On 23 July 2025, the International Court of Justice (ICJ) delivered its long-awaited Advisory Opinion Obligations of States in Respect of Climate Change.[1] The opinion is widely described as a “landmark”[2] because it does two things at once: (i) it clarifies what international law already requires of States on climate change, across multiple legal regimes, and (ii) it explains how State responsibility and remedies can operate when those obligations are breached.
UN’s Request
In its resolution 77/276 dated 29 March 2023, the UN General Assembly asked the Court to identify the following:
- States’ obligations under international law to protect the climate system from anthropogenic greenhouse gas emissions; and
- The legal consequences when States breach those obligations.[3]
ICJ advisory opinions are not binding judgments, but they are highly authoritative statements of international law and are regularly relied on by governments, international organisations, and domestic and international courts. The Court’s opinion is therefore influential not because it creates new obligations, but because it clarifies how many climate-related duties are already legally binding under treaties, customary international law, the law of the sea, and international human rights law.
Key Substantive Duties Clarified by the ICJ
Duty to Prevent Significant Harm Applies to Climate Change
The Court confirms that the customary obligation to prevent significant environmental harm extends to harm to the climate system, not just classic bilateral transboundary scenarios. In other words, the Court confirms that climate change falls within the scope of the customary prevention obligation, even though it concerns a shared, global environmental interest.[4]
The Prevention Obligation Is One of Due Diligence
The ICJ frames prevention as an obligation of conduct, not a guarantee of results. States must exercise due diligence using “all means reasonably available” to prevent significant harm.[5] In the climate context, the Court indicates this standard is demanding, requiring sustained vigilance and action commensurate with the risk. [6]
Appropriate Measures
Due diligence translates into concrete substantive requirements. States must adopt and enforce adequate regulatory and policy measures, including mitigation and adaptation measures that are science-based and capable, in practice, of delivering deep, rapid, and sustained emissions reductions and effective climate resilience. Such measures may include adopting a national legal framework, administrative procedures, and enforcement mechanisms.[7] The Court also stresses that regulation must extend to private actors within a State’s jurisdiction.[8]
Customary Duty to Co-operate
The Court accepts that the level of due diligence can vary depending on capabilities and national circumstances, and that expectations can increase over time.[9] The ICJ further recalls that States have a customary obligation to co-operate in addressing global environmental problems. It stresses that this obligation is especially important for climate change, given its collective-action nature. As the ICJ points out, “co-operation is not a matter of choice for States but a pressing need and a legal obligation”.[10] In practice, this co-operation includes financial support, technology transfer, and capacity-building.[11]
Paris Agreement Threshold
The Court emphasises that the 1.5°C threshold in Article 2 of the Paris Agreement is not merely aspirational, but the agreed primary temperature goal against which the adequacy of Parties’ mitigation efforts must be assessed. It recalls that Parties have agreed to keep warming well below 2°C while pursuing efforts to limit it to 1.5°C.[12] The Court notes that 1.5°C has become the scientifically grounded “consensus target” and primary temperature goal under the Agreement.[13]
The Court further explains that while Parties retain discretion in determining their nationally determined contributions (NDCs), that discretion is constrained by due diligence and best efforts. In particular, NDCs must be formulated and implemented through domestic frameworks capable, in aggregate, of realising the Paris Agreement’s objectives, including limiting global warming to 1.5°C.[14] Referring to the Intergovernmental Panel on Climate Change (IPCC), the Court underscores that every increment of warming increases harm,[15] and that limiting global warming to 1.5°C requires deep, rapid and sustained emissions reductions and a swift transition toward net zero.[16]
The Court says that while Article 7(9) of the Paris Agreement does not mandate any single adaptation measure, it sets out the types of steps Parties can take. It adds that compliance is judged by due diligence, requiring Parties to adopt science-based measures capable of enhancing adaptive capacity, strengthening resilience, and reducing vulnerability, with examples including ecosystem restoration, early warning systems, resilient infrastructure, and other practical risk-reduction measures.[17]
Consequences of a Breach
The Court makes clear that where a State breaches its international climate-related obligations, the entire panoply of legal consequences under the law of State responsibility may follow.[18]
These consequences include duties of cessation and guarantee of non-repetition, regardless of whether harm is established. They also include the duty to make full reparation, through restitution, compensation and/or satisfaction,[19] depending on what is required to wipe out the consequences of the wrongful act.[20] However, a breach does not extinguish the responsible State’s continuing duty to perform the obligation it violated.[21]
At the same time, the Court does not allocate responsibility or quantify remedies in the abstract. Instead, it emphasises that any award of compensation depends on demonstrating a “sufficiently direct and certain causal nexus” between the wrongful act and the harm claimed.[22]
Conclusion
The ICJ’s 23 July 2025 advisory opinion marks a practical landmark by translating climate ambition into justiciable legal standards. States’ obligations under the Paris Agreement and customary international law are assessed against a due-diligence standard, requiring concrete mitigation and adaptation measures to prevent significant harm to the climate system. The Court also treats the 1.5°C threshold as the benchmark for assessing the adequacy of nationally determined contributions and clarifies the legal framework governing breach, State responsibility, and remedies. In doing so, it significantly narrows the scope for “policy discretion” arguments and strengthens pathways to accountability and climate-related negotiations. Against this evolving legal landscape, Aceris Law provides pro bono public international law advice to developing States addressing complex issues of climate change, State responsibility, and international adjudication.
[1] ICJ, Press Release dated 23 July 2025.
[2] See, e.g., IISD, Historic International Court of Justice Opinion Confirms States’ Climate Obligations (28 July 2025), available at: https://www.iisd.org/articles/deep-dive/icj-advisory-opinion-climate-change.
[3] Resolution 77/276 adopted by the General Assembly on 29 March 2023, p. 3.
[4] ICJ Advisory Opinion dated 23 July 2025, para. 139.
[5] ICJ Advisory Opinion dated 23 July 2025, para. 135.
[6] ICJ Advisory Opinion dated 23 July 2025, paras. 136-138.
[7] ICJ Advisory Opinion dated 23 July 2025, paras. 250-253, 282.
[8] ICJ Advisory Opinion dated 23 July 2025, para. 282.
[9] ICJ Advisory Opinion dated 23 July 2025, para. 226.
[10] ICJ Advisory Opinion dated 23 July 2025, paras. 301-308; see also paras. 140-142.
[11] ICJ Advisory Opinion dated 23 July 2025, para. 227.
[12] ICJ Advisory Opinion dated 23 July 2025, para. 223.
[13] ICJ Advisory Opinion dated 23 July 2025, paras. 224, 242.
[14] ICJ Advisory Opinion dated 23 July 2025, para. 245.
[15] ICJ Advisory Opinion dated 23 July 2025, paras. 82-83.
[16] ICJ Advisory Opinion dated 23 July 2025, para. 243.
[17] ICJ Advisory Opinion dated 23 July 2025, paras. 257-258.
[18] ICJ Advisory Opinion dated 23 July 2025, para. 445.
[19] ICJ Advisory Opinion dated 23 July 2025, para. 445.
[20] ICJ Advisory Opinion dated 23 July 2025, paras. 450-455.
[21] ICJ Advisory Opinion dated 23 July 2025, para. 445.
[22] ICJ Advisory Opinion dated 23 July 2025, para. 452, 457.