“Stringent Due Diligence”, Duties of Cooperation and Assistance to Climate Vulnerable States, and the Selective Integration of External Rules in the ITLOS Advisory Opinion on Climate Change and International Law

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The International Tribunal on the Law of the Sea’s  (ITLOS) 153-page 21 May 2024 Advisory Opinion on Climate Change and International Law is the first decision issued in the trifecta of advisory proceedings on climate change pending in international courts (the other two being the International Court of Justice and the Inter-American Court of Human Rights).  ITLOS declared at the outset that its Advisory Opinion would “confine itself to primary obligations” (Advisory Opinion, para. 148), and not the legal consequences arising from any breach of such primary obligations, such that the Tribunal would only refer to responsibility and liability in the Advisory Opinion only “to the extent necessary to clarify the scope and nature of primary obligations” (Advisory Opinion, para. 148, last sentence).  To recall, the Commission of Small Island States (COSIS) to posed the following two questions to ITLOS for its advisory opinion:

“What are the specific obligations of State Parties to the United Nations Convention on the Law of the Sea (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?” (Advisory Opinion, para. 102. Emphasis added.)

In this post, I emphasize two substantive features of the Advisory Opinion (e.g. the Tribunal’s articulation of the “stringent due diligence” obligation in relation to the pollution of the marine environment from excessive anthropogenic greenhouse gas emissions; and the Tribunal’s elaboration of various duties of cooperation throughout UNCLOS as well as forms of assistance required for climate vulnerable States), and the Tribunal’s methodological approach of integrating external rules into the interpretation of UNCLOS as a “living instrument” (Advisory Opinion, para. 130) that “coordinates and harmonizes” between UNCLOS and external rules. I conclude with observations on the Advisory Opinion’s silence as to other possible external rules — such as international human rights law —- that could have informed the Tribunal’s interpretation of duties of cooperation and assistance for climate vulnerable States, considering that the Tribunal already expressly declared that “climate change represents an existential threat and raises human rights concerns“. (Advisory Opinion, para. 66). Significantly, ITLOS stated at the outset that it is “mindful of the fact that climate change is recognized internationally as a common concern of humankind…[it] is also conscious of the deleterious effects climate change has on the marine environment and the devastating consequences it has and will continue to have on small island States, considered to be among the most vulnerable to such impacts.” (Advisory Opinion, para. 122).

Excess Anthropogenic Greenhouse Gas Emissions (GHGs) as Pollution of the Marine Environment

The Tribunal affirmed scientific findings of the Intergovernmental Panel on Climate Change (IPCC), and declared that “the introduction of excess heat (energy) into the marine environment due to the accumulation of GHGs in the atmosphere results in ocean warming…Anthropogenic GHG emissions thereby cause climate change, which includes ocean warming and sea level rise.  The introduction of anthropogenic GHGs into the marine environment also causes ocean acidification…interacting with other climatic and non-climatic factors [also] produce multiple deleterious effects on the marine environment and beyond.” (Advisory Opinion, para. 175. Italics added.). The characterization of excess anthropogenic GHGs as marine environmental pollution enabled the application of UNCLOS provisions (especially Part XII, Articles 192, 193, and 194) on the obligations of States to prevent, reduce, and control such pollution coming from any source. (Advisory Opinion, para. 189).  The Tribunal articulated three main obligations in relation to Article 194:  “first, the obligation under paragraph 1 to take necessary measures to prevent, reduce, and control marine pollution; second, the obligation under paragraph 2 to take necessary measures to ensure that certain situations relating to pollution do not occur; and third, the obligation under paragraph 5 to take necessary measures to protect and preserve rare or fragile ecosystems as well as the habitat of depleted, threatened or endangered species and other forms of marine life.” (Advisory Opinion, para. 195).  With respect to the first obligation, while the Tribunal recognized that “the best available science is found in the works of the IPCC which reflect the scientific consensus” (Advisory Opinion, para. 208), it also cautioned that “this does not mean that the science alone should determine the content of necessary measures…there are other relevant factors that should be considered and weighed togehter with the best available science” (Advisory Opinion, para. 212).  The Tribunal was silent as to what these other relevant factors are, and instead went on to declare that “while the precautionary approach is not explicitly referred to [in UNCLOS], such approach is implicit in the very notion of pollution of the marine environment, which encompasses potential deleterious effects.” (Advisory Opinion, para. 213).  The Tribunal also affirmed that “the global temperature goal and the timeline for emission pathways set forth in the Paris Agreement [are] particularly relevant”. (Advisory Opinion, para. 215). 

