For the last six years, together with a handful of colleagues and friends from around the world, we have worked with the Government of Vanuatu to develop and implement a legal strategy to turn the aspiration of a group of students from the Pacific into a reality, bringing climate change before the International Court of Justice (ICJ). The road has been filled with hope, encouragement and support, but also alas with much scepticism, sometimes even sarcasm, and unrelenting pushback. Yet, on 23 July of this year, the aspiration turned into a reality. The Court rendered a far-reaching Advisory Opinion on Obligations of States in respect of Climate Change, and it did so unanimously.

What was under legal scrutiny, as the Court expressly recognized (para. 94), was a certain conduct, namely the acts and omissions that are the cause of climate change. The Court was asked to identify the obligations most directly governing this conduct and the legal consequences under such obligations. Along the way, the Court took the time to engage with science, characterize – even expand – the ‘relevant conduct’, address a wide body of obligations of treaty and customary international law, well beyond the climate change treaties, and formulate a legal theory of responsibility for climate harm that rests on the International Law Commission’s Draft Articles on State Responsibility for Internationally Wrongful Acts (ARSIWA). It did not – and it was not asked to – declare that a specific State or group thereof had breached international law, nor did it take – although it was asked to do so – a clear legal stance on the core factual issue at the heart of climate justice, as Judge Yusuf, Judge Sebutinde and Judge Charlesworth regretted.

But seen from the perspective of the many close calls that we faced over the years, first in the complex negotiation process of drafting of the request, and then in the preparation of written and oral submissions, as well as in the wider coordination efforts, what stands out is that we feel the Court managed to find a high common denominator (see also Judge Tladi’s declaration, paras. 2-3 & 10). The advisory opinion defines the relevant conduct with exceptional breadth, rejects every doctrinal argument that might have shielded large emitters and producers from responsibility, and affirms that breaches of climate‑related obligations give rise to the full panoply of legal consequences recognized by the general international law of State responsibility. By characterizing those obligations as erga omnes and emphasizing that individuals as well as States may potentially invoke responsibility, the Opinion supplies the normative foundations that Vanuatu, the Melanesian Spearhead Group (MSG), the Organisation of African, Caribbean and Pacific States (OACPS) and many other participants had urged.

In this brief post, we analyze how the Court reaches these conclusions, where it leaves questions open, and what the Opinion may mean for the future of climate litigation and diplomacy.

Setting the Scene

Adopted by consensus after an historic youth‑led campaign spearheaded by Vanuatu, General Assembly resolution 77/276 asked the Court to clarify both the obligations of States to ensure the protection of the climate system and other parts of the environment and the legal consequences under those obligations for States ‘where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment’, with respect to (i) ‘States, including, in particular, small island developing States, which due to their geographical circumstances and level of development, are injured or specially affected by or are particularly vulnerable to the adverse effects of climate change’ and (ii) ‘Peoples and individuals of the present and future generations affected by the adverse effects of climate change’.

The Court agreed to render its Opinion, noting the General Assembly’s characterization of climate change as an ‘unprecedented challenge of civilizational proportions’ (para. 95) and relying on the Intergovernmental Panel on Climate Change (IPCC)’s scientific consensus as a factual foundation. The Advisory Opinion spans 132 pages, with an additional 100+ pages in the form of separate opinions (six) and declarations (six), and it has instantly generated extensive commentary (see, e.g. among a sea of other commentaries, here, here and here).

Of particular note, for this ‘record’, the process was unlike any other so far before the Court. It brought many new voices from all corners of the World, who addressed the Court firmly and decidedly, with the authority that only genuine conviction and first-hand experience of a situation can give. And these are the voices that carried the day, overcoming and overpowering those of some highly experienced counsel who, at times, did not appear to fully grasp what was happening before the Court.

