On May 21, 2024, the International Tribunal for the Law of the Sea (ITLOS) issued the first-ever opinion by an international court or tribunal articulating States’ obligations with respect to climate change. The unanimous advisory opinion held that all 169 State parties to the United Nations Convention on the Law of the Sea (UNCLOS) have concrete legal obligations to undertake efforts to mitigate climate change, including through the reduction of greenhouse gas emissions.

ITLOS clarified the relationship between the 2015 Paris Agreement and UNCLOS. It found that mere compliance with the Paris Agreement is not necessarily sufficient to satisfy a State’s obligations under UNCLOS to prevent, reduce, and control pollution of the marine environment. A key consideration is what comes next as States begin to implement the decision and respond to the threat of climate change.

Origins of the Case and Legal Issues

The request for an advisory opinion was submitted to ITLOS on Dec. 12, 2022, by the Commission of Small Island States on Climate Change and International Law (COSIS). It was originally founded in 2021 by just two State parties, Tuvalu and Antigua and Barbuda. During the first day of oral hearings, the Prime Minister of Antigua and Barbuda, Gaston Browne, explained COSIS’s motivation for seeking an advisory opinion from the Tribunal:

Year after year, we listened as promises to mitigate climate change were made, and year after year, we watched as those promises went unfulfilled.

We have patiently listened and waited. We have ardently urged and pleaded, but with little avail.

But we were not willing to resign our peoples to this death sentence, occasioned by the continuing failure to take effective action against climate change.

COSIS now has nine member States and membership is open to all 39 members of the Alliance of Small Island States (AOSIS).

The request garnered significant participation beyond COSIS member States, with over 30 State parties to UNCLOS and nearly 10 intergovernmental organizations filing written statements in the case, with a similar level of participation from States and intergovernmental organizations in the oral hearings last September.

Small island States are particularly vulnerable to climate change. Four States, including founding member of COSIS, Tuvalu, are at risk of losing their entire territory due to sea level rise. These States have long provided leadership on climate change, including by spearheading the efforts for the advisory opinion requests to the International Court of Justice (ICJ, also known as the “World Court”) and ITLOS. These requests are one of the ways in which small island States are deploying international law to confront the “asymmetr[ies] of power” between States with “significant effect.”

The ITLOS advisory opinion is one of three advisory opinion requests on climate change submitted to international courts and tribunals in the last few years. Public hearings in the request for an advisory opinion from the Inter-American Court of Human Rights (IACtHR) began in April 2024 in Barbados and continued at the end of May in Brazil. A decision in that case may be issued by the end of the year. A third request on “the Obligations of States in respect of Climate Change” is pending before the ICJ. A historic 91 written statements have been filed with the World Court in that case, the only United Nations General Assembly request to the ICJ for an advisory opinion to ever be adopted by consensus. A decision is not expected from the ICJ before the end of the year, as responses to the initial written statements are due in August and the Court has yet to announce a date for anticipated public hearings.

Key Takeaways from the Opinion

The substance of the request from COSIS was for an advisory opinion on the obligation of State parties to UNCLOS to (a) “prevent, reduce and control pollution of the marine environment” from the effects of climate change, including through ocean warming and sea level rise, and ocean acidification, which are caused by greenhouse gas emissions into the atmosphere, and (b) “to protect and preserve the marine environment in relation to climate change impacts, including ocean warming and sea level rise, and ocean acidification.”

UNCLOS is considered “the constitution of the oceans.” Despite the fact that marine ecosystems serve as “sinks and reservoirs of greenhouse gasses,” the text of UNCLOS does not mention the words “greenhouse gasses” or “climate change.” This is because the Convention was negotiated between 1973-1982, before climate change was recognized by the General Assembly as a “common concern” in 1988. It was adopted in 1982, well before the establishment in 1988 of the United Nations body charged with assessing the science related to climate change, the Intergovernmental Panel on Climate Change (IPCC), or even the foundational climate treaty, the United Nations Framework Convention on Climate Change (UNFCCC), in 1992. Thus, it was an open question as to whether greenhouse gas emissions constituted “pollution of the marine environment” within the meaning of UNCLOS, and what specific obligations flowed from that threshold determination.

