Rising to the Occasion

Climate change litigation has, at last, reached the World Court, with the International Court of Justice (ICJ) issuing its highly anticipated Advisory Opinion on the Obligations of States in respect of Climate Change. In the lead-up, skeptics had raised concerns about the merits of addressing climate change through this judicial avenue, pointing to the multiple pitfalls and political minefields of such an Opinion (see Bodansky; Guo, Li and Tian; Tigre and Rocha). We argue in this blog post that the ICJ has largely risen to the occasion by defining its role as architect of systemic integration in international climate law, forging a harmonious international legal framework on climate change that integrates the diverse sources of international law and prior rulings of international courts and tribunals into a coherent legal architecture.

The ICJ and the Advisory Process as a Global Public Forum

The ICJ is institutionally and procedurally well-positioned to fulfill this architectural role. As opposed to other international courts adjudicating specific bodies of international law, such as the United Nations Convention on the Law of the Sea (“UNCLOS”), in the case of ITLOS, or regional human rights treaties, in the case of the IACHR and the ECtHR, the ICJ covers international law generally. The ICJ was, accordingly, tasked with comprehensively laying down the obligations of states with respect to climate change under public international law by the General Assembly (“GA”). For this purpose, the procedural build-up to advisory opinions in general – and to the climate change proceedings in particular – confers valuable “process-based legitimacy” on the ICJ (Bodansky 2023:191). The GA’s request seeking the Opinion was bolstered with the authority of a majority vote, being co-sponsored by 132 states. In total, 96 states delivered oral statements during the widely covered public hearings before the ICJ in December 2024 – the broadest participation in the Court’s history. Finally, the Court furthered the authoritative nature of its opinion by adopting it unanimously – as only the fifth of all 29 advisory opinions delivered by the ICJ so far.

The broad participation of the community of states in these advisory proceedings “unlike any that have previously come before the Court” (¶456) and its institutional character as “the principal judicial organ of the United Nations” (Art. 92 UN Charter) vested the Court with distinct authority and legitimacy. Against this background, the Court was predestined to act as a global public forum on the climate change obligations of states and to establish a coherent legal framework integrating diverse state perspectives ranging from historically high-emitting states to particularly vulnerable small island nations. An added advantage of the advisory process was that its legally non-binding outcome spared the Court from the higher stakes and jurisdictional hurdles of legally binding contentious proceedings.

Systemic Integration and Global Minimum Baseline

The main interpretive tool utilized by the ICJ was the principle of systemic integration – though not explicitly referred to as such. With its help, the Court convincingly rejected the submission of big emitters, such as the US (¶4.1) and Japan (¶47), that the climate change treaties constitute leges speciales. Underscoring the importance of harmonious interpretation, the Court noted “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). Consequently, state obligations under, for example, the Paris Agreement continue to be flanked by customary international law. As the rejected lex specialis approach would have entrenched an atomistic fragmentation between specialized climate change law and general international law, this harmonious interpretation systematically integrates the climate treaties into the workings of the international legal order, thus maintaining its coherence and integrity. While the underlying interpretive principle of unity of the legal order is well-established in legal hermeneutics, the Court’s holistic integration of treaty and custom, of primary and secondary rules of international law as well as of international human rights and environmental law in the Opinion, is likely to produce significant legal consequences yet to be discerned in legal discourse in the following months and years.

Furthermore, the ICJ found itself in the challenging position of issuing its advisory opinion after a range of international adjudicatory bodies – the UN Human Rights Committee, ECtHR, ITLOS, IACHR – had already ruled on matters of climate change. Of course, multiple decisions of judicial bodies with parallel jurisdictions over the same issue inevitably entail the risk of conflicting legal interpretations. One of the concerns preceding the ICJ’s climate change opinion was that it might contradict the previous international rulings, creating confusion and undermining the credibility and authority of international law (Boisson de Chazournes).

The Court was clearly aware of this risk and thus meticulously avoided contradictory pronouncements. While stating not to be bound by the ITLOS’ interpretations, it generously quoted its Advisory Opinion on Climate Change, building on its foundation to achieve clarity, consistency, and legal security (¶338). For example, it transposed the Tribunal’s notion of “stringent due diligence” from the narrower context of maritime pollution to the broader field of climate change law, applying it, for instance, to the obligations under the Paris Agreement (¶252). It not only reaffirmed the findings of this previous ruling but also universalized its contents for non-members of the UNCLOS, productively cross-fertilizing the law of the sea and the international climate law regime.

In a similar vein, the Court recognized the IACHR’s Advisory Opinion No. 32, issued only a few weeks ago, albeit without engaging with its substance. Although some of the Inter-American Court’s findings – inter alia irreversible harm to the environment being a ius cogens crime (Advisory Opinion No. 32, ¶293) or the expansion of extraterritorial jurisdiction for transboundary human rights violations (Advisory Opinion No. 32, ¶278) – are substantially more progressive than those of the Court, the latter neither reasserted nor negated them. The ICJ instead opted for a global minimum baseline-approach that avoided pronouncements on the precise scope of regional or specialized treaty commitments. It thus kept the door open to regional groups of states – such as those of the American Convention – to adopt a higher standard of climate protection confined to their jurisdictions. This approach acknowledges the distinct normative space occupied by regional and specialized regimes, while integrating them into a broader global framework of climate obligations.

