Treaty and Custom in the ICJ’s Climate Change Opinion
Summary:
- ICJ opinion could influence global climate litigation
- U.N. treaties should guide responsibilities, rich countries say
- South, small island states seek firm measures to curb emissions
THE HAGUE—The United Nations` highest court on Wednesday told wealthy countries they must comply with their international commitments to curb pollution or risk having to pay compensation to nations hard hit by climate change.
In an opinion hailed by small island states and environmental groups as a legal stepping stone to make big polluters accountable, the International Court of Justice said countries must address the "urgent and existential threat" of climate change. The International Court of Justice has released its long-awaited Advisory Opinion on Obligations of States in respect of Climate Change. Delivered yesterday by a unanimous Court, the Opinion marks a turning point in the international law governing climate change. It follows several significant rulings and advisory opinions from regional human rights courts and the International Tribunal for the Law of the Sea (see, for example, here and here). The Opinion addresses complex legal issues, including the interaction between different sources of international law, state responsibility for harms with diffuse and cumulative causes (such as greenhouse gas emissions), the scope of due diligence obligations, the implications for statehood, and reparations. These questions will be explored in greater depth here on EJIL: Talk! over the coming days.
In this post, we focus on one foundational issue: the Court’s treatment of the sources of international law governing climate change and, crucially, how these sources interact. In what follows, we first consider (I) the Court’s identification and interpretation of the applicable law. We then turn to the Court’s analysis of the interaction between treaties and customary international law, focusing on three key dimensions: (II) the application of the lex specialis principle; (III) the co-constitutive relationship between treaties and custom; and (IV) the role of presumptions in operationalizing this relationship.
(I) Applicable Law Governing Question (a) and Customary Obligations in Respect of Climate Change
In the UN General Assembly’s request for an advisory opinion, its first question (a) asked: What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases for States and for present and future generations? In identifying the applicable law, the Court focused on those rules it considered ‘most directly relevant’ to the question before it (¶114). It then proceeded to assess, in detail, the obligations arising from those sources (¶¶174–404).
The Court identified as the most directly relevant applicable law: (i) the UN Charter; (ii) the UNFCCC, Kyoto Protocol, and Paris Agreement (‘the climate change treaties’); (iii) the UN Convention on the Law of the Sea (UNCLOS); (iv) other environmental treaties; (v) customary international law, with special attention to (a) the duty to prevent significant harm to the environment and (b) the duty to co-operate for the protection of the environment; (vi) core human rights treaties and human rights recognized under customary international law; (vii) and a set of principles applicable insofar as they guide ‘the interpretation and application of the most directly relevant legal rules’ (¶161).
We will not delve into the details of how the Court interpreted the obligations arising from all of these applicable sources of law. We expect that analysis to follow in subsequent commentary. It suffices to say here that the Court interpreted these treaty commitments robustly. A particularly significant example is its interpretation of the Paris Agreement’s seemingly aspirational goal of limiting the average global temperature to 1.5ºC. The Court treated this goal as a hard legal requirement on the basis of state parties’ subsequent agreement on the meaning of Articles 2 and 4 of that treaty (¶224, 242). However, it is worth dwelling for a moment on the Court’s identification of obligations arising under customary international law, whose provenance and contents are less self-evident.
The Court found that two primary rules of customary international law impose obligations on states in relation to climate change: the duty to prevent significant harm to the environment (‘duty to prevent’) and the duty to cooperate for the protection of the environment (‘duty to cooperate’). While most States accept the existence of these obligations in general terms, the Court explicitly extended them to the context of climate change.
To begin with the duty to prevent, some states had argued that the obligation was confined to preventing direct cross-border harm. The Court rejected that view, finding the duty to be broader in scope. Drawing on Pulp Mills and Legality of the Threat or Use of Nuclear Weapons, the Court stated that:
…the duty to prevent significant harm to the environment is not confined to instances of direct cross-border harm and that it applies to global environmental concerns. Therefore, the customary duty to prevent significant harm to the environment also applies with respect to the climate system and other parts of the environment. (¶134)
The Court also affirmed the robustness of the duty. While it is an obligation of ‘conduct’ rather than ‘result’, it is one that requires all states to ‘…employ all means reasonably available to them, so as to prevent [harm] so far as possible”’ (¶135). This entails a standard of due diligence, with several elements, including:
…States taking, to the best of their ability, appropriate and, if necessary, precautionary measures, which take account of scientific and technological information, as well as relevant rules and international standards, and which vary depending on each State’s respective capabilities. Other elements of the required conduct include undertaking risk assessments and notifying and consulting other States, as appropriate. (¶136; further elaborated in ¶¶280–300)
The Court insisted, finally, that ‘the standard of due diligence for preventing significant harm to the climate system is stringent’, and requires not only the adoption of appropriate climate rules and measures but also heightened vigilance in their implementation and enforcement (¶138).