UNCLOS is considered a “living treaty” with flexible rules of interpretation that can be read with reference to the text of the instrument itself in addition to other relevant “external rules” of international law, as confirmed by the Tribunal in the advisory opinion (paras. 130-131). The Tribunal considered that the global climate change regime, including the UNFCCC and the Paris Agreement, were particularly relevant external rules in responding to the request (para. 137).

The Tribunal determined, making reference to these external rules, that human-made greenhouse gas emissions constitute “pollution of the marine environment” under UNCLOS (para. 179). This is significant as State parties to UNCLOS have an obligation to “prevent, reduce and control pollution of the marine environment” under Article 194 of the treaty.

In interpreting the obligations under Article 194, the Tribunal found that States have “specific obligations to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic [greenhouse gas] emissions” (para. 243). The Tribunal held that the obligation is one of “due diligence” and the standard “is stringent, given the high risks of serious and irreversible harm to the marine environment” from greenhouse gas emissions (para. 243). The Tribunal limited States’ discretion in determining what constitutes “necessary measures,” stating that this “should be determined objectively.” (para. 206).

It also found that States have specific obligations to ensure that greenhouse gas emissions within their jurisdiction and control do not cause transboundary harm. While acknowledging the concerns about the difficulty of discerning how greenhouse gas emissions from one State cause harm in another State, the Tribunal distinguished between challenges in establishing causation and the existence of the obligation. The standard of due diligence applicable to this obligation was “even more stringent…because of the nature of transboundary pollution,” in that pollution from one State that spreads beyond its jurisdiction has the potential to “cause damage to other States and their environment” (para. 258).

The Tribunal further elaborated on obligations to adopt national legislation and establish international rules and standards, and the obligations to enforce those standards, relevant to specific sources of pollution, such as pollution caused by land-based sources or vessels; that States have an obligation to cooperate to prevent, reduce, and control marine pollution from greenhouse gas emissions (para. 321); that States have “specific obligations to assist developing States…in their efforts to address marine pollution from” greenhouse gas emissions (para. 339);  and that States have an obligation to conduct environmental impact assessments for activities that might “cause substantial pollution to the marine environment or significant and harmful changes” from greenhouse gas emissions, among other obligations (para. 367).

Regarding question (b) and the obligation to protect and preserve the marine environment under Article 192 of the Convention, the Tribunal found that this obligation “has a broad scope, encompassing any type of harm or threat to the marine environment” (op. para. 4(b)). States must “take measures necessary to protect and preserve the marine environment” and the obligation “requires States to ensure that non-State actors under their jurisdiction or control comply with such measures” (para. 396). The measures must be “as far-reaching and efficacious as possible to prevent or reduce the deleterious effects of climate change and ocean acidification on the marine environment” (para. 399). Here too, the obligation of due diligence was a “stringent” one. (para. 399).

The content and scope of the obligations announced by the Tribunal are elaborate and specific. Additional commentary is already being written and recorded.

Significance of the Opinion

Advisory opinions are not legally binding, even for the entity or organ requesting them. However, they are considered “authoritative statement[s] of international law” with “legal effect” (paras. 202-05). Judge Kriangsak Kittichaisaree, in his separate Declaration, explained his view of the authoritative nature of advisory opinions, writing that they “carry no less weight and authority than those in its judgments in contentious cases because they are made with the same rigour and scrutiny by the principal judicial organ set up by the Convention itself” (para. 3).

The Tribunal appeared keenly aware of the significance of the opinion and the necessity for urgent action on climate change. It highlighted several conclusions of the IPCC, including that many changes to the ocean and sea level due to greenhouse emissions are “irreversible for centuries to millennia” (para. 62). The Tribunal also observed that “climate change represents an existential threat” (para. 66).

The opinion will have particular significance for the 169 State parties to UNCLOS, all of whom, with the exception of Yemen, are also parties to the Paris Agreement. The opinion clarifies the relationship between the Paris Agreement and UNCLOS. While the Tribunal found that the UNFCCC and the Paris Agreement were “relevant” as the “primary legal instruments addressing” climate change, the analysis of the obligations of State parties to UNCLOS could not be confined to those or other climate agreements (para. 222). Indeed, the Tribunal found that a State would not necessarily satisfy their obligations under Article 194 of UNCLOS “simply by complying with the obligations and commitments under the Paris Agreement” (para. 223).

