1. Introduction
On 21 May 2024, the International Tribunal for the Law of the Sea (ITLOS, Tribunal) delivered its Advisory Opinion on Climate Change and International Law (Advisory Opinion) regarding the obligations of the States Parties to the United Nations Convention on the Law of the Sea (UNCLOS) in respect of climate change.Footnote 1 This was a much-anticipated decision for several reasons,Footnote 2 including that it was the first in a chain of three advisory opinions by international tribunals concerning the obligations of States under international law in the context of global warming. The Inter-American Court of Human Rights followed ITLOS, notifying its Advisory Opinion on the Climate Emergency and Human Rights on 3 July 2025.Footnote 3 Twenty days later, the International Court of Justice completed the sequence by handing down its own Advisory Opinion on the Obligations of States in respect of Climate Change. Footnote 4 In addition, the African Court on Human and Peoples’ Rights is expected to undertake a similar task, having recently received a request for an advisory opinion on the subject.Footnote 5
This article focuses on the Advisory Opinion requested by the Commission of Small Islands States on Climate Change and International Law (COSIS) in December 2022. At its core, the submission concerned the following interpretative issue: could UNCLOS—a treaty adopted in 1982—be used to tackle an urgent contemporary challenge that did not have the same significance at the time of its adoption as it does today?Footnote 6 More specifically, COSIS sought clarification on whether the obligations of States under UNCLOS to prevent, reduce and control marine pollution, and to protect and preserve the marine environment, could be applied in the context of the deleterious effects of climate change.Footnote 7
Ultimately, ITLOS addressed these concerns with an affirmative response. The Tribunal unanimously decided that States Parties to UNCLOS have obligations in relation to the harmful effects of climate change. To reach this conclusion, ITLOS made several methodological choices, but none were more decisive than its extensive use of external legal and non-legal materials to support its findings. Indeed, the Tribunal highlighted the importance of external instruments in various paragraphs throughout the Advisory Opinion. For example, it stated that ‘coordination and harmonization between the Convention and external rules are important to clarify, and to inform the meaning of, the provisions of the Convention and to ensure that the Convention serves as a living instrument’.Footnote 8 Similarly, the Tribunal repeatedly incorporated scientific information originating outside the UNCLOS regime into its analysis and, while there is an attempt to justify the use of external rules and scientific materials, the specific ways in which ITLOS did so open up a number of questions whose significance exceeds the advisory proceeding itself.
In this context, this article provides an examination of the use of legal and scientific external resources in the Advisory Opinion and analyses relevant issues that emerge from this approach. Section 2 provides an overview of the Advisory Opinion, focusing on methodological aspects. Section 3 examines external materials in general as discussed in the Advisory Opinion, while Section 4 addresses the role that these instruments play in supporting the Tribunal’s findings. Section 5 discusses unresolved questions raised by this interpretative approach, considering, first, concerns about the criteria for using external rules under the principle of systemic integration and, second, the justification for relying on scientific knowledge in a normative way. Finally, Section 6 provides concluding remarks.
2. Overview of the Advisory Opinion
On 12 December 2022, COSIS submitted a letter to ITLOS requesting an advisory opinion on the obligations of States Parties to UNCLOS in relation to the effects of climate change. The Tribunal had previously issued two advisory opinions involving interpretations of UNCLOS.Footnote 9 In particular, COSIS submitted the following questions:
What are the specific obligations of State Parties to the United Nations Convention on the Law of the Sea (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?Footnote 10
After dealing with jurisdiction and discretion,Footnote 11 the Advisory Opinion turned to the applicable law,Footnote 12 the interpretation of UNCLOS in relation to external rulesFootnote 13 and the scope of the request.Footnote 14 The first two issues are closely related to the main topic of this article and will be discussed in subsequent sections. Regarding the scope of the questions posed by COSIS, the Tribunal made four methodological decisions that ultimately framed its use of external rules and scientific materials to interpret UNCLOS.
