On 23 July 2025, the International Court of Justice (ICJ) issued its landmark Advisory Opinion on Obligations of States in Respect of Climate Change, a defining moment in the legal response to the climate crisis.
Requested by the UN General Assembly at the initiative of Vanuatu (resolution 77/276), the Opinion follows recent advisory opinions by the International Tribunal for the Law of the Sea (ITLOS) and the Inter-American Court of Human Rights (IACtHR) and drew historic engagement, with 91 written submissions and 54 comments from States and international organisations.
The Court was asked to clarify:
1. What are the obligations of States under international law to ensure the protection of the climate system for present and future generations?
2. What are the legal consequences for States that have caused significant harm through their acts or omissions?
The ICJ responded with a unanimous Opinion that articulates a comprehensive legal framework rooted in treaty law, customary international law, and human rights law. Withers' Public International Law team participated in the ICJ and IACtHR proceedings and brings extensive experience advising States on international environmental law.
States' legal obligations
Climate Treaties
The ICJ confirmed that the United Nations Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, and Paris Agreement form the core treaty framework governing State obligations on climate change, which include:
- Stringent binding obligations: The climate treaties impose stringent binding legal obligations to mitigate GHG emissions, adapt to climate change, and cooperate internationally.
- Progressive targets: States are required to submit Nationally Determined Contributions (NDCs) that become progressively more ambitious over time, consistent with the goal of limiting global warming to 1.5°C.
- Interpretative principles: These treaties are interpreted in light of the foundational principles set out in Article 3 of the UNFCCC, including common but differentiated responsibilities and respective capabilities, the precautionary approach, sustainable development, equity, and intergenerational equity.
These principles are legally relevant and guide the interpretation and implementation of treaty obligations. The Court also held that decisions adopted by Conferences of the Parties (COPs) may carry binding or interpretive weight, depending on the terms of the treaty.
Customary International Law
The Court identified three foundational duties that apply to all States under customary international law:
- Duty to prevent significant harm: This duty is triggered not only by actual harm but also by the risk of significant transboundary environmental harm. The Court found that the accumulation of GHG emissions meets this threshold.
- Duty of due diligence: States must take all necessary legislative, regulatory, administrative, and enforcement measures—using all means at their disposal—to prevent harm. This includes regulating both public and private actors through effective monitoring and enforcement mechanisms. States must adopt rules designed to achieve the 'deep, rapid, and sustained reductions' in GHG emissions required to prevent significant harm.
- Duty to cooperate: The Court affirmed that cooperation is not optional but a binding legal obligation. States must engage in good faith to achieve mitigation goals, share information and technologies, and coordinate actions across borders.
Human Rights Law
The Court recognised that climate change can impair the enjoyment of fundamental human rights, including:
- The right to life;
- The right to health;
- The right to an adequate standard of living;
- The right to privacy, family life, and home;
- The right to a clean, healthy, and sustainable environment.
States must avoid discriminatory impacts, particularly on women, children, and Indigenous Peoples. The Court also confirmed that, in situations where individuals face a real risk of irreparable climate-related harm upon return to their country of origin, the principle of non-refoulement applies. The Court declined to provide a definitive view on the extraterritorial application of human rights obligations, noting that such application depends on the terms of the relevant treaty and specific factual circumstances.
Law of the Sea and Statehood
The ICJ aligned its reasoning with that of the ITLOS and recognised the grave implications of sea-level rise for small island States and low-lying coastal nations. It held that loss of territory due to climate impacts does not necessarily result in loss of statehood. The Court stressed the legal obligation of all States to cooperate in achieving equitable outcomes and preserving maritime rights and sovereignty in accordance with the UN Convention on the Law of the Sea (UNCLOS).
Armed Conflict and Military Activities
Although the Court did not address armed conflict explicitly in its Opinion, Judge Sarah Cleveland, in her separate Declaration, emphasised that State obligations to assess, report and mitigate GHG emissions extend to armed conflict and other military activities. Failing to account for such emissions undermines compliance with international obligations to protect the climate system.
Legal consequences and accountability
While the ICJ did not determine the responsibility of specific States, it confirmed that the general framework under customary international law applies to assess legal consequences as follows:
- Attribution: A State may be held responsible for failing to comply with its climate obligations, including failing to regulate private entities under its jurisdiction that contribute significantly to GHG emissions.
- Causation: The Court confirmed that a 'sufficiently direct and certain causal nexus' must exist between the wrongful act and the injury. This standard is appropriately flexible to accommodate the cumulative and diffuse nature of climate change.
- Erga omnes obligations: Certain obligations—such as those under the UNFCCC and Paris Agreement—are owed to the international community as a whole. A non-injured State may invoke responsibility for cessation and guarantees of non-repetition but cannot claim reparations for itself.
- Forms of reparation: Breaches may give rise to the full range of consequences under the law of State responsibility, including cessation, restitution, compensation, satisfaction, and assurances of non-repetition.
Conclusion
The ICJ's Advisory Opinion affirms that States have binding legal obligations under climate treaties, customary international law, and human rights law. These obligations apply at all times—including during armed conflict—and extend to the conduct of private actors. Together with recent opinions from ITLOS and the IACtHR, the decision marks a significant advance in the legal architecture for climate accountability.
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