A Middle Ground Between Backlash-Provoking Specificity and Issue-Skirting Generality

The ICJ also deserves credit for establishing concrete and robust climate change obligations without controversially venturing into how states ought to specifically design their climate change policies. As Bodansky laid down (2023:192), the Court had to thread the needle between overly specific, highly ambitious and innovative duties, and cautious general holdings that fail to advance the legal landscape. 

For the most part, the ICJ succeeded to do so by basing the international climate change framework on pre-existing and well-established legal principles rather than introducing new and avant-gardist notions exclusively tailored to the specifics of climate change. Although the phenomenon of climate change requires profound modifications of “our habits, comforts and current way of life” (¶456), it is doubtful that a bold act of judicial disruption by the ICJ would have served the global climate movement well. It would have likely provoked strong backlash by high-emitting states and stretched the legal and legitimacy-related limits of the World Court. Instead, its judicial role is better served by reconciling the legal adjustments necessitated by climate change with the need for legal stability and continuity. In the final paragraph of its Opinion, the Court reflects its institutional constraints accordingly, noting “the limits of its judicial function” and the “important but ultimately limited role [of international law] in resolving this [climate change] problem” (¶456).

This self-reflection informs the Court’s doctrinal reasoning, for instance, as the ICJ expands the long-established customary rule to prevent significant (transboundary) harm to the environment. Some states indeed submitted that this duty remains limited to situations of direct cross-border harm (¶133). The Court, however, broadened its scope, holding that “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). Moreover, the Court affirmed in a reasoned manner that violations of climate protection obligations compel states to provide reparations (¶¶449-450), despite some states arguing that the diffuse nature of climate change would preclude the general application of the rules of reparation (¶434). At the same time, the ICJ confined itself to establishing the general framework of state responsibility in relation to climate change, while reserving an “in concreto assessment” of the responsibility of individual or groups of states to a “case-by-case” analysis to be undertaken in future contentious litigation (¶106). Consistent with its judicial function, the Court thus kept to sound but evolutive legal reasoning. It wisely refrained from imposing more specific state obligations, such as enumerating concrete measures to reduce greenhouse gas emissions as part of states’ nationally determined contributions (NDCs), spelling out how damages from floodings in small-island states are to be calculated, or following the more ambitious paths preferred by some individual judges (see Separate opinions of Vice-President Sebutinde, Judge Yusuf, Judge Bhandari, and Joint declaration of Judges Bhandari and Cleveland).

The Battle over Interpretive Authority 

Two examples of these more ambitious paths are the right to a clean, healthy, and sustainable environment and the international legal obligations relating to fossil fuels. Judge Bhandari asserts that a right to a clean, healthy and sustainable environment exists under customary international law (Separate opinion of Judge Bhandari, ¶3) and criticizes the Court for failing to “ultimately affirm[]” its existence or to “clarify its normative status” (Id.). Although the Court appears to view such a right as emerging, but not yet established, custom, the ambivalence in the Court’s reasoning, which is likely a reflection of a formulaic compromise between the majority and the more ambitious minority of judges, leaves the door open for more progressive understandings of the ICJ’s Opinion. Not surprisingly, Judge Bhandari takes the defensible, albeit not convincing legal position “that the Opinion recognizes the existence of this right under customary international law” (Id.).

In the second example, Judges Cleveland and Bhandari argue in their Joint declaration that the obligation to mitigate the potential harms of climate change would require states to phase out fossil fuel production and dependency (¶23), criticizing the Court for not being “more forceful” (¶4). In the aftermath of the Opinion, commentators have nonetheless argued that the Court’s more specific conclusion that “fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses or the provision of fossil fuel subsidies […] may constitute an internationally wrongful act” (¶427, emphasis added) would effectively amount to “a cease-and-desist notice to fossil fuel producers” in the domestic sphere. However, such a reading misinterprets the Court’s treatment of fossil energy. In our view, instead of suggesting a sweeping prohibition, the Court used fossil fuels as a salient example to illustrate how certain state conduct may breach existing obligations under international law – depending on the specific mitigation framework chosen by a particular state. It emphasizes that a breach of a state’s international obligations hinges not on the use of fossil fuels per se, but on whether the state meets its due diligence duties overall (¶427). Even when relying on fossil energy, a state may consequently counterbalance the ensuing greenhouse gas emissions through other climate change mitigation policies.

What these two examples instead demonstrate is that the battle over the interpretive authority of the ICJ’s Climate Change Opinion and how to best understand its legal consequences has already begun. In his Separate declaration, Judge Nolte raised the anxious question whether the Advisory Opinion will “stand the test of time”, suggesting that it will fail to do so if “it is understood as encouraging litigation to compensate for a lack of political will on the part of the community of States to protect against climate change” (Declaration of Judge Nolte ¶34). We consider the Opinion more accurately understood as affirming neither an international obligation to phase out fossil fuels nor a new customary right to a clean, healthy, and sustainable environment. To have done otherwise would have, arguably, crossed the line into lawmaking. Not only had this exceeded the bounds of the Court’s judicial function; it would have also disregarded the inescapable reality that the ICJ crucially depends on – and cannot substitute for – a broad political consensus among States on international climate action.

Conclusion

The ICJ has – for the most part – demonstrated the necessary self-awareness of its institutional role and limits. Its emphasis on setting up a harmonious international climate law framework is doctrinally sound, yet sufficiently bold. In so doing, the Court has reaffirmed its role as an architect of an integrated legal order capable of confronting the “ongoing climate crisis” (¶456).