Introduction
On July 23, 2025, the International Court of Justice ("ICJ") released a highly anticipated advisory opinion1 addressing the obligations of States concerning climate change under international law (the "Advisory Opinion"). This significant decision is the result of a referral from the United Nations General Assembly , which requested the ICJ's advisory opinion on two specific questions:
- What are the obligations of States under international law to protect the climate and environment from anthropogenic greenhouse gasses for both other States and present and future generations; and
- What are the legal consequences under such obligations for States that have, through their acts or omissions, caused significant harm to the climate and environment, with respect to other States and peoples and individuals affected by climate change?
While the Advisory Opinion is not itself legally binding, the ICJ's conclusions are authoritative statements regarding the content of international law. In that light, in the Canadian context the Advisory Opinion is relevant to the various stakeholders (including government organs, private parties, or NGOs) that determine or to seek to influence law and policy in areas related to climate change.
Key Takeaways from the Advisory Opinion
State obligations in respect of Climate Change
The ICJ concluded that States have various obligations to protect the climate and environment from anthropogenic greenhouse gas emissions. These obligations can be owed to other States or to peoples or individuals.
The sources of these obligations are varied. They include key climate-related international instruments (such as the UN Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol and the Paris Agreement, among others), which impose obligations such as:
- implementing mitigations with respect to the presence of greenhouse gasses in the atmosphere
- adapting State practices in order to moderate harm or exploit opportunities in light of climate change
- cooperation between States, such as with respect to information and technology-sharing
Beyond the specific obligations found in international instruments, the ICJ also determined that State obligations with respect to climate change also arise from customary international law. This finding is significant because it means that States have international law obligations regarding climate change other than what they have expressly agreed to in treaties that they have signed and ratified. In particular, the ICJ found two obligations arise under customary international law:
- The duty to prevent significant harm to the environment ('duty to prevent'): This is an obligation to act with due diligence that itself consists of several components, such as "precautionary measures, which take account of scientific and technological information, as well as relevant rules and international standards, and which vary depending on each State's respective capabilities".
- The duty to cooperate for the protection of the environment ('duty to cooperate'): This requires States to work together to prevent significant climate and environmental harm, on the basis that uncoordinated efforts may not garner useful results.
The ICJ noted that other environment-related treaties and international human rights law also define States' climate change obligations.
Additionally, the ICJ concluded that principles such as sustainable development, common but differentiated responsibilities and respective capabilities, equity, intergenerational equity, and the precautionary approach must be considered when determining the applicable law.
The Advisory Opinion also addresses the nature of certain obligations as erga omnes (which refers to obligations that are not owed as between specific parties, but to the international community as a whole).
Consequences of a Breach of State Obligations
On the second question, the ICJ observed that breaches by a State of its climate and environment obligations "may give rise to the entire panoply of legal consequence provided for under the law of State responsibility." Those consequences include having to:
- perform the relevant obligation, notwithstanding the breach;
- immediately cease any ongoing breach and implement measures to prevent any repeat of the breach; and
- provide reparation for harm caused. Reparations may be accomplished by: (i) restitution – that is, restoring the situation to its original state prior to the wrongful act (although the ICJ acknowledged this "may prove difficult or unfeasible in the case of environmental harm"); (ii) offering financial compensation; or (iii) undertaking other appropriate actions, such as issuing an apology or making a public statement.
Since issues of both legal and factual attribution and causation will be critical to the determination of any specific case, the ICJ sought to address these in its analysis of the legal consequences of a breach. While acknowledging the complexity of such considerations, the ICJ concluded that they do not, in principle, exclude the possibility of establishing State responsibility within the context of climate change.
Conclusion
The Advisory Opinion may influence government actions related to the climate and environment across multiple sectors. With its impact potentially extending to policies, laws and executive decisions in areas like energy, infrastructure, and transportation, the Advisory Opinion raises issues relevant to all stakeholders in these sectors, including public, private, and non-governmental groups.
Given the nature of the Advisory Opinion, its contents are also likely to be raised by various stakeholders in the course of litigation, whether before domestic or international courts and tribunals. As States and other stakeholders begin to operationalize the ICJ's guidance, each application of the Advisory Opinion will help clarify key issues that were not fully addressed by the ICJ—such as attribution, causation, and the specific thresholds for harm, and may also set precedent regarding the scope and limits of climate-related obligations.
Footnote
1. A summary of the Advisory Opinion is also available at: https://www.icj-cij.org/sites/default/files/case-related/187/187-20250723-pre-01-00-en.pdf
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