ICJ issues landmark Advisory Opinion on States’ legal obligations in respect of climate change

As previously reported, on 23 July 2025, the International Court of Justice (ICJ) issued its long-awaited advisory opinion on the obligations of States under international law in relation to climate change (the Advisory Opinion). 

In summary, the Advisory Opinion recognises that:

1. States have an obligation to protect the climate system from the harmful effects of greenhouse gas (GHG) emissions, including by regulating the activities of private actors.
2. These obligations arise under both climate change treaties and other sources of international law, including environmental treaties, human rights treaties, and customary international law.
3. The legal consequences for breaching climate-related obligations could potentially include the cessation of harmful activities and compensation for damages. 

The various separate opinions and declarations of ICJ judges have also addressed, in more detail, some of the more difficult issues that were before the Court, including the duty of due diligence, the responsibilities of developed and developing States, the effect of sea-level rise on statehood, and causation of loss caused by climate change.

While it is not legally binding on States, the Advisory Opinion—which was adopted unanimously by the ICJ’s 15 judges—is an authoritative interpretation of international law obligations that carries significant weight. It is expected to significantly influence international environmental governance and inform future climate-related litigation and policy at both the international and domestic levels, including as regards the regulation of private companies and industries.

Background

The UN General Assembly requested the Advisory Opinion in March 2023 (resolution 77/276), asking the Court to clarify:

• the obligations of States under international law to protect the climate system and other parts of the environment from anthropogenic GHG emissions; and
• the legal consequences for States whose acts or omissions have caused significant harm to the climate system and other parts of the environment, including with respect to particularly vulnerable States and to present and future generations.

The request was supported by 132 UN Member States, and the opinion followed extensive written and oral submissions from States and international organisations. It follows closely on the heels of recent advisory opinions by the International Tribunal for the Law of the Sea (issued on 21 May 2024) and the Inter-American Court of Human Rights (issued on 29 May 2025 and notified on 3 July 2025), both of which examined States’ obligations regarding climate change within their respective mandates.

Key Takeaways

1. Obligations of States in respect of climate change

The Court affirmed that States’ climate change-related obligations are not exclusively found in climate change treaties—i.e., the UN Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, and Paris Agreement. Rather, relevant obligations are found in a range of other international law sources, including customary international law, the law of the sea, and environmental and human rights treaties. 

a) Climate change treaty obligations 

The Court confirmed that the UNFCCC and Paris Agreement impose binding obligations on State Parties to these treaties, which include the vast majority of States. These obligations include:

• Mitigation: State Parties to the Paris Agreement are required to prepare, communicate, and maintain nationally determined contributions (NDCs) that reflect their “highest possible ambition” and, when taken together, are capable of achieving the temperature goal of limiting global warming to 1.5°C above pre-industrial levels. This is a binding obligation of conduct, subject to a stringent due diligence standard that varies according to a State’s capabilities and responsibilities.
• Adaptation: State Parties to the UNFCCC and the Paris Agreement must plan for and implement measures to adapt to the impacts of climate change. Developed countries have additional obligations to support adaptation efforts in developing and vulnerable States.
• Support and cooperation: State Parties to the UNFCCC and the Paris Agreement must cooperate in good faith, including through information sharing and transparency. Developed States are required to provide financial assistance, technology transfer, and capacity-building support to developing States.
• Accountability: State Parties to the UNFCCC and the Paris Agreement must regularly report on progress, including through the regular publication of national inventories of GHG emissions, the formulation of national programmes and the publication of information to the Conference of the Parties (COP) (the body mandated with promoting the effective implementation of the UNFCCC).

b) Customary international law obligations

The Court reaffirmed several key customary international law obligations that are binding on all States, regardless of which treaties they have ratified. The Court therefore dismissed the contention advanced by several States that the climate change treaties established a distinct legal regime, displacing other rules of international law with respect to climate change issues. The Court also clarified that compliance in full and in good faith by a State with the climate change treaties suggests, but does not necessarily mean, that the State has fulfilled its obligations under customary international law. In other words, States’ customary obligations may require conduct beyond that required by treaties. 

These customary international law obligations include:  

• Duty to prevent significant harm: States must use all means at their disposal to avoid activities—whether public or private—that cause significant environmental harm, including cumulative and transboundary GHG emissions. This obligation applies even where it is difficult to attribute a precise share of responsibility to any individual State. States must also take into account the environmental impact of their activities in the broader context of cumulative effects, including those arising from activities beyond their jurisdiction. 

In complying with this obligation, States must act with due diligence. The standard of care is context-specific and evolves with scientific knowledge. It involves both substantive and procedural elements. States with greater capabilities are expected to take more robust measures. The level of risk associated with potential harm informs the degree of diligence required under this standard. 