First, ITLOS clarified that its Advisory Opinion would concern ‘specific obligations of States Parties under the Convention’.Footnote 15 This approach dismissed the view that these proceedings were an opportunity to address States’ obligations regarding climate change under general international law.Footnote 16 Second, ITLOS explained that it could examine UNCLOS obligations located in any part of the treaty.Footnote 17 Third, the Tribunal stated that, unlike its previous advisory opinions, it would not engage in questions of responsibility and liability since they were not part of the request, and it would only address these matters if necessary to interpret primary obligations.Footnote 18 Finally, ITLOS affirmed that question (b) of COSIS’ request was broader in scope and encompassed question (a),Footnote 19 making parts of the response to question (a) applicable to question (b).Footnote 20
Based on these methodological decisions and on the use of external materials—which will be examined in Sections 3 and 4—ITLOS answered COSIS’ questions. The Tribunal affirmed in the operative clause of the Advisory Opinion that States Parties to UNCLOS are obligated to take measures to prevent, reduce and control marine pollution—including the emission of anthropogenic greenhouse gases (GHG)—and to protect and preserve the marine environment from the harmful effects of climate change. ITLOS specified that these are due diligence obligations, the standard of which is ‘stringent’ due to the seriousness of the threat that climate change poses to the marine environment. Furthermore, the Tribunal identified obligations of States Parties to ensure that GHG emissions produced within their jurisdiction and under their control do not spread or cause harm outside of the ‘areas where they exercise sovereign rights’.Footnote 21 In addition, ITLOS found that States Parties have specific obligations depending on the sources of the GHG emissions and also have duties to: cooperate and assist developing States in dealing with the effects of climate change; conduct environmental impact assessments; protect and preserve fragile ecosystems; and take measures to conserve living marine resources.Footnote 22
3. External rules and materials discussed in the Advisory Opinion
Among the methodological decisions that ITLOS made in order to reach its conclusions, the most influential one was its choice to interpret UNCLOS by relying heavily on external rules and scientific materials.Footnote 23 Section 3.1 identifies these rules and materials and Section 3.2 examines how the Tribunal justified their use.
3.1. Background: identification of the relevant external rules and materials
Immediately after laying out the chronology of the procedure, the Tribunal turned to the ‘background’ to the request.Footnote 24 In the first part of this review, the Tribunal analysed scientific aspects of climate change, identifying the Intergovernmental Panel on Climate Change (IPCC) as the relevant authority in the field. The Tribunal gave a detailed account of its procedures, stressing the role that member countries play in reviewing and endorsing the adoption of reports.Footnote 25 For the purposes of the case, ITLOS considered a series of IPCC assessment reports on climate change as the most relevant materials. For example, it mentioned the 2018 Special Report on Global Warming of 1.5°C and several other working groups’ reports.Footnote 26 The Tribunal relied on these documents throughout the Advisory Opinion in order to find definitions for scientific conceptsFootnote 27 and to provide evidence of scientific knowledge,Footnote 28 regarding them as the ‘best available science’.Footnote 29 Notably, ITLOS only used IPCC materials as a source of scientific knowledge and highlighted that the authoritative value of the IPCC reports was not challenged by any of the participants in the proceedings.Footnote 30
The second part of the background review analysed the main international legal instruments addressing climate change.Footnote 31 This was a critical step, as the Tribunal subsequently relied on them to interpret UNCLOS. The Advisory Opinion recognised the United Nations Framework Convention on Climate Change (UNFCCC) as the cornerstone of the climate legal framework, emphasising its objective of stabilising GHG concentrations in order to prevent the disruption of the climate system due to human causes.Footnote 32 The Advisory Opinion further identified definitions and relevant concepts contained in the UNFCCC, such as emissions, greenhouse gases, common but differentiated responsibilities, precautionary measures, sustainable development and cooperation.Footnote 33 It also outlined the States Parties’ commitments in relation to GHG emissions in light of the Kyoto Protocol and the Doha Amendment.Footnote 34
The Advisory Opinion then examined the Paris Agreement.Footnote 35 It highlighted the temperature goal set in this convention and the methodology established to achieve it based on Nationally Determined Contributions (NDCs) that States must submit periodically reflecting their highest possible ambitions to reduce GHG emissions.Footnote 36 The Advisory Opinion included the resolutions adopted by the Conferences of the Parties (COP) to the UNFCCC, Kyoto Protocol and Paris Agreement as part of the normative framework relevant to COSIS’ questions.Footnote 37
Finally, the Advisory Opinion concluded its analysis of the legal background by identifying a mix of binding and non-binding instruments adopted by the International Civil Aviation Organisation (ICAO), the International Maritime Organization (IMO) and in relation to the Montreal Protocol.Footnote 38 The Tribunal did not mention other conventions in this part of the Advisory Opinion, reflecting a strict approach to what it considered to be international instruments addressing climate change. By contrast, in other sections of the Advisory Opinion, ITLOS referred to the Convention on Biological Diversity (CBD),Footnote 39 the Convention on International Trade of Endangered Species of Wild Fauna and Flora (CITES)Footnote 40 and even to the Agreement under the United Nations Convention on the Law of the Sea on the Conservation and Sustainable Use of Marine Biological Diversity of Areas Beyond National Jurisdiction (BBNJ Agreement), which is not yet in force.Footnote 41
3.2. Justification for the use of external rules and materials
ITLOS dedicated two sections of the Advisory Opinion to discussing the legal basis for the use of external rules in its reasoning. These parts are entitled ‘Applicable law’ and ‘Interpretation of the Convention and the relationship between the Convention and external rules’.Footnote 42
Regarding the determination of the applicable law, the Advisory Opinion identified Article 293 UNCLOS as the relevant rule allowing for the use of external materials. This provision states that ‘[a] court or tribunal having jurisdiction under this section shall apply this Convention and other rules of international law not incompatible with this Convention’.Footnote 43 Consequently, the Tribunal declared that UNCLOS, the COSIS AgreementFootnote 44 and any other rules of international law that were ‘not incompatible’ with UNCLOS were applicable to the questions posed in the proceedings.Footnote 45 Regrettably, ITLOS did not specify the criteria for determining compatibility, leaving the concept open and flexible.
