Historical ICJ opinion on state obligations in respect of climate change

Introduction

On 23 July 2025, the International Court of Justice (hereafter ‘ICJ’) delivered a highly anticipated advisory opinion clarifying the legal obligations of States under international law to address climate change.

The Court unequivocally affirmed that States have binding obligations to protect the climate system and other parts of the environment from greenhouse gas (GHG) emissions. Importantly, the ICJ held that a breach of these obligations constitutes an internationally wrongful act, thereby engaging the international responsibility of the State concerned.

This opinion provides authoritative legal guidance with far-reaching implications for States, policymakers and stakeholders, and could reshape the landscape of climate litigation and international climate governance.

The advisory opinion of 23 July 2025

1. Key legal findings

1.1. Obligation of states

The obligations of States regarding climate change are grounded in a comprehensive framework of international law, including treaty law, customary international law, and guiding legal principles. 

Treaty obligations: 

States are bound by the UNFCCC, Kyoto Protocol and Paris Agreement to adopt and update mitigation and adaptation measures, pursue ambitious Nationally Determined Contributions (NDCs), and cooperate through finance and technology transfers. 

Customary international law:  

The Court further clarified that, irrespective of treaty participation, all States are bound by customary international law to: 

• Prevent significant transboundary environmental harm, including harm to the climate system; 
• Exercise due diligence in regulating activities that may contribute to climate change, with a particularly stringent standard given the existential risks involved; 
• Cooperate in good faith with other States to address climate risks and implement effective climate action.

Human rights nexus:  

The ICJ recognised the intrinsic link between climate change and the enjoyment of fundamental human rights, including the rights to life, health, and a clean, healthy and sustainable environment. States are required to take necessary measures to protect these rights by addressing climate change, both through mitigation and adaptation.

1.2. Consequences of breach 

The Court held that a breach of the identified climate obligations constitutes an internationally wrongful act, triggering the international responsibility of the State. The legal consequences include: 

• A continuing duty to perform the breached obligation; 
• Cessation of the wrongful conduct and guarantees of non-repetition; 
• Providing assurances and guarantees of non-repetition of wrongful actions or omissions, if circumstances so require; 
• Full reparation to injured States, which may take the form of restitution, compensation and satisfaction, provided a sufficiently direct and certain causal nexus can be established between the wrongful act and the injury. 

According to the Court, the principle of international law that attributes the actions of any State organ to the State itself applies to climate change matters. If a State does not take measures to address greenhouse gas emissions – through activities such as fossil fuel production or consumption, issuing exploration licences or granting subsidies – this may be considered an internationally wrongful act attributable to that State. 

With respect to private actors, the Court notes that States are obligated to exercise due diligence by regulating private sector activities accordingly. Consequently, a State may be held accountable if it fails to implement necessary regulatory or legislative measures to control emissions generated by private entities within its jurisdiction. 

The Court states that any injured State has the right to independently invoke the responsibility of each State that has committed an internationally wrongful act resulting in harm to the climate system or other environmental components. Additionally, if multiple States are responsible for the same act, the responsibility of each State may be invoked regarding that act. 

The ICJ also underscored that these obligations are erga omnes (owed to the international community as a whole), reinforcing the collective interest in the protection of the global climate system. 

2. Implications

This advisory opinion is a watershed moment for international climate law and policy. It significantly strengthens the legal basis for holding States accountable for insufficient climate action, both in international and domestic fora. The opinion is expected to:

• Catalyse a new wave of climate litigation, providing courts and tribunals with clear legal standards for assessing State conduct;
• Empower vulnerable States, small island developing States, and civil society actors in their pursuit of climate justice;
• Influence the development and implementation of national and EU climate policies, with heightened scrutiny of State compliance with both treaty and customary obligations;
• Reinforce the integration of human rights considerations into climate action and environmental governance.