{"id":25780,"date":"2025-08-06T14:00:49","date_gmt":"2025-08-06T12:00:49","guid":{"rendered":"https:\/\/voelkerrechtsblog.org\/?p=25780"},"modified":"2025-11-14T15:06:51","modified_gmt":"2025-11-14T14:06:51","slug":"the-advisory-opinion-on-obligations-of-states-in-respect-of-climate-change","status":"publish","type":"post","link":"https:\/\/voelkerrechtsblog.org\/de\/the-advisory-opinion-on-obligations-of-states-in-respect-of-climate-change\/","title":{"rendered":"The Advisory Opinion on Obligations of States in Respect of Climate Change"},"content":{"rendered":"

Introduction<\/strong><\/p>\n

The International Court of Justice (ICJ) has delivered an Advisory Opinion on <\/span>Obligations in Respect of Climate Change<\/span><\/i><\/a> that meets the moment. That the members of the Court felt the weight of that moment on their shoulders \u2013 with respect to the role of the ICJ in the context of the current pressures on international legality and the existential threat of climate change \u2013 is evident from the fact that the Court issued an opinion that is not only robust but also unanimous on all aspects of the opinion\u2019s operative part.\u00a0<\/span><\/p>\n

The Advisory Opinion covers the wide spectrum of issues raised by the <\/span>UN General Assembly request of March 2023<\/span><\/a>. While there is much to be said, my focus is on the Court\u2019s highly significant pronouncements on States\u2019 environmental harm prevention obligation under customary international law. In this short contribution, I highlight some of the most important ways in which the opinion reinforces, clarifies and augments the Court\u2019s previous decisions on this pivotal obligation. In so doing, the opinion also speaks to debates around the structure and, according to some (see e.g. <\/span>Koskenniemi 2007<\/span><\/a>; <\/span>Klabbers 2023<\/span><\/a> at 291), \u2018proceduralization\u2019 of international (environmental) law. Far from being a relic of the past, customary international law emerges as a highly relevant, strong normative framework for grappling with global environmental concerns.<\/span><\/p>\n

Five Take-Aways (One in Five Parts \u2026)<\/strong><\/p>\n

<\/b>1.<\/strong> Invoking its 1996 Advisory Opinion on <\/span>Legality of Nuclear Weapons<\/span><\/i><\/a> (para. 29), the Court confirms that <\/span>States\u2019 harm prevention obligations under customary law apply to global environmental concerns<\/strong>. The ICJ is explicit that the salient obligations are \u201cnot confined to instances of direct cross-border harm\u201d (para. 134).\u00a0 Since all of the Court\u2019s judgments on the harm prevention rule were rendered in the context of disputes involving transboundary environmental impacts (<\/span>Gab\u010d\u00edkovo-Nagymaros<\/span><\/i><\/a>, para. 53; <\/span>Pulp Mills<\/span><\/i><\/a>, para. 193; <\/span>Certain Activities\/Construction of a Road<\/span><\/i><\/a>, para. 104), this is a very welcome statement. The Court had previously confirmed that the harm prevention rule applied to the \u201cenvironment of other States or of areas beyond national control\u201d (<\/span>Legality of Nuclear Weapons<\/span><\/i><\/a> (para. 29); <\/span>Gab\u010d\u00edkovo-Nagymaros<\/span><\/i><\/a>, para. 53; <\/span>Pulp Mills<\/span><\/i><\/a>, para. 193). Its unequivocal affirmation in <\/span>Obligations in Respect of Climate Change<\/span><\/i><\/a> that the rule also applies to the climate system is all the more significant because several States had argued that it did not apply, given the cumulative and global nature of climate change (paras. 133, 134, 273).<\/span><\/p>\n