The Tribunal stated that its opinion was confined to primary obligations and not the responsibility and liability of State parties for their acts or omissions. However, it noted it “may have to refer to responsibility and liability” if “necessary to clarify the scope and nature of primary obligations” (para.148). The Tribunal referred to how State responsibility would be engaged for breaches of their obligations several times in the opinion. For example, if a State “fails to comply” with the obligation to “take all necessary measures to prevent, reduce and control marine pollution from anthropogenic [greenhouse gas] emissions, including measures to reduce such emissions” then the Tribunal stated that “international responsibility would be engaged for that State.” (emphasis added) (para. 286). As will be explained below, these references to State responsibility potentially open the door to a future advisory opinion request or litigation related to liability and State responsibility.

This was reinforced in the Tribunal’s elaboration on the obligation to cooperate. The Tribunal said that it was not sufficient for a State to simply adopt and join a particular treaty, such as the UNFCCC or the Paris Agreement. Rather, States are required to engage in an ongoing effort to cooperate “to participate meaningfully in the formulation and elaboration of rules, standards and recommended practices and procedures” based on scientific knowledge “for the protection and preservation of the marine environment”  (para. 307). While the obligation to cooperate is one of conduct, the Tribunal said that the results of any such cooperation “may be relevant in assessing States’ compliance with the obligation to cooperate” (para. 310).

There are aspects of the opinion that at first glance seem rather flexible. For example, in fulfilling their obligations to prevent, reduce and control pollution of the marine environment under Article 194, States are to use “the best practicable means at their disposal and in accordance with their capabilities,” a standard the Tribunal said was similar to the concept of “common but differentiated responsibilities and respective capabilities” in the Paris Agreement (paras. 226-27). The Tribunal said that this “injects a certain degree of flexibility” in how States can meet their obligation and it is meant to “lessen the excessive burden” in implementing the obligation on certain States. However, the Tribunal went on to say that “the reference to available means and capabilities should not be used as an excuse to unduly postpone, or even be exempt from” the obligations under Article 194. Lest there be any doubt, the Tribunal stated that “[a]ll States must make mitigation efforts”(para. 229).

The opinion is ambitious in many ways. While the Tribunal did not find that States must immediately cease marine pollution from greenhouse gas emissions (para. 199), it said that States should take into account “the timeline for emissions pathways” to achieve the Paris Agreement’s goal of limiting increases in global warming to 1.5 degrees Celsius (para. 243). It stated that “the Convention requires States to take all necessary measures with a view to reducing and controlling existing marine pollution from such emissions and eventually preventing such pollution from occurring at all” (para. 199). It held that any measures taken by States to fulfill their obligations under Article 194 should take into account “the best available science and relevant international rules and standards” contained in climate change treaties such as the UNFCCC and the Paris Agreement (para. 243).

This is not to say that there is no room for critique of the opinion. While noting the opinion is “truly remarkable,” law professor Christina Voigt observed that the Tribunal missed the opportunity to further incorporate references to the Paris Agreement, such as the requirement that Nationally Determined Contributions (NDCs) be successive, progressive, reflective of each Party’s “highest possible ambition,” and informed by the global stocktake. Legal scholar Tim Stevens commented that while the opinion has “a great deal of value as an authoritative summary of the science and of the effects of climate change on the oceans,” it is lacking in nuance and clarity as to “precisely what the obligation is under Article 194” (a problem that has long-plagued international environmental law) (see comments beginning at 40:00). Nor does the Tribunal articulate “exactly by how much or by when” States need to reduce greenhouse gas emissions (see comments at 46:00).

Looking Ahead to What Comes Next

As mentioned, this is the first of three anticipated advisory opinions on climate change to be issued by international courts and tribunals in the coming year or so. As I have previously written, there were concerns that the three requests for advisory opinions on climate change, submitted in short succession within a four-month time-span, would potentially lead to conflicting decisions. However, the concern is lessened as ITLOS moved expeditiously in the consideration of this case. Its advisory opinion was issued less than a year and a half after COSIS submitted the initial request to the Tribunal and there appears to be ample opportunity for the IACtHR and the ICJ to consider the ITLOS opinion.

