ICJ Advisory Opinion on States’ Climate-related Obligations: Implications for Companies

On 23 July 2025, the world’s highest court, the International Court of Justice (ICJ), handed down its long-awaited Advisory Opinion (the Opinion) on States’ legal obligations in respect of climate change. 

The Opinion is the third advisory opinion issued by an international court in relation to climate change, with the International Tribunal for the Law of the Sea (ITLOS) and the Inter-American Court of Human Rights publishing their opinions in May 2024 and July 2025 respectively. 

The Opinion provides important clarification on the duties of States in relation to climate change. Although advisory opinions are not binding on States, they exert highly persuasive authority. The ICJ’s Opinion is likely to be particularly influential due to its unanimous adoption by the fifteen judges of the ICJ (only the fifth time it has done so in its almost 80-year history) and the high levels of participation by States and international organisations (the highest ever) in the ICJ proceedings – 96 States and 11 international organisations presented oral statements at the hearing.

Although the Opinion applies to States only, the ICJ’s findings are relevant to companies due to their potential implications for State-level regulation and scope to bolster arguments advanced by climate and environmental groups in domestic strategic climate litigation.

The ask

The Opinion follows a March 2023 request by the United Nations General Assembly driven by a group of small island developing States. The General Assembly asked the ICJ to clarify (a) States’ obligations under international law to ensure protection for the climate system and environment against anthropogenic greenhouse gas (GHG) emissions, and (b) the legal consequences for States of causing significant harm to the climate system and environment. Read more in our previous blog post on the ICJ advisory proceedings.

Question (a) - State responsibility

The ICJ has confirmed that States have legal obligations to take appropriate action to protect against climate change. In doing so, the ICJ found that States’ obligations in relation to climate change are not limited to those set out in climate treaties (e.g. the UN Framework Convention on Climate Change, the Kyoto Protocol and the Paris Agreement). In the ICJ’s view, whilst the climate treaties are the primary legal instruments regulating the international response to climate change, they do not constitute a ‘lex specialis’ regime that takes precedence over obligations arising under other international law rules. As a result, the ICJ opined that States’ legal obligations in relation to climate change also arise under customary international law, the law of the sea and international human rights law, each of which establish independent and sometimes overlapping obligations. Therefore, even States which are not party to the climate treaties (e.g. the US has taken preliminary steps to withdraw from the Paris Agreement for the second time) are required to prevent harm significant harm to the climate.

Importantly, the Opinion represents the first time the Paris Agreement has been interpreted by an international court and therefore marks a crucial step in clarifying the Agreement’s expectations on States, including the latitude afforded States in applying its provisions. For example, the ICJ opined that, although the Paris Agreement aimed to limit temperature increases to at least 2^oC above pre-industrial levels and encouraged additional efforts towards 1.5^oC, based on current scientific consensus, 1.5^oC is now the “agreed primary temperature goal” in the Paris Agreement. In addition, in relation to States’ mitigation obligations, the ICJ highlighted that nationally determined contributions (NDCs) must be successive and become progressively more demanding over time to reflect each State’s “highest possible ambition”. Notably, States are expected to submit new NDCs ahead of COP 30 which is due to be held in Brazil in November.

Question (b) - Legal consequences 

Attribution and the role of private actors

The ICJ found that the failure by a State to meet its legal obligations to take appropriate action to protect against climate change (discussed above under question (a)) – for example by failing to regulate GHG emissions under States’ duty to exercise due diligence under customary law – may amount to an “internationally wrongful act” attributable to that State. 

The ICJ also observed that while climate change is caused by cumulative GHG emissions, it is scientifically possible to quantify an individual State’s contribution to global emissions. The Opinion does not specify how to perform such an exercise and in his separate Declaration, Judge Nolte stressed that science cannot determine the extent of wrongfulness and, in particular, whether a State has made its “best efforts” to reduce GHG emissions. Even so, the ICJ took that view that, in principle, each injured State may separately invoke the responsibility of each and every State which has carried out an internationally wrongful act resulting in damage to the climate system to bring claims against one another..

In addition, whilst the Opinion concerns the obligations of States, the ICJ delivers important observations concerning private actors. Indeed, the ICJ conceived its inquiry as encompassing not merely State acts and omissions, but equally those of non-State actors within States’ jurisdiction or effective control. The ICJ also endorsed the ITLOS advisory opinion which took the position that due diligence duties are most relevant where activities are “mostly carried out by private persons or entities”. Yet, the relationship proves more nuanced than simple attribution might suggest, as private actors (including high emitters) simultaneously pioneer transformative solutions to mitigate against and adapt to climate change. 

