Clifford Chance

The ICJ’s advisory opinion articulates the obligations of States under international law in relation to climate change and concludes, inter alia, that the inadequate regulation of private actors may breach those obligations.

Introduction

On 23 July, the International Court of Justice (ICJ or Court) delivered its advisory opinion following two weeks of hearings and the largest participation of States and international organisations in the history of the Court.

The opinion responds to a request from the UN General Assembly for the Court to consider States' legal obligations to protect the environment from the effects of climate change and the legal consequences under international law for failing to do so¹.

The ICJ concluded that:

• States have obligations to ensure the protection of the climate system and other parts of the environment from anthropogenic greenhouse gas (GHG) emissions.
• These obligations arise under climate change treaties and under customary international law (CIL).
• Any breach of such obligations constitutes an internationally wrongful act.

The operative paragraphs of the 133-page opinion are stated to be unanimous which is significant, not least because unanimity is rare for the Court. However, 11 of the 15 judges deliberating the request chose to clarify, elaborate on and, in some instances, disagree with elements of the Court's opinion, submitting separate opinions and declarations. This indicates the importance and complexity of the issues opined upon by the Court.

In this blog, we summarise the Court's views on selected key issues and highlight issues of relevance to business.

A broad range of applicable law is relevant

A foundational consideration for the Court concerned what law it should apply to the questions before it. Some participants argued that the Court should refer exclusively to climate change treaties which, they contended, constitute a lex specialis – applicable to the "general exclusion by the climate change treaties of other rules of international law" [171] thereby displacing the application of any other international law.

Rejecting this position, the Court considered that the most directly relevant law applicable to the opinion was not limited to climate change treaties and includes the UN Charter, UNCLOS and other environmental treaties, CIL, and international human rights law as well as principles to guide the interpretation and application of relevant legal rules (i.e. sustainable development, common but differentiated responsibilities, equity, intergenerational equity, and the precautionary principle – but notably, not the polluter pays principle). The Court expressly stated that the list was without prejudice to the application of other rules where relevant, such as international trade law, international investment law and international humanitarian law.

State obligations under climate change treaties

A significant proportion of the ICJ's opinion is devoted to identifying and clarifying the main obligations of States under the UN Framework Convention on Climate Change, the Kyoto Protocol and the Paris Agreement (Climate Change Treaties), which the Court interpreted in accordance with international law, including the guiding principles mentioned above.

The Court concluded that the Climate Change Treaties "establish stringent obligations on States to ensure the protection of the climate system and other parts of the environment" [268], placing mitigation and adaptation obligations on States.

The Court's analysis of the Paris Agreement (on which most participants focused their submissions) involved identifying State obligations in relation to climate change, 3 of which are discussed below.

1.5°C temperature goal

The Court's interpretation of the temperature goal in the Paris Agreement is notable. While acknowledging the Agreement’s reference to limiting warming to well below 2°C, the Court observed that 1.5°C has become "the scientifically based consensus target." Therefore, the Court considered 1.5°C as "the parties’ agreed primary temperature goal for limiting the global average temperature increase" [224].

"Limited" discretion in the preparation of States' Nationally Determined Contributions (NDCs)

Significant also is the Court's detailed analysis of States' obligations under the Paris Agreement to mitigate - reflected in the obligation to prepare, communicate and maintain NDCs.

The Court's view, contrary to some participants' submissions, was that the content of NDCs is not "self-defined" [237]. Rather, each State's NDCs: (i) must be progressively more demanding every 5 years, (ii) must reflect that State’s highest possible ambition, and (iii) must be "capable of making an adequate contribution to the achievement of the temperature goal" [242, 270].

In addition, the Court found that all NDCs set by Parties must collectively be capable of achieving the objectives in Article 2, including the temperature goal.

States must implement NDCs and domestic mitigation measures

The Court concluded that Parties are required to act with due diligence in taking necessary measures to achieve the objectives set out in their NDCs (including through the regulation of private actors, see below) and must undertake best efforts to achieve the content of their NDCs.

State obligations under CIL

The Court turned to international environmental law to identify CIL obligations on States in the context of climate change.

First, the Court interpreted and applied the existing customary duty of States to prevent significant harm to the environment, to climate change. It came to the view that "the diffuse and multifaceted nature of various forms of conduct which contribute to anthropogenic climate change does not preclude the application of the duty to prevent significant harm to the climate system and other parts of the environment" [279] (emphasis added).