2.<\/strong> The Court notes that the duty to prevent<\/strong> under customary law arises when there is \u201ca risk of significant harm to the environment<\/strong>\u201d<\/strong> (para. 274). This simple, intuitive statement provides a crucial clarification of some of the Court\u2019s previous jurisprudence on the harm prevention rule. Its decisions in the Pulp Mills<\/i> case and, in particular, in the Certain Activities\/Construction of a Road<\/i> cases, appeared to suggest that the Court considered that a failure to take the requisite prevention measures, notably procedural steps, would not constitute a violation of the rule unless harm had been caused (for a discussion see Brunn\u00e9e 2020<\/a>, at 88-95). However, harm causation is not in fact an element of the primary rule, which is concerned precisely with harm prevention <\/i>(ibid. at 101-104). In Obligations in Respect of Climate Change<\/i>, the ICJ could not have been more explicit on this important point, stating that the \u201c[f]ailure of a State to take appropriate action to protect the climate system \u2026 may constitute and internationally wrongful act\u201d (para. 427), \u201cwhether that act causes harm or not\u201d (para. 433). Hence, unless the desired remedy is compensation for harm, proof of harm causation is not required when States invoke the responsibility of others for failures to address climate change. So long as it can be established that a State is not taking measures commensurate with the risk of climate harm, it may incur legal consequences under the law of State responsibility (Brunn\u00e9e 2020, at 103-104), including the \u201cobligations of cessation and non-repetition, which \u2026 apply irrespective of the existence of harm\u201d (Obligations in Respect of Climate Change<\/i>, para. 445).<\/p>\n

3. <\/strong>What then is required of States in discharging their harm prevention obligation? As the ICJ previously decided, States must exercise due diligence (Pulp Mills<\/i>, paras. 101, 197), the requirements of which in a given situation, according to the ICJ in Obligations in Respect of Climate Change<\/i>, \u201cshould be determined objectively\u201d (para. 300). The ICJ previously held that due diligence \u201centails not only the adoption of appropriate rules and measures, but also a certain level of vigilance in their enforcement and the exercise of administrative control applicable to public and private operators\u201d (Pulp Mills<\/i>, para. 197). In Obligations in Respect of Climate Change<\/i>, the Court provides a series of important clarifications regarding the \u201celements\u201d that determine the \u201cconduct required by due diligence\u201d<\/strong> (para. 136). Here, I highlight only the most significant of these clarifications.<\/p>\n

i.<\/strong> Naturally, the \u201cdegree of a given risk of harm<\/strong> is always an important element\u201d (para. 275). Aligning itself with the conceptualization employed by the International Tribunal on the Law of the Sea (ITLOS) in its 2024 Advisory Opinion on Climate Change and International Law<\/i><\/a>, the ICJ finds that \u201c[w]hether an activity constitutes a risk of significant harm\u201d depends upon a combination of \u201cboth the probability or foreseeablility of the occurrence of harm and its severity or magnitude\u201d (ibid.). Therefore, \u201cthe higher the probability and the seriousness of possible harm, the more demanding the required standard of conduct\u201d (ibid.).<\/p>\n

Indeed, with respect to threats of serious or irreversible harm, the ICJ agrees with ITLOS\u2019 conclusion \u201cthat \u2018where there are plausible indications of potential risks,\u2019 a State \u2018would not meet its obligation of due diligence if it disregarded those risks\u2019\u201d (para. 294, citing to <\/span>Climate Change Obligations and International Law<\/span><\/i>, para. 131). Thus, significantly, the ICJ agrees with ITLOS that \u201c<\/span>the \u2018precautionary approach is also an integral part of the general obligation of due diligence<\/strong>\u2019 under the duty to prevent significant harm to the environment\u201d (ibid.).<\/span><\/p>\n

ii.<\/strong> The ICJ also echoes ITLOS\u2019 conclusion that the requisite risk can exist in situations involving \u201cthe cumulative effect of different acts undertaken by various States and by private actors subject to their jurisdiction or control\u201d (para. 276). In short, <\/span>the fact that risks of harm to the climate system result from the multiple States\u2019 and private actors\u2019 contributions does not relieve individual States from their obligation<\/strong> to take appropriate preventive measures. This conclusion is in line with other judicial decisions to the effect that no State should \u201cevade its responsibility by pointing to the responsibility of other States\u201d and that \u201ceach State has its own responsibilities within its own territorial jurisdiction in respect of climate change\u201d (<\/span>KlimaSeniorinnen v. Switzerland<\/span><\/i><\/a>, paras. 442-443). Given the growing international and national judicial consensus on this issue, the \u2018drop in the ocean\u2019 argument seems to have \u2026 run dry.<\/span><\/p>\n