Identifying the ‘Relevant Conduct’

At the outset the Court framed its task broadly. It reasoned that the material scope of its inquiry ‘encompass[ed] the full range of human activities that contribute to climate change, including both consumption and production activities’ (para. 94). Significantly, it expressly identified ‘the relevant conduct’ for the purposes of the advisory proceedings, stipulating that it ‘is not limited to conduct that, itself, directly results in GHG emissions, but rather comprises all actions or omissions of States which result in the climate system and other parts of the environment being adversely affected by anthropogenic GHG emissions’ (para. 94). That construction mirrors the submissions of Vanuatu, which defined the ‘relevant conduct’ as ‘acts and omissions of individual States – and of a specific group thereof – that have resulted over time in a level of anthropogenic GHG emissions from activities within their jurisdiction or control, which have interfered with the climate system and other parts of the environment to an extent which amounts to at least significant harm to the latter, whether or not the anthropogenic GHG emissions of a given State over time are the only or the main cause of climate change, and whether or not they are the only or the main cause of the specific harm suffered by another State, people or individual’ (written statement, para. 4). The Court’s understanding of the relevant conduct was further specified in its discussion on attribution, where it explicated that ‘fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies … may constitute an internationally wrongful act’ (para. 427; see further the joint declaration of Judges Bhandari and Cleveland, para. 2). Judge Cleveland further observed that the conduct under assessment must include emissions resulting from armed conflicts and other military activities, given their significant contribution to global warming (para. 18). By linking the notion of conduct to significant harm to the climate system rather than – upstream – to any specific emitter or facility or – downstream – to any specific extreme event or material consequence thereof, the Court lays the normative groundwork for attributing responsibility for significant harm to the climate system itself – and other parts of the environment – even where multiple actors have contributed over time.

Temporal Dimensions of Obligation and Breach

The Court devoted a separate section to ‘temporal issues’, recognizing that the dates on which the applicable obligations crystallized – and the period over which breaches continue – will be central to any in concreto assessment of State responsibility (para. 97). It acknowledged that customary and treaty rules may have entered into force at different moments, and that questions of non‑retroactivity or scientific knowledge can arise, but it stressed that none of these complexities prevent a finding of breach where the facts so warrant. Significantly, the Court’s treatment of the Kyoto Protocol – stressing the relevance of now-expired emission reduction commitments for assessing compliance with States’ obligations (para. 221) – confirms the importance of temporality for determining breaches. The Court emphasized this point by adding that ‘[t]hus, [past] non-compliance with [previously applicable] emission reduction commitments by a State may constitute an internationally wrongful act’ (para. 221). The detailed discussion of obligations arising from the Paris Agreement subsequently undertaken by the Court must be understood against the backdrop of this temporal sensibility, confirming that States can be held accountable under international law for their historical contributions to climate change.

No Safe Harbours: Lex Specialis, Attribution and Causation

During the proceedings, the argumentative fault-lines around questions (a) and (b) quickly became discernible. At the level of applicable law – and later on with respect to the operation of specific obligations – the main bone of contention was whether the relevant conduct is governed essentially by a – highly permissive – lex specialis (a certain interpretation of the climate change treaties), both at the level of primary rules of obligation and secondary rules in case of breach, or by a much wider – and much tighter – body of obligations which, if breached, trigger the legal consequences provided for under the general international law of State responsibility. The Court clearly sided with the latter view (see e.g. paras. 171 and 420), taking the Charter of the United Nations as the starting point for its examination of ‘the most directly relevant applicable law’ (para. 115) and proceeding to revisit a range of obligations specifically addressing climate change, the law of the sea, other related environmental agreements, some rules of customary international law, human rights law, and several principles. The examination of each of these areas – and specifically of the obligations identified – is in itself a major contribution, sometimes breaking new ground, as is the case of the recognition of a right to clean, healthy and sustainable environment (paras. 387-393) or the observations regarding sea level rise, maritime spaces and continued Statehood (paras. 355-365). These two aspects were widely discussed in the separate opinions and declarations of judges (Bhandari, para. 3; Charlesworth, paras. 8-12; Tladi, paras. 24-33; Aurescu, paras. 27-46 and 2-25 respectively; Tomka, paras. 2-11; Sebutinde, para. 8)

But an additional complexity remained in relation to the interactions between treaty and custom. This interaction is examined in the post by Uriburu and Arato. A particularly important aspect of it concerned the relations between the climate change treaties (the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol and the Paris Agreement) and what the Court now calls, without ambiguity, the ‘duty to prevent significant harm to the environment’ (paras. 132-139). The compromise language enabling unanimity seems to be located in paragraph 314 where 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 … 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’. The continued application of customary law, particularly of prevention, was emphasized in the joint declaration of Judges Charlesworth, Brant, Cleveland and Aurescu (see para. 5). Other judges also made reference to this issue. Of note, Judge Tladi rightly recalled that only compliance with the climate change treaties ‘as interpreted by the Court’ would have effects (see para. 22).

