The International Court of Justice confirms the obligations and consequences for Nation States to address climate change under international law

Background

In September 2021, a consortium of Small Island Developing States led by the Republic of Vanuatu sought an advisory opinion from the ICJ regarding the obligations of States under international law with respect to climate change.

With the support of other UN Member States, the United Nations General Assembly passed a resolution (resolution 77/276) on 29 March 2023 seeking an advisory opinion from the ICJ on two questions:

1. 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”?⁴
2. What are the legal consequences under these obligations for States where their “acts or omissions”⁵ have caused significant harm to the climate system and other parts of the environment?

The ICJ’s advisory opinion, although non-binding, carries significant legal, political and moral weight. It involved the highest level of participation in a proceeding in the history of the ICJ (including its predecessor) with:

• 91 written statements and 62 written comments filed by States and international organisations during the written phase of the proceedings; and
• 96 States and 11 international organisations presenting oral statements during the public hearings in the proceedings from 2 to 13 December 2024.

The ICJ is one of several international courts tackling major climate-related advisory opinions. In May, the International Tribunal of the Law of the Sea confirmed in its advisory opinion that ocean-absorbed GHG emissions constitute marine pollution, requiring States to act to protect the sea.

Obligations of States in respect of Climate Change

The ICJ identified several key sources of international law as the “most directly relevant applicable law”⁶ concerning States’ obligations in respect of climate change.⁷ These include: the Charter of the United Nations and the climate change treaty framework comprising the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol and the Paris Agreement which together form the principal legal instruments governing the international response to climate change.

In addition to these treaties, the ICJ confirmed that relevant obligations are found in a range of other sources of international law including the law of the sea, environmental treaties, international human rights law and customary international law.

We address each of the sources of States’ key obligations in respect of climate change below:

1. Climate Change Treaties
   
   The ICJ held that the three climate change treaties are mutually supportive and do not present any incompatibilities. The Kyoto Protocol and the Paris Agreement provide further detail and specificity to the general obligations contained in the UNFCCC.
   
   1. UNFCCC
      
      Article 2 of the UNFCCC describes that the ultimate objective of this treaty is the stabilisation of GHG concentrations in the atmosphere at a level that prevents dangerous anthropogenic interference with the climate system. It also requires that this is achieved in a timeframe that allows ecosystems to adapt to climate change, safeguards food production and supports sustainable economic development. According to the ICJ, the other UNFCCC provisions are to be interpreted and applied in accordance with this overriding objective.
      
      States parties’ obligations under the UNFCCC include:
      
      i. Mitigation: The Court confirmed that Article 4 of the UNFCCC imposes a binding obligation on States to develop, communicate and implement measures to mitigate the impact of GHG emissions, and that they must cooperate with other States and parties in doing so.
      
      ii. Adaptation: The UNFCCC places a binding obligation on States to prepare to adapt to the impacts of climate change and to assist developing parties that are particularly vulnerable to such effects in meeting the costs of doing so. Although States have some discretion as to how they implement these commitments, they remain binding.
      
      iii. Co-operation and assistance: States party to the UNFCCC co-operate with other States to protect the environment. This is an obligation of conduct to be assessed against a standard of due diligence.
   2. Kyoto Protocol
      
      Despite no commitment targets being renewed in 2020, the ICJ clarified that the Kyto Protocol retains its legal effect insofar as it provides “(i) interpretative aids for the identification of obligations under the climate change treaty framework and (ii) substantive provisions to assess the compliance of Annex I parties in Annex B of the Protocol with applicable emission reduction targets during the relevant commitment period.” Accordingly, the Court confirmed that a State’s failure to meet the emission reduction commitments agreed under the Protocol may constitute an internationally wrongful act.
   3. Paris Agreement
      
      The Court confirmed several important obligations under the Paris Agreement, namely that the 1.5°C threshold is the parties’ agreed primary temperature goal for limiting the global average temperature increase and that the UNFCCC’s overall objective constitutes the object and purpose of the Paris Agreement, with the temperature goal defined therein providing a means for achieving this object and purpose.

   i. Mitigation: The Court considered that Article 4 establishes a binding obligation to prepare, communicate and maintain successive national determined contributions (NDCs), to account for them and to register them. Further, it clarified the content of NDCs must reflect a State’s “highest possible ambition” capable of “making an adequate contribution” towards limiting global warming to 1.5°C above pre-industrial levels. Owing to the seriousness of the threat posed by climate change, stringent standards of due diligence apply to the preparation and communication of NDCs. Finally, it clarified that the obligation to pursue domestic mitigation measures to meet the objectives of successive NDCs is an obligation of conduct. This means that a State’s compliance with this obligation is to be assessed based on whether it exercised due diligence in taking domestic mitigation measures, including in regulating the activities of third parties. It is not an obligation of result.

   ii. Adaptation: Similarly to the UNFCCC, the Paris Agreement places binding adaptation obligations on States. The fulfilment of these adaptation obligations is to be assessed against a standard of due diligence and supplement the mitigation obligations in preventing and reducing the harmful consequences of climate change.

   iii. Co-operation and assistance: The Court noted that whilst States are free to select the means of co-operation, this is subject to the obligation of good faith and due diligence. Further, the Paris Agreement prescribes the principal forms of cooperation for Developed States to assist Developing States via the provision of financial assistance, technology transfer and capacity building.

2. United Nations Convention on the Law of the Sea:
   
   The Court made the following important clarifications regarding States’ obligations under the UN Convention on the Law of the Sea (UNCLOS) in relation to the protection of the climate system:
   
   • Pollution and the marine environment under UNCLOS: The Court confirmed that anthropogenic GHG emissions constitute pollution of the marine environment within the meaning of UNCLOS. State parties have a positive obligation to take all necessary measures to prevent, reduce and control marine pollution. These obligations are subject to a stringent due diligence standard.
   
