As climate litigation continues to rise, a pivotal and unresolved legal question emerges in the law of State responsibility: how to allocate responsibility for injuries that result from the cumulative conduct of multiple actors.
Climate-related injury derives from the aggregate and diffuse effect of anthropogenic activities, as well as natural events, in which the actions of both States and non-State actors across extended temporal horizons are involved –as recently highlighted by both the IACtHR in its Advisory Opinion of 29 May 2025 on the Climate Emergency and Human Rights, ¶¶44 et seq, and the ICJ in its Advisory Opinion on Obligations of States in respect of Climate Change (“ICJ Advisory Opinion”), ¶421–. Such a reality may render precise causal tracing between the conduct and harm inherently difficult or even impossible.
While the ICJ has acknowledged that the rules on State responsibility are, in principle, “capable of addressing situations in which there exists a plurality of injured or responsible States” in the context of climate change (ICJ Advisory Opinion, ¶430), it leaves many open questions to be resolved on a case-by-case basis. The multi-actor, multi-temporal nature of climate harm may make attribution and causation standards in international law ill-suited to safeguard the victims’ right to full reparation.
This post proposes joint and several liability, a well-known principle drawn from domestic torts, as a potential legal solution to address these challenges in the law of State responsibility. By shifting the burden of attribution away from victims, this doctrine may arguably promote more accessible remedies and greater accountability.
The law of State responsibility in cases of multiple causation
The ICJ Advisory Opinion considers that the rules on State responsibility under customary international law sufficiently address the issue of multiple States contributing to the same damage (¶430). Yet it acknowledges that the most pressing questions on State responsibility for breaches of climate change obligations –namely, issues of attribution, causation and apportionment–, necessitate in concreto assessment (¶¶97, 108, 423, 437). The overly simplistic approach taken by the ICJ Advisory Opinion in this regard has been criticized by Judges Yusuf (separate opinion, §§II and VI) and Nolte (declaration, §II). (For a review of the ICJ Advisory Opinion’s view on the legal consequences of breaching climate change obligations under the law of State responsibility, see here and here).
The law of State responsibility, as codified in the International Law Commission’s Articles on State Responsibility for Internationally Wrongful Acts (“ARSIWA”), only partially addresses the problem of multiple causation. Article 47 states that where multiple States are responsible for the same internationally wrongful act, each may be held responsible for the act as a whole, thus preventing any State from evading responsibility due to a minimal contribution (as asserted by the ICJ Advisory Opinion, ¶431). On its part, Article 31 affirms that a responsible State must make full reparation for the injury caused by its wrongful act.
The International Law Commission’s commentaries clarify that factual causation “is a necessary but not a sufficient condition for reparation”, and that other elements –such as foreseeability, proximity, or the scope of the breached obligation– may influence the right to reparation [commentary (10)]. The commentaries also further that where injury arises from concurrent causes, reparation should still be granted unless the injury is too remote or can be severed [commentary (13)], and that State practice does not support the attenuation of the reparation merely because there are other contributory causes, except in cases of contributory fault under Article 39 ARSIWA [commentary (12)].
However, international practice has not consistently applied such an approach. In several cases, international courts and tribunals have imposed restrictive causation requirements for awarding reparation, limiting compensation to injuries that are directly and clearly linked to a State’s conduct (see, e.g. some decisions issued by the UN Compensation Commission panels following Iraq’s unlawful invasion and occupation of Kuwait). In the 2022 Armed Activities reparations judgment, the ICJ reaffirmed its traditional causation standard of a “sufficiently direct and certain causal nexus” between the wrongful act and the injury (¶93), while acknowledging the evidentiary limits for apportioning certain types of damages for which factual causality was unclear (e.g., ¶¶177, 221, 253). (For a deeper view on how international judicial practice approaches causation, see Plakokefalos [2015], Lanovoy [2022] and Nollkaemper [2024].)
