An advisory opinion of the ICJ

An advisory opinion of the Court of Justice in The Hague (ICJ) is a formal legal interpretation issued at the request of certain United Nations organs and specialised agencies, which themselves cannot be parties to contentious proceedings before the ICJ. Although advisory proceedings resemble contentious cases in structure, they are distinct in purpose and nature, serving to clarify legal questions rather than resolve disputes between States.

While advisory opinions are not legally binding, they carry substantial legal weight and moral authority. In domestic civil proceedings, such opinions may be cited as persuasive authority, particularly in cases involving questions of public international law, treaty interpretation, or the legal status of international institutions. Courts may refer to them to support legal reasoning, especially where national law intersects with international obligations or norms.

Key findings of the ICJ: legal climate obligations of States

The United Nations General Assembly requested an advisory opinion, following a global campaign led by the pacific island nation of Vanuatu, supported by over 130 countries and numerous civil society groups. This support was founded in growing concern over the inadequacy of current climate action and the need for legal accountability. While treaties like the UN Framework Convention on Climate Change, the Kyoto Protocol and the Paris Agreement set out political commitments, enforceable legal obligations or clear consequences for non-compliance remained unclear.

The advisory opinion that was issued on 23 July 2025 by the ICJ (the Advisory Opinion), provides authoritative guidance on two questions posed by the UN General Assembly:

  1. Question (a): What are the obligations of States under international law to protect the climate system and other parts of the environment from anthropogenic greenhouse gas (GHG) emissions?

  2. Question (b): What are the legal consequences for States that fail to fulfil these obligations where significant climate-related harm to other States, peoples and individuals of the present and future generations affected by the adverse effects of climate change?

With regard to the question (a)the ICJ unanimously held that States have binding obligations under international law – including inter alia human rights – to protect the climate system from anthropogenic GHG emissions (paras. 112–404). These obligations include a duty to prevent significant environmental harm. This duty of States is triggered when there is a risk of such harm and must be fulfilled with due diligence. That means States must adopt appropriate legislative and administrative measures, use the best available scientific and technological knowledge, and act proactively. Given the global and potentially irreversible nature of climate harm, the ICJ emphasised that the due diligence standard imposed on States in this context is particularly stringent (paras. 135-139 and 280-300).

These obligations of States also extend to the regulation of private actors. States must ensure that activities carried out by private actors within their jurisdiction shall not undermine climate protection. This requires effective legislation and oversight – especially in jurisdictions where private actors (such as companies) are major emitters (paras. 252-253). For example, a State may be responsible where it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the GHG emissions caused by private actors under its jurisdiction (para. 428). States cannot hide behind the autonomy of private actors or regulatory gaps (paras, 270, 444-455).

Furthermore, the ICJ reaffirmed that obligations must reflect each State’s capabilities and historical GHG contributions (paras. 179-182 and 241-242). The due diligence standard itself is differentiated: what is expected depends on the State’s capacity and GHG emissions profile (paras. 290-292). This helps ensure fairness between States and provides a legal basis for determining whether they are meeting their climate obligations.

Together, these findings confirm that climate protection is a legal duty and not a matter of policy discretion. This duty includes regulating private actors by taking the necessary regulatory and legislative measures and must be interpreted considering principles of international law including equity, due diligence and cooperation between States.

Key findings of the ICJ: legal consequences of breaching climate obligations of States

With regard to question (b) the ICJ unanimously held that a breach by a State of any of the obligations identified in response to question (a) constitutes an internationally wrongful act (paras. 444-445). This includes, for example, a failure to adopt or implement mitigation measures, or a failure to exercise due diligence in preventing significant environmental harm (paras. 409 and 445).

Such a breach triggers the international responsibility of the State concerned. Notably, the ICJ confirmed that the general rules of State responsibility under customary international law apply fully in the climate context and are not displaced by (specific) climate treaties, unless those treaties contain clearly expressed and specific rules to the contrary (paras. 410–411). In the absence of such lex specialis, the general rules – including the duty to cease the wrongful act and to make full reparation – apply equally to all States, regardless of whether they are parties to specific climate treaties (paras. 408-409). Therefore, States can be held internationally liable for climate-related harm under general laws, even when the breach arises from conduct not explicitly addressed in a specific climate treaty.

Returning to question (b), according to the ICJ the legal consequences of a breach include three core obligations for the State concerned:

  • Duty of performance: the State has an ongoing duty to fulfil the breached obligation – for instance, to adopt or reinforce mitigation measures (para. 446);

  • Duty of cessation and guarantees of non-repetition: the State must cease the wrongful conduct and, where appropriate, provide assurances and guarantees of non-repetition (paras. 447-448); and,

  • Duty to make reparation: the State must make full reparation for the injury caused. This may take the form of restitution (restoring the situation to its previous State), compensation for quantifiable damage and/or satisfaction for non-material harm (paras. 449-4455).

