Introduction
As we have previously noted, in March 2023, the United Nations General Assembly requested the International Court of Justice (ICJ) to opine on the following questions in accordance with Article 65 of the ICJ Statute:
“(a) What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases [(GHG)] for States and for present and future generations;
(b) What are the legal consequences under these obligations for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment, with respect to:
(i) States, including, in particular, small island developing States, which due to their geographical circumstances and level of development, are injured or specially affected by or are particularly vulnerable to the adverse effects of climate change?
(ii) Peoples and individuals of the present and future generations affected by the adverse effects of climate change?”
This was followed by a written phase of proceedings, where 153 written statements and comments were filed by States and international organisations. Public hearings were conducted from 2 to 13 December 2024, during which 107 States and international organisations presented oral statements, reflecting the highest level of participation in a proceeding in the history of the ICJ.
On 23 July 2025, the ICJ issued an advisory opinion on these matters. The operative clause of the opinion was adopted unanimously, although several judges appended separate opinions on aspects of the advisory opinion.
The obligations of States under international law to ensure the protection of the climate system
Climate change treaties
The ICJ examined the obligations of States under the climate change treaty framework, considering the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol and the Paris Agreement to be the principal legal instruments regulating the international response to the global problem of climate change. The UNFCCC establishes the overarching objectives and principles, while the Kyoto Protocol and Paris Agreement set out more detailed and interrelated obligations.
Under these treaties, States have mitigation, adaptation and co-operation and assistance obligations:
- Mitigation: States must take steps such as limiting GHG emissions by sources and enhancing sinks and reservoirs of GHGs.
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Adaptation: States must adjust to actual or expected climate change and its effects in order to moderate harm or exploit beneficial opportunities.
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Co-operation and assistance: States must work together in good faith (e.g. by financial transfers, technology transfers and capacity-building).
The ICJ observed that there are two categories of mitigation obligations under the climate change treaties. First, obligations of conduct, which require States that are signatories to those treaties to use best efforts and exercise due diligence to achieve certain aims, such as an obligation to cooperate to develop technology that reduces GHG. A State acts wrongfully if it fails to use all means at its disposal to bring about the objective envisaged. Secondly, obligations of result, which require States that are signatories to those treaties to achieve specific outcomes, such as an obligation to adopt national policies and take corresponding measures on the mitigation of climate change. Simply adopting a policy as a mere formality will not satisfy the obligation. The specific policy adopted must be able to achieve the required goal to discharge the relevant obligation of result.
Regarding the Paris Agreement, the ICJ also noted that States are to determine their nationally determined contributions (NDCs) which reflect their “highest possible ambition”. This is not a matter of complete discretion; States must ensure that their NDCs are capable of adequately contributing to the achievement of the temperature goal (1.5°C above pre-industrial levels).
Customary international law
While certain States argued that the application of customary international law was excluded as climate change treaties should be regarded as lex specialis, the ICJ disagreed, noting that there are no inconsistencies between the treaties and other relevant rules and principles of international law. Under customary international law, States have a duty to prevent significant harm to the climate system and must act with due diligence, the standard of which is stringent. This duty applies to all States, including those that are not signatories to the climate change treaties. Such duty comprises substantive elements (e.g. adopting mitigation and adaptation measures to regulate the conduct of public and private operators within the States’ jurisdiction or control, together with effective enforcement and monitoring to ensure their implementation) and procedural elements (e.g. notifying and consulting other States). States also have a pressing obligation to co-operate. While States have some discretion to determine the means for regulating GHG emissions, this cannot serve as an excuse for States to present their efforts as an entirely voluntary contribution that cannot be subjected to scrutiny.
Other treaties and principles
The ICJ also considered relevant obligations under other treaties, such as the Ozone Layer Convention and Montreal Protocol, and the United Nations Convention on the Law of the Sea. It affirmed that there is a human right to a clean, healthy and sustainable environment, and that States must take into account their human rights obligations when implementing their climate-related obligations.
Legal consequences for States where they have caused significant harm to the climate system and other parts of the environment
The ICJ considered that the general rules on State responsibility apply to climate-related obligations under the climate treaties and customary international law.
States may breach their obligations by failing to take appropriate action to protect the climate system from GHG, such as through fossil fuel production, consumption and the granting of exploration licences or the provision of fossil fuel subsidies. States can also be held responsible for failing to regulate private actors within their jurisdiction who contribute to GHG emissions.
The ICJ acknowledged the potential complexity of determining whether an individual State’s breach of its climate-related obligations caused significant harm to the climate system when there are multiple contributors. However, it considered that responsibility for such injury may still be allocated among the possible contributors. In particular, each injured State may separately invoke the responsibility of every State which has committed an internationally wrongful act resulting in damage to the climate system.
In principle, a breach of climate-related obligations may result in obligations of cessation and non-repetition (e.g. revoking any offending legislation) and full reparation, including restitution (e.g. restoring a damaged ecosystem), compensation (e.g. including a global sum taking into account equitable considerations) and/or satisfaction (e.g. formal apologies). A non-injured State may seek cessation and non-repetition from an offending State, but only performance of reparation in the interest of an injured State or of the beneficiaries of the obligation breached (not for itself).
Comment
The ICJ’s advisory opinion unequivocally recognises the binding nature of States’ climate-related obligations for the first time, including obligations arising under customary international law and therefore applicable to non-signatories of climate treaties. It also provides a clear call to action, emphasising the importance and necessity of co-operation between States in light of the existential threat of climate change and the pressing need to exercise due diligence in accordance with the best available science.
Although not formally binding on States, it carries “great weight and moral authority” and provides leverage for States that have consented to ICJ jurisdiction to potentially bring inter-State proceedings before the ICJ in relation to breaches of climate-related obligations, including failures to mitigate, adapt or co-operate.
While the advisory opinion does not directly address the obligations on companies and other private parties, the emphasis on a State’s regulatory duty to limit GHG emissions within its jurisdiction, including to regulate private parties, may lead to more stringent domestic regulations over private sector emissions.
Public and private parties involved in disputes before other courts (international, regional or domestic, including in existing and future litigation regarding the energy transition from fossil fuels) may also seek to rely on the opinion. However, this does not necessarily mean that such courts will take the opinion into account when determining the relevant dispute. Similarly, parties to proceedings before arbitration tribunals may also seek to rely on the opinion, for instance, as an interpretative tool for investment treaty obligations considered in investor-State arbitrations (as noted in Judge Cleveland’s separate opinion).