To fulfil the duty, States must act with due diligence, a "stringent" standard, which encompasses both the requirement to take substantive measures and also procedural steps (i.e. a requirement to assess risks to the environment). In the context of climate change which entails transboundary harms, any assessment of this kind must take the specific character of the respective risk into account.

The role of science was also emphasised: it must be taken into account in any assessment of significant harm, and informs the standard of due diligence. The Court found that the IPCC reports constituted the "best available science" on climate change [74].

Second, the Court identified a duty on States to co-operate with each other in good faith to prevent significant harm to the climate system and other parts of the environment, requiring sustained and continuous forms of co-operation by States when taking measures to prevent such harm.

International Human Rights Law

A growing number of regional and international human rights bodies such as the UN Human Rights Committee, the Inter-American Court of Human Rights and the European Court of Human Rights have established that international human rights instruments impose obligations on States to protect the climate system. However, in these proceedings, participants were divided on the issue.

The ICJ concluded that climate change is a human rights issue and that the adverse effects of climate change may impair the effective enjoyment of human rights.

The Court acknowledged the relevance and applicability of human rights treaties in climate-related proceedings – subject to the specific provisions of each treaty. The Court also found also that under international law the human right to a "clean, healthy and sustainable environment is essential for the enjoyment of other human rights" [393].

State responsibility for breaches and establishing causation

The Court concluded that any State which breaches its climate obligations commits an internationally wrongful act.

For a State to incur responsibility for an internationally wrongful act, the breach of the relevant obligation must be attributable to it. The Court considered participants' submissions that attribution is difficult in the context of climate change, but came to the view that the analysis should be based on criteria determined by established international law.

Accordingly, the Court reiterated that the internationally wrongful act in question is not the emission of GHGs per se, but the breach of States' obligations relating to the protection of the climate system from significant harm resulting from GHGs.

On this basis, the Court concluded that States are responsible for failing to take appropriate action in relation to fossil fuel activities, and that failures to regulate private actors may breach States' climate-related obligations:

• The "[f]ailure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies — may constitute an internationally wrongful act which is attributable to that State" [427].
• Further, since States' obligations include regulating the activities of private actors, a State may be responsible for failure to exercise due diligence by not taking regulatory/legislative action to limit emissions by private actors under its jurisdiction.

Causation is relevant to the determination of any reparations if an internationally wrongful act (or omission) has occurred. For reparations to be made, a causal link must be established between the wrongful act and damages suffered by an injured State, or (in the case of international human rights violations) injured people.

The Court opined that the established test for causation under international law may be applied in relation to breaches of climate-related obligations. Accordingly, the existence of a "sufficiently direct and certain causal nexus" between the wrongful act and the injury suffered is required [436]. The Court accepted that the link between the wrongful actions or omissions of a State and the harm arising from climate change is "more tenuous" than in the context of localised pollution, but considered that it was not impossible to identify such a link[438]. Indeed, the Court acknowledged that science can assist in evidencing links between climatic events and climate change, even if damage suffered needs to be considered against the facts of each case.

The Court clarified that under established principles of international law and in the usual way, a State's responsibility for breaching its climate-related obligations may engage legal consequences, inter alia, that it cease wrongful acts, provide assurances and guarantees of non-repetition and / or make reparations in the form of restitution, compensation, or satisfaction.

The Court also made other findings of particular relevance to States, including that their climate-related obligations, in particular the obligation to prevent significant transboundary harm under CIL, are obligations erga omnes – these are obligations that are owed to the international community as a whole and that all States (not just the harmed State) have an interest in protecting.

Significance of the Opinion

The ICJ's opinion comes as the latest in a series of binding and non-binding pronouncements by regional and international bodies identifying various State obligations of States in the context of climate change. This includes the Inter-American Court of Human Rights' advisory opinion on climate change, issued on 3 July 2025.

As the term suggests, an advisory opinion constitutes advice and does not legally bind the requesting entity or any other body or State to take any specific action pursuant to the opinion. Emanating from the UN's principal judicial body, the ICJ's opinion is however, the most authoritative and potentially influential of recent pronouncements.

Small island states such as Vanuatu, which played a key role in spearheading the request by the UN General Assembly for the opinion, will be able to deploy the Court's findings at the upcoming COP 30 in support of their calls for greater State action on climate.

Climate activists will also likely utilise the ICJ's opinion to place pressure on States to improve their NDCs and climate policies and strategies, as well as to increase regulation of the private sector's GHG emissions.

The ICJ's views on the scope of States' climate-related obligations are likely to be deployed by claimants and their representatives in domestic litigation against States and State entities.

¹See ICJ Advisory Opinion here.