In the ICJ’s view, the “sufficiently direct and certain” causation standard is “flexible enough” to address the challenges arising from complex, multi-causal contexts, such as climate change (ICJ Advisory Opinion, ¶436), while the apportionment of responsibility must be resolved on a case-by-case basis (ibid., ¶432). However, such an assessment deliberately ignores the limits of State responsibility.
As was pointed out by Judges Yusuf (separate opinion, ¶42) and Nolte (declaration, ¶16), a State’s duty to make reparation under ARSIWA arises only when an internationally wrongful act can be established. Yet many injurious acts or omissions contributing to climate change may not meet that threshold. Moreover, climate change derives from the uncoordinated and cumulative actions and omissions of both State and non-State actors over time. Such a scenario, which has traditionally been contemplated in strict liability schemes for hazardous activities (see, e.g., the ILC preparatory work of the 2006 Draft principles on the allocation of loss in the case of transboundary harm arising out of hazardous activities), stands in sharp contrast to concerted practices that may lead to the responsibility of multiple State actors as envisaged under Article 47 ARSIWA.
This situation risks creating a significant accountability gap and depriving victims of full reparation under the current model of international responsibility. In this context, the application of joint and several liability seeks to bridge some of these gaps in scenarios of indivisible injury and complex causation.
The joint-and-several approach to climate harm
Judge Simma, in his separate opinion in the 2003 Oil Platforms case, identified the principle of joint and several liability as a potential solution for the problem of overdetermination in the law of State responsibility. In his view, the doctrine amounted to a general principle of international law drawn from domestic practice, whereby multiple wrongdoers contributing to an indivisible injury may each be held liable for the full extent of the injury, even if their individual conduct alone would not have led to the injury. This reasoning is particularly relevant to climate change reparations, where losses and damages are often non-severable and no single State is solely responsible for the injurious outcome.
The principle of joint and several liability has already found expression in certain international legal regimes. The 1972 Convention on International Liability for Damage Caused by Space Objects (“1972 Space Liability Convention”) provides that where “two or more States jointly launch a space object, they shall be jointly and severally liable for any damage caused” by the objective, and thus a claimant may seek full compensation from any of the launching States. Similarly, Annex VI to the Protocol on Environmental Protection to the Antarctic Treaty –although not yet in force– proposes joint and several liability for environmental emergencies involving multiple operators, unless it can be shown that the environmental emergency is severable. These examples reflect a normative effort toward ensuring full redress for victims in cases involving hazardous activities with diffuse causation.
The application of the doctrine of joint and several liability in the context of climate change offers several potential benefits:
- Facilitate full reparation for victims: Joint and several liability would significantly ease the procedural burden on those seeking compensatory relief, thus making it more likely for victims of climate harm to get full redress. Victims would be able to recover the full breadth of damages from any responsible State, even where individual contributions cannot be precisely determined or quantified. This is crucial in climate change litigation, where evidentiary limitations imposed by the current state of science and diffuse causal link may prevent victims from adequately demonstrating attribution or causation for allocating a specific share of damages to the emissions of one or another State. A telling example is the case of Lliuya v. RWE, in which the first instance decision shows the high burden claimants face when proving the share of damages attributable to each emitter, despite the nuanced view taken by the Higher Regional Court of Hamm on RWE’s objections on climate liability (see an analysis here).
- Enhanced deterrence: Holding each State potentially liable for 100% of climate-related damage may create an incentive to reduce emissions. States can no longer downplay their role by invoking their minor share of global emissions –the so-called “drop-in-the-ocean” defense– as each could be held fully accountable for the damage. This may foster climate action and greater compliance with mitigation, reduction and adaptation strategies.
Therefore, the application of joint and several liability in the domain of State responsibility for climate reparations represents a potentially transformative approach for ensuring climate justice in the face of collective injury and the victims’ right to full redress.