iii.<\/strong> What measures are appropriate to prevent harm is dependent, in particular, upon \u201c<\/span>scientific and technological information, as well as upon relevant rules and international standards<\/strong>,\u201d including those adopted under the auspices of the UN climate regime (<\/span>Obligations in Respect of Climate Change<\/span><\/i>, paras. 88-89; also 283-288). As is true for the risk to which States\u2019 measures must respond, these factors underscore that the <\/span>standard of conduct can evolve over time<\/strong>, for example in the light of \u201cnew scientific or technological knowledge\u201d (para. 284). The ICJ notes at various points that, given the indisputable scientific evidence of the universal and urgent risks of significant harm to the climate system, \u201c<\/span>the standard of due diligence is stringent<\/strong>,\u201d requiring \u201ca heightened degree of vigilance and prevention\u201d (para. 138), that States do their \u201cutmost\u201d (para. 246), and that they take measures that are \u201cdesigned to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system\u201d (para. 282).<\/span><\/p>\n

iv.<\/strong> As the Court confirms, \u201cwhat is required by due diligence ultimately \u2018calls for an assessment in concreto<\/i>\u2019 of what is reasonable under the specific circumstances in which a State finds itself\u201d (para. 137, citing to Bosnia and Herzegovina v Serbia and Montenegro<\/i><\/a>, para. 430), such that the standard \u201cwill vary depending on each State\u2019s respective capabilities\u201d (para. 136). For the Court, this conclusion follows because the obligation requires a given State to \u201cuse all means at its disposal\u201d (para. 290). I do not have space here to comment on the Court\u2019s various observations regarding the principle of common but differentiated responsibilities and respective capabilities<\/strong>.<\/strong> Suffice it to say for present purposes that it considers the principle to be relevant to the \u201cdetermination of the applicable standard of due diligence in a particular situation<\/strong>\u201d<\/b> (ibid.).<\/p>\n

v. Due diligence entails substantive and procedural requirements<\/strong>. Previously, notwithstanding acknowledgement of their \u201cfunctional\u201d linkages (para. 289, citing to Pulp Mills<\/i>, para. 79), the Court appeared to separate these requirements legally speaking, finding a procedural violation while refraining, in the absence of harm causation, from finding a substantive violation (see discussion of take-away #2 above). In Climate Change Obligations<\/i>, the ICJ seems intent on overcoming this much criticized, including by members of the Court (Pulp Mills<\/i>, Joint Dissenting Opinion Al-Khasawneh and Simma<\/a>; and Certain Activities\/Construction of a Road<\/i>, Separate Opinion Donoghue<\/a>), and conceptually unconvincing approach to the relationship between procedure and substance (for a discussion see Brunn\u00e9e 2020, at 88-95). After all, in the context of the harm prevention rule, procedural elements, such as the obligations to undertake risk or environmental impact assessments, or to notify and consult potentially affected States, are tightly intertwined with the substantive goal of harm prevention, such that the latter could not be achieved without the former. Obligations in Respect of Climate Change<\/i> reflects this reality, noting that the \u201cduty to prevent significant harm to the environment consists of substantive elements \u2026 and procedural elements, through both of which States fulfill their duty of due diligence\u201d (at 289; see also at 136, 295, 299).\u00a0 Accordingly, a State\u2019s failure, for example, to assess the risks of harm to the climate system occasioned by activities under its jurisdiction, could amount to a violation of the harm prevention rule. That certainly would be the logical consequence of the Court\u2019s observation in Pulp Mills<\/i> that \u201cdue diligence \u2026 would not be considered to have been exercised, if a party \u2026 did not undertake an environmental impact assessment on the potential effects\u201d of activities that may have a significant adverse impact (Pulp Mills<\/i>, para. 204).<\/p>\n