ITLOS then addressed the interpretation of UNCLOS in light of Articles 31 to 33 of the Vienna Convention on the Law of Treaties (VCLT).Footnote 46 In this context, the Tribunal stated that ‘[t]he relationship between Part XII of the Convention, entitled “Protection and Preservation of the Marine Environment”, and external rules is of particular relevance in this case’.Footnote 47 Furthermore, the Tribunal noted that the synchronised application of UNCLOS and external rules clarifies the provisions of UNCLOS, and ensures ‘that the Convention also serves as a living instrument’.Footnote 48
ITLOS then proceeded to point out three ‘mechanisms through which a relationship between the provisions of Part XII of the Convention and external rules is formed’.Footnote 49 These mechanisms provide legal justification for, and enabled the Tribunal to rely on, external rules in its Advisory Opinion. The first mechanism is the ‘rules of reference’ contained in UNCLOS.Footnote 50 These are provisions that explicitly refer to external rules and will be discussed in Section 4.2.2. The second mechanism is Article 237 UNCLOS. This provision requires, in the words of the Tribunal, ‘consistency and mutual supportiveness’ between the rules contained in Part XII UNCLOS and other rules of international law on the protection and the preservation of the marine environment.Footnote 51 An example of the operation of this mechanism in the Advisory Opinion is the invocation of the BBNJ Agreement.Footnote 52 Finally, the third mechanism that ITLOS identified was Article 31(3)(c) VCLT.Footnote 53 This rule is known as the principle of systemic integration,Footnote 54 and it requires that the interpretation process takes into account, together with the context of the provision, ‘any relevant rules of international law applicable in the relations between the parties’.Footnote 55 This method is built on the premise that treaties exist within the international legal system and not in isolation, meaning that they must be interpreted and applied in light of their broader legal context.Footnote 56 The Tribunal endorsed this idea,Footnote 57 and mentioned the International Law Commission’s (ILC) Report on the Fragmentation of International Law, which asserts that multiple international rules addressing the same topic should be interpreted in a way that enables a coherent set of obligations.Footnote 58 In this case, the Tribunal considered that the interpretation of UNCLOS had to be compatible with the legal instruments dealing with climate change identified in Section 3.1.
These passages of the Advisory Opinion represent a significant effort by the Tribunal to articulate the legal basis for its extensive reliance on external rules. However, as Sections 4 and 5 will explore, the Advisory Opinion falls short of adequately explaining how these materials are integrated into its legal reasoning. In particular, the Advisory Opinion did not clearly specify how the various external instruments cited fit within the different parts of Article 31 VCLT. Although ITLOS quoted the general rule of interpretation in Article 31 VCLT in full, it only engaged with Article 31(3)(c), leaving other relevant elements of the provision unaddressed. Additionally, while ITLOS explicitly identified the grounds for its use of external legal instruments, it did not provide a similar explanation of the basis for incorporating scientific sources—such as IPCC reports—into the interpretive process. This omission is relevant as the Advisory Opinion treated legal norms and scientific findings alike as part of the contextual background for interpreting UNCLOS. Finally, the invocation of the principle of systemic integration under Article 31(3)(c) warranted a deeper engagement. After all, as Bianchi and Zarbiyev explain, Article 31(3)(c) requires interpretation in itself, and this interpretation can be variable.Footnote 59 Furthermore, as McLachlan observes, almost every word in the provision is open for debate.Footnote 60 These are all important issues and will be analysed in Section 4.
4. The role played by external rules and materials in the Advisory Opinion
This section explores the specific ways in which the Tribunal used external materials to support its findings. First, it discusses the role these instruments played in the determination of the meaning of terms and expressions (Section 4.1). Second, it examines how ITLOS used external materials to determine the content of UNCLOS obligations as applied to climate change (Section 4.2).
4.1. Interpretation of COSIS’ questions and UNCLOS provisions
The first use that ITLOS gave to external materials was to interpret the meaning of terms and expressions contained in both COSIS’ questions and UNCLOS provisions. The answer to each of COSIS’ enquiries began with a section dedicated to the clarification of terms.Footnote 61 In them, the Tribunal discussed the meaning of concepts such as ‘climate change’, ‘greenhouse gas emissions’ and ‘ocean acidification’, stating that it would understand these notions in the way that they are defined in the legal instruments and authoritative IPCC reports identified earlier in the Advisory Opinion.Footnote 62
The most relevant interpretative question faced by the Tribunal was whether anthropogenic GHG emissions constitute ‘pollution of the marine environment’ under UNCLOS. This was a critical question because the obligation established in Article 194 to prevent, reduce and control pollution of the marine environment only becomes relevant in the context of climate change if GHG emissions are considered to constitute marine pollution.