Significantly, the Tribunal declared that “the [UN Framework Convention on Climate Change or UNFCCC] and the Paris Agreement, as the primary legal instruments addressing the global problem of climate change, are relevant in interpreting and applying the Convention with respect to marine pollution from anthropogenic GHG emissions” (Advisory Opinion, para. 222), but also stressed that “the Paris Agreement is not lex specialis to the Convention.” (Advisory Opinion, para. 222). The Tribunal significantly contrasted the mitigation targets under the Paris Agreement and corresponding Nationally Determined Contributions (NDCs) of States under this Agreement that were all voluntary, with the legal effect of a State’s failure to comply with the obligation under UNCLOS Article 194 paragraph 1 (e.g. taking all necessary measures to prevent, reduce, and control marine pollution from anthropogenic GHG emissions, including meausres to reduce such emissions):  “international responsibility would be engaged for that State.” (Advisory Opinion, para. 223).  The Tribunal further held that while the principle of common but differentiated responsibilities and respective capabilities is not found in UNCLOS, the scope of measures under UNCLOS Article 194 “may differ between developed and developing States” (Advisory Opinion, para. 229), and “the scope and content of necessary measures may vary depending on the means available to States and their capabilities, such as their scientific, technical, economic, and financial capabilities.” (Advisory Opinion, para. 226). The Tribunal further declared that States have the obligations under UNCLOS Article 194(1) to “make every effort to harmonize their [policies on prevention, reduction, and control of marine pollution]…but are not required to achieve such harmonization.” (Advisory Opinion, para. 230). Marine geoengineering “would be contrary to [UNCLOS Article 195] if it has the consequence of transforming one type of pollution into another.” (Advisory Opinion, para. 231).

The Tribunal reserved its analysis of measures necessary to protect and preserve rare or fragile ecosystems and habitats through the second question in the request for advisory opinion, namely, on the broader obligations to protect and preserve the marine environment in relation to climate change impacts and ocean acidification.  (Advisory Opinion, paras. 385-388).  The Tribunal stressed that climate resilience and adaptation actions “as described in climate change treaties” are part of the corpus of necessary measures under UNCLOS Article 192, to protect and preserve the marine environment in relation to climate change impacts and ocean acidification. The Tribunal then cited Articles 2 and 7 of the Paris Agreement “are compatible with the obligations of [UNCLOS] and exemplify how science and other relevant considerations are taken into account by States in implementing adaptation measures.” (Advisory Opinion, para. 394).

“Stringent” Due Diligence in Relation to Marine Environmental Pollution from Anthropogenic GHG Emissions

The unique feature in the Tribunal’s interpretation of State obligations in relation to marine environmental pollution through excess anthropogenic GHGs is found in its characterization of the due diligence obligation (previously recognized in the Tribunal’s Area Advisory Opinion of 2011 and its 2015 Advisory Opinion).  ITLOS declared that “the standard of due diligence States must exercise in relation to marine pollution from anthropogenic GHG emissions needs to be stringent.  However, its implementation may vary according to States’ capabilities and available resources.  Such implementation requires a State with greater capabilities and sufficient resources to do more than a State not so well placed.” (Advisory Opinion, para. 241).  Notably, ITLOS did not specify what “stringent” due diligence entails in regard to the duty to prevent, reduce, and control marine environmental pollution from excess anthropogenic GHG emissions, other than to link the due diligence obligation with the precautionary approach:  “States would not meet their obligation of due diligence under Article 194, paragraph 1 of [UNCLOS] if they disregarded or did not adequately account for the risks involved in the activities under their jurisdiction or control.  This is so, even if scientific evidence as to the probability and severity of harm to the marine environment of such activities were insufficient.” (Advisory Opinion, para. 242.  Italics added.)