The Court’s interpretation of the UNFCCC, the Kyoto Protocol and the Paris Agreement, particularly its understanding of the stringent due diligence obligations laid out in these treaties (para. 268), even as regards the content of nationally determined contributions (paras. 245-246, 270), is, in our view, fundamental for the COP process. Indeed, in the context of significant, and not unwarranted, scepticism regarding the operation of ongoing climate negotiations, the Court’s stringent interpretation of the climate change treaties breathes new life into an ailing system in dire need of credibility. As called for by Cynthia Houniuhi, in her pleadings on behalf of Vanuatu and the Melanesian Spearhead Group, this interpretation seeks a return to the ‘spirit’ of the climate change treaties, away from one which turns them into ‘a polluter safe-harbour and a trap for everyone else’.

At the level of secondary rules, the Court had no problem in recognizing the application of the general international law of State responsibility. This is where a strategy advanced by certain States with substantial GHG emissions seems to have backfired. During the negotiations of the Paris Agreement, the proposal from climate-vulnerable States to include a standalone article on loss and damage generated massive pushback from major emitters, due to the potential risks of liability that recognition of this action area could entail for them. Paragraph 51 was therefore added in the COP decision adopting the Paris Agreement to stress that ‘Article 8 of the Agreement does not involve or provide a basis for any liability of compensation’. The purpose was of course to dissociate the expression ‘loss and damage’ from any understanding of compensable damage. During the proceedings before the ICJ, paragraph 51 was cited by many major emitters who sought to argue that the Paris Agreement contained special secondary rules excluding the rules reflected in ARSIWA. Yet, not only did the Court reject this argument, but – in an unexpected (at least by those making the argument) turn – it drew on paragraph 51 to bring ‘loss and damage’ directly under the framework of State responsibility, thus undoing years of efforts by large emitting States to the contrary: ‘the Court concludes that responsibility for breaches of obligations under the climate change treaties, and in relation to the loss and damage associated with the adverse effects of climate change, is to be determined by applying the well-established rules on State responsibility under customary international law’ (para. 420).

The Court then developed the foundational elements of a theory of State responsibility for wrongful climate-related harm, clarifying the operation of the general rules in the context of climate change (this aspect is specifically examined in this post by Paddeu and Jackson). It should be noted here that the Court did not address the legal consequences arising from harmful but lawful conduct, a matter raised by Judges Yusuf, Nolte and Bhandari from different perspectives (Yusuf, paras. 41-48; Nolte, paras. 15-17; Bhandari, para. 2). This could be important in case the conduct causing significant harm to the climate system and other parts of the environment might be deemed lawful, at least in part or for certain periods of time or for certain specific acts (justified or excused). The Court made clear that it is not the GHG emissions themselves but the relevant conduct of States that requires legal scrutiny (para. 427).

On attribution, the Court insisted that the ordinary criteria contained in Articles 4–11 of ARSIWA are fully apt for climate cases. Failure to regulate private emitters counts as an attributable omission, and attribution difficulties do not shield States from responsibility (paras. 427-428). The need to take into account cumulative emissions, the plurality of responsible and injured States, and the challenges in disentangling the share of responsibility of each State may add scientific complexity, but they create no legal impediment under the law of State responsibility (paras. 429-431).

With respect to causation, the Court rightly noted that this is not a requirement for establishing responsibility but a matter pertaining to reparation of harm caused (para. 433). Based on its own jurisprudence, it considered that while a ‘sufficiently direct and certain causal nexus’ must be shown for reparation (para. 436), this standard is ‘not static’ and ‘flexible enough to address the challenges’ related to climate change (para. 436). For example, multiple concurrent causes do not exonerate any State from the duty to make reparation (para. 435). Far from insulating historic emitters, the complex nature of climate harm calls for careful case‑by‑case analysis (para. 436). In this context, the point made by Judge Yusuf regarding the complementarity of the legal regimes relating to wrongful conduct and lawful conduct appears pertinent: ‘In fact, the two regimes, namely State responsibility, for which wrongfulness is a necessary element but not injury, and international liability, for which injury is indispensable but not wrongfulness, are not mutually exclusive, but are intertwined in certain circumstances’ (para. 42). Yet, there is also a risk that resort to the rules on injurious consequences of lawful activities may conjure up the spectre of strict liability in international law (as noted by Judge Nolte, para. 20). The approach followed by the Court, which is limited to wrongful acts, may be explained by such reluctance, but it can also be explained by the terminology of the request, which specifically relied on ARSIWA in question (b).