Legal and practical challenges in the application of joint and several liability to climate reparations
While joint and several liability holds promise as a legal tool for climate reparations, its application in the domain of State responsibility may face doctrinal and practical challenges:
- Fairness and equity concerns in the allocation of liability: Holding a State fully liable for the entirety of climate harm –regardless of its relatively modest contribution to climate change– may be viewed as disproportionate and may counter basic principles such as the “common but differentiated responsibilities” embedded in climate law. Also, as previously stated, the injurious outcome may result from uncoordinated acts not prohibited by international law. Applying joint and several liability in this setting would mean accepting that States may be held liable for injuries not directly caused by their own wrongful actions or omissions. Accordingly, the application of the joint-and-several approach would likely require the incorporation of a risk-based liability system –i.e. being liability assigned based on the inherent risk associated with an activity, rather than based on fault or negligence– to balance equity considerations.
- Lack of recourse among States: While Article 47 ARSIWA acknowledges the right of recourse against other responsible States, the international legal order currently offers no adequate legal avenues for seeking contribution against other co-responsible parties. Without such a mechanism, a State held liable to pay for climate damages might be unable to recover the share of damages attributable to other States. To make joint and several responsibility viable, legal development may be needed to enable contribution claims between co-responsible States, which may require ad hoc contribution agreements between States. Notably, the 1972 Space Liability Convention anticipated this issue: it explicitly allows a launching State that paid compensation to seek contribution from other launching States involved.
- Procedural and jurisdictional hurdles in international adjudication: There might be a number of jurisdictional and admissibility challenges when bringing claims based on the joint and several liability doctrine, such as the indispensable third-party problem. In this regard, international courts may be reluctant to rule on a State’s responsibility on a joint-and-several basis if third States –absent from the proceedings– may also bear responsibility vis-à-vis the victims. However, in the Certain Phosphate Lands in Nauru case, the ICJ rejected Austalia’s objection that the United Kingdom and New Zealand were indispensable third parties, holding that the claim against Australia could proceed independently (¶55). This reasoning suggests a degree of flexibility in adjudicating claims involving multiple potentially responsible States, even under a joint and several framework.
- Risk of political backlash: The perception that joint and several liability threatens States’ sovereignty or encroaches upon their economic interests may create a political backlash against participation in international agreements or international adjudication, which may ultimately undermine international cooperation (already in decline).
Conclusion
The growing push for climate reparations has exposed a critical gap in the law of State responsibility: the allocation of losses resulting from the cumulative, often lawful and uncoordinated conduct of many States. While the ICJ Advisory Opinion considers that the law of State responsibility adequately addresses these gaps, it leaves many critical issues to unfold as this area of practice progresses. The ICJ Advisory Opinion offers little guidance on how to interpret questions of attribution, causation or apportionment of responsibility in the climate change context. In this regard, the doctrine of joint and several liability could present a meaningful way forward to address this impasse.
Joint and several liability would allow each responsible State to be held accountable for the full extent of indivisible injury. By lifting the burden from victims to prove the apportioned share of damages attributable to each State, it would facilitate access to compensation even when some actors cannot be brought before a court or their exact contribution to the injury suffered is uncertain. It also may incentivize States to reduce emissions rather than rely on the shield of shared blame.
But this proposal is not without challenges. It raises fairness concerns and risks imposing liability on States that have not acted in a legally wrongful way. Additionally, there is currently no clear international adjudicatory system that enables contribution claims, which could lead to grievances among co-responsible States. These challenges are real, but not insurmountable. They highlight the need for procedural innovation, legal development, and inter-State cooperation to climate justice.
Applying joint and several liability to climate harm is a forward-looking proposal. It does not offer a perfect solution, but a principled starting point for reimagining how international law can respond to collective and cumulative injury. It could help reshape how we think about responsibility in a world where harm is shared and so must be the duty to repair it.
* The views expressed in this blog post are those of the author and do not necessarily reflect the views of the author’s affiliated institution. The author is solely responsible for the content, and the institution is not responsible for any errors or omissions. This post is intended for academic purposes only and should not be taken as the institution’s position on the topic.
María de Arcos Tejerizo
María de Arcos Tejerizo is a Spanish lawyer specializing in litigation and international arbitration at Pérez-Llorca in Madrid. She holds an Advanced LL.M. in Public International Law from Leiden University.