The denotative meanings of “stringent” are “marked by rigor, strictness, or severity, especially with regard to rule or standard” (Merriam-Webster Dictionary’s definition), or “having a severe effect, or being extremely limiting” (Cambridge Dictionary’s definition) or “very strict and that must be obeyed” (Oxford Dictionary’s definition).  At best, the Tribunal elaborates on this ‘stringent’ due diligence obligation in relation to marine pollution from anthropogenic GHG emissions as “prescrib[ing] not only the required conduct of States but also the intended objective or result of such conduct.  Whether this obligation is that of conduct or of result depends on whether States are required to achieve the intended objective or result, i.e., prevention, reduction and control of marine pollution….what is required under Article 194, paragraph 1 is NOT to achieve the prevention, reduction, and control of marine pollution, but to take all necessary measures to that end.” (Advisory Opinion, para. 238. Italics added.). With respect to UNCLOS Article 194, paragraph 2 (e.g. requiring States to take all necessary measures to ensure that activities under their jurisdiction or control do not cause transboundary harm, or that pollution arising from incidents or activities under their jurisdiction or control do not spread beyond areas where they exercise sovereign rights), the Tribunal also applied the same “stringent due diligence” obligation: “with respect to transboundary pollution affecting the environment of other States, the standard of due diligence can be even more stringent.” (Advisory Opinion, para. 256).  This stands in contrast with the specificity of the Tribunal’s declaration of the obligation of all States to adopt laws and regulations and take necessary measures to implement rules and standards set out in climate change treaties and other relevant instruments (in relation to UNCLOS Articles 213 and 222), where if a State Party to UNCLOS that is also bound by such treaties fails to take such measures, “its international responsibility would be engaged for breach of the obligations under [UNCLOS] articles 213 or 222.” (Advisory Opinion, para. 286).  Likewise, the Tribunal again imposed the “stringent due diligence” threshold for the fulfilment of obligations under the much broader UNCLOS Article 192, “to take measures as far-reaching and efficacious as possible to prevent or reduce the deleterious effects of climate change and ocean acidification on the marine environment.  The standard of due diligence under article 192 is…stringent given the high risks of serious and irreversible harm to the marine environment by climate change impacts and ocean acidification.” (Advisory Opinion, para. 399).

The ambiguity of the “stringent due diligence” standard as articulated in the ITLOS 2024 Advisory Opinion on Climate Change and International Law creates simultaneous hazards of overbreadth and underinclusion — given that the Tribunal had already indicated that international responsibility would be engaged if a State fails to comply with UNCLOS Article 194 paragraph 1 in relation to taking all necessary measures to prevent, reduce, and control marine pollution from anthropogenic GHG emissions, that failure would be tested against the nebulous metric of “stringent due diligence”. The ambiguity is equally problematic for specific obligations of States in UNCLOS Article 194 paragraph 2 (on taking all necessary measures to ensure that activities under their jurisdiction or control do not cause transboundary harm), as well as for UNCLOS Article 192 (on the broader obligation to protect and preserve the marine environment).  While ITLOS already affirmed that the best available science would not necessarily be singularly determinative of the content of such ‘necessary measures’, it also did not indicate who was ultimately authoritative in determining the appropriate precautionary approach in regard to marine environmental pollution arising from excess anthropogenic GHG emissions; whether the IPCC or other related institutions to the UNFCCC or the Paris Agreement or other environmental agreements and treaties are more significantly authoritative in deciding the appropriate precautionary approach in light of the best available science; or whether future judicial review by ITLOS of any precautionary approach taken as alleged “stringent due diligence” would more heavily privilege the “best available science” (a phrase referred to seventeen times throughout the Tribunal’s Advisory Opinion), alongside what other factors could be deemed “relevant” if any, as ITLOS briefly referred to in this Advisory Opinion but did not elaborate. 

Duties of Cooperation and Assistance to Climate Vulnerable States and the Human Rights Gap in Applicable Law

The Tribunal emphasized that “the duty to cooperate is reflected in and permeates the entirety of Part XII of UNCLOS” (Advisory Opinion, para. 297), and the core obligation of cooperation is indicated in UNCLOS Article 197, which is “aimed at the formulation and elaboration of rules, standards, and practices and procedures for the protection and preservation of the marine environment, and is characterized by a large degree of flexibility.” (Advisory Opinion, para. 302).  The Tribunal clarified that UNCLOS Article 197 “does not oblige States to achieve a normative outcome but to participate meaningfully in the formulation and elaboration of rules, standards and recommended practices and procedures for the protection and preservation of the marine environment”. (Advisory Opinion, para. 307, Italics added.).  Here, the Tribunal again did not clarify what it meant by meaningful participation, but instead reiterated its 2015 Advisory Opinion describing due diligence obligations as those requiring States to consult with one another in good faith, where “the consultations should be meaningful in the sense that substantial effort should be made by all States concerned, with a view to adopting effective measures necessary to coordinate and ensure the conservation and development of shared stocks.” (Advisory Opinion, para. 308, see also the Tribunal’s discussion of the same obligation to cooperate in para. 423 with respect to climate change impacts and ocean acidification).  Other duties to cooperate refer to UNCLOS Articles 200 and 201, on the promotion of studies, research programmes, information exchange, and establishment of appropriate scientific criteria for regulations. (Advisory Opinion, paras. 312-320), on a “good faith, continuous efforts” basis, either directly or through competent international organizations. (Advisory Opinion, para. 319).

