The ICJ's Advisory Opinion on Climate Change: A Landmark Decision

Understanding the Advisory Opinion

On July 23, 2025, the International Court of Justice (ICJ) issued its long-awaited Advisory Opinion on climate change. The ICJ, often referred to as the “World Court,” is the principal judicial organ of the United Nations. It has the authority to settle legal disputes between states and provide non-binding Advisory Opinions on legal questions referred to it by authorized UN bodies. While these opinions are not legally binding, they carry substantial authority and can influence both international and domestic legal proceedings. To date, the ICJ has issued 29 Advisory Opinions. [1]

In its Advisory Opinion on climate change, the ICJ clarified two legal questions referred to it by the United Nations General Assembly in March 2023:

What legal obligations do States have under international law to ensure the protection of the climate system from greenhouse gas (GHG) emissions?
What are the legal consequences under these obligations for States whose acts or omissions have caused significant harm to the climate system affecting other States, peoples, and individuals?

The case drew unprecedented engagement. Ninety-six States and several international organizations participated in the written and oral proceedings, marking the highest level of participation in ICJ history. [2] The Court delivered its Advisory Opinion unanimously, reflecting a strong consensus among the judges on the legal framework. Several judges included separate declarations elaborating on their individual reasoning.

Legal Obligations of States

The Advisory Opinion begins by outlining the relevant scientific context. It emphasizes the damaging effects of fossil fuels, citing the Intergovernmental Panel on Climate Change’s (IPCC) finding that fossil fuels are responsible for approximately 64% of global GHG emissions. This scientific backdrop shapes the Court’s substantive legal analysis later on.

International Treaties and Customary Law

The ICJ confirmed that states’ conduct regarding climate change is governed by both international treaty law and customary international law. Key climate treaties include the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol, and the Paris Agreement. Together, these treaties form a binding legal framework requiring States to protect the climate system from GHG emissions.

The Paris Agreement receives particular attention. It requires states to adopt and communicate Nationally Determined Contributions (NDCs) to limit GHG emissions and collectively remain within the 1.5°C temperature increase threshold. The level of ambition in these NDCs is not left to the discretion of the States. They must reflect the “highest possible ambition in order to realize the objectives of the agreement.”[3] *This ambition corresponds with historical contributions to GHG emissions, level of development, and national circumstances. In other words, developed States and major emitters bear a proportionately greater duty to reduce emissions.

Implementation of Ambitions

These ambitions need to go beyond mere words. States have an obligation to implement them as well. This means that states must proactively adopt regulatory measures—including regulation of private actors—to implement the targets set out in their NDCs.

States not party to these climate treaties are subject to the same stringent standard. The duty to prevent significant harm to the environment is also a principle of customary international law. This means all States—whether or not parties to the climate treaties—must act with due diligence and must “use all means at their disposal to prevent activities carried out within their jurisdiction or control from causing significant harm to the environment.”[4]

There is no conflict of norms; treaty and customary law obligations must be read as complementary and mutually reinforcing. This requires States to act to prevent significant harm to the environment.

Legal Consequences of Breaches

The ICJ concludes that breaches of these obligations constitute an internationally wrongful act. This is a well-established rule in international law, but the ICJ now explicitly recognizes its applicability in the context of climate change. The ICJ stresses that the failure to prepare, communicate, or implement concrete NDCs as required by the Paris Agreement may give rise to State responsibility. The Court is crystal clear:

“Failure of a state to take appropriate action to prevent the climate system from GHG emissions—including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses, or the provision of fossil fuel subsidies—may constitute an internationally wrongful act which is attributable to that State.”[5]

Responsibility of States

The fact that GHG emissions are largely caused by private actors is not an excuse. States have a duty to regulate. The ICJ explicitly mentions that this includes the obligation to take necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.

The ICJ clarifies that while climate change is caused by cumulative GHG emissions of multiple States, it is scientifically possible to determine each State’s total contribution to global emissions. As a result, any injured State may invoke the international responsibility of any emitting State (or group of States) that has breached its obligations.

In terms of causation, the obligation to cut GHG emissions is an obligation towards the international community as a whole (erga omnes) and to some States in particular under some of the climate treaties (erga omnes partes).

General Legal Consequences

The legal consequences will depend on the case and the nature of the particular harm. However, as general guidance, the ICJ repeats the general legal consequences that apply in the context of international wrongful acts, which include:

The duty to perform their obligations. For example, in the case of a State party setting an inadequate NDC, a competent court or tribunal could order that State to improve and present an adequate NDC.
Ceasing the wrongful conduct and offering assurances and guarantees of non-repetition. For instance, the ICJ mentions that States could be required to end all administrative, legislative, and other measures contributing to its GHG emissions.
Providing full reparation to injured States in the form of restitution, compensation, and satisfaction, provided there is a sufficiently direct and certain causal link between the wrongful act and the injury suffered.

Implications of the Advisory Opinion

While not legally binding, this Advisory Opinion may have significant legal and political impacts. The ICJ made it explicitly clear that States not only have a right but indeed a duty under international law to regulate and implement measures that effectively reduce GHG emissions. States must adopt ambitious climate plans tailored to their historical emissions and level of development. In this context, developed States and major emitters bear a proportionately greater responsibility to curb emissions.

Crucially, these plans are not merely aspirational; they must be implemented through concrete legal and regulatory action. A failure to do so may constitute an internationally wrongful act. The ICJ also firmly rejected the notion that emissions from private actors absolve states of responsibility. In doing so, the Advisory Opinion confirms that states are under a legal obligation to regulate the fossil fuel industry and must take active steps to limit harmful activities within their jurisdiction, whether carried out by public or private entities.

Increased Exposure to Litigation

This clarification of States’ legal obligations significantly increases their exposure to litigation by other States for failure to act. This is particularly true because these obligations are erga omnes and, under some climate treaties, to some States in particular (erga omnes partes). At the same time, efforts to reform or tighten regulation—especially in relation to the fossil fuel industry—may give rise to claims by private investors, including under international investment treaties.

At Sovereign Arbitration Advisors, we support States in navigating legal risks. Whether you are designing climate measures, defending against investor-State claims, or preparing for international proceedings, our team is ready to assist. Please don’t hesitate to contact us for tailored legal advice.

[1] See International Court of Justice, Obligations of States in respect of Climate Change, Press Release No 2025/36 (23 July 2025).

[2] ibid.

[3] International Court of Justice, Advisory Opinion, Obligations of States in respect of Climate Change (23 July 2025), para. 81.

[4] ib., para. 457.

[5] Ib., para. 427.