On 23 July 2025, the International Court of Justice ("ICJ") gave an advisory opinion at the request of the United Nations General Assembly. The opinion explains what States must do under international law to protect the climate and environment from greenhouse gas ("GHG") emissions, and what happens if they fail. The opinion brings together treaty law, customary law, and human rights law, and marks a major step in international environmental law.
- The Context and Scope of the Advisory Opinion
The ICJ responded to General Assembly resolution 77/276, which asked:
- What are States' obligations under international law to protect the climate and environment from GHG emissions for current and future generations?
- What are the legal consequences for States whose actions or failures cause significant harm, especially to vulnerable States and people? The Court said the climate crisis is a global, existential problem. It confirmed its jurisdiction and said the questions were legal, requiring it to interpret all relevant international law.
- Applicable Law
The ICJ used a broad legal framework, including the UN Charter, UNFCCC, Kyoto Protocol, Paris Agreement, UNCLOS, Ozone and Biodiversity Conventions, customary international law, and human rights law.1 The Court said climate treaties do not exclude other international law rules: "the principle of lex specialis does not lead to a general exclusion by the climate change treaties of other rules of international law".2
- Treaty Obligations: The UNFCCC, Kyoto Protocol, and Paris Agreement
The ICJ explained the main climate treaties. The UNFCCC aims to stabilise GHGs.3 The Kyoto Protocol sets emission targets for developed States. The Paris Agreement requires all parties to set "nationally determined contributions" ("NDCs"). The Court said these treaties create binding obligations: "the climate change treaties set forth binding obligations for States parties to ensure the protection of the climate system and other parts of the environment from anthropogenic greenhouse gas emissions".4 The Paris Agreement's temperature goal (well below 2°C, aiming for 1.5°C) is the main target.5 The obligation to prepare, communicate, and maintain NDCs is not just procedural: "All NDCs prepared, communicated and maintained by parties under the Paris Agreement must, when taken together, be capable of realizing the objectives of the Agreement which are set out in Article 2".6 The standard of due diligence is "stringent".7
- Customary International Law
The Court said States must, under customary law, prevent significant harm to the environment and co-operate in good faith. This duty covers global issues like climate change. "The existence of the general obligation of States to ensure that activities within their jurisdiction and control respect the environment of other States or of areas beyond national control is now part of the corpus of international law relating to the environment".8 The due diligence standard is "stringent". States must "use all means at their disposal to prevent activities carried out within their jurisdiction or control from causing significant harm to the climate system and other parts of the environment, in accordance with their common but differentiated responsibilities and respective capabilities".9 Due diligence includes making rules, enforcing them, doing environmental impact assessments, and following the precautionary principle. The duty to co-operate means States must work together continuously to prevent harm.
- The Interdependence of Environmental Protection and Human Rights
The opinion recognises that protecting the environment is necessary for enjoying human rights. "the protection of the environment is a precondition for the enjoyment of human rights, whose promotion is one of the purposes of the United Nations as set out in Article 1, paragraph 3, of the Charter".10 Climate change harms rights like life, health, and an adequate standard of living. States must take steps to protect the climate and environment to ensure human rights, including mitigation, adaptation, and regulation.11
The ICJ, for instance, dedicates only a single, brief paragraph to this pressing matter,12 despite its profound implications for millions of people forced to flee their homes due to climate change. The ICJ's recognition of climate displacement under the principle of non-refoulement is nonetheless significant. In its opinion, the Court affirms that states must not return individuals to situations where there are substantial grounds to believe there is a real risk of irreparable harm to their right to life, as protected by Article 6 of the International Covenant on Civil and Political Rights. This position draws directly from the Human Rights Committee's decision in Teitiota v. New Zealand,13 which established that climate change can create conditions that trigger non-return obligations under international law. By adopting a "real risk" standard rather than requiring imminence of harm, the ICJ clarifies a key ambiguity in international protection law. This clarification is crucial, as misunderstanding the threshold for protection has previously led to restrictive interpretations and limited the scope for legal protection of those displaced by climate impacts.
Despite this progress, the ICJ's opinion is marked by notable omissions and a cautious tone. The Court fails to engage with the more comprehensive and differentiated approach to climate displacement adopted by the Inter-American Court of Human Rights,14 which explicitly requires states to establish effective legal and administrative mechanisms for the protection of cross-border displaced persons. The ICJ also does not address the specific vulnerabilities of certain groups, such as children or those facing involuntary immobility, nor does it fully acknowledge the present and ongoing reality of climate-induced displacement as highlighted by the IPCC and the UN General Assembly.
