Authored By: Wade Harris
Deakin University
I Introduction
On 23 July 2025, the International Court of Justice held that States are required to protect the climate system by international law.[1]A difficult legal question arose from this judgement: when cumulative global emissions cause harm to the climate, can any individual State be assigned responsibility under customary international law?
The Court confirmed that States can not fail to exercise due diligence to prevent harm to the environment, cooperate internationally, and protect humanity now and in the future. A connection was also made between climate obligations and pre-existing principles governing breach, causation, reparation, and attribution. The findings highlight a major development; however, fail to impose liability on any State. A contentious claim must establish jurisdiction, proof of breach, and a highly probative causal connection between the harm and conduct of the State.
Thus, an inquiry into State responsibility should be triggered following significant climate harm but should lack the ability to create automatic liability. Where a State materially contributes to significant environmental harm, they should be held accountable. This analysis proceeds by examining the customary no-harm rule, the operation of general principles of State responsibility, and the obstacles future litigation face.
II Advisory Opinion to Accountability
A Customary International Law Recognising a Climate Due-Diligence Obligation
The no-harm rule is the starting point of the customary international law. The rule requires States to ensure that activities in their jurisdiction or control do not lead to significant damage to other States or areas beyond national jurisdiction.[2] The rule developed through internation practice, including the Trail Smelter arbitration and following judicial decisions concerning boundaries to environmental harm.[3]
The International Court’s July 2025 Advisory Opinion applied the principle to climate change directly. In that decision, anthropogenic greenhouse-gas emissions were treated as capable of causing significant harm to the climate system and additional parts of the environment. Prevention was characterised by the Court as an obligation of due diligence, as opposed to an obligation guaranteeing a specific result.[4]
Due diligence would require reasonable measures applied to the surrounding facts known, or ought to have known, to the State. Relevant considerations include the seriousness of the risk, scientific knowledge, regulatory capacity, available technology, and the State’s contribution to the danger. Thus, climate science informs the legal standard, as opposed to supplying background evidence.
This customary obligation has been reinforced by the Paris Agreement and United Nations Framework Convention on Climate Change. Their provisions in respect to mitigation, cooperation, prevention, and climate-system protection implement treaty-based duties partnered with the no-harm rule.[5]The Advisory Opinion also recognised the intersection between climate obligations and international human rights obligations, despite the exact consequences of the intersection remaining legally undetermined.
Catastrophic injury should not be required to constitute “significant harm” before making liability possible. The threshold prevents liability for pure speculative or trivial effects, but climate change presents foreseeable risks when the seriousness of the risks is scientifically established. The failure of a state to regulate major emissions may therefore breach due diligence before an extreme event can be proven.
The importance of the central legal distinction remains the same. Significant harm identifies the scale of environmental harm relevant to the no-harm rule; it does not, of itself, establish a breach. A party that brings a claim against a State must still establish that the State did not adopt reasonable measure in light of the risk.
B State Responsibility Requiring More Than Harm
The Articles on State responsibility provide the framework for translating the Advisory Opinion into liability. A wrongful international act requires conduct attributable to the State and conduct inconsistent with an international obligation binding upon that state.[6]This framework applies equally to emissions directly produced by States and regulatory emissions attributable to public authorities.
Accordingly, climate litigation will focus heavily on breach. A claimant may challenge licensing decisions, fossil-fuel subsidies, production approvals, inadequate mitigation plans, or failures to enforce emissions standards. The relevant question will not merely ask whether the State emitted greenhouse gases. It will ask whether the State took reasonably steps to reduce a known and serious risk.
Climate causation creates the most prominent difficulty. No single state caused climate change independently, and many injuries result from emissions released over decades by multiple public and private actors. However, responsibility should not be defeated due to multiple causes. The Advisory Opinion treated scientific complexity as a problem of proof and legal assessment, rather than as a complete defence.[7]
A sufficiently direct causal connection between the State’s wrongful conduct and the claimed injury would need to be established. That connection may involve the State’s contribution to atmospheric concentrations, the increased probability of a particular event, or the State’s exacerbation of existing climate vulnerability. Central evidentiary tools would be attribution science, emissions inventories, climate modelling, and counterfactual analysis.
The law should reject both extremes. Requiring proof that the State was the sole cause of an entire climate disaster would make the customary obligation practically meaningless. Treating every State contribution as automatically compensable would instead convert due diligence into strict liability.
The better approach recognises contribution without disregarding causation. A State should bear liability where its unlawful conduct materially increased the risk or severity of significant harm. The remedy that follows may reflect the contribution of the State, the nature of the harm, concurrent causes, and the evidentiary limits.