To determine whether GHG emissions fall within the category of ‘pollution of the marine environment’, ITLOS looked at the definition contained in Article 1(1)(4) UNCLOS and interpreted its terms relying heavily on external rules and scientific materials. For the purposes of determining the meaning of the words ‘substance’ and ‘energy’, it recalled the ILC’s 2021 draft Guidelines on the Protection of the Atmosphere.Footnote 63 In relation to the concepts ‘gas’, ‘greenhouse gases’ and ‘anthropogenic emissions’, it referenced the UNFCCC and IPCC reports.Footnote 64 Regarding ‘ecosystem’, it relied on the CBD and IPCC outcomes.Footnote 65 For the meaning of ‘marine environment’, it quoted the International Seabed Authority Regulations.Footnote 66 Finally, to support the deleterious effects of climate change, it invoked the UNFCCC preamble and provisions, and their further elaboration in the Kyoto Protocol and the Paris Agreement.Footnote 67 All of this led the Tribunal to its most critical finding: the emission of GHGs constitutes pollution of the marine environment.Footnote 68 This conclusion, fundamentally supported by external instruments, ultimately enabled the application of Article 194 UNCLOS in the context of climate change.
It is possible to find similar interpretative exercises in other parts of the Advisory Opinion. For example, in determining the measures that States must take to comply with their obligation in Article 192 to protect and preserve the marine environment,Footnote 69 the Tribunal discussed the meaning of the expression ‘adaptation measures’. While it acknowledged that the expression is not contained in UNCLOS, the Tribunal relied on both IPCC reports and legal instruments such as the UNFCCC and the Paris Agreement to elucidate its meaning.Footnote 70 Another example is found in the discussion of the obligation of States 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’.Footnote 71 ITLOS noted that the expressions ‘rare or fragile ecosystems’ and ‘habitat of depleted, threatened or endangered species and other forms of marine life’ are not defined in UNCLOS, in relation to the latter referring to the CBD to determine a generally accepted definition of ‘habitat’, and the categorisation of species under the CITES regime in order to guide its identification of species depleted, threatened or endangered.Footnote 72
4.2. Determination of the content of climate change obligations under UNCLOS
The second way in which the Tribunal used external materials was to specify the content of the UNCLOS obligations which, according to the Advisory Opinion, are applicable to climate change. These newly identified ‘UNCLOS climate change’ obligations mostly demand that States adopt measures to achieve particular goals. They are found in Articles 192, 194 and in other provisions spread across UNCLOS, that can be mostly categorised as rules of reference. This section analyses the use that the Tribunal gave to external instruments in order to explain the content of these UNCLOS climate change obligations.
4.2.1. The content of Articles 192 and 194 UNCLOS
The Tribunal identified the obligation set out in the first paragraph of Article 194 as the most relevant one relating to question (a) of COSIS’ request. This obligation entails the adoption of ‘all necessary measures’ to prevent, reduce and control pollution of the marine environment.Footnote 73 The discussion then centres around what constitutes ‘necessary measures’ and their content.Footnote 74 The Tribunal adopted a broad interpretation of the necessity requirement,Footnote 75 and stated that although UNCLOS grants States discretion in selecting measures to prevent, reduce and control marine pollution, this discretion is not unlimited; rather, the measures adopted must meet an objective standard of necessity.Footnote 76 Importantly, the Tribunal added that scientific knowledge and international rules and standards are relevant to the objective determination of the measures that States must adopt.Footnote 77
This finding led the Tribunal to a discussion of the role that other treaties and instruments play in the objective determination of the measures that States Parties must take to comply with their obligation under Article 194.Footnote 78 In this context, the Tribunal delivered a key passage addressing the principle of lex specialis and its potential application to the relationship between the obligations under UNCLOS and the commitments established by the Paris Agreement.Footnote 79
Some participants argued that if States comply with their obligations under the Paris Agreement, they would simultaneously satisfy their UNCLOS duties.Footnote 80 In other words, the Paris Agreement would be lex specialis and, accordingly, should take preference over the provisions of UNCLOS.Footnote 81 By contrast, other participants argued that the Paris Agreement represents a ‘minimum standard’ of compliance with UNCLOS. Consequently, the Tribunal was entitled to go ‘beyond the Paris Agreement’ as this treaty would not be lex specialis since there was no conflict of norms to be solved.Footnote 82
In response to these arguments, the Tribunal stated that while the Paris Agreement’s temperature goal and timelines inform the measures to be taken by States under Article 194 UNCLOS, these are essentially different instruments. ITLOS established that the provisions of the Paris Agreement should not take precedence over those of UNCLOS,Footnote 83 and explained that the Paris Agreement neither modifies UNCLOS nor qualifies as lex specialis in relation to it, and that, in this context, the lex specialis principle has no relevance in the interpretation of UNCLOS.Footnote 84