Most significantly, in relation to UNCLOS Articles 202 and 203 on technical assistance to developing States, ITLOS took the view that “scientific, technical, educational and other assistance to developing States that are particularly vulnerable to the adverse effects of climate change is a means of addressing an inequitable situation.” (Advisory Opinion, para. 327).  It recalled that the fifth paragraph of the Preamble to UNCLOS refers to the achievement of the goals of this Convention as contributing “to the realization of a just and equitable international economic order which takes into account…the special interests and needs of developing countries.” (Advisory Opinion, para. 328.) Assistance measures that were identified by ITLOS for developing countries fall under three categories all subject to the discretion of States: (1) promotion of programmes of scientific, educational, technical, and other assistance to developing States; (2) provision of appropriate assistance in order to minimize the effects of major incidents which may cause serious marine pollution; and (3) provision of “appropriate assistance” on the preparation of environmental assessments. (Advisory Opinion, paras. 332-335).  Most significantly, the Tribunal affirmed that “other assistance” in UNCLOS Article 202(a) could include “financial assistance aimed at providing developing States with assistance to promote the programmes and undertake the activities indicated in [UNCLOS Article 202].” (Advisory Opinion, para. 336). However, the Tribunal did not draw on any external rules to interpret these duties of cooperation and assistance towards climate vulnerable states.

It is important to note that while ITLOS considered that climate change “raises human rights concerns” (Advisory Opinion, para. 66) and also elaborated on “preferential treatment to developing States” as required under UNCLOS Article 203 (Advisory Opinion, paras. 324 to 339), the Tribunal also stopped well short of including international human rights law as part of the external rules that it would consider as applicable law in the interpretation of UNCLOS. This would have made a significant difference, in light of the fifth paragraph of the Preamble to UNCLOS, and the well-established legal nexus between human rights and environmental protection through the human right to a healthy, safe, and sustainable environment. Duties of cooperation and assistance are also elaborated in Articles 1(2), 2(1), 11, and 15 of the International Covenant on Economic, Social and Cultural Rights (ICESCR), and whose Article 1(2) strictly proscribes that “in no case may a people be deprived of its own means of subsistence.” Small island developing States (SIDS) are precisely not just the most climate vulnerable populations, but they are peoples who literally stand to be deprived of their own means of subsistence due to unabated sealevel rise, the increased frequency and atmospheric hazards induced by climate change, and now, ever worsening problems of accessing international development financing and international private financing to enable building their urgently needed climate resilience and adaptation projects, due to their uninsurability as a result of climate risks they did not author, unsustainable debt levels with some traceable to colonization-imposed debts, and the persistent neglect of the international system for over 63 million persons (including women, children, persons with disabilities, indigenous peoples, persons of Afro-descent and other vulnerable groups) in the SIDS countries that are either rendered homeless, displaced, perishing, or perpetually at risk by climate-induced natural disasters. ITLOS’ silence on the obligations of States in the face of small island developing States’ experiences of urgent and pressing marine environmental pollution, sealevel rise, and integrated human rights concerns for the very existence of small island developing States, notwithstanding its many references to ‘the most climate vulnerable’, was the most glaring gap — and a completely missed opportunity to be consistent with ITLOS’ interpretation of UNCLOS as a “living instrument” that liberally draws on external rules to evolve its interpretation — in this Advisory Opinion.


There are many noteworthy legal and interpretive developments from the ITLOS Advisory Opinion on Climate Change and International Law. ITLOS famously ascribes a liberal and evolutive interpretation to UNCLOS, widely drawing on external rules in the interpretation of relevant provisions of UNCLOS. The Tribunal did so with respect to climate change treaties and some aspects of general international law, but chose not to do so with respect to international human rights treaty (or customary) law. The Advisory Opinion does not explain the omission, even if the Tribunal had concluded at the outset that UNCLOS, the COSIS Agreement, “and other relevant rules of international law not incompatible with the Convention constitute the applicable law in this case.” (Advisory Opinion, para. 127).  The request for this advisory opinion was brought by the Commission of Small Island States (COSIS), whose legal experts form working sub-committees on marine environment, loss and damage, sealevel rise, human rights, and litigation management.  It cannot be inferred that human rights was meant to just stand as one sentence in one paragraph of this Advisory Opinion, or that international human rights law was meant to be isolated away from how we understand duties of international cooperation and assistance to developing countries. Ultimately, interpretive ambiguity and the selective (and non-transparent) integration of external rules in the interpretation of UNCLOS provisions will not advance the objects and purposes of this Convention to create “a legal order for the seas and oceans which will facilitate international communication, and will promote the peaceful uses of the seas and oceans, the equitable and efficient utilization of their resources, the conservation of their living resources, and the study, protection and preservation of the marine environment.” (UNCLOS Preamble, fourth paragraph).  COSIS is to be commended for leading the Herculean effort at ITLOS.  One can hope that the remaining advisory opinion proceedings at the Inter-American Court of Human Rights and the International Court of Justice will address the human rights law deficit in the ITLOS Advisory Opinion.

