When the International Court of Justice (ICJ) delivered its Advisory Opinion on Obligations of States in Respect of Climate Change on 23 July 2025, commentators rightly called it a watershed for international environmental law. The Court unanimously confirmed that the customary rules of State responsibility, as reflected in many respects in the ILC’s Articles on State Responsibility (ARSIWA) (para. 407), apply in full to climate harm, rejecting the argument that the climate treaties displace general international law (paras. 171, 420) (see Uriburu and Arato’s discussion of this holding here). It also recognised States’ obligations to protect the climate system as erga omnes, opening standing to invoke responsibility to every State, not only those specially affected (para. 442) (for a critical reading, see here).
Much less settled, in the immediate wake of the Opinion, was how the Court’s framework would cope with the fact that makes climate change different from almost any other internationally wrongful act: it results from two centuries of cumulative, geographically dispersed conduct by dozens of States, whose respective contributions must somehow be disentangled if any individual State is to bear a defined share of responsibility. Reading only the headline holdings, it is tempting to think the Court avoided this problem. It did not, but it left the hardest part, translating a sound principle into a workable method, for others to solve.
What the Court Actually Said About Historical Emissions
The Opinion does not treat historical emissions as legally irrelevant. In its discussion of temporal issues, the Court accepted that non-compliance with earlier commitments, including expired Kyoto Protocol targets, “may constitute an internationally wrongful act” (para. 221), confirming that States can be held accountable for their historical contributions to climate change. On attribution, the Court went further, holding that it is scientifically possible to determine each State’s total contribution to global emissions, taking into account both historical and current emissions (para. 429), and that the existence of multiple wrongdoing States “creates no legal impediment” to responsibility (paras. 429–431). Citing its own Reparations Judgment in Armed Activities on the Territory of the Congo, the Court accepted that where multiple causes attributable to different actors combine to produce an injury, “responsibility for part of such injury should [be] allocated among [the] actors” (para. 430).
This is a significant finding, and one that a growing body of EJIL:Talk! commentary on the Opinion has already mapped in detail (see here, here and here). What the Court did not do, as Federica Paddeu and Miles Jackson observe, is develop the allocation of responsibility among multiple actors, which remains under-developed in the practice of international law even after the Opinion (see here). On causation, the Court held that a sufficiently direct and certain causal nexus must still be shown for reparation, described as flexible enough to accommodate the diffuse, cumulative character of climate harm (para. 436), and operationalised through a two-step inquiry: first, whether a climatic event or trend is attributable to anthropogenic climate change in general; second, whether the resulting damage is attributable to a particular State (para. 437).
The picture that emerges is not one of silence but of deliberate deferral. The Court affirmed, as a matter of principle, that historical and cumulative emissions may ground both a variable due diligence standard under the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC), already discussed on this blog in relation to NDCs (here), and an apportioned reparations claim. What it withheld was any indication of how the two-step causation test is actually to be operationalised once a case reaches a tribunal – and, unexpectedly, an older separate opinion supplies a useful analytical structure for exactly that step.
A Doctrinal Bridge from Oil Platforms
In Oil Platforms, the United States brought a counterclaim against Iran for endangering Gulf shipping during the Iran–Iraq War, partly through specific attacks traceable to Iran alone and partly through a general climate of danger to which both Iran and Iraq contributed without coordinating. Judge Simma’s separate opinion treated these as two distinct problems. For the specific incidents, he held that ARSIWA Article 47, which applies where several States have committed “the same internationally wrongful act”, could not apply, since Iran and Iraq had not acted in concert with respect to any single attack (para. 76 of the opinion). But for the generic claim, the broader deterioration of a safe shipping environment, he held that Article 47 did apply, because at that level of generality the wrongful act, creating dangerous conditions, was properly attributable to both States jointly, even absent coordination (para. 77). Independently of Article 47, he also identified, from a comparative survey of common law, French, Swiss and German tort law, a general principle that where multiple tortfeasors cause an indivisible harm that cannot be apportioned between them, each may be held liable for the whole, with the burden shifting to the tortfeasor to disprove its own contribution (paras. 66–74).
This specific/generic distinction maps closely onto the two-step causation test the Court itself later adopted for climate change (para. 437). On Simma’s reasoning, the first step, whether large-scale climate disruption is attributable to anthropogenic conduct in general, is the generic wrongful act: the collective, if uncoordinated, conduct of many emitting States, which Article 47 (or the comparative-law principle of liability for indivisible harm) can support treating as attributable to the plurality of emitting States as a class. It is only the second step, translating that collective responsibility into what a specific respondent State owes a specific claimant, that requires a further, quantifying method. Proportional or “market-share” apportionment, of the kind Nataša Nedeski and André Nollkaemper set out on this blog three years before the Opinion, drawing on extreme-event attribution science and on Saúl Luciano Lliuya’s ongoing claim against RWE before the Higher Regional Court of Hamm (see here), is best understood not as a rival to Article 47 or joint-and-several liability but as the tool that performs this second, quantifying step once collective responsibility for the first is established. The 2025 Opinion’s citation of Armed Activities, endorsing allocation of responsibility for part of such injury among multiple actors (para. 430) rather than the alternative, mentioned in the same judgment, of a single actor bearing full reparation, points in exactly this direction, and gives the market-share proposal a firmer doctrinal anchor than it had in 2022.
Why This Matters for Bangladesh and Other Vulnerable States
For a State such as Bangladesh, responsible for under 0.5% of global greenhouse gas emissions on its own government’s figures (see Bangladesh’s NDC 3.0), yet ranked among the countries most exposed to sea-level rise and cyclonic storms, the two-step architecture matters in practice, not just in theory. The first step, general attribution of climate disruption to anthropogenic conduct, is by now scientifically uncontroversial and, on the Simma/Article 47 analogy, gives collective responsibility a doctrinal foothold that goes beyond the Opinion’s own brief treatment of multiple wrongdoing States. The second step, quantifying what a particular respondent State owes for a particular loss, for instance the erosion of a stretch of Bangladeshi coastline, is the harder one. Attribution science can already estimate an individual State’s marginal contribution to specific extreme-weather probabilities, but the uncertainty ranges attached to such estimates remain wide, and translating a probabilistic increase in likelihood into a compensable share of a concrete loss is a step the Opinion leaves entirely to future litigants and tribunals to work out, consistently with the case-by-case method it prescribed (para. 432).
Conclusion
The ICJ’s Advisory Opinion does not leave historical responsibility unaddressed; it leaves it unquantified. Paragraphs 429 to 437 confirm that cumulative and historical emissions are legally relevant to both the standard of due diligence and the calculation of reparation, and the Court’s own two-step causation test, together with the doctrinal resources already available in Judge Simma’s Oil Platforms opinion and in this blog’s own scholarship on collective causation, supplies more of a method than is usually credited. What remains for litigants before the Inter-American Court of Human Rights, domestic courts, or a future contentious case at the ICJ itself is the more modest but no less consequential task of building the evidentiary bridge between that method and an actual award. Until that bridge is built, the fundamental inequity of climate change, that those who contributed least bear the earliest and heaviest losses, will remain, in the Court’s own vocabulary, a matter for the case-by-case future rather than the doctrinal present.