The Court grounded the related duty to cooperate on the UN Charter (Art. 1) and on the Declaration on Friendly Relations, the adoption of which confirmed the duty’s customary status in general. Here, the Court found that the duty extends to cooperation relating to the environment, citing, in particular, the duty’s expression in many binding and non-binding instruments related to the environment (¶140).
Finally, the Court noted that the duty to prevent and the duty to cooperate are intrinsically linked. In its view, it would be difficult for states to discharge the duty to prevent without cooperating. Rather, ‘uncoordinated individual efforts by States may not lead to a meaningful result’ (¶141).
The Court’s identification of two robust norms of customary international law in relation to climate change raises a significant question: how do these rules interact with the major multilateral treaties on climate change? Do treaties like the UNFCCC, the Kyoto Protocol, and the Paris Agreement displace (or ‘contract out’ of) the customary duties of prevention and cooperation? Or do these treaties and customary norms exist harmoniously, perhaps even influencing one another?
(II) Lex Specialis
As part of its analysis of how the different obligations should interact, the Court addressed the question of whether some of the applicable rules of law might take precedence over others – specifically, whether certain rules might be displaced by others as lex specialis (¶162). Several states had argued that the rules set out in the climate change treaties should generally take precedence over customary international law and/or other rules of international law on these grounds. (See, for example, the positions taken by Japan, Russia, and the United States in their written statements.)
The Court recalled the two elements that must be present in order for the lex specialis principle to apply: (i) there must be ‘some actual inconsistency’ between the provisions in question; or (ii) there must be ‘a discernible intention that one provision is to exclude the other’ (¶167).
The Court found that neither condition was met (¶¶168–170). It held that there are no actual inconsistencies between the climate change treaties and other rules or principles of international law, including the customary duties of prevention and cooperation. Nor could it discern an intention by states parties to those treaties generally to displace other possibly applicable rules or principles. In support of this conclusion, the Court considered the object and purpose of the climate change treaties. It found that these goals indicate no conflict with broader rules of international law, nor any intent to clear the field of other rules or principles. Similarly, the fact that the treaties were carefully negotiated and form a ‘calibrated set of interrelated rules’ did not, in itself, indicate an intention to displace other rules.
(III) Systemic Integration and Co-Constitution
Given its position that the customary duties of prevention and cooperation exist alongside multilateral climate change treaties, the Court considered the nature of their coexistence. The principle of ‘systemic integration’ plays a central role in the Opinion. At the outset, the Court notes that ‘it is a generally recognized principle that, when several rules bear on a single issue, they should, to the extent possible, be interpreted so as to give rise to a single set of compatible obligations’ (¶165). Because neither the treaties nor the customary duties displace one another, they should thus be interpreted harmoniously.
In the Court’s view, the systemic integration of treaties and custom is a bi-directional process: ‘the obligations arising from the climate change treaties, as interpreted herein, and State practice in implementing them inform the general customary obligations, just as the general customary obligations provide guidance for the interpretation of the climate change treaties’ (¶313). Both aspects of this interplay are worth exploring.
On the one hand, the VCLT itself calls for treaties to be interpreted by taking into account ‘any relevant rules of international law that are applicable in the relations between the parties’ (Art. 31(3)(c), VCLT) – a category that clearly includes generally applicable customary international law. The Court indicates that this interpretive principle holds ‘in particular [for] multilateral environmental treaties’ (¶311). Although the Court does not explain why systemic integration should apply with special force to environmental treaties, its approach is consistent with a broader judicial trend: international courts and tribunals often rely on systemic integration and evolutive interpretation when dealing with collective or integral obligations, as opposed to strictly reciprocal ones. (See generally Arato, ‘Accounting for Difference in Treaty Interpretation Over Time’).
On the other hand, the identification of rules of customary international law and the determination of their content can both be shaped by treaties. Not for the first time, the Court notes that treaties can help identify extant rules of customary international law and can shed light on their content. Indeed, drawing on its case law, the Court says that ‘customary principles may even develop under the influence of the treaty, to such an extent that a number of rules contained in the treaty acquire a status independent of it’ (¶312). This is especially the case when, as with rules pertaining to climate change, ‘the relevant customary and treaty rules flow from a common fundamental principle’ (¶312).
The discussion of the interaction between treaty and custom may initially appear abstract, but the Court draws at least two subtle but important practical consequences from it. First, the Court relies on the interaction of these sources to make both the customary and conventional obligations more robust, most notably by linking the customary duty of due diligence to the treaty provisions. The Court says that ‘the climate change treaties establish standards that may enable or facilitate the identification and application of the diligence that is due in specific instances [under customary international law]’ (¶313). In this way, the Court takes the treaty and customary obligations to enrich one another.