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Freddy Sourgens says

June 5, 2024

What an excellent post!

I wanted to add a couple of observations: What I address here is what does ‘stringent’ diligence mean from an energy law perspective? What does it requires from domestic regulators? The decision offers different potential solutions..

The international law due diligence standard is well established. It requires diligent conduct - reasonable preparedness. Did ITLOS intended to modify this international legal standard?

By way of analogy, the distinction between ordinary and stringent diligence might be comparable ordinary negligence vs. strict liability. While strict liability is not absolute - it is still measured against unreasonable risk - it imposes a seemingly more ‘stringent’ standard. If this is what ‘stringent’ diligence means, it would impose a duty to avoid an unreasonable risk of climate change. This closes in on the boundary between obligations of conduct and obligations of conduct. (The analogy is not fully well drawn as diligence follows a nuisance rather than negligence paradigm.)

On balance, it does not seem that ITLOS intended this result. Had it intended such a shift, it likely would have elaborated it meaning more fully.

This means the word ‘stringent’ does not a change in the applicable rules outright. Rather, ITLOS wished to communicate that climate change is particularly important - important because, in the law of the sea context alone, it touches millions of people and also deeply affects ecosystems.

We could again take this in two ways. We could outright say that climate change is just more important than other considerations. ‘Stringent’ diligence means to give a kind of super preference to climate impacts over other impacts that policies wish to achieve.

Alternatively, we can understand ‘stringent’ as a factual modifier, not a legal one. Climate change deserves greater weight as a matter of factual inputs into legal analysis.

This second approach would be a more precautionary approach to climate impacts rather than one that would give a greater preference. Science does not know with certainty what the impacts will be. It provides a range of potential impact. As we look at diligence we could therefore look at a requirement that more serious impact be weighed more heavily. This would be an approach that would lend itself to a greater harmonization across regimes.

What is the domestic impact of ITLOS’ stringent diligence standard? The domestic application of due diligence is a task for domestic lawyers. They conduct diligence as part of domestic regulatory processes.

How should a state attempt to implement the ITLOS advisory opinion in its own domestic regulatory proceedings? There are two ways that one might look at this. First, ITLOS could effectively require that states change their legislation to be stringently diligent.

To the extent that stringent diligence imposes a climate super preference, domestic legislation must reflect it. Most legal systems do not reflect such a super preference. Rather, regulators take climate impacts into accounts as one factor. Still, that does not impose some kind of veto or super preference as against other policy ideals. It is an important, but not an ‘overridingly’ important consideration. If this is what ITLOS ‘stringent’ diligence were to require, it is pragmatically likely to fail. Such legislation is unlikely to be forthcoming, judging the global political landscape.

This would leave a more modest approach. This approach would tend to lineup with a more precautionary approach. We must use the mechanisms that are available under existing domestic legislation in a different manner. Most if not all domestic legislative systems will impose some form of social cost of carbon measure as part of energy diligence. While the social cost of carbon measures are, of course, contentious they have been accepted as a principal matter in many if not most jurisdictions. Most jurisdictions also measure climate impacts in other ways as part of broader environmental diligence.

If we were to assume that we are to implement the new ‘stringent’ diligence standard in this regulatory context we would simply have to change our assessment of the value of the social cost of carbon and of likely global climate impacts. Again, by reference to a tort analogy, if harm increases, so does the duty of care. This likely does not require legislative action but is a matter of regulatory determination.

This would align with a precautionary approach to ‘stringent’ diligence. This precautionary approach would say that the social cost of carbon, and global climate impacts, may be undervalued. We could thus increase social cost of carbon measures, and likely climate impacts, in light of a different appreciation of the science. Still, this does not require a specific result. A regulator can still adopt a greenhouse gas intensive policy because it lacks reasonable alternative to realize other essential rights of its residents. It would do so after appropriate deliberations.

This approach would still tilt the balance in favor of more climate friendly policies and projects by virtue of deploying existing regulatory approaches. It thus seems more balanced and realistic it. This would be the more reasonable approach for ITLOS to have intended and provides a more ready map to assess its implementation.