Regarding question (b), the Tribunal identified Article 192 UNCLOS as the most relevant rule.Footnote 85 This provision, which obliges States Parties to protect and preserve the marine environment, was found by ITLOS to be applicable to any kind of threat or form of degradation, including those emerging from climate change.Footnote 86 On this basis, the Advisory Opinion elaborated on what the obligation to protect and preserve the marine environment means in the context of the negative effects of ocean warming, sea-level rise and ocean acidification.Footnote 87 To this end, the Tribunal once again relied on external materials to determine the content of the measures that States must take to fulfil this obligation. It stated that external instruments inform such measures and pointed to the UNFCCC, the Paris Agreement and other more specific instruments, such as the Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of 10 December 1982 relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks, as well as the CBD.Footnote 88 The Tribunal explained that all these instruments may assist in the determination of protection and preservation measures.Footnote 89
4.2.2. Rules of reference
As part of its response to question (a), ITLOS interpreted a number of UNCLOS provisions that contain what are commonly known as ‘rules of reference’.Footnote 90 In the words of the Tribunal, these norms are ‘provisions […] that refer to external rules’ and which ‘employ different terms and have both a different scope and legal effect’.Footnote 91 The Tribunal discussed which external rules should be considered to implement the relevant UNCLOS rules of reference and the role they must play in such implementation.
For example, the Tribunal interpreted Articles 207, 211 and 212 UNCLOS as applied in the context of GHG emissions.Footnote 92 These provisions establish obligations on States Parties to adopt international rules and national legislation to prevent, reduce and control pollution of the marine environment emanating from land, vessels and the atmosphere. The Tribunal explained the role that international rules and standards play in the determination of the national legislation that States must adopt under these provisions.Footnote 93
Articles 207 and 212 establish the obligation on States Parties to adopt national legislation to prevent, reduce and control pollution of the marine environment from land-based sources and originating from or through the atmosphere ‘taking into account internationally agreed rules, standards and recommended practices and procedures’.Footnote 94 The Tribunal elaborated on the meaning of the last sentence, clarifying that the international instruments that States must consider can be binding or non-binding.Footnote 95 For example, it mentioned the UNFCCC, the Paris Agreement, Annex 16 to the Chicago Convention,Footnote 96 the IMO amendments to Annex VI to the International Convention for the Prevention of Pollution from ShipsFootnote 97 and the IMO’s 2023 GHG Strategy.Footnote 98 Regarding the meaning of the expression ‘taking into account’, ITLOS explained that it does not impose an obligation to implement the international rules and standards into domestic legislation but to give them due regard, in good faith. ITLOS added that ‘[i]n any case, States must comply with internationally agreed rules and standards, which are binding upon them’.Footnote 99
Article 211 deals with the obligation to adopt national legislation to prevent, reduce and control pollution from vessels. In this case, international rules and regulations play a stricter role than in Articles 207 and 212. As opposed to ‘taking them into account’, national legislation dealing with GHG emissions from vessels must ‘have the same effect as that of generally accepted international rules and standards established through the competent international organization or general diplomatic conference’.Footnote 100 Thus, the Tribunal considered that the international rules and standards which are generally accepted and which originated within the relevant international organisation constitute a minimum level that domestic legislation must meet in order to comply with Article 211.Footnote 101
5. Questions raised by the use of external rules and materials
As noted above, ITLOS attempted to provide a justification for its extensive reliance on external instruments to interpret UNCLOS. This is commendable in its own right. However, it should not exclude any scrutiny of the practice. Christina Voigt, for instance, argues that ITLOS ‘fell short of a comprehensive and consistent approach to determining which other treaty norms would be relevant to the interpretation of UNCLOS and how’.Footnote 102 Similarly, Ellen Hey suggests that ‘ITLOS’ engagement with external rules in treaty instruments could have been more profound’.Footnote 103 Sections 5.1 and 5.2 explore additional issues that arise from the use of external materials in the Advisory Opinion.
5.1. Unresolved questions regarding the use of external rules
The Tribunal’s use of external rules in its Advisory Opinion gives rise to several interpretive concerns. As discussed in Section 2.2, ITLOS mostly justified their use in light of the principle of systemic integration, as expressed in Article 31(3)(c) VCLT. While this is appropriate to determine the content of the new UNCLOS climate change obligations, it is less suitable when external rules are used to determine the meaning of particular terms and expressions. This is significant because ITLOS generally referred to other treaties—predominantly the UNFCCC and the Paris Agreement—for this purpose. Despite this, the Advisory Opinion did not address the relevance of Article 31(1) VCLT, which concerns the interpretation of treaty terms in accordance with their ordinary meaning. This omission is notable, as the literature had already considered the possibility of treating external rules as a kind of ‘elaborate law dictionary’,Footnote 104 and would have allowed the Tribunal to avoid complex questions about the criteria for using external rules based on Article 31(3)(c) VCLT. As a result of the silence on Article 31(1) VCLT, the reader is left to assume that almost every reference to external rules—except for those used in the context of rules of reference—is an application of the principle of systemic integration. This, in turn, raises even more complicated interpretive challenges.