Second, the Court’s conclusion that the customary duties are substantively informed (and thus made more robust) by the climate treaties and the state practice around them can, effectively, extend the treaties’ reach and entrench their legal effects. For one thing, tying customary and conventional norms together helps to bring non-party states closer to the treaty provisions. Likewise, this can potentially mitigate the (legal) effects of a large state like the United States exiting the treaty-based system – because the concomitant customary obligations would remain in force.
(IV) Interaction in Practice: What Role for Presumptions?
Finally, the Court concludes its considerations on the relationship between customary and conventional rules with two important paragraphs. In paragraph 314, the Court states:
As it is difficult to determine in the abstract the extent to which the climate change treaties and their implementation practice influence the proper understanding of the relevant customary obligations and their application, the Court considers that, at the present stage, compliance in full and in good faith by a State with the climate change treaties, as interpreted by the Court (see paragraphs 174-270 above), suggests that this State substantially complies with the general customary duties to prevent significant environmental harm and to co-operate. This does not mean, however, that the customary obligations would be fulfilled simply by States complying with their obligations under the climate change treaties (see Climate Change, Advisory Opinion, ITLOS Reports 2024, pp. 85-86, para. 223). While the treaties and customary international law inform each other, they establish independent obligations that do not necessarily overlap.
The Court goes on to add:
…it is possible that a non-party State which co-operates with the community of States parties to the three climate change treaties in a way that is equivalent to that of a State party, may, in certain instances, be considered to fulfil its customary obligations through practice that comports with the required conduct of States under the climate change treaties. However, if a non-party State does not co-operate in such a way, it has the full burden of demonstrating that its policies and practices are in conformity with its customary obligations. (¶315)
The Court makes two important points here concerning presumptions. First, it says that compliance ‘in full and in good faith’ by a state with the climate change treaties, ‘as interpreted by the Court’, suggests substantial compliance with the general customary duties to prevent significant environmental harm and to co-operate. This is not the happiest formulation, particularly given that some states have argued that their conduct should be assessed solely under treaty obligations, implicitly suggesting that compliance with treaty rules insulates them from customary international law.
In our view, such positions were never tenable as a matter of international law, and are even less so following the Advisory Opinion. As the Court made clear, the climate change treaties do not constitute lex specialis in relation to customary rules (¶171; see also the joint declaration of Judges Charlesworth, Brant, Cleveland, and Aurescu, ¶2).
The Court also emphasised that treaty compliance must be full, in good faith, and consistent with its own interpretation of those provisions. Most importantly, the Court reiterates that compliance with treaty obligations does not, in itself, discharge customary law obligations. This reflects the Court’s longstanding jurisprudence, according to which the existence and applicability of customary norms remain analytically distinct from those of treaty obligations (see again the joint declaration of Judges Charlesworth et al.). Judge Nolte further clarifies what this presumption does and does not mean:
This approach is not lex specialis by another name. Rather, it is a way of achieving a harmonious interpretation of, and maintaining a proper relationship between, the climate change treaties and customary international law. This presumption is qualified and gives room for nuance and complexity. It can also take account of the different character of the respective sources of obligations and of possible future scenarios. (Declaration of Judge Nolte, ¶13)
The Court’s second presumption relates to non-party states; it says that non-party states cooperating with state parties to the three climate change treaties ‘in a way that is equivalent to that of a State party, may, in certain instances, be considered to fulfil its customary obligations’. As noted above, this can be read as a mechanism to bring non-party states closer to treaty standards by anchoring customary law in prevailing treaty practice. Conversely, non-parties that do not cooperate in such a manner bear the ‘full burden’ of demonstrating that their domestic policies and practices conform to customary law. Functionally, then, the Court seems to envision the presumption’s protective bubble as an incentive to cooperate – and perhaps a useful litigation tool when the time comes.
(V) Conclusion
This is not the place for a comprehensive evaluation of an advisory opinion of this magnitude. However, one key takeaway is clear: the Court has provided a detailed legal catalogue of obligations relating to climate change from diverse sources of international law – and clarified how these interact – in ways that will prove valuable to vulnerable states and communities, including in domestic litigation. As Judge Cleveland notes in her declaration, these obligations can also inform international dispute resolution in other fora, particularly in the context of investor-state dispute settlement.
One can hope that the Court’s elaboration of the legal architecture of climate change could serve to guide the behaviour of major emitters, including potential outliers such as the United States. At the same time, the Court’s methodology is important in its own right. It is striking that the Court leaned heavily on systemic integration rather than the principle of lex specialis. This is not a conservative opinion from the perspective of the doctrine of sources, and the Court’s approach to the sources of international law here will likely provide insight and inspiration far beyond the confines of this case.
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Photo: Palace of Justice, The Hague. Source: EJIL Talk!
