Climate Change and International Law: Unpacking the Landmark ICJ Advisory Opinion and Its Implications for International Arbitration

On July 23, 2025, the International Court of Justice ("ICJ" or "Court") delivered a unanimous advisory opinion on the Obligations of States in respect of Climate Change that is poised to influence the interpretation of international law on climate change for years to come.

The ICJ’s legal opinion follows several decisions by other regional or specialized international courts and tribunals that recently made similar determinations, such as the Advisory Opinion on Climate Change of 21 May 2024 issued by the International Tribunal for the Law of the Sea ("ITLOS"), Advisory Opinion on Climate Emergency and Human Rights of 29 May 2025 issued by the Inter-American Court of Human Rights and Verein KlimaSeniorinnen Schweiz and Others v. Switzerland, judgment of 9 April 2024 rendered by Grand Chamber of the European Court of Human Rights ("ECtHR"), (for previous coverage, see here and here).

Broadly, the advisory opinion addresses two main questions put forward by the United Nations General Assembly Resolution 77/276: (a) the legal obligations of States under international law to ensure the protection of the climate system from anthropogenic, i.e. human-made, greenhouse gas ("GHG") emissions; and (b) the legal consequences arising from the breach of these obligations.

While the advisory opinion engages with numerous significant issues, this blog post necessarily focuses on a few selected aspects. The omission of other issues should not be interpreted as suggesting they are of lesser importance.

Obligations of States Under (Customary) International Law

Answering question (a), the ICJ identified a breadth of obligations of States related to climate change creating a comprehensive legal framework comprising not only climate change treaties but also other areas of international law. To name just a few, the Court's reasoning provided important clarifications regarding the nature and content of mitigation, adaptation, and cooperation and assistance obligations under the climate change treaties (paras. 174-270). It further placed an emphasis on States' obligations under the United Nations Convention on the Law of the Sea ("UNCLOS"), confirming, amongst others, key findings of the Advisory Opinion on Climate Change issued by ITLOS, including by reaffirming that GHG emissions constitute marine pollution under UNCLOS (paras. 339-340) and that States accordingly are obligated to 'protect and preserve the marine environment' and to take 'all necessary measures' to reduce and control such pollution under a stringent due diligence standard (paras. 336-368). The Court also noted that the human right to a healthy environment is essential for the enjoyment of a range of other human rights (paras. 369-404).

The Court’s deliberations with respect to the well-established customary obligation to prevent significant transboundary harm to the environment (cf. Trail Smelter and Legality of the Threat or Use of Nuclear Weapons) merit particular attention. The ICJ identified this obligation as a central duty of States in relation to climate change, reasoning that the global climate system is a vital part of the environment and accordingly falls within the scope of the obligation and must be protected for present and future generations (para. 273). Having reviewed what the Court considered to be the best available science (i.e. reports of the Intergovernmental Panel on Climate Change ("IPCC")), the Court considered that the adverse effects of climate change are grave, causing significant harm to the climate system (paras. 277-279). Crucially, the Court took the view that States cannot rely on the fact that it may be difficult to attribute a specific share of climate harm to an individual State. In this respect, States must consider the possible cumulative effects of their actions and the planned activities under their jurisdiction or control (para. 276). It naturally follows that the Court considered the duty of States to co-operate for the protection of the environment to equally have customary international law character (paras. 301-306).

The Court rejected the 'lex specialis' argument advanced by several participants in the proceedings, finding that climate change treaties are not a self-contained regime that displaces other rules of (customary) international law in the context of States’ climate change obligations (paras. 162-171). Instead, the Court found that climate change treaties and customary international law continue to exist independently, while each may inform the identification and content of obligations under the other (para. 310).

Regarding the scope of the obligation to prevent significant harm to the environment, the ICJ concluded that the required standard of conduct is one of due diligence (para. 280). The Court identified several factors that are particularly relevant in assessing whether a State’s conduct satisfies that standard in a given context. Such factors are, inter alia, the adoption of regulatory mechanisms (paras. 281-282), the availability and assessment of scientific information (paras. 283-286), the need for developed States to take more demanding measures within their capabilities (paras. 290-292) and environmental impact assessments for significant activities (paras. 289-298).

Actions That Can Be Classified as Breaches of International Law

In determining State responsibility in the climate change context under question (b), the ICJ acknowledged the 'multifaceted' nature of climate change involving responsibilities for multiple States over long periods of time. This in turn gives rise to particular issues in relation to the application of the customary rules on State responsibility, considering that States' GHG emissions vary considerably as do the effects of climate change on States (para. 421).

