Violetta Sefkow-Werner, PhD candidate at the University of Zurich
On 24 July 2025, the ICJ issued its long-awaited Advisory Opinion on the Obligations of States in respect of Climate Change. This blog post comments on the Opinion with respect to the ICJ’s findings (or non-findings) on the interaction between treaty and customary law and the missed opportunity to include human rights law.
Important Clarifications on the Applicable Law
The ICJ first clarified that the relevant customary international law obligations are the duty to prevent significant harm to the environment and the duty to cooperate. These duties apply in the context of climate change. They are not limited to bilateral relations or to direct cross-border harm (para. 134) and they are not displaced by the climate change treaties via the lex specialis principle (para. 171). This resolves a long-standing point of contention concerning the applicability and scope of customary environmental law in the context of climate change. It also confirms that the harm prevention rule applies to the global commons – in this case the climate system – and is an obligation erga omnes (para. 440) although the exact implications of this remain to be explored (see, e.g., Pezzano, EJIL:Talk!, 8 August 2025, and the Interim Report by the Institut de Droit International).
Interaction Between Treaty and Customary Law: Nudging, Presumption and a Gordian Knot
The ICJ further explained that customary international law and treaty law inform each other. The climate treaties provide substance to the due diligence required under customary law and the customary obligations provide guidance for the interpretation of the climate treaties (paras. 311 ff., Article 31(3)(c) VCLT). The ICJ referred to its previous case-law as well as the ITLOS’ Advisory Opinion on Climate Change and held 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. The Court also considers that 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.” (para. 313)
Moreover, there is a presumption of compliance with customary obligations (prevention duty and cooperation) if treaty obligations are fulfilled. This presumption ultimately applies to all States, regardless of whether they are parties to the climate treaties or not:
“[At] the present stage, compliance in full and in good faith by a State with the climate change treaties, as interpreted by the Court (…), suggests that this State substantially complies with the general customary duties to prevent significant environmental harm and to co-operate.” (para. 314)
“[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.” (para. 315)
This can be seen as a form of nudging by the ICJ, pushing States to act in line with the international climate treaties, and immunizes climate obligations from a State’s treaty exit. It also suggests that the treaty obligations are included in the customary obligations and that the latter go further. Namely, the ICJ clarified:
“This does not mean, however, that the customary obligations would be fulfilled simply by States complying with their obligations under the climate change treaties (…). While the treaties and customary international law inform each other, they establish independent obligations that do not necessarily overlap.” (para. 314)
This raises the question of what is or will be found in the catch-all called customary international law. Judge Nolte notes that the added value of the conceptualization of treaty and customary law is that it provides room for nuance and complexity (Declaration of Judge Nolte, para. 13). With regard to the duty to cooperate, the ICJ held that climate treaties only contain some means of cooperation (para. 314), i.e. other forms remain possible and further collective action may potentially be necessary (para. 307). Moreover, States do not discharge their duty to cooperate by concluding and fulfilling treaties (para. 314). Consequently, there may be additional obligations under customary law, i.e. obligations that go beyond that required by the climate treaties (see also Joint Declaration of Judges Charlesworth, Brant, Cleveland and Aurescu, paras 5 and 9 f.).
What is more, a closer look at the specific obligations under treaty and customary law may reveal small discrepancies. For example, in the realm of customary law, the ICJ considers the different capabilities of States not according to their status as developed or developing countries but based on an individual assessment (para. 292). This assessment might depart from the categorization under the UNFCCC (Annex I, II and Non-Annex Parties) which results in differing commitments.
Finally, the customary duty of due diligence provides a yardstick for compliance with treaty obligations, although applying that yardstick might prove tricky. For instance, with regard to the assessment whether a State has acted with the required diligence when discharging its obligation under Article 3 of the Paris Agreement, there are in fact multiple offers for what diligence means: (1) the “stringent” standard as formulated by the ICJ in para. 138 and elaborated in paras. 246 f.; (2) the customary standard established in paras. 136 and 280 ff.; (3) the referral to Articles 4, 7, 9, 10, 11 and 13 of the Paris Agreement by Judge Xue in her Separate Opinion (paras. 48 f.). Thus, the ICJ’s pronouncements on the interplay between treaty and custom is only the neat-looking tip of a big and messy iceberg. Or, in more positive terms, a playground for academics and comfortably squishy interpretative gateway for judges.
