On 23 July 2025, the International Court of Justice (ICJ) issued its long-awaited advisory opinion on climate change, following a request made two years ago by the United Nations General Assembly at the initiative of the island nation of Vanuatu.
The questions underpinning this request were notably broad: What are the legal obligations of States under international law regarding climate change? And what are the legal consequences under these obligations for States where they, by their acts and omissions, have caused harm to the climate system?
The resulting opinion, lengthy and detailed, sets out a powerful legal position from the world’s highest judicial authority. It marks a significant milestone in a time when climate law is rapidly evolving. Although not legally binding, it has the potential to have significant legal implications, including for domestic litigation in countries such as Italy.
The context: an increasingly clear body of international jurisprudence
The opinion of the ICJ is part of a rapidly evolving legal landscape. In just over a year, three other international courts have ruled on climate-related issues:
- The European Court of Human Rights, in the Klimaseniorinnen v. Switzerland ruling of April 2024, recognised that climate inaction can violate the right to private life, and condemned the Swiss state for failing to adequately mitigate climate risks that threaten the older generation.
- The International Tribunal for the Law of the Sea, in an advisory opinion published in May 2024 in response to a request from the Commission of Small Island States on Climate Change and International Law, established that States have an obligation to protect the marine environment from climate change.
- The Inter-American Court of Human Rights, in an advisory opinion issued just weeks ago, clarified that the right to a healthy climate is a human right, and that States must adopt all necessary measures to prevent irreversible damage and reduce any risks to the climate system, including those arising from fossil fuel production and consumption.
The ICJ has now added its voice to this chorus, bringing with it both legal weight and symbolic authority as the principal judicial organ of the United Nations. Its opinion is likely to serve as a key reference point for future regulatory and judicial developments.
Concrete legal obligations on states
Among the most significant elements of the ICJ’s opinion is its recognition of concrete legal obligations on States in relation to climate change, both in terms of mitigation – in line with the target of keeping the temperature rise below 1.5°C – and in terms of adaptation and loss and damage.
The ruling acknowledges that, in the face of climate-related legal obligations, the treaties that specifically address climate change (namely the Paris Agreement, the Kyoto Protocol, and the UN Framework Convention on Climate Change) are not the only applicable legal foundations.
The Court instead identifies a range of sources, in addition to climate treaties, from which these obligations derive:
- norms of customary international law, binding even for those States that are not party to specific treaties. The customary norms considered binding by the Court are the duty to prevent significant environmental harm and the duty of cooperation among States in environmental matters;
- a set of international treaties other than environmental ones, including the UN Charter, the UN Convention on the Law of the Sea, the Montreal Protocol on the ozone and the Convention to Combat Desertification;
- international human rights law, with particular emphasis on the right to a clean, healthy and sustainable environment as a precondition inherent to the enjoyment of all other human rights.
In doing so, the ICJ affirms that climate change is not an “exceptional” issue, nor is it separate from international law (so-called lex specialis) – as some States have argued in an attempt to limit their obligations to those outlined in climate treaties – but is instead fully embedded within the broader legal framework. As such, it demands consistency, accountability and action.
Guiding principles in the interpretation of climate-relevant norms
The Court gives legal value and interpretive function to various principles of international environmental law, including:
- the precautionary principle,
- the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC),
- the polluter pays principle,
- the principles of equity and intergenerational equity.
This last of these is particularly relevant in the Italian context. Following a 2022 constitutional reform, Italian law explicitly introduced environmental protection “in the interest of future generations” into Article 9 of the Constitution. The ICJ’s reference to this principle reinforces the constitutional legitimacy of ambitious climate policies and strengthens the legal foundation underlying their enforceability in court.
Fossil fuels: support may constitute an unlawful act
A key message emerging from the ICJ’s advisory opinion is that States must regulate fossil fuels to prevent harm to the climate system – and may be held accountable if they fail to do so. The Court notes that:
- The production, consumption, issuance of exploratory licences or provision of subsidies for fossil fuels may constitute internationally wrongful acts if they result in significant harm to the climate system.
- States must act with prudent diligence to prevent such harm: these appear to be obligations of conduct, not of immediate result, but violating them may give rise to international liability and and an obligation to take reparative action.
What really changes, and what it means for Italy
The Court’s opinion is advisory, and therefore not legally binding on States. The Court has jurisdiction to resolve disputes between States, and climate disputes of this kind remain rare – with negotiations, such as those at the COPs, still the preferred venue for international climate diplomacy.
However, the scope of the opinion is not measured solely in terms of formal obligation. As often happens in international law, its effectiveness lies in its ability to shape practices, interpretations and judgments, even at the national level.
The growing body of international climate jurisprudence is building a shared legal framework that national judges can – and in many cases must – take into account. In Italy, where international law tends to take precedence over ordinary legislation, the contents of this opinion could be invoked in domestic climate litigation.
Thanks to a recent ruling by Italy’s Court of Cassation, this scenario is no longer merely theoretical. Coinciding with the reading of the ICJ’s advisory opinion, the Joint Civil Chambers of the Court of Cassation issued a historic ruling (no. 20381/2025), recognising, for the first time in Italy, that ordinary courts have jurisdiction over claims for climate-related damages.
This marks a significant turning point, opening the door to concrete climate litigation in Italy. The ICJ’s advisory opinion – by reinforcing the international legal basis for such claims – will serve as a key tool in supporting their strength and legitimacy.
This article offers an initial overview of the International Court of Justice’s advisory opinion, to be followed by a more in-depth analysis to help fully understand the legal and political implications of this new piece in the puzzle of global climate law.
Photo by Tingey Injury Law Firm