To establish responsibility and claim reparation, the ICJ mentions three elements that must be demonstrated by a claimant vis-à-vis a State:

  1. Breach of a climate obligation by a State – as identified in response to question (a) – constitutes an internationally wrongful act or omission if the State fails to comply with its binding climate obligations (paras. 208, 409 and 445-446);

  2. The wrongful act or omission must be attributable to the State. This includes both active conduct and passive failure, such as the State’s omission to regulate private actors operating within its jurisdiction (paras. 422-429); and,

  3. There must be a causal link between the State’s wrongful act or omission, and the damage suffered by the claimant. The ICJ reaffirmed that the applicable standard is a “sufficiently direct and certain causal nexus” (para. 436), which must be assessed on a case-by-case basis. Importantly, the ICJ clarified that the cumulative and diffuse nature of climate harm — resulting from multiple sources and over time — does not preclude the attribution of legal responsibility (paras. 276–277, 429).

In this context, the ICJ explicitly rejected the so-called 'drop in the ocean' defence. The ICJ held that a State may still be held responsible for its contribution to global GHG emissions, even if this contribution is small (paras. 77–83, 276–277). In other words, while a State's emissions may appear insignificant in isolation, they can have serious consequences when considered alongside emissions from other States (para. 276). The ICJ emphasised that it is the total amount of GHG emissions from all such individual States that contributes to anthropogenic climate change (para. 277). Therefore, all States have a duty to act, regardless of their relative GHG emissions, and cannot rely on the global or collective nature of the problem to avoid legal responsibility.

Potential implications of the Advisory Opinion

The ICJ Advisory Opinion fits within a broader legal trend that increasingly frames climate inaction as a matter of legal – and in particular human rights – responsibility (please be referred to our earlier trend report on this). Last year, the Europe Court of Human Rights (ECHR) ruled that Switzerland ad violated the rights of members of the KlimaSeniorinnen association by failing to take adequate and timely measures to address climate change (please be referred to our earlier blog). The ECHR found that Switzerland had not fulfilled its positive obligations under Article 8 of the European Convention on Human Rights, which protects the right to respect for private and family life. The ECHR also emphasised that States must act with due diligence to prevent environmental harm that threatens human rights, including by establishing effective legislative and administrative frameworks (paras. 538–540 of the KlimaSeniorinnen-ruling). It emphasized that States must act with due diligence to prevent environmental harm that threatens human rights, including by establishing effective legislative and administrative frameworks (paras. 538–540 of the KlimaSeniorinnen-ruling).

The ICJ echoed this reasoning in the Advisory Opinion, confirming that States have binding obligations under international law – including human rights law – to prevent significant environmental harm. This includes a duty of due diligence, requiring proactive legislative and administrative measures. Both the Advisory Opinion and the KlimaSeniorinnen-ruling reflect a growing legal consensus: failure to act on climate change is no longer just a policy gap; it increasingly constitutes a breach of legal responsibility.

The Advisory Opinion may also influence how EU climate legislation – such as the CSDDD – is interpreted, particularly in relation to corporate accountability (please be referred to our earlier blog). In the Netherlands, where climate litigation is already shaping legal standards, the recent Shell-ruling by the Court of Appeal of The Hague illustrates how courts are applying international norms. In the Shell case, the Court of Appeal affirmed that major GHG emitters like Shell have a general duty of care grounded in both human rights and soft law instruments such as the OECD Guidelines and the UN Guiding Principles on Business and Human Rights – even in the absence of explicit statutory mandates (please be referred to our case analysis). Article 6:162 of the Dutch Civil Code, which provides an open-ended social standard of care, offers a legal basis to enforce such corporate duties in relation to climate change.

The ICJ Advisory Opinion may further support this line of reasoning. By rejecting the ‘drop in the ocean’ defence and recognising the cumulative and diffuse nature of climate harm (paras. 276–277, 429), the ICJ affirmed that even relatively small contributions to global GHG emissions can give rise to legal responsibility. This may help claimants overcome the complex causality challenges that often arise in ESG litigation, both against States and, potentially, against companies A recent example is the Lliuya v. RWE-case in Germany, where the court acknowledged that companies can, in principle, be held liable for their quantifiable contribution to climate-related harm, despite the transboundary and multifactorial nature of the damage (please be referred to our earlier blog).

Although the Advisory Opinion is not legally binding, it reinforces the principle that States must act with due diligence to prevent environmental harm, including through the regulation of private entities. This interpretation may influence how courts assess the scope of corporate climate responsibilities under general tort law. NGOs such as Oxfam Novib and Amnesty International deemed the Advisory Opinion as a landmark development which can be used to further determine accountability of States in relation to climate change. The Advisory Opinion could potentially trigger a new wave of climate change litigation against States. To be continued.

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