- The Law of the Sea and Other Environmental Treaties
The ICJ said States under UNCLOS must protect the marine environment from climate change and co-operate in good faith.15 This includes environmental impact assessments for activities that may harm the marine environment. Other treaties, like the Ozone and Biodiversity Conventions, also create obligations to protect the climate.
- Legal Consequences of Breach: State Responsibility and Remedies
The Court applied the rules of State responsibility. If a State breaches its obligations, it commits an internationally wrongful act.16 Legal consequences include:
- Duty of performance: States must still perform their obligations even after a breach.17
- Duty of cessation and guarantees of non-repetition: States must stop wrongful acts and prevent them from happening again.18
- Duty to make reparation: States must make full reparation for damage, including restitution, compensation, and satisfaction, if there is a direct and certain link between the act and the injury.19 The Court recognised that climate harm is complex, but the rules on State responsibility can address situations with many responsible or injured States.
- Obligations Erga Omnes and the Collective Interest
The opinion says climate protection obligations are Erga Omnes owed to the whole international community. "States' obligations pertaining to the protection of the climate system and other parts of the environment from anthropogenic GHG emissions, in particular the obligation to prevent significant transboundary harm under customary international law, are obligations Erga Omnes".20 Any State can invoke responsibility for breaches.
- Comparison with Major Jurisdictions and Pre-existing Laws
The opinion builds on existing law but goes further. Before, the main sources were the UNFCCC, Kyoto Protocol, and Paris Agreement, but the binding nature of these, especially the Paris Agreement's NDCs, was debated. In the EU, courts have started recognising climate obligations as justiciable. The European Court of Human Rights in 2024 found that inadequate climate action can violate the right to private and family life.21 The Inter-American Court of Human Rights in 2025 recognised the right to a healthy environment.22 The ICJ's opinion supports these trends but goes further by unifying treaty, customary, and human rights law, and clarifying the high standard of due diligence. The recognition of obligations Erga Omnes and the link between environmental protection and human rights are major advances.
- Transformative Changes and Implications
The opinion brings several key changes:
- Clarification of Binding Obligations: The opinion confirms that treaty and customary law obligations are binding, and States must take effective action with a high standard of due diligence.
- Integration of Human Rights and Environmental Law: The opinion gives a strong legal basis for individuals and groups to claim protection from climate harms under human rights law.
- Obligations Erga Omnes: All States can hold each other accountable, opening the door to more international litigation and enforcement.
- Stringent Due Diligence and Limited Discretion: States do not have full discretion in setting NDCs or choosing measures. NDCs must be able to achieve the Paris Agreement's temperature goal, and States must aim for their highest possible ambition.
- Remedies and Reparation: The opinion confirms that cessation, guarantees of non-repetition, and full reparation, including compensation, are available where there is a direct and certain link to the damage.
- Conclusion
The ICJ's opinion clarifies how treaty and customary law obligations work together in climate change. "compliance in full and in good faith by a State with the climate change treaties, as interpreted by the Court... suggests that this State substantially complies with the general customary duties to prevent significant environmental harm and to co-operate"23 but this is a rebuttable presumption, not a blanket rule. Treaty compliance does not shield States from broader customary obligations. The Court also addressed non-party States. If a non-party State "co-operates with the community of States parties to the three climate change treaties in a way that is equivalent to that of a State party, may, in certain instances, be considered to fulfil its customary obligations", but those who do not must show their policies meet customary law. This approach encourages all States to align with treaty norms and ensures that customary law remains enforceable for all.
Footnotes
1 ICJ advisory, para. 172.
2 ICJ advisory, para. 171.
3 UNFCCC, Article 2; ICJ advisory, para. 197.
4 ICJ advisory, para. 457(3)(A).
5 ICJ advisory, para. 224.
6 ICJ advisory, para. 249.
7 ICJ advisory, para. 246.
8 ICJ advisory, para. 134.
9 ICJ advisory, para. 457(3)(B)(a).
10 ICJ advisory, para. 373.
11 ICJ advisory, para. 409.
12 ICJ advisory, para. 378.
13 Teitiota v New Zealand, UNHRC, Comm No 2278/2016, UN Doc CCPR/C/127/D/2728/2016 (2019).
14 IACtHR Advisory Opinion OC-32/25.
15 ICJ advisory, para. 457(3)(D).
16 ICJ advisory, para. 457(4).
17 ICJ advisory, para. 446.
18 ICJ advisory, para. 447.
19 ICJ advisory, para. 457(4)(c).
20 ICJ advisory, para. 440.
21 Verein KlimaSeniorinnen Schweiz and Others v. Switzerland, App. No(s). 53600/20.
22 Inter-American Court of Human Rights, in its 2025 advisory opinion (OC-32/25).
23 ICJ advisory, para. 314.
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