A legal turning point? The ICJ's advisory opinion on climate change and the evolution of international responsibility

Olga Lablache, PhD Candidate at NLS

The International Court of Justice's (ICJ) advisory opinion on "Obligations of States in respect of Climate Change," delivered on 23 July 2025, marks a decisive moment in the evolution of international law. For the first time, the ICJ has articulated, in authoritative terms, the scope of States' legal obligations to mitigate and prevent the adverse effects of climate change, as well as the legal consequences of failing to do so. Although advisory opinions are not legally binding, they constitute authoritative statements of international law and carry considerable persuasive weight. This opinion consolidates fragmented principles into a coherent framework of responsibility. It reaffirms that climate protection is no longer a matter of policy preference, but a legal duty grounded in customary international law.

The request to the ICJ was initiated politically by a coalition of Pacific Island States led by Vanuatu, and then formally made by the United Nations General Assembly in resolution 77/276, with broad support from Small Island Developing States (SIDS). For these States, the request was not theoretical but a response to a genuine existential threat. SIDS face disproportionate risks from climate change, including sea-level rise, coastal erosion, freshwater salinisation and, ultimately, the potential loss of habitable territory. Their economies and societies are closely tied to the ocean, leaving them acutely vulnerable to marine and environmental degradation associated with global warming. Although they are among the least responsible for global greenhouse gas emissions, they experience some of the most severe and immediate impacts of climate change. Judicial clarification was therefore sought not merely as a symbolic act, but as a means to strengthen their diplomatic and legal position. The ICJ's response now provides a detailed exposition of States' obligations, which vindicates the legal concerns long raised by these vulnerable nations. In its opinion, the Court affirmed that States are bound by existing obligations under both treaty and customary international law to protect the climate system. It held that the duty to prevent significant transboundary harm, and the duty to cooperate in addressing global environmental risks are established norms of general international law. The ICJ thus rejected the argument that climate obligations exist solely within the treaty framework of the United Nations Framework Convention on Climate Change and the Paris Agreement. Instead, it confirmed that those treaties operate alongside and in reinforcement of broader customary principles. This reasoning prevents States from evading responsibility through narrow treaty interpretation or non-participation.

The Court's formulation of the due diligence standard is particularly significant. It requires States to take appropriate measures, using all the means at their disposal and in light of the best available science, to prevent or minimise climate-related harm. This standard encompasses both substantive and procedural elements, including risk assessment, mitigation, adaptation planning, and cooperation. The ICJ's emphasis on capacity reflects the principle of common but differentiated responsibilities, aligning obligations with each State's resources and level of development. Equally important is the Court's treatment of state responsibility. It affirmed that failure to exercise due diligence constitutes an internationally wrongful act engaging the general law of state responsibility. Breaching States are required to cease wrongful conduct, offer assurances of non-repetition, and make full reparation through restitution, compensation, or satisfaction. The Court acknowledged the practical difficulties in quantifying environmental harm and the irreversibility of some climate impacts, but it made clear that these challenges do not negate the existence of liability. One of the most striking elements of the opinion is the recognition of the 1.5 °C temperature goal from the 2015 Paris Agreement as a legally relevant benchmark. The Court described it as a "primary temperature goal" to which national contributions must be oriented. This effectively imports scientific thresholds into legal reasoning, signalling a shift towards empirically informed jurisprudence. It also gives domestic and regional courts a concrete reference point when assessing the adequacy of State action.

The implications of this opinion are far-reaching. It consolidates what some scholars describe as a "constitutional moment" in international climate change law, integrating climate protection with human rights and intergenerational equity. It provides an authoritative statement that obligations to protect the environment are erga omnes, owed to the international community as a whole. This conceptual move situates climate change alongside other global concerns, such as genocide and torture, where all States have a legal interest in compliance. The opinions recognition of obligations towards future generations further expands the temporal reach of State responsibility.

Nevertheless, significant limitations remain. Advisory opinions are non-binding, and their effectiveness depends on political will and interpretive uptake. The opinion leaves unresolved the complex issue of attribution, particularly the identification of causal links between specific States' emissions and discrete harms. Moreover, whilst the Court emphasised States' duty to regulate private actors, it refrained from addressing the liability of corporations and financial institutions that drive emissions. Questions of equity also persist, balancing historical responsibility against present capacity, and continue to divide developed and developing States. Despite these limitations, the advisory opinion marks a crucial step in transforming climate protection from a moral or political aspiration into a legally enforceable obligation. It offers future claimants, whether States, individuals or communities, a stronger doctrinal foundation upon which to base arguments of responsibility and reparation. Domestic courts are likely to rely on its reasoning when interpreting environmental or constitutional rights, whilst regional and international tribunals may follow its approach in applying principles of prevention, due diligence and cooperation.  

The opinion's authority, therefore, lies not in any binding force but in its character as an authoritative statement of the law, capable of guiding judicial reasoning and informing institutional practice. It stands as a declaration that the failure to take adequate climate action constitutes not only a moral shortcoming but a breach of international law, and it may, in time, prove to be one of the most significant legal developments in the struggle against climate change.

Further Reading:

Stephanie van den Berg and Alison Withers, ‘ Top UN court says treaties compel wealthy nations to curb global warming’ (Reuters 23 July 2025) <https://www.reuters.com/sustainability/cop/top-un-court-says-treaties-compel-wealthy-nations-curb-global-warming-2025-07-23/>

Marisa McVey and Annalisa Savaresi, ‘The ICJ Advisory Opinion on Climate Change: A Business and Human Rights Perspective’ (Opinio Juris 4 August 2025) <https://opiniojuris.org/2025/08/04/the-icj-advisory-opinion-on-climate-change-a-business-and-human-rights-perspective/>

‘Obligations of States in respect of Climate Change’ (International Court of Justice 2025) <https://www.icj-cij.org/case/187>

Panpailin Jantarasombat and Icarus Chan, ‘ICJ Climate Change Advisory Opinion: Peoples and Individuals as Obligees’ (EJIL: Talk! 17 October 2025) <https://www.ejiltalk.org/icj-climate-change-advisory-opinion-peoples-and-individuals-as-obligees/>