The ICJ’s analysis extends further, establishing that failure by a State to address emissions, “including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies” can amount to internationally wrongful conduct attributable to the State. Through an illustrative example of what might amount to a breach of due diligence obligations, the ICJ crystallises a fundamental principle that States must actively regulate the climate-related impacts of the private sector through domestic regulation and legislation, bolstered by effective enforcement and monitoring mechanisms. With this, the ICJ reiterates its earlier judgment in Pulp Mills on the River Uruguay (Argentina v. Uruguay), in which it held that the obligation to act with due diligence entailed “a certain level of vigilance” in enforcing and exercising administrative control on public and private operators, including monitoring such operators. Through this reasoning, the ICJ clarifies that attribution of private conduct to States is unnecessary for establishing state responsibility, as an equally relevant cause of action lies in States’ failure to adequately regulate private actors. 

Causation

On the question of causation, the ICJ observed that it is not necessary to establish causation of damage when determining a State’s responsibility for an internationally wrongful act, unless reparation (e.g. compensation) is sought. The ICJ also rejected the submissions of some States that it is impossible to establish causation due to the diffuse nature of climate change. Relying on its earlier (non-climate) case law, the ICJ affirmed that damage resulting from concurrent causes does not exempt States from responsibility or legal consequence (Democratic Republic of the Congo v. Uganda). 

The relevant legal standard for causation is “a sufficiently direct and certain causal nexus” between the alleged wrongful action or omission and the alleged damage (as established in Democratic Republic of the Congo v. Uganda and Costa Rica v. Nicaragua). To identify a causal link between a State’s wrongful act or omission and the climate-related damage it is necessary to consider:

1. Whether, according to the climate science, the particular “climactic event or trend” can be attributed to anthropogenic climate change; and
2. Whether, based on the facts of each case, the damage caused by climate change is attributable to the relevant State. 

The Opinion acknowledges that the nature of climate change means that the causal link between a wrongful act or omission and the harm arising from climate change is more tenuous than in other cases e.g. harms caused by local sources of pollution. However, the ICJ considered that it is still possible to establish a link by looking at the facts and circumstances on a case-by-case basis (an ‘in concreto’ assessment). This is an important finding which climate activists may attempt to rely on to support domestic climate litigation claims.

Looking ahead

The ICJ’s Opinion sets out a roadmap for States in navigating their conventional and customary climate-related obligations which, while not binding in nature, may have significant implications for States found in breach of their climate obligations. The ICJ confirmed that international law remedies may in principle be available against offending States found to have committed an “internationally wrongful act” in breach of their climate obligations, which could extend to a requirement to compensate injured States. The ICJ’s Opinion can also be expected to enliven new arguments and strategies in future climate negotiations, particularly in light of the ICJ’s finding that COP decisions may create legally binding obligations for the involved parties, which may be significant for the upcoming COP 30. For example, the ICJ’s discussion of environmental impact assessments may chart new cartographies of corporate accountability. Its insistence that “possible specific climate-related effects must be assessed […] at the level of proposed individual activities” finds further expression through Judges Bhandari and Cleveland’s Joint Declaration, which recognises that fossil fuel projects must now contemplate their emissions’ entire lifecycle. That may include tracing emissions from extraction through to combustion by distant end users, regardless of jurisdictional boundaries. Their reasoning draws upon domestic case law, including the UK Supreme Court’s 2024 ruling on Scope 3 emissions in Finch v. Surrey County Council, yet it suggests that customary international law itself demands a similarly expansive analytical lens across all jurisdictions. This recognition may promise to fundamentally reshape the terrain upon which fossil fuel expansion unfolds.

Whilst the Opinion may have less immediate direct impact at the domestic level, particularly in dualist systems like the UK, where international law is not automatically incorporated into domestic law (unlike monist systems, such as France and the Netherlands), its influence could have a trickle-down effect. Climate and environmental groups are likely to find ways to leverage the Opinion’s findings in pursuit of their climate goals, including in support of strategic climate litigation against States and the private sector. In the UK, for example, climate groups may seek to rely on the Opinion in support of judicial review of State or public body decisions, such as development consents and licences for high-emitting projects which were jump-started by Finch. We have already seen this in the context of the ITLOS opinion which has featured in arguments in support of a High Court challenge to North Sea oil and gas licences on grounds including that the Government failed to consider the impact on marine life, for example.

Further, the ICJ’s observations on attribution and causation have scope both to constrict causes of action brought by private actors and to expand causes brought against them. Its findings may, on one hand, encourage formal recognition of governments' rights to regulate climate impacts under customary international law. Such an acknowledgement, by courts and tribunals in particular, could modify investment law so as to limit the ability of private actors to bring claims against governments under investment treaties (including the Energy Charter Treaty), notwithstanding any amendment or sunset clauses. Conversely, the ICJ’s finding that concurrent causes of harm to the environment do not preclude individual State responsibility could be used to bolster similar arguments advanced by claimants in corporate climate litigation in some jurisdictions. In May this year, a German appeal court found that an energy company may in principle be liable for climate-related harms. Though it remains to be seen how this argument might successfully be deployed in other jurisdictions, climate groups may seek to rely on the Opinion’s findings on causation to argue an extension to private actors.