities and resources must “do whatever it can in accordance with its capabilities and available resources to prevent, reduce and control marine pollution from anthropogenic GHG emissions” (para. 241).

Obligations to protect and preserve the marine environment 

The tribunal found that article 192 of UNCLOS “imposes a general obligation on States Parties to protect and preserve the marine environment” (para. 400). This obligation “applies to all maritime areas and can be invoked to combat any form of degradation of the marine environment, including climate change impacts, such as ocean warming and sea level rise, and ocean acidification” (para. 400).  The obligation “may include restoring marine habitats and ecosystems” where the marine environment has been degraded (emphasis added) (para. 386). Moreover, the obligation to take measures necessary to protect and preserve the marine environment mandates that States ensure non-State actors under their jurisdiction or control adhere to these measures (para 396). Similar to the obligation to prevent, reduce, and control pollution, the obligation involves a due diligence standard, with similar contours as described above. 

The Role of the Paris Agreement

One dispute between those filing submissions was over the role of the Paris Agreement. Some State submissions argued that the Paris Agreement occupies the field of climate change, and UNCLOS imposes no additional obligations. However, the majority of submissions took a different view, arguing that the Paris Agreement is not lex specialis and that the provisions of UNCLOS impose separate, specific obligations on States. The tribunal sided with the majority, writing:

The Tribunal does not consider that the obligation under article 194, paragraph 1, of the Convention would be satisfied simply by complying with the obligations and commitments under the Paris Agreement. The Convention and the Paris Agreement are separate agreements, with separate sets of obligations. While the Paris Agreement complements the Convention in relation to the obligation to regulate marine pollution from anthropogenic GHG emissions, the former does not supersede the latter. (para. 223).

Best Available Science

A key question that is likely to come before many courts in climate litigation is which scientific evidence constitutes the “best available science.” As noted above, ITLOS concluded that States must take all “necessary measures” to control marine pollution associated with anthropogenic GHG emissions and said that what constitutes necessary measures “should be determined objectively” taking into account, among other things, “the best available science” (para 243). The tribunal did not elaborate on the meaning of that term, but noted that “[w]ith regard to climate change and ocean acidification, the best available science is found in the works of the IPCC which reflect the scientific consensus” (para. 208). The tribunal also highlighted the duty in the UNCLOS convention to cooperate with regard to scientific research and that this duty also applies in the context of marine pollution from anthropogenic GHG emissions (para. 312 – 320). 

Implications of the Advisory Opinion

The advisory opinion has global implications as 168 States are Parties to UNCLOS. Even states that are not party to UNCLOS (e.g., the US) recognize many of its provisions as forming part of customary international law, and the applicability of the Court’s pronouncements to these countries may be an interesting legal development in the coming years. 

The opinion is also relevant beyond the UNCLOS context, and is part of a growing body of international and regional law relevant to climate change. Just last month, the European Court of Human Rights found that the European Convention on Human Rights requires “that each Contracting State undertake measures for the substantial and progressive reduction of their respective GHG emission levels, with a view to reaching net neutrality within, in principle, the next three decades.” And, as mentioned above, both the Inter-American Court of Human Rights and the International Court of Justice will issue advisory opinions on State obligations with respect to climate change in the coming year or two. These upcoming decisions will likely consider, and draw from, the ITLOS opinion, and from the growing consensus that States have legal obligations to take swift action to combat climate change.

This blog symposium will delve further into specific aspects of the ITLOS opinion and situate it in the wider context of climate and environmental litigation. The goal is to provide deeper context and rich, early analysis of this important development in international climate change law. 

Related Work: The Sabin Center and Verfassungsblog recently partnered on a similar blog symposium, exploring three key rulings from the European Court of Human Rights on States’ obligations to combat climate change. The introduction to that joint blog symposium is available here