The Court’s environmental jurisprudence supports this approach. In Certain Activities, the Court recognised environmental damage as capable of valuation and compensation, while preserving the requirement that harm result from intentional wrongful conduct.[8]Climate cases can develop reasoning without creating a new theory of absolute liability.
C Contentious Litigation Will Test the Advisory Opinion’s Practical Reach
The Advisory Opinion carries substantial legal authority; however, it does not bind States in the same manner as a judgement in contentious proceedings. Article 59 of the Statute of the International Court of Justice limits binding force to decisions between parties and the particular case, while Article 65 governs advisory jurisdiction.[9]The Opinion consequently clarifies applicable law without removing the requirement for State consent to contentious jurisdiction.
A future complainant must identify a jurisdictional basis, such as a treat compromissory clause, a declaration accepting compulsory jurisdiction, or a special agreement. They must then also identify the applicable obligation and establish standing. Article 48 of the Articles on State Responsibility allows invocation by a State other than an injured State where the obligation protects a collective interest or is owed to the international community as a whole.[10]
The Advisory Opinion may strengthen claims brought by particularly affected States. It confirms that climate obligations protect shared interests and that breaches can engage the ordinary consequences of State liability. The International Tribunal for the Law of the Sea’s 2024 Advisory Opinion similarly recognised stringent climate due diligence obligations, although that Opinion lacks direct legally binding force.[11]
III Conclusion
The ICJ’s July 2025 Advisory Opinion strengthens the view that customary international law requires States to exercise reasonable care against foreseeable, significant climate harm. However, serious harm alone does not establish responsibility. Attributable conduct, breach, injury, and a sufficiently persuasive causal link must be proven. Cumulative emissions complicate the ability to prove this, but do not make it impossible. Future courts should assess scientific evidence carefully, distinguish contribution from strict liability, and tailor remedies to loss, concurrent causes, and each State’s contribution.
BIBLIOGRAPHY
Certain Activity Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua) [2018] ICJ Rep 15
International Court of Justice, Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion) [1996] ICJ Rep 226
International Law Commission, Draft Articles on Responsibility of States for Internationally Wrongful Act, UN Doc A/56/10 (2001)
Obligations of States in Respect of Climate Change (Advisory Opinion) (International Court of Justice, 23 July 2025)
Paris Agreement, opened for signature 22 April 2016, 3156 UNTS 79 (entered into force 4 November 2016)
Pulp Mills on the River Uruguay (Argentina v Uruguay) [2010] ICJ Rep 14
Request for an Advisory Opinion Submitted by the Commission of Small Island States on Climate Change and International Law (Advisory Opinion) (International Tribunal for the Law of the Sea, 21 May 2024)
Statute of the International Court of Justice, 26 June 1945, 33 UNTS 993 (entered into force 24 October 1945)
Trail Smelter (United States v. Canada) (Arbitral Tribunal) (1938, 1941) 3 RIAA 1905
United Nations Framework Convention on Climate Change, opened for signature 9 May 1992, 1771 UNTS 107 (entered into force 21 March 1994)
[1] Obligations of States in Respect of Climate Change (Advisory Opinion) (International Court of Justice, 23 July 2025) (The Advisory Opinion).
[2] Ibid; United Nations Framework Convention on Climate Change, opened for signature 9 May 1992, 1771 UNTS 107 (entered into force 21 March 1994) arts 2-3.
[3] Trail Smelter (United States v. Canada) (Arbitral Tribunal) (1938, 1941) 3 RIAA 1905, 1965; Pulp Mills on the River Uruguay (Argentina v Uruguay) [2010] ICJ Rep 14, 55-56.
[4] The Advisory Opinion (n 1).
[5] Paris Agreement, opened for signature 22 April 2016, 3156 UNTS 79 (entered into force 4 November 2016) arts 2, 4; United Nations Framework Convention on Climate Change, opened for signature 9 May 1992, 1771 UNTS 107 (entered into force 21 March 1994) arts 3, 4.
[6] International Law Commission, Draft Articles on Responsibility of States for Internationally Wrongful Acts, UN Doc A/56/10 (2001) arts 1-2.
[7] The Advisory Opinion (n 1).
[8] Certain Activity Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua) [2018] ICJ Rep 15, 44-46.
[9] Statute of the International Court of Justice, 26 June 1945, 33 UNTS 993 (entered into force 24 October 1945) arts 36, 59, 65; International Court of Justice, Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion) [1996] ICJ Rep 226, 236.
[10] International Law Commission, Draft Articles on Responsibility of States for Internationally Wrongful Act, UN Doc A/56/10 (2001) art 48.
[11] Request for an Advisory Opinion Submitted by the Commission of Small Island States on Climate Change and International Law (Advisory Opinion) (International Tribunal for the Law of the Sea, 21 May 2024).
