Great Expectations

While the ink is still fresh, general media as well as experts’ assessment seem to be unanimous that the 23 July 2025 Advisory Opinion of the International Court of Justice (‘ICJ’ or ‘Court’) on Obligations of States in respect of Climate Change (in the following, I cite to passages from this Opinion as ‘AO, para. …’) is a historic, ‘landmark opinion’. Indeed, expectations were high before its delivery and remain so in its aftermath. However, a glance at the Court’s own jurisprudence constante seems to pour cold water on any expected relevant impact of this, as in fact any, advisory opinion. Since 1950, the Court consistently underlines that ‘as such, [an advisory opinion] has no binding force’ (Peace Treaties (I), p. 71). Accordingly, when the Special Chamber of the International Tribunal for the Law of the Sea (‘ITLOS’) in Mauritius v. Maldives recently sought to grant the ICJ’s Chagos opinion de facto res judicata effect (paras. 202-205), this received wide-spread criticism (e.g. Eichberger, pp. 14 et seq.).

Yet, when invoking prior ‘case-law’ as persuasive international precedent, the Court and other international judicial bodies refer to advisory opinions and decisions in contentious proceedings in an indiscriminate fashion, as the International Law Commission (‘ILC’) recently noted (p. 93, para. 278). So, where does such legal impact come from and how should we assess the impact of this ‘landmark’ opinion? I submit that the lasting legal impact and influence of the Advisory Opinion of 23 July – like that of any of the Court’s opinions – depends to  a considerable degree on how well the Court succeeds in managing and responding to expectations held prior to its delivery. This harkens back, and pertains to, the World Court’s judicial function, no less (see on the latter Hernández). 

Expectations

In a recent article (Kulick, pp. 37-51), I argue that stakeholders pursue advisory opinions before the ICJ with one or several of a total of six different expectations in mind, ranging from ‘advice as advice’ to ‘advice as miracle’. Their ‘persuasiveness’ and thus their relevance and impact regarding future case law depends to a considerable degree on how well the Court succeeds in managing these expectations in its Opinion (see ibid., 56-60). With respect to the Climate Change Advisory Opinion, two of these expectations are of particular interest. 

1. Stakeholders may seek an advisory opinion to have the Court ascertain and develop the law on abstract and general legal questions. It was expected that the ICJ would provide ‘advice as law-development’ pertaining to a whole panoply of complex doctrinal issues, including: the relationship of the UNFCCC and the Paris Agreement (‘PA’) as well as of climate change treaties and customary international law; the elements and scope of due diligence standards; the potential existence of a human right to a clean and healthy environment; as well as various matters of state responsibility, causation and compensation among them.
2. In addition, advice may be sought from the Court to resolve a grand societal debate: ‘Advice as miracle’. Arguably, some expectations attached to the present Opinion concerned its potential to single-handedly transform worldwide climate policy. I submit that only once before has the Court been confronted with expectations of a similar magnitude – and fared rather badly in handling them: In its Nuclear Weapons Advisory Opinion (para. 105 (2) E.) it had no clear answer to the fundamental societal question whether nuclear weapons and their use were to be banned by international law tout court.

Expectation Management and the Court’s Judicial Function

When discussing admissibility, the Court underlined its ‘judicial function’, citing Nuclear Weapons (para. 18) to stress that, when issuing an advisory opinion, it ‘states existing law and does not legislate’ (see AO, para. 48). It echoes a similar point later, quoting from Fisheries Jurisdiction (pp. 23-24, para. 53) that ‘the Court, as a court of law, cannot render judgments sub specie legis ferendae.’ This pertains to both expectations at play regarding the Opinion of 23 July 2025.

With respect to the ‘advice as miracle’ expectation, the Opinion and the Judges’ separate opinions and declarations (e.g. Nolte,  para. 1) evince unequivocally how much the Court and its Members were aware of the high hopes attached to these proceedings and their outcome. This much becomes apparent not only in the frequent references to the Court’s judicial function (AO, paras. 45, 48, 100, 338 and 456). The ICJ, further, felt compelled to add a very last paragraph before the dispositif. Here, it addresses the enormity of the challenges posed by climate change thus implicitly accounting for the enormous expectations held vis-à-vis the Court’s Advisory Opinion: ‘[T]he questions posed … represent more than a legal question: they concern an existential problem of planetary proportions.’ (ibid., para. 456). The Court, so it underlines, can only make a modest contribution to the solution: ‘The Court, as a court of law, can do no more than address the questions put to it’ with respect to ‘international law’, which ‘has an important but ultimately limited role in resolving this problem.’ (ibid., see also the Declaration of Judge Tladi, para. 38). Here, the spectre of Nuclear Weapons seems to loom (note the frequent references to the 1996 opinion in the text: AO, paras. 48, 98, 114, 134, 141, 155, 272, 373).

