A Comprehensive Analysis of the ICJ Climate Advisory Opinion

The International Court of Justice’s advisory opinion on climate change, issued July 23, offers a chance to create a unified understanding of states’ climate obligations, transcending specific treaties and regional arrangements and cutting across the entire spectrum of international law. It provides authoritative guidance on how international law applies more broadly to the climate crises.

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This article is part 3 of a three-part series looking at landmark, climate-related advisory opinions issued by international courts. Read part 1 (ITLOS) and part 2 (IACtHR).

Initiated by the Pacific Island nation of Vanuatu, the United Nations General Assembly (UNGA) Resolution A/77/L.58 – jointly sponsored by 132 developed and developing countries – formally requested an advisory opinion from the International Court of Justice (ICJ) concerning states’ legal duties on climate change. 

It was the culmination of a six-year campaign initiated by the Pacific Island Students Fighting Climate Change (PISFCC), who, when faced with the existential threat of sea level rise, sought legal clarity on climate obligations through the world’s highest court.

The UNGA resolution specifically posed the following questions:

(a) What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases for States and for present and future generations? 

(b) What are the legal consequences under these obligations for States   where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment, with respect to: 

(i) States, including, in particular, small island developing States, which due to their geographical circumstances and level of  development, are injured or specially affected by or are  particularly vulnerable to the adverse effects of climate change? 

(ii) Peoples and individuals of the present and future generations affected by the adverse effects of climate change?

More on the topic: Climate Change Is an ‘Existential Threat of Planetary Proportions’, Says World’s Top Court in Historic Ruling

ICJ Opinion: Highlights

The advisory opinion was hailed as a historic turning point in climate justice and accountability, with the ICJ unanimously affirming the entirety of Vanuatu’s submissions. Below are some key highlights.

Climate as an existential threat of anthropogenic nature 

The court underscores that the consequences of climate change are “severe and far-reaching,” affecting both natural ecosystems and human populations. Climate change, it asserts, is an “urgent and existential threat” to humanity. 

Moreover, the court confirms that greenhouse gas emissions are “unequivocally caused by human activities,” and observes that all parties to the proceedings accept the Intergovernmental Panel on Climate Change (IPCC) as the most authoritative source of scientific knowledge on the “causes, nature and consequences” of climate change.

The Paris Agreement goal of 1.5C

The ICJ confirms that limiting global warming to 1.5C is the primary temperature goal collectively embraced by Paris Agreement parties. This elevates 1.5C from an aspirational goal to a definitive legal benchmark governing the ambition, scope, and due-diligence duties of every state under both the Paris accord and general international law.

On the interpretative principle of lex specialis 

During the ICJ proceedings, a coalition of predominantly high-emitting, industrialized states advanced the theory of climate-treaty exceptionalism – namely that the UNFCCC, Kyoto Protocol, and Paris Agreement constitute a self-contained lex specialis framework superseding the broader body of international law.

The ICJ opinion rejects this narrow framing, affirming that climate treaties neither displace nor exhaust states’ wider obligations under general international law and other treaty laws.

Directly relevant applicable law

The ICJ considers the entire corpus of international law to form part of the most directly relevant applicable law.

This includes but is not limited to: the Charter of the United Nations; the  three climate change treaties (UNFCCC, Kyoto Protocol, Paris Agreement); the UN Convention on the Law of the Sea (UNCLOS); international human rights law; customary international law relating to climate change, as well as other environmental treaties, including the Ozone Layer Convention, the Montreal Protocol, the Biodiversity Convention, and the Desertification Convention. This also includes the principles of sustainable development, common but differentiated responsibilities and respective capabilities, equity, intergenerational equity and the precautionary approach as guiding principles, that are also part of the applicable law.

On climate treaties

The ICJ unanimously held that the UNFCCC, the Kyoto Protocol and the Paris Agreement are complementary and mutually reinforcing. The judges found no conflict among them, making it unnecessary to invoke the later-in-time lex posterior rule under the Vienna Convention, an idea advanced by a coalition of predominantly high-emitting, industrialized states. 

It also determined that states’ discretion in formulating their Nationally Determined Contributions (NDCs) is not unlimited. In setting NDCs, parties must exercise due diligence so that, collectively, their commitments can limit warming to 1.5C above pre-industrial levels and stabilize greenhouse-gas concentrations at a level that prevents dangerous human-induced interference with the climate system.

