On 23 July 2025, the International Court of Justice ("ICJ") delivered its long-awaited Advisory Opinion on Climate Change. The Court noted that climate change—driven primarily by anthropogenic greenhouse gas ("GHG") emissions—poses grave risks to humanity and the environment, and opined that States are subject to binding obligations under international treaties (e.g. the UNFCCC, Kyoto Protocol, and Paris Agreement), customary international law, and human rights law to mitigate climate change, cooperate and implement adaptation measures. This Advisory Opinion represents a milestone in international law, where climate change takes the spotlight. While it has direct implications for States, in recognising their regulatory role as part of discharging their international law obligations, it also has ramifications for private actors across all sectors, both as the subject of such regulation and in an increased risk of climate-change related litigation.
Background
On 29 March 2023, the UN General Assembly issued Resolution 77/276 requesting the ICJ to deliver an Advisory Opinion under Article 65(1) of the ICJ Statute regarding: (a) States' legal obligations under international law to protect the climate system; and (b) the consequences of failing these obligations with respect to other States (in particular, small island and developing States) and people of the present and future generations. Following an unprecedented level of participation from States and international organisations, the ICJ delivered its opinion on 23 July 2025.
Obligations of States regarding Climate Change
Sources of Obligations of States
The ICJ considered that the international legal framework relevant to climate change consists of treaty law such as the UN Charter, the UNFCCC, the Kyoto Protocol, the Paris Agreement, UNCLOS, and certain other environmental and human rights treaties. It noted that, under international law, the human right to a clean, healthy and sustainable environment is essential for the enjoyment of other human rights. It further identified certain customary international law duties - specifically, the duty to prevent environmental harm and the duty to cooperate - and highlighted guiding principles such as sustainable development, equity, intergenerational equity, the precautionary principle, and common but differentiated responsibilities, which also form part of the relevant legal framework on climate change. The Court concluded that these sources collectively form the applicable law for addressing climate change, without any one set of rules excluding the others.
Substantive Obligations of States
The ICJ characterised climate change obligations by their normative function: mitigation, adaptation or cooperation.
Mitigation Obligations
Mitigation obligations require States to reduce GHG emissions and enhance carbon sinks. The ICJ emphasised that these obligations are central to the climate regime, and their content has become increasingly detailed across the UNFCCC, Kyoto Protocol, and Paris Agreement. However, their design and scope vary from treaty to treaty. For instance, under the UNFCCC, all parties are required to develop and periodically update national inventories of GHG emissions and formulate mitigation programs (Article 4(1)). These are a combination of obligations of result (e.g., reporting) and obligations of conduct (e.g., best efforts in technology cooperation).
Adaptation Obligations
Adaptation obligations require States to take proactive measures to reduce vulnerability to the adverse effects of climate change and to enhance resilience. These obligations are recognised as legally binding under both the UNFCCC and the Paris Agreement.
Obligations of Cooperation
The duty to cooperate is a fundamental principle of international environmental law and is reflected across all major climate instruments. The ICJ reaffirmed that this duty exists under both customary international law and treaty law.
Erga Omnes Character of Climate Obligations
The ICJ considered that obligations related to the protection of the climate system - particularly the duty to prevent significant transboundary harm- are obligations erga omnes, owed to the international community as a whole. This reflects the common interest of all States in safeguarding global environmental commons such as the atmosphere and the high seas.
In the treaty context, the UNFCCC and Paris Agreement recognise climate change as a "common concern of humankind", requiring a global response. Accordingly, the obligations under these treaties are erga omnes partes, meaning that any State party has a legal interest in their fulfilment and may invoke the responsibility of another State for their breach.
This may be viewed as expanding the scope for legal standing in climate-related disputes, allowing States to act not only in defence of their own interests but also in protection of the global climate system.
State Responsibility in the Climate Change Context
The ICJ found that the customary rules on State responsibility are applicable in the context of climate change, notwithstanding its complexity, and are capable of addressing the legal consequences of breaches of international obligations related to the protection of the climate system.
Attribution of Conduct
It is well-established under international law that the conduct of any State organ - including legislative, executive, and administrative bodies - is attributable to the State. Notably, the ICJ stated that "failure of a State to take appropriate action to protect the climate from GHG emissions - including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies - may constitute an internationally wrongful act which is attributable to that State".
The internationally wrongful act would not be the emission of GHGs per se, but the breach of obligations - whether treaty-based or customary - pertaining to the protection of the climate system. The Court also underscored that a State "may be responsible where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction".
Causation
Causation is not required to establish responsibility, however, it is essential for determining reparation. The ICJ rejected both the presumption of causation and the argument that causation is impossible due to the diffuse nature of climate change. Instead, the Court upheld the standard of a "sufficiently direct and certain causal nexus" between the wrongful act and the damage suffered. This standard involves a two-step analysis: (i) determining whether a climatic event or trend is linked to anthropogenic climate change (scientific attribution); and (ii) assessing whether the damage can be linked to the conduct of a particular State or group of States (legal attribution).
Legal Consequences of Wrongful Acts
The ICJ considred that every internationally wrongful act entails the international responsibility of the State. In the climate change context, wrongful acts may include, for instance, breaches of treaty obligations (e.g., failure to submit or implement NDCs under the Paris Agreement); breaches of customary law obligations (e.g., failure to regulate emissions or conduct appropriate environmental impact assessments); or breaches of obligations of international human rights law (e.g., failure to respect the human right to a clean and healthy environment).
Notably, the ICJ opined that a State may violate international climate change obligations under customary law without being a signatory of climate treaties. The legal consequences of such breaches include the duty of performance, the duty of cessation and non-repetition and the duty to make reparation. The ICJ affirmed that compensation may be owed where a causal nexus is established between wrongful acts and climate-related harm.
The significance of the Opinion
The ICJ noted that international law has an important but ultimately limited role in resolving climate change. In a particularly resonant final passage, the ICJ moved beyond legal doctrine to underscore the scale and complexity of the climate crisis. The Court observed that:
"A complete solution to this daunting, and self-inflicted, problem requires the contribution of all fields of human knowledge, whether law, science, economics or any other. Above all, a lasting and satisfactory solution requires human will and wisdom — at the individual, social and political levels — to change our habits, comforts and current way of life in order to secure a future for ourselves and those who are yet to come."
While the ICJ's Advisory Opinion on Climate Change is not formally binding, its relevance for international law cannot be ignored. Courts and arbitral tribunals around the world are very likely to refer to it when grappling with climate-related disputes, particularly those involving emissions regulation and related policies. Moreover, in recognising the role of States in regulating private actors as part of discharging their international law obligations, this Advisory Opinion (along with the opinions given by the Inter-American Court of Human Rights and the International Tribunal for the Law of the Sea, discussed here and here, could give significant impetus to States to increase regulation of activities which contribute to climate change; to impose or strengthen due diligence obligations; to incentivise or to compel GHG emissions reduction; and to introduce more significant monitoring and enforcement mechanisms. This changing landscape brings both challenges and opportunities across multiple sectors, in which private actors will need to review proactively their own operations, policies and standards, and anticipate and adapt to climate change-related regulatory changes and increased litigation risk.
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