4.<\/strong> In Obligations in Respect of Climate Change<\/i> the ICJ confirms that \u201cStates\u2019 obligations pertaining to the protection of the climate system and other parts of the environment<\/strong> \u2026, in particular the obligation to prevent significant transboundary harm under customary international law, are obligations erga omnes<\/i><\/strong>\u201d (para. 440). Since \u201cobligations erga omnes<\/i>, are \u2018[b]y their very nature \u2026 the concern of all States\u201d (para. 441, citing to Barcelona Traction<\/i><\/a>, para. 33), it follows that, under the rules on State responsibility, \u201cclimate mitigation obligations \u2026 may be invoked by any State when such obligations arise under general international law\u201d (Obligations in Respect of Climate Change<\/i>, para. 442), including a State \u201cother than an injured State\u201d (ibid., citing to Article 48(1)() ARSIWA). This pronouncement marks the first time that the ICJ affirmed the erga omnes<\/i> nature of obligations regarding the prevention of harm to the environment beyond national jurisdiction. Given the Court\u2019s reference to \u201cobligations<\/i> pertaining to the protection of the climate system\u201d (para. 440; emphasis added), it stands to reason that all harm prevention obligations are owed erga omnes<\/i>. This would include the procedural requirements detailed by the Court, as well as the all-important obligation to cooperate in good faith (paras. 140-142), which for reasons of space I cannot discuss here.<\/p>\n

5.<\/strong> There is much that could be said about the Court\u2019s engagement with States\u2019 commitments under the UN Framework Convention on Climate Change (FCCC) and, in particular, the Paris Agreement (see Voigt<\/a>, in this symposium). For present purposes, the key point is that the ICJ is clear that, contrary to the views expressed by several States, these treaties do not displace States\u2019 harm prevention obligations under customary law<\/strong> (paras. 162-171). On the contrary \u2013 according to the ICJ, the due diligence requirements discussed above inform the interpretation of States\u2019 \u201cobligations of conduct\u201d under the Paris Agreement, including their obligations regarding the mitigation of greenhouse gas emissions (paras. 228-229; 241-242; 245-246; 252; 254).<\/p>\n

Conclusion<\/strong><\/p>\n

As the ICJ notes in its concluding observations: climate change is \u201cmore than a legal problem;\u201d it is \u201can existential problem of planetary proportions that imperils all forms of life and the very health of our planet\u201d (para. 456). \u00a0 Conscious that international law \u201chas an important but ultimately limited role\u201d in resolving the climate crisis, the Court expressed the hope that \u201cits conclusions will allow the law to inform and guide social and political action to address the ongoing climate crisis\u201d (ibid.). In my assessment, the ICJ has made a significant contribution to these ends. As I hope to have illustrated, its opinion on <\/span>Climate Change Obligations<\/span><\/i> reinforces, clarifies and augments its previous decisions on international environmental law\u2019s cornerstone obligation. The Court\u2019s observations confirm that, in terms of the structure of international law, the harm prevention obligation has emerged as a pivotal part of the international legal regime applicable to the climate crisis. Not only does it provide standards that bind every State, whether party to climate agreements or not, it also provides a robust normative framework. In this framework, the proceduralization of international environmental law that some observers have detected would seem to be a strength rather than a weakness. Through the harm prevention obligation\u2019s due diligence standard, substance and procedure are tightly intertwined to provide adaptable, stringent and remarkably concrete requirements for the conduct of all States in the face of the climate crisis.\u00a0<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Introduction The International Court of Justice (ICJ) has delivered an Advisory Opinion on Obligations in Respect of Climate Change that meets the moment. That the members of the Court felt the weight of that moment on their shoulders \u2013 with respect to the role of the ICJ in the context of the current pressures on […]<\/p>\n","protected":false},"author":36,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[6639],"tags":[3792,4003,7521],"authors":[7669],"article-categories":[3572],"doi":[],"class_list":["post-25780","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-climate-change","tag-customary-international-law","tag-obligations-erga-omnes","authors-jutta-brunnee","article-categories-symposium"],"acf":{"subline":"Harm Prevention under Customary International Law"},"meta_box":{"doi":"10.17176\/20250806-122343-0"},"_links":{"self":[{"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/posts\/25780","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/users\/36"}],"replies":[{"embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/comments?post=25780"}],"version-history":[{"count":6,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/posts\/25780\/revisions"}],"predecessor-version":[{"id":25820,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/posts\/25780\/revisions\/25820"}],"wp:attachment":[{"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/media?parent=25780"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/categories?post=25780"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/tags?post=25780"},{"taxonomy":"authors","embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/authors?post=25780"},{"taxonomy":"article-categories","embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/article-categories?post=25780"},{"taxonomy":"doi","embeddable":true,"href":"https:\/\/voelkerrechtsblog.org\/de\/wp-json\/wp\/v2\/doi?post=25780"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}