• Duty to cooperate: States must engage in sustained, good faith cooperation to address climate change, including through collective action and treaty implementation. The Court noted that “climate change is a common concern. Co-operation is not a matter of choice for States but a pressing need and a legal obligation”.

c) Law of the Sea and related obligations

The Court made significant observations on the obligations of States under the United Nations Convention on the Law of the Sea (UNCLOS) to ensure the protection of the climate system and address sea level rise:

• GHG emissions and marine pollution: The Court held that GHG emissions constitute marine pollution under UNCLOS, triggering State Parties’ obligations to prevent, reduce, and control such pollution. These obligations are subject to a stringent due diligence standard and apply regardless of whether emissions are direct or cumulative.
• Sea level rise: The Court found that UNCLOS does not require States to update maritime baselines or outer limit coordinates due to physical changes. Once established and publicised in accordance with the Convention, such baselines remain valid. The Court also clarified that the disappearance of a State’s territory due to sea level rise does not necessarily entail the loss of statehood under international law. This is a significant finding for a number of States whose territory is threatened by sea level rise, and it is the first time that the Court has taken a position on this important question. 
• International cooperation: The Court underscored the importance of international cooperation in addressing the legal and humanitarian consequences of sea level rise, including through equitable solutions that respect the rights of affected States and their populations.

d) Environmental and human rights obligations

With respect to environmental and human rights obligations:

• The Court found that obligations under environmental treaties, including the Ozone Layer treaties, Biodiversity Convention, and the Convention to Combat Desertification, establish relevant, complementary obligations that interact with climate change obligations and must be observed in good faith.
• The Court also recognised the interdependence between environmental protection and the enjoyment of human rights. Climate change adversely affects rights to life, health, housing and a clean, healthy and sustainable environment. States must take appropriate measures to safeguard these rights, including through climate action.

2. Legal consequences of breach

The Court confirmed that the general rules of State responsibility apply to breaches of climate-related obligations. These include the duty to perform, the duty to cease wrongful conduct and prevent its recurrence, and the duty to make full reparation—including restitution, compensation and satisfaction. 

Further, the Court characterised certain obligations, such as those relating to the protection of the climate system, as erga omnes or erga omnes partes—meaning they are owed to the international community as a whole. Thus, any State, not just those directly injured by the breach, may invoke responsibility for their breach.

a) Attribution

The Court confirmed that it was possible to attribute actions or omissions to a State in the context of climate change.  For example, it noted that the failure of a State to take appropriate action to protect the climate system from GHG emissions—“including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses or the prevention of fossil fuel subsidies”—may constitute an internationally wrongful act which is attributable to that State. The internationally wrongful act would not be the emission of GHGs per se, but the breach of applicable treaty or customary obligations.

The Court also commented that a State may be responsible for failing to exercise due diligence regarding the activities of private actors including, for example, by not taking the necessary regulatory and legislative measures to limit the GHG emissions caused by private actors within its jurisdiction.

b) Causation

The Court clarified that causation of damage is not required to establish responsibility—only for determining reparations. It adopted a flexible approach to causation in the climate context, breaking it down into two elements: (1) whether a climatic event can be linked to climate change, based on scientific evidence, and (2) whether the resulting harm can be tied to a particular State or group of States, which must be shown in concreto. Importantly, with respect to the first element, the Court found that the scientific evidence adduced before it “establishes that significant harm to the climate system and other parts of the environment has been caused as a result of anthropogenic GHG emissions”. 

Conclusion

The ICJ’s Advisory Opinion represents a significant development in international climate law. At the international level, it clarifies the general legal framework for States—and potentially for courts and tribunals—to assess compliance with international obligations relating to climate change.  It also contributes to the progressive development of international law on complex issues, such as the impact of sea level rise and potential disappearance of territory on statehood.  At the domestic level, the Advisory Opinion could have far-reaching implications for national legal systems by shaping legislative and policy responses, influencing judicial reasoning in climate change litigation, and informing domestic courts’ approaches to critical questions such as causation and attribution.

Yet, many difficult questions about the application of the framework set forth in the Advisory Opinion to specific factual contexts remain unresolved.  These include important issues of causation and attribution which the Court did not address (including how causation could be established concretely to tie the harm to a specific State), and the interplay of the various relevant bodies of law, such as human rights law and climate change treaties. The concrete implications of the Advisory Opinion must therefore be considered on a case-by-case basis – but follow-on litigation between States and efforts to amend the international climate change legal framework (e.g., via agreements seeking to give effect to the advisory opinion) should be expected.