As noted in Section 3.2, Article 31(3)(c) VCLT establishes that, in the interpretation of a treaty, ‘[t]here shall be taken into account, together with the context: … (c) any relevant rules of international law applicable in the relations between the parties’. Throughout the Advisory Opinion, ITLOS invoked a number of international instruments, but it did not clarify why they belong to the category set out in this provision. In other words, ITLOS did not directly address the debates concerning the three requirements that external rules must meet to be invoked under Article 31(3)(c) VCLT.
First, a central requirement under Article 31(3)(c) is that the external rules are ‘relevant’. Scholars have proposed different approaches to determining relevance in this context. Merkouris suggests a ‘proximity’ test based on four criteria to determine whether the rule is relevant, while McLachlan supports a more context-specific analysis.Footnote 105 In the Advisory Opinion, ITLOS stated that, for the purposes of the case, relevant external rules are found particularly in the treaties addressing climate change.Footnote 106 However, it did not engage in the debate by explaining what makes them ‘relevant’ for the purposes of Article 31(3)(c). This silence prevents a better understanding of this requirement and raises questions, such as why other potentially pertinent instruments were overlooked.
The most noteworthy example is the exclusion of human rights conventions, even though a number of participants argued in favour of taking them into account.Footnote 107 Some scholars consider this as a missed opportunity, suggesting that ITLOS was conservativeFootnote 108 or engaged in ‘selective integration of external rules’.Footnote 109 Others, however, argue that references to human rights instruments were unnecessary to deliver a comprehensive response to the specific questions submitted by COSIS.Footnote 110 A few judges addressed this issue in declarations stating that they would have preferred human rights considerations to be considered in the Advisory Opinion.Footnote 111 Regardless of whether their inclusion was warranted in this case, the omission highlights the following issue: what are the criteria for choosing which external instruments are ‘relevant’? The Advisory Opinion offers little guidance in this regard, which might reinforce the view that the principle of systemic integration can allow both ‘bold or cautious interpretative moves’.Footnote 112
The second question concerning the criteria for applying the principle of systemic integration contained in Article 31(3)(c) involves the need for the external instruments to be ‘rules’. McLachlan argues that this excludes soft law instruments,Footnote 113 while Gardiner acknowledges that the issue is subject to debate.Footnote 114 In its Advisory Opinion, ITLOS referred to a broad range of external sources—conventions, protocols, annexes, COP resolutions and decisions of international organisations—without addressing their differing legal nature.Footnote 115 This raises the question of whether the legal status of these instruments affects both their admissibility and the manner in which they may be used in treaty interpretation. In particular, does it make a difference if the instruments are binding or non-binding? The Advisory Opinion offers limited guidance on this issue.
ITLOS explicitly acknowledged the existence of the binding and non-binding categories.Footnote 116 However, it only considered the difference between them in certain parts of its analysis. For example, ITLOS dealt diligently with the issue of the legal status of external instruments in its examination of rules of reference. Regarding the obligation established in Articles 207 and 212, the Tribunal clarified that international rules and standards that States must take into account include both binding and non-binding norms.Footnote 117 The inclusion of the latter makes sense, given that the obligation is limited to ‘taking into account’ such rules and standards, and not to directly implementing them. In line with this approach, when the Tribunal considered the obligation of States under Article 213, it explained that this only includes rules that are binding on States.Footnote 118 Once again, this is reasonable given that an obligation to implement international instruments domestically can only refer to those of a binding nature.
By contrast, the Tribunal’s distinction between binding and non-binding instruments is less rigorous in its consideration of external materials by application of Article 31(3)(c) VCLT. While most of the instruments that ITLOS used to guide its interpretation of UNCLOS are binding, the Tribunal also relied on non-binding instruments—such as COP resolutions and standards adopted by international organisations—without offering a specific justification.Footnote 119 Arguably such instruments do not qualify as ‘rules’ and therefore fall outside the scope of Article 31(3)(c). Nevertheless, ITLOS referenced them in the same way as it referenced binding legal instruments, raising the question of what qualifies as a ‘rule’ under this provision. Maybe the language used in UNCLOS or other specific characteristics of the interpreted provisions was relevant to determine in the particular case what counted as applicable rules. However, ITLOS was silent in this regard. This omission is significant both in theoretical and practical terms. On a theoretical level, it is relevant because it might affect the soundness of the interpretation achieved using non-binding external materials. On a practical level, if non-binding outcomes adopted in international organisations or COPs are used to interpret the content of other treaties, this could create concerns among States and risk constraining their negotiations.