There is still a chance that the forthcoming advisory opinions will create some fragmentation in the law due to the overlapping nature of the requests. There is likely to be less overlap between the ITLOS decision and the forthcoming decision by the IACtHR, a court with a focus on the obligations of States under human rights law (although three judges discussed human rights in their separate declarations, with Judge Kittichaisaree considering that human rights were “duly borne in mind” by the Tribunal, Judge Stanisław Pawlak preferring ITLOS to incorporate human rights considerations in its decision, and Judge María Teresa Infante Caffi writing that the Tribunal could have benefitted from further argumentation on human rights). That is not the case for the ICJ request. The question posed to the ICJ refers to several overlapping instruments and principles of international law relied on in the ITLOS advisory opinion, specifically UNCLOS, the obligation of due diligence, and the duty to protect and preserve the marine environment, among others.

The opinion opens the door to future international litigation and arbitration. The late law professor Alan Boyle and others have argued that UNCLOS provides a viable pathway for State-to-State dispute resolution regarding the obligation to protect and preserve the marine environment from the effects of climate change. This avenue is particularly appealing due in part to the compulsory dispute resolution regime in UNCLOS and because “State-as-polluter” litigation is generally a well-worn path in international law. Prior to the advisory opinion, one barrier to pursuing such a case was the uncertainty as to whether a tribunal would find that greenhouse gas emissions constituted pollution of the marine environment within the meaning of UNCLOS. Now that the advisory opinion has clarified this point and provided a substantive link between the climate regime and the law of the sea, any subsequent case will have a lower barrier to entry.

While State responsibility and liability were not directly addressed by ITLOS in the advisory opinion, it may be easier in the future for a State to bring a case seeking liability for another State’s alleged failures to comply with their obligations under UNCLOS given the language in the opinion on when issues of international responsibility may arise with respect to the emission of greenhouse gasses. Stevens noted the opinion may also provide a basis for a future advisory opinion on State responsibility under UNCLOS. As he explained, should the BBNJ Agreement, which concerns conservation and sustainable use of marine biological diversity in areas beyond national jurisdictions, come into force, it provides an avenue for the parties to that Agreement to seek an advisory opinion from ITLOS under Article 47(7). The ITLOS opinion may also influence the ICJ’s consideration of State responsibility in its forthcoming climate change advisory opinion as that request specifically asks “[w]hat are the legal consequences under these obligations for States where they, by their acts and omissions, have caused significant harm” to the climate and the environment? In the words of Julian Aguon, counsel to Vanuatu, this part of the ICJ request is “the home for reparations.”

The decision is also likely to influence regional and domestic courts. The recent decision by the European Court of Human Rights (ECtHR), Verein KlimaSeniorinnen Schweiz and Others v. Switzerland, took note of the advisory opinion request to ITLOS. It ruled that a State has a positive duty to adopt effective measures to mitigate existing and future effects of climate change. There has been significant growth in domestic climate change litigation. Domestic courts are increasingly citing to international law in reaching conclusions on if the government of a State is sufficiently confronting the threat of climate change. Domestic litigants are likely to be particularly interested the requirement that States ensure that non-State actors within their jurisdiction or control comply with measures to protect and preserve the marine environment as this connects State responsibility to corporate emissions. This and the subsequent advisory opinions are likely to be cited in future domestic cases due to their specific relevance on the obligations of States.

It is difficult to overstate the significance of this opinion. As it is the first issued by an international tribunal, it will have an enormous influence on subsequent cases brought before international courts and tribunals. The opinion found that States cannot simply abide by their commitments in the Paris Agreement to fulfill their obligations under UNCLOS. It is likely that the ICJ will similarly follow suit and look directly to the relevant bodies of law in determining the content and scope of States’ obligations, rather than simply confining itself to the climate regime. For those that have been dissatisfied with the pace of progress under the Paris Agreement, this is a positive outcome.

This case is not just about whose argument won the day. Many COSIS States are running out of options and running out of time to avoid the most catastrophic impacts of climate change. They formed this organization and sought an advisory opinion after decades of advocacy and diplomacy had failed to produce sufficiently ambitious results. They succeeded in securing an opinion that drastically shifts the Overton Window in the climate discourse. Its impact will reverberate for years to come.

IMAGE: The emblem of the International Tribual for the Law of the Sea (ITLOS) is shown at its offices in Hamburg, Germany, on Sept. 11, 2023. (Photo by GREGOR FISCHER/AFP via Getty Images)