The International Court of Justice issued an Advisory Opinion on obligations of the States in relation to climate change
Special Bulletin from the Department of Environmental Law and Climate Change | The International Court of Justice issued an Advisory Opinion on obligations of the States in relation to climate change
The Advisory Opinion of the International Court of Justice (“ICJ”) on obligations of the States in relation to climate change (“AO”), issued on July 23rd, 2025, represents a milestone in consolidating and clarifying the international legal framework applicable to the protection of the climate and the environment from anthropogenic greenhouse gas (“GHG”) emissions. This opinion was requested by the United Nations General Assembly through Resolution No. 77/276.
Relevance
The ICJ’s AO was issued following a two-year procedure initiated by a UN resolution in response to a motion submitted in 2023 by Vanuatu, a small island state in the Pacific Ocean.
The AO is significant because it confirms the legal obligations of States regarding climate change. Although it is not legally binding, the opinion carries considerable weight and is likely to influence the increase of climate litigation, shaping the future of climate law.
Notably, it was adopted unanimously by all ICJ members.
The main points of the AO are analysed below.
Obligations of the States in general
The ICJ identified the following as relevant applicable law: the Charter of the United Nations, the United Nations Framework Convention on Climate Change (“UNFCCC”), the Kyoto Protocol, the Paris Agreement, the United Nations Convention on the Law of the Sea, the treaties on the ozone layer, the Convention on Biological Diversity, the Convention to Combat Desertification, customary international law (in particular, the duty to prevent significant environmental harm and the duty to cooperate), international human rights law, and certain guiding principles such as sustainable development, equity, intergenerational equity, the precautionary principle, and the principle of common but differentiated responsibilities and respective capabilities.
The ICJ emphasized that these principles not only guide the interpretation of obligations but also inform their concrete application, including determining the standard of due diligence and the equitable distribution of burdens among States according to their historical circumstances and current capabilities.
The ICJ explicitly rejected the notion that climate change treaties constitute an exclusive lex specialis with respect to other relevant international norms, affirming that treaties and customary law coexist and inform each other, each maintaining its own scope of application.
Obligations of States under climate change treaties
With respect to substantive obligations, the ICJ stated that climate change treaties impose binding obligations on State parties to ensure the protection of the climate system and other parts of the environment from anthropogenic GHG emissions. It also affirmed that COP decisions may create legally binding obligations for parties and may constitute subsequent agreements under Article 31(3)(a) of the Vienna Convention on the Law of Treaties, insofar as such decisions express substantial agreement among the parties regarding the interpretation of the relevant treaty and thus must be considered as interpretative tools for climate treaties.
Regarding the UNFCCC, whose objective is to stabilize GHG concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system, the ICJ held that:
1- Mitigation is at the heart of the Convention.
2- Mitigation-related commitments constitute legally binding obligations that may result in State responsibility for non-compliance.
3- To fulfil this result-oriented obligation, the policies and measures adopted must be capable of achieving the required objective. Mere formal adoption of policies and related measures is insufficient to comply with the obligation.
Regarding the Kyoto Protocol, in the absence of a new commitment period, several participants argued that it is no longer relevant and need not be addressed in this proceeding. In fact, it is not listed among the instruments in the heading of the questions posed by the General Assembly. However, the ICJ held that the absence of a new commitment period does not deprive the Kyoto Protocol of its legal effect and that it continues to form part of the applicable law.
Regarding the Paris Agreement, the ICJ stated that:
1- The “temperature goal” (limiting the global average temperature increase to 1.5°C above pre-industrial levels, Article 2) constitutes a specification and quantification of the overall objective set by the UNFCCC.
2- The Agreement’s key features are its obligations regarding mitigation (Articles 3 to 6), adaptation (Article 7), loss and damage (Article 8), and cooperation through financial, technological, and capacity-building support (Articles 9 to 11).
3- To fulfil their mitigation obligations (Article 4), all parties must adopt measures that constitute an adequate contribution toward the collective temperature goal. These measures must be reflected in each party’s Nationally Determined Contributions (“NDCs”), which must be updated to increase ambition every five years; and the NDCs must, collectively, be capable of achieving the temperature goal and the purposes of the Agreement.
4- The obligation to prepare, communicate, and maintain successive NDCs is procedural in nature but constitutes a result-oriented obligation. Therefore, failure to prepare, communicate, and maintain NDCs, or to account for and register them, would amount to a breach of obligations. Mere formal compliance is not sufficient to meet the requirements of Article 4. The content of the NDCs is also relevant in determining compliance.
5- Although the scope and content of measures included in NDCs may vary according to each party’s available means and capacities, parties do not have unfettered discretion in formulating their NDCs. Each party is obligated to act with due diligence to ensure that the NDCs it submits represent its highest possible level of ambition with a view to achieving the Agreement’s objectives.
6- Parties must make best efforts to implement the contents of their NDCs. This means enacting legislation, administrative procedures, enforcement mechanisms, and monitoring systems to ensure the achievement of the NDC targets.
7- Adaptation obligations (Article 7) must be assessed based on a standard of due diligence.