Customary law

The “duty to prevent significant harm to the environment” embodies an obligation to act with due diligence. This duty, construed expansively in the ICJ advisory opinion on the Legality of the Threat or Use of Nuclear Weapons as one that applies to “global environmental concerns,” is therefore extended to include “the climate system and other parts of the environment.”

Due diligence requires assessments, in concreto, of what measures are reasonable in the specific circumstances of a state, while also adhering to general conduct standards proportionate to the character of the risk, as is especially clear given the indisputable threats posed by climate change.

The “duty to cooperate”, which lies at the core of the UN Charter, is central to climate change treaties and other environmental treaties. It is intrinsically linked to the duty to prevent significant harm to the environment.

Human right to a clean, healthy and sustainable environment

The ICJ recognizes the interdependence between human rights and the protection of the environment. It affirms that every person has a human right to a “clean, healthy and sustainable” environment, which is a prerequisite for enjoying foundational rights – such as life, health, food, water and housing. States cannot meet their existing human-rights obligations unless they also protect the environment. The court treats the right as firmly grounded in international, regional and national law, pointing to its widespread recognition by United Nations resolutions, human-rights treaties and more than 100 constitutions.

Judge Iwasawa Yuji, President of the International Court of Justice. Photo: UN Photo/ICJ-CIJ/Frank van Beek. Courtesy of the ICJ.

Breach of legal responsibility

In its opinion, the ICJ held that states failing to honor their climate obligations commit “an internationally wrongful act.” Judge Yuji Iwasawa, the court’s President, underscored that omitting necessary measures to safeguard the climate system amounts to a breach of international law, with corresponding legal consequences.

The court reinforces the principle that any conduct – whether action or omission – by a state organ is attributable to the state itself. It goes on to specify that a state’s failure to regulate greenhouse gas emissions – including through fossil-fuel production, consumption, licensing, or subsidies – constitutes such an internationally wrongful act. 

Furthermore, the offending state must continue to fulfil the breached obligation and, where applicable, cease the harmful conduct, offer guarantees of non-repetition, and provide full reparation – including restitution, compensation and satisfaction – to injured states, provided a sufficiently direct causal link to the harm can be established under the rules of state responsibility.

The court further determined that any breach of obligations related to loss and damage from climate change must be assessed under the established customary international law rules on state responsibility.

Climate finance

The ICJ confirmed that under the Paris Agreement parties incur binding obligations to provide financial assistance, technology transfer and capacity-building to vulnerable states.

Protection of statehood for Small Island States

The ICJ underscored that rising sea levels threaten the physical territory – and, by extension, the very existence – of Small Island States. Nonetheless, it stresses that even if an entire landmass were lost and its people displaced, the state should still be presumed to continue as a legal entity; once statehood is achieved, losing a component such as territory does not automatically strip a state of its sovereign status.

People watch the live stream of the ICJ advisory opinion delivery outside of the Peace Palace in The Hague on July 23, 2025. Photo: Holland Park Media.

Implications

Although advisory opinions are not formally legally binding, they still possess considerable legal and moral weight and play a crucial role in shaping and clarifying international law by outlining states’ legal responsibilities. 

While lacking direct enforceability under Article 59 of the Court’s Statute, they are among the most persuasive sources of international legal interpretation. Historically, they have often altered state behavior, driven legislative change, and guided national courts because their reasoning clarifies existing international law and helps crystallize emerging customs.

In this climate advisory opinion, the ICJ confirms that states do have legal duties – including a due-diligence obligation to regulate private actors – to prevent, mitigate, and remedy climate harm. Those duties flow from treaty obligations (e.g., UNFCCC, Paris Agreement) and customary international law, including a right to a clean, healthy, and sustainable environment.

As well-respected Christiana Figueres – former executive secretary of the UNFCCC and a key architect of the Paris Agreement – observes “This is without a doubt, the most far-reaching, comprehensive and the most consequential legal Opinion we have ever had.” 

As put by Jennifer Robinson, Barrister for Vanuatu and the Marshall Islands, the opinion marks a pivotal shift for climate negotiators and litigators, poised to serve as a powerful tool in strategic legal proceedings before domestic and regional courts worldwide. 

Very importantly, this advisory opinion has advanced the cause of climate justice. 

To conclude with the words of the ICJ judges: “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.”

Featured image: UN Photo/ICJ-CIJ/Frank van Beek. Courtesy of the ICJ.