   • Sea level rise: The Court considered that UNCLOS does not require State parties to update their charts, or lists of geographical coordinates that show the baselines and outer limit lines of their maritime zones resulting from climate-change related sea level rise. In the Court’s view, the disappearance of an established State’s territory would not necessarily result in the loss of statehood.
3. Environmental Treaties
   
   States which are parties to the Vienna Convention for the Protection of the Ozone Layer, the Montreal Protocol on Substances that Deplete the Ozone Layer and its Kigali Amendment, the Convention on Biological Diversity and the UN Convention to Combat Desertification are required to take measures which mitigate the harms of anthropogenic GHG emissions. These environmental treaties establish complementary obligations in addition to their obligations under climate change treaties and under customary international law which must be complied with in good faith.
4. Customary International Law
   
   Perhaps the most significant clarification from the ICJ was that, whilst obligations under climate change treaties and customary obligations give rise to a single set of compatible obligations which inform each other, they establish independent obligations that do not necessarily overlap. This means that States cannot comply with their customary obligations by fulfilling their obligations under climate change treaties. Conversely, States cannot escape their customary obligations by withdrawing from climate change treaties – the Court made it expressly clear that customary obligations are the same for all States and apply regardless of whether a State is party to the climate change treaties.
   
   The two customary international law obligations considered by the Court include:
   
   • Duty to prevent significant harm: States are obliged to prevent activities within their jurisdiction from causing significant harm to the climate and other parts of the environment, in a manner proportionate to their capacity to reduce such harms. States must fulfil this duty by acting with due diligence.
   
   • Duty to co-operate: States are obliged to cooperate for the protection of the environment also in a manner proportionate to their capacity, recognising this duty is the foundation of meaningful international efforts with respect to climate change.
5. International Human Rights Law
   
   The Court also considered the interdependence between human rights and the protection of the environment, namely that the full enjoyment of human rights such as the right to life, the right to health and the right to an adequate standard of living, cannot be ensured without safeguarding the climate. Therefore, the Court found that the protection of the right to a clean, healthy and sustainable environment constitutes a human right under international law, essential for the enjoyment of many other human rights.

Legal Consequences for State Breaches of International Law

The ICJ was clear that States must act in accordance with the agreements to which they are parties or face the legal consequence for their acts or omissions for failure to comply with their obligations, which may include:

a. the cessation of wrongful actions or omissions;
b. provision of assurances and guarantees of non-repetition of wrongful actions or omissions; and/or
c. full reparations from offending States to injured States.

1. Attribution
   
   The ICJ confirmed that it is possible to attribute actions or omissions to a State in the context of climate change, a necessary element to establish State liability under international law. The Court noted, for example, that the failure of a State to take appropriate action to protect the climate system from GHG emissions may constitute an internationally wrongful act which is attributable to that State. In this context, the wrongful act is not the emission of GHGs, but the breach of the obligations on the State to protect the climate system from the harmful effects of anthropogenic emissions.
   
   The Court was clear that the cumulative impact of GHG emissions on climate systems over an extended period did not impact the legal principle of attribution: the rules on State responsibility are capable of addressing situations where multiple States have engaged in wrongful conduct. In reaching this conclusion, the ICJ also considered up to date climate science at a high level to conclude that, practically, “it is scientifically possible to determine each State’s total contribution to global emissions”.
   
   The Court also clarified that emissions from private actors can be attributed to States on the basis that the obligation on States to protect the climate system includes to act with due diligence in relation to the activities of private actors. This includes, for example, emissions caused by failure to implement sufficient regulatory and legislative standards to limit private sector emissions.
2. Causation
   
   The ICJ confirmed that causation of damage is not required to establish liability for damage caused by GHG emissions but is necessary in determining reparations. In this context, the Court:
   1. confirmed that the existing standard of causation which requires the existence of “a sufficiently direct and certain causal nexus between the wrongful act...and the injury” is sufficiently flexible to apply in the context of damage caused by States in respect of climate change; and
   2. further broke the standard into two elements:
      
      i. whether a climate event can be attributed to climate change. The Court stated that this element could be established by science and that “the scientific evidence adduced in these proceedings establishes that significant harm to the climate system and other part of the environment has been caused as a result of anthropogenic GHG emissions”; and
      
      ii. to what extent the damage caused by climate change can be attributed to a particular State or a group of States.

Key Takeaways

This is an opinion which lends significant weight to cases by States suffering acutely from the effects of climate change. These States can rely on the legal framework at the nation-State level to seek reparations from other States for damage caused by climate change. The ICJ’s opinion makes States expressly responsible for combatting climate change proportionately to their capabilities. Economically developed States which have contributed the most to GHG emissions may seek to balance adhering to international agreements and laws while also ensuring businesses who are major contributors to GHG emissions remain competitive. The call for cooperation in good faith may prompt for consistent regulation across affected countries.

The ICJ has made clear that States must take active steps towards fulfilling their obligations or risk the legal consequences of inaction, and such obligations are not confined to treaties and can be found under other sources of international law. Withdrawal from agreements offers no safeguard against accountability at the international level.

However, the opinion gives little guidance about how the framework that it has established should be applied in practice. For example, questions remain unanswered about how to attribute liability in respect of GHG emissions and how to evidence causation by a particular State. Despite relying on various bodies of international law, including human rights law and environmental law, the Court also did not explain how these obligations should interact. The practical implications of the opinion, and how it will be applied in each case, therefore remain unclear.

The ICJ’s advisory opinion dated 23 July 2025 is accessible here.