8- With respect to developed countries’ obligation to provide financial resources and assist developing countries (Article 9), parties must fulfil this obligation in a manner and at a level that enables the achievement of the objectives set forth in Article 2. This level may be assessed based on factors including the capacity of developed States and the needs of developing States.
9- Regarding the duty to cooperate (Article 12), States are free to choose the means of cooperation, provided they are consistent with the obligations of good faith and due diligence.
Obligations of the States under customary international law
The ICJ notes that the duty to prevent significant harm to the environment also applies to the climate system, which is an integral and vitally important part of the environment and must be protected for present and future generations.
Furthermore, the ICJ emphasizes that cooperation is not a matter of choice for States, but a pressing need and a legal obligation. States must cooperate to achieve concrete emission reduction targets or to develop a methodology for determining each State’s contributions, including with respect to compliance with any collective temperature goal. The duty to cooperate applies to all States, although its scope may vary based on additional criteria, primarily the principle of common but differentiated responsibilities and respective capabilities.
Customary obligations are the same for all States and exist independently of whether a State is party to climate change treaties. On this basis, the ICJ considers that a non-party State that cooperates with the community of States parties to the three climate treaties in a manner equivalent to that of a party may, in certain cases, be regarded as complying with its customary obligations through conduct aligned with that required under the treaties. However, where a non-party State does not cooperate in this way, the burden is entirely on that State to demonstrate that its policies and practices are in conformity with its customary obligations.
Obligations of the States under international human rights law
The ICJ considers that the full enjoyment of human rights cannot be ensured without the protection of the climate system and other parts of the environment. In order to guarantee the effective enjoyment of human rights, States must adopt measures to protect the climate system and other parts of the environment. These measures may include, among others, mitigation and adaptation actions that take into due account the protection of human rights, the adoption of rules and legislation, and the regulation of private actors’ activities.
Therefore, under international human rights law, States are required to adopt the necessary measures in this regard.
Obligations of the States arising from acts and omissions causing significant harm to the climate system
The ICJ observes that it has identified a series of legal obligations under the UNFCCC, the Kyoto Protocol, and the Paris Agreement concerning the protection of the climate system from anthropogenic GHG emissions. In the absence of special rules to the contrary, a party’s responsibility may be engaged under the rules on State responsibility where an obligation is breached.
With respect to customary international law, the ICJ notes that the primary and most important obligation of States regarding climate change is the duty to prevent significant harm to the climate system and other parts of the environment, which applies to all States, including those that are not parties to one or more climate change treaties. Under this obligation, a State does not incur responsibility merely because the desired outcome is not achieved. Rather, it incurs responsibility if it fails to take all measures within its power to prevent significant harm. In this regard, the notion of due diligence—which requires an in concreto assessment—constitutes the standard for determining compliance. Therefore, a State that fails to exercise due diligence in fulfilling its primary obligation to prevent significant environmental harm, including harm to the climate system, commits an internationally wrongful act that entails responsibility.
Determining State responsibility in the context of climate change
The ICJ considers that any injured State may individually invoke the responsibility of any State that has committed an internationally wrongful act causing damage to the climate system and other parts of the environment. And where multiple States are responsible for the same wrongful act, the responsibility of each may be invoked in relation to that act.
As to the application of the standard of causation in the context of climate change, the ICJ observes that causation entails two distinct elements. First, whether a given climate-related event or trend can be attributed to anthropogenic climate change; and second, the extent to which the damage caused by climate change can be attributed to a particular State or group of States. While the second element must be established in concreto in relation to specific claims brought by States concerning damage, in many cases the first element may be addressed through recourse to science.
The ICJ clarified that the obligations to protect the climate and the environment are erga omnes in nature—that is, they are obligations owed to the international community as a whole—and that any State may invoke the responsibility of another State for their breach.
Legal consequences of wrongful acts
The ICJ notes that violations of States’ obligations may give rise to a range of legal consequences under the law on State responsibility. These include:
1- Obligations of cessation and non-repetition, which apply regardless of whether actual harm has occurred; and
2- Consequences requiring full reparation, including restitution, compensation, and/or satisfaction.
The ICJ also notes that violations of a State’s obligations do not affect the ongoing duty of the responsible State to comply with the breached obligation.
Fossil Fuels. Private Actors
The ICJ emphasizes that a State’s failure to adopt appropriate measures to protect the climate system from GHG emissions—including, among others, the production and consumption of fossil fuels, licensing for exploration, or granting subsidies for such fuels—may constitute an internationally wrongful act attributable to that State.
This is on the understanding that the internationally wrongful act at issue is not the GHG emissions themselves, but rather the failure to comply with the conventional and customary obligations identified throughout the AO relating to the protection of the climate system from significant harm resulting from anthropogenic emissions of such gases.
Additionally, in relation to private actors, the ICJ observes that the obligations identified in the AO include the duty of States to regulate the activities of such actors as part of the obligation to exercise due diligence. Therefore, attribution in this context involves holding the State accountable for its own acts or omissions that constitute a failure to exercise regulatory due diligence. In such circumstances, the question is not one of directly attributing the conduct of private actors to the State. Thus, a State may be held responsible, for example, if it has failed to exercise due diligence by not adopting the necessary regulatory and legislative measures to limit the amount of emissions generated by private actors under its jurisdiction.