The Court considered that (any) failure of a State to take appropriate climate action may constitute an internationally wrongful act attributable to that State, naming fossil fuel production and consumption, whether by the State itself or private actors, as possible examples. A State's failure to take regulatory action, for example by continuing to grant fossil fuel exploration licenses or providing fossil fuel subsidies, can thus equally entail a State's international responsibility. The Court was however careful to clarify that not the emission as such is the internationally wrongful act, but failure to take the requisite measures to protect the climate system from the harm resulting from excessive anthropogenic emissions (para. 427). In relation to private actors, the Court held that State responsibility may be engaged where a State has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the GHG emissions caused by private actors under its jurisdiction (para. 428).

The ICJ also noted that the customary international law rules on State responsibility are capable of addressing a situation in which there exists a plurality of injured or responsible States. Drawing on Armed Activities on the Territory of the Congo, the Court concluded that in such circumstances each injured State may separately invoke the responsibility of another State (paras. 430-431).

The Court further found the existing legal standard for causation – 'a sufficiently direct and certain causal nexus' between a wrongful action or omission and the damage – to be capable of being applied in the climate change context. According to the Court, the approach is two-tiered: one must first examine whether a given climatic event or trend can be attributed to anthropogenic climate change – in the Court’s view a question of science. Second, it must be established to what extent damage caused by climate change can be attributed to a particular State or group of States. In this respect, the Court acknowledged that such a causal link must be established in each case in concreto (para. 437).

Lastly, the Court recalled that all States have a common interest in the preservation of the climate system, thus certain obligations in this respect, and particularly, the obligation to prevent significant harm under customary international law, are erga omnes obligations (para. 440). As such, they can be invoked by each State to demand cessation of the harmful activity in breach of climate change-related obligations.

Legal Consequences of Breaches

At the outset, the ICJ considered that 'legal consequences' are identified and addressed through the application of the secondary rules of international law concerning State responsibility (paras. 104, 420). The Court, reiterating the two-tiered test for causation outlined above, highlighted that while reparations require establishing a causal link between the breach and the resulting harm, the obligations of cessation and non-repetition arise irrespective of any actual damage. A violation of international law suffices to trigger these duties. This conclusion must be read in light of the Court’s finding that climate change-related obligations are erga omnes, implying that every State is entitled to invoke a breach.

In terms of the legal consequences following established breaches of the identified climate change-related obligations, the Court considered that these may give rise to 'the entire panoply of legal consequences' under State responsibility, including:

• A duty of cessation and guarantees of non-repetition, e.g. requiring a State to revoke all administrative, legislative and other measures that constitute an internationally wrongful act or to employ all means at its disposal to reduce their GHG emissions (paras. 447-448);
• A duty to make reparations (paras. 449-450);
• Restitution, e.g. reconstructing damaged or destroyed infrastructure, and restoring ecosystems and biodiversity (para. 451);
• Compensation, corresponding to financially assessable damage (paras. 452-455); and
• Satisfaction, e.g. through formal apologies (para. 455).

Potential Implications for International Arbitration

While the practical ramifications of the advisory opinion – whether legal, policy-related, or diplomatic – have yet to unfold, the ICJ's analysis, although legally non-binding, provides a clarification of existing obligations of States in the context of climate change.

One area where its impact could be particularly significant is international investment law. The advisory opinion raises critical questions for investor-state dispute settlement, as highlighted in Judge Cleveland's separate declaration which suggests that the interpretation of investment instruments should be informed by States' climate change obligations under international law. It will be interesting to see to what extent States will seek to invoke conflicting obligations identified by the ICJ under international law to justify certain State-conduct towards foreign investors.

One may further ask whether States could seek to rely on the Court’s opinion to justify the suspension of certain international obligations, such as protections under investment treaties, towards other States by way of countermeasures. The erga omnes character of climate change-related obligations confirmed by the Court adds further complexity to this question and bears a significant potential for disputes.

Cross-border commercial disputes, most often resolved through international arbitration, are also likely to be affected. As States’ international law obligations are regularly transformed into domestic law, and many States consider customary international law to form part of their domestic legal order, it is plausible that the ICJ’s advisory opinion will shape the interpretation of issues such as force majeure under domestic law. This in turn may be relevant in commercial disputes resolved through arbitration if an underlying obligation is impeded by climate change-related circumstances or regulation.