The third and final concern regarding the Tribunal’s application of the principle of systemic integration is its lack of engagement with the traditional question of who qualifies as ‘the parties’, as stated in Article 31(3)(c) VCLT. This is problematic because cases where multilateral treaties are used to interpret other multilateral treaties—such as the Advisory Opinion—are the ones where this issue becomes most pressing.Footnote 120
There are two main views in the literature regarding who ‘the parties’ are.Footnote 121 The more restrictive approach holds that an external rule can only be used in the interpretation of a treaty when all parties to the latter are also parties to the former.Footnote 122 In contrast, the broader view maintains that using an external rule to interpret a treaty is justified when the parties to a particular dispute concerning such interpretation are bound by both instruments.Footnote 123 McLachlan notes that the second approach risks being too detached from State consent, whereas the first one leads to the paradox that the more widely ratified a treaty is, the more isolated it may become, due to the lower likelihood of shared membership with other conventions.Footnote 124
Although ITLOS did not address the issue directly, the choice of conventions it used to interpret UNCLOS suggests that it adopted the broader approach. Merkouris and Bartenstein support this view, arguing that only the broader approach allows an interpretation that is not disconnected from the wider international legal system.Footnote 125 However, the advisory nature of the proceedings arguably required the Tribunal to engage more directly with the meaning of ‘the parties’, since the broader view presupposes the existence of a dispute, which is, by definition, excluded from the purview of advisory opinions.Footnote 126
As Linderfalk argues, Article 31(3)(c) VCLT should apply not only in the presence of a dispute, but generally when any party seeks clarification regarding the interpretation of a provision.Footnote 127 In such situations, similar to an advisory proceeding, there is no dispute and therefore no ‘parties to a dispute’. This makes it all the more important to clarify who qualifies as ‘the parties’ for the purposes of Article 31(3)(c), yet ITLOS did not provide an explanation. In one instance, however, when the Tribunal referred to CITES to interpret the term ‘depleted’, it noted that the treaty enjoys ‘near-universal adherence’.Footnote 128 This may suggest an attempt to justify its use. However, ITLOS did not make similar remarks in other instances where it relied on external instruments to interpret UNCLOS. This leaves unresolved, once again, an important question concerning the application of Article 31(3)(c), particularly in the context of an advisory opinion.
5.2. Unresolved questions regarding the use of scientific materials
Climate change is a fundamentally scientific phenomenon. As such, judicial cases concerning its effects cannot elude scientific debates. In its Advisory Opinion, ITLOS relied on the IPCC reports, considering them as authoritative and reflective of the ‘scientific consensus’.Footnote 129 While science played a central role in the Advisory Opinion,Footnote 130 the legal justification for relying on scientific resources to interpret UNCLOS leaves open several questions.
This is not related to using IPCC reports to understand the meaning of scientific concepts, as the Tribunal did with ‘greenhouse gases’, ‘anthropogenic emissions’ and other expressions discussed in Section 4.1. Greater clarity would have been achieved had ITLOS stated that it used scientific reports for the purposes of Article 31(1) VCLT. However, even in the absence of such clarification, the use of scientific information to identify the meaning of words fits within the mandate of that provision, which requires treaties to be interpreted according to the ordinary meaning of their terms in their context.
The more difficult question concerns the normative use of scientific materials, that is, the derivation of legal consequences from scientific knowledge. This is important because, as Sulyok observes, science does not itself answer legal questions, and the ways in which tribunals consider, or fail to consider, scientific information can affect the legitimacy of their decisions in the eyes of the public.Footnote 131 Sulyok develops this idea, arguing that the essence of the judicial function lies in turning scientifically legitimate information into legally legitimate outcomes. She claims that this prevents courts from adopting decisions that contradict a generally accepted scientific consensus.
While this is a strong and compelling argument, it seems aimed at tribunals that are reluctant to incorporate scientific insights into their reasoning. That is not the case here: ITLOS demonstrated a clear willingness to base its reasoning on the best available science. For this reason, the issue raised by this Advisory Opinion is different. The key question becomes: how should a tribunal that is open to engaging with science justify drawing legal conclusions from scientific findings in a way that remains consistent with the doctrine of sources of international law?