Widening the Circle of Customary Law: The Relevance of Non-binding Norms
The standard of due diligence is not only informed by treaty law but also by soft law. The ICJ held:
“Current standards may arise from binding and non-binding norms. Such standards may therefore not only be contained in treaties and in customary international law, but they may also be reflected in certain decisions of the COPs to the climate change treaties and in recommended technical norms and practices, as appropriate.” (para. 287)
This is a fairly open statement. First, the ICJ did not specify what other non-binding norms or standards could play a role and how or to what extent they are to be taken into account. For example, do they have less normative force than binding rules? Second, one may wonder whether this interpretative approach will lead to differences between customary and treaty law. In principle, systemic interpretation is possible both for treaty and customary international law (Fortuna, in: Fortuna et al (eds), Customary International Law and Its Interpretation by International Courts, CUP 2024, Ch. 11). Article 31(3) VCLT enlists not only “relevant rules” (lit. c, confined to binding rules) as relevant sources for interpretation but also subsequent “agreements” (lit. a) and subsequent “practice” (lit. b). Hence, the interpretative sources may vary, but whether this is also true for the result remains to be seen. Third, concerns might arise regarding the reliance on soft law to interpret climate-related obligations. The ECtHR has already received backlash for integrating the climate treaties into human rights obligations, as this was perceived to circumvent the exclusion of enforcement mechanisms by the parties to the climate law regime. The ICJ’s findings on the applicability of the law of State responsibility and its rejection of the lex specialis argument in that context (paras. 410 ff.) has taken the wind out of that criticism. If – hypothetically – soft law is used in such a way that the hard law obligation essentially mirrors the soft law standard, it is de facto upgraded to hard law while, formally, still being referred to as soft law. This might raise legitimacy concerns, although it may ultimately remain a theoretical rather than a practical problem.
The Downer: Scarce Integration of Human Rights
While the ICJ was quite integrative and explicit in establishing a close link between customary and treaty obligations with respect to climate change, it was less so with regard to the interplay between treaty- or custom-based climate law and human rights. In para. 402, it stated that the (territorial?) scope of customary international law and human rights are distinct. This might be an implicit rejection of the 2017 IACtHR’s Advisory Opinion where the IACtHR established an extraterritorial human rights-based duty not to cause harm to the environment of other States (cf. IACtHR, OC-23/17 of 15 November 2017, para. 101; Brunnée, Procedure and Substance in International Environmental Law, 2020, p. 145). Moreover, unlike the IACtHR, the ICJ did not integrate a mandatory comprehensive human rights perspective into the relevant legal framework relating to climate change (IACtHR, AO-32/25 of 29 May 2025, Part VI). Similar criticism has also been made with regard to the ITLOS Advisory Opinion on Climate Change (Desierto, EJIL:Talk!, 3 June 2024).
The suggested interaction between human rights law and climate law is only weak. According to the ICJ, States are obliged under human rights law to take the necessary measures to protect the climate system and other parts of the environment. These may include, inter alia, “mitigation and adaption measures, with due account given to the protection of human rights, the adoption of standards and legislation of the activities of private actors” (para. 403, emphasis added). This reads like reiteration of the treaty- and custom-based obligations with a pinch of human rights protection. Moreover, the part on customary international law obligations does not mention human rights at all. Thus, where the ICJ Opinion is strong in harmonizing the climate treaties with customary environmental law, it does not use the full potential of systemic integration with respect to human rights law. Maybe the ICJ wanted to leave the interpretation of human rights law to the specialized treaty bodies and regional courts (cf. the ICJ’s references to the case law and General Comments of the UN human rights organs and the Inter-American and European Court of Human Rights, paras. 372 ff., 385). What is more, bringing human rights law together with climate and environmental law might prove more difficult upon closer examination due to the structural differences between the respective regimes concerning the object of protection and holders and beneficiaries of the obligations.