Legal Consequences: Erga Omnes Obligations and the ‘Entire Panoply’ of Legal Consequences

One of the Opinion’s most consequential findings is the characterization of obligations to protect the climate system as erga omnes, owed to the international community as a whole. Drawing explicitly, for the first time, on Article 48 ARSIWA as well as on its own jurisprudence, the Court explains that both injured and non‑injured States may invoke responsibility, although only injured States may demand reparation for their own benefit.

The Court then states that breaches ‘may give rise to the entire panoply of legal consequences provided for under the law of State responsibility’, expressly listing cessation, non‑repetition, restitution, compensation and satisfaction (para. 445). It adds that individuals and peoples may potentially invoke responsibility where human rights obligations are breached, thereby extending the Opinion’s reach beyond the inter‑State plane. This point, and the type of remedies that may be adequate for cessation (including ‘to revoke all administrative, legislative and other measures that constitute an internationally wrongful act of that State’, para. 447) provides a far-reaching basis for domestic and international litigation by individuals, peoples and their representatives (e.g. NGOs, when this is allowed) both under human rights law and under domestic administrative law (e.g. scope of environmental impact assessments in licensing systems, see e.g. UK Supreme Court, Finch, as recalled by Judges Bhandari and Cleveland at paragraph 16 of their joint declaration).

Judge Tladi’s declaration points out that, unlike the Wall and Occupied Palestinian Territory Opinions, the Opinion stops short of articulating the special consequences that follow from breaches of erga omnes obligations of a jus cogens character. Yet, the open‑ended language used in paragraph 445 (‘these include…’) together with the Court’s reference to self-determination as part of the applicable law (para. 357; see further Judge Sebutinde’s separate opinion, para. 8) strongly suggest that non‑recognition and cooperative countermeasures remain available. Moreover, silence on fleshing out consequences arising from aggravated responsibility is not a limitation when such consequences have been recognized for breaches of other obligations as a result of their erga omnes character.

Significantly, in paragraph 111 – notably criticized by Judge Sebutinde (para. 6) for conflating locus standi and substantive rights – the Court appears to suggest that the Opinion’s broad legal framework applies mutatis mutandis to claims by peoples and individuals suffering climate harms. That acknowledgment not only meshes with, but bolsters evolving human rights jurisprudence. More broadly, the growing field of rights-based climate litigation will benefit from the Court’s careful consideration and persuasive dismissal of some of the most commonly cited defences in such cases.

Concluding Reflections

The Advisory Opinion does not award damages to any State, nor does it apportion the carbon budget. It does what only the Court could do – and it does it unanimously: it sets a new legal baseline under which the conduct responsible for climate change is not assumed to be lawful but reframed through the prism of internationally wrongful acts, with the full panoply of ensuing legal consequences. It is, effectively, a new baseline where such conduct is deemed in principle unlawful, unless a display of consistently stringent due diligence can be shown. That is a fundamental reset of the entire understanding of how international law governs such conduct, and it was the core outcome pursued by many countries, organizations and groups. By weaving together a wide range of obligations which, all too often, remain prisoner of ‘branch-based’ mindset, and placing them as possible precursors of State responsibility, the Court closes doctrinal escape routes that major emitters and producers have relied upon for decades. The challenge now shifts to litigators, negotiators and policymakers: to translate the Court’s authoritative statement of principled unlawfulness into concrete pathways for deep decarbonization, adequate finance, and effective remedies. Future cases – before the ICJ in contentious mode, before the International Tribunal for the Law of the Sea (ITLOS), human rights courts and treaty bodies, trade panels and courts, arbitral tribunals of different sorts and domestic courts – will supply the in concreto findings that this Opinion deliberately reserves.

Thanks to a group of Pacific students who, to paraphrase Mark Twain, did not know it was impossible, the ICJ’s Advisory Opinion on climate change is now a reality, and conduct responsible for climate change can no longer hide in plain legal sight.