In the Advisory Opinion, ‘ITLOS is obviously comforted by the scientific rigor with which these [IPCC] reports are produced and by their wide recognition as “authoritative assessments of the scientific knowledge on climate change”’.Footnote 132 However, the Tribunal appeared to anticipate concerns about the decisive impact of scientific information on its findings by providing a detailed account of the procedures that the IPCC uses to adopt its outcomes. It highlighted that these instruments are reviewed and endorsed by States,Footnote 133 and even explained the system of ‘confidence levels’ used in them.Footnote 134 This arguably responds to the need to provide justification for the extensive use of IPCC reports, by establishing their epistemic authority,Footnote 135 and, perhaps, implicitly suggesting that these reports—like international legal instruments—can in some way be linked to State consent. Nonetheless, this is only conjecture and, in any case, the procedures for adopting scientific outcomes differ from those leading to the adoption of legal instruments.Footnote 136 In terms of legal consequences, endorsing a scientific report is not the same as consenting to an international obligation, or even to the adoption of a non-binding instrument that might eventually be used in the interpretation of a convention as a subsequent agreement, in accordance with Article 31(3)(a) VCLT. For that reason, the legal consequences drawn from scientific sources require deeper justification, especially given their potential significance. The question thus arises: are affirmations of the IPCC’s epistemic authority and the tenuous connection of its reports to State consent sufficient to justify deriving legal conclusions from them?
This is important because in the Advisory Opinion ITLOS drew key legal conclusions on the basis of scientific evidence. The main one was the Tribunal’s determination that the standard of due diligence applicable to States Parties under Articles 194 and 192 UNCLOS is a ‘stringent’ one.Footnote 137 This critical finding rests on the Tribunal’s assessment of IPCC reports,Footnote 138 which demonstrate the ‘serious and irreversible harm to the marine environment by climate change and ocean acidification’.Footnote 139 Given the centrality of this conclusion to global efforts to combat climate change, the need to firmly establish its foundations and reinforce its legitimacy in the eyes of the public, but perhaps more importantly, in the eyes of the States Parties, cannot be underestimated.
While ITLOS linked the ‘stringent’ standard of due diligence to its reasoning in its Advisory Opinion on Responsibilities and Obligations of States Sponsoring Persons and Entities with respect to Activities in the Area,Footnote 140 as well as to the principle of prevention and the precautionary approach,Footnote 141 the importance of this finding would have warranted a more detailed examination of both the legal basis of the standard and the ways in which science informs its content. ITLOS observed that the standard of due diligence depends on a range of factors and circumstances, including ‘scientific and technological information, relevant international rules and standards, the risk of harm and the urgency involved’.Footnote 142 However, it did not elaborate on the interplay among these elements. Similarly, the Tribunal affirmed that while science is crucial in designing effective measures to comply with Article 194(1) UNCLOS, there are also ‘other relevant factors that should be considered and weighed together with the best available science’.Footnote 143 Here again, however, ITLOS provided little guidance on how such factors are to be considered or weighed. As a result, the Advisory Opinion raises questions about the balance between science and other factors in shaping legal obligations. This becomes particularly pressing considering that the international responsibility of States might be engaged if they fail to meet such predominantly science-based obligations.Footnote 144
6. Conclusions
The Advisory Opinion is remarkable for many reasons. This article dives deep into the Tribunal’s extensive use of external rules and scientific materials to interpret UNCLOS and to respond to the questions submitted by COSIS.
Sections 2 and 3 addressed the identification of the relevant external instruments in the Advisory Opinion, the legal basis provided by the Tribunal for their use and the specific roles they played at various stages of the decision. These sections demonstrated the significance that ITLOS gave to the instruments that formed the normative and scientific background of the request, as well as the complexity of its engagement with external materials. They also showed that these materials were not only used to identify the meanings of terms and expressions, but also to determine the content of the new UNCLOS climate change obligations.
However, given the importance of ITLOS’ interpretative approach and its potential far-reaching effects on future cases and climate negotiations,Footnote 145 it is perhaps even more important to examine its limitations in order to clarify its reach and scope. To this end, Section 5 accordingly identified several unresolved questions regarding the role played by external rules and scientific materials in the interpretative process. These issues are ultimately linked to the need for a sound theoretical foundation for the use of external legal and non-legal materials in treaty interpretation. This is critical because the success of international law in tackling the climate crisis requires States implementing its norms—such as the newly identified UNCLOS climate change obligations—and this requires States to accept such norms as legal obligations.
Overall, the Advisory Opinion has the undeniable merit of applying an open and contextual approach to interpretation with care and rigour. This makes it an indispensable reference for anyone interested in treaty interpretation, the principle of systemic integration and the sources of international law in general. However, the Advisory Opinion also leaves outstanding questions that deserve continued attention. Addressing these issues will be essential for strengthening the legal response to climate change and to any other challenges that require international law to operate as a system.
Acknowledgements
I began working on this article during a research visit to the Institute for International Law and the Humanities at Melbourne Law School, University of Melbourne, in July 2024. I am grateful to Vicenta Jorquera for her assistance with reference checking, and to León Carmona, Benjamín Salas, Diego Zapata and especially Tejas Rao for their comments on earlier drafts. I also wish to thank Co-Editor Professor Richard Barnes and the two anonymous reviewers for their valuable feedback. All errors are my own.
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