Focusing in the following on expectations pertaining to ‘advice as law-development’, among the almost infinite number and variety of legal issues potentially to be addressed and clarified regarding states’ obligations in respect of climate change, the Court arguably did not shy away from tackling many of the most pertinent matters. Some of the most important contributions to the development of international climate change law are doubtless to be found in its analysis of the climate change treaties as well as their interrelationship and their interaction with customary international law. The Court emphasizes that the PA is neither lex specialis regarding the UNFCCC nor are the climate treaties leges speciales vis-à-vis customary standards such as the principles of prevention or co-operation (AO, paras. 162-171, 187-195 and 309-315). Similarly, the ICJ clarifies that ‘discretion’ regarding the PA Contracting Parties’ NDCs under Art. 4(2) is not unlimited but rather guided by a requirement of progression and the obligation of a 1.5° Celsius threshold (AO, paras. 237 et seq.), the latter deriving from the interpretation of Art. 2(1) PA (AO, paras. 222 et seq.). Moreover, it engages in a detailed and differentiated discussion of the contours and conditions of the due diligence standards in both the PA (e.g. AO, paras. 246, 258) and especially under the customary prevention principle (AO, paras. 280-300). Overall, these sections, as most parts of the Advisory Opinion, are exceptionally well-reasoned, displaying an impressive attention to doctrinal precision and detail (see also, with similar assessment, Arato and Uriburu).

Nonetheless, as is almost inevitable in such a long opinion addressing a wide range of matters, there are some passages less convincingly argued than others. Among those, the Court’s identification of a ‘human right to a clear, healthy and sustainable environment’ stands out as probably most wanting: it remains rather opaque whether the Court regards it as a self-standing right or rather accessory to other treaty provisions and it is equally unclear whether the opinion asserts this right as a rule of custom (as asserted by Judges Aurescu, paras. 28 et seq., and Tladi, paras. 29 et seq.) or rather derives it from necessary implication (see AO, para. 393: ‘precondition’, ‘inherent’) or functionality (see AO, paras. 391 and 393: ‘importance’, ‘essential’). 

Similarly, the distinction between customary principles (AO, paras. 131-142) and ‘other principles’ (AO, paras. 146-161) merely ‘guiding … the interpretation and application of the most directly relevant legal rules’ (AO, para. 161) will probably raise more questions than it answers (Are they ‘general principles of law’ or rather ‘general principles of law formed within the international legal system’, in the parlance of the ILC, pp. 26 et seq. – or are they rather interpretative maxims similar to effet utile or contra proferentem, but special to the climate law regime (exclusively or also beyond? Only regarding treaty interpretation or custom interpretation as well?)? I could go on). 

Finally, the Court is most curt where the highest potential for litigation lies: While the discussion of Question (b) contains many important doctrinal pronouncements, I agree with Judge Nolte (paras. 18 et seq.) in his assessment that the comparatively short statements on reparation, especially compensation, (AO, paras. 449-455) leave a lot room for interpretation and thus will likely entail considerable contentious litigation – whereas I do not share his opinion that such litigation may necessarily be ‘counterproductive’, ibid., para. 34.

Conclusions

The lasting effect of this Advisory Opinion will remain in its response to the expectation of and thereby its contribution to law-development. The proof is in the pudding of the ‘persuasiveness’ (Kulick, pp. 56 et seq.) of the Court’s argumentation regarding the individual legal pronouncements. While the Court and the individual Judges, for good political reasons, need to emphasize that the ICJ can only state, analyse and apply, but never create the law, the Court undoubtedly attains a law-developing function – however, within the confines of thorough legal argumentation. 

Through what arguably falls squarely within its judicial function, the Opinion of 23 July 2025 has indeed made an important contribution to the climate change debate, not least because it relied for the most part on robust legal reasoning. Even most of the more progressive and innovative findings are thoroughly reasoned and backed up by doctrine, especially by extensive references to case law. It is of note here that this Court, which usually is rather sparse in citing other international courts and other adjudicatory bodies, in this Opinion summons an impressive amount of other judicial authorities – from ITLOS to regional human rights courts or the Human Rights Committee – to corroborate its legal analysis. ITLOS alone is cited no less than 28 times in the Opinion.

The potentially high relevance of the Court’s pronouncements on central legal questions pertaining to states’ obligations in respect of climate change, finally, results from its careful expectation management. The ICJ emphasises its important but limited role in a debate that eventually can only find resolution through ‘human will and wisdom’ (AO, para. 456): law-development: yes; miracle-working: no. This is not at all to say that litigation and arbitration based on many of the Court’s doctrinal pronouncements could not and should not be pursued by claimants. Especially the findings on Art. 2(1) and 4(2) PA as well as the customary requirements of the prevention principle and its due diligence standards, particularly vis-à-vis fossil fuel exploration, production and consumption (AO, para. 427), provide avenues for contentious proceedings before the ICJ and beyond. The Court exercised its judicial function precisely with the view to future law application, including through adjudication. Nonetheless, it makes clear that eventually, it is the political process that needs to find the final responses to the grand societal debate on, as the Court puts it, this ‘existential problem of planetary proportions’ (ibid.). In Judge Tladi’s words: ‘It requires those in decision-making positions to make the right choices for the sake of the future of our planet.’  (para. 38). Hopefully, they will be guided by the legal requirements, standards and guidelines so aptly clarified in this ‘landmark opinion’ – and maybe also by future case law inspired by and building upon it.