The ICJ's Advisory Opinion on Climate Change: Insights and Impact

On July 23, 2025, the International Court of Justice issued a historic advisory opinion on climate change, marking only the fifth instance in which the Court issued a unanimous opinion in nearly eight decades. The World Court’s opinion explicitly affirmed that states are obliged to ensure the protection of the climate system and prevent environmental harm, particularly with regard to existing provisions laid out in the Paris Agreement and the United Nations Framework Convention on Climate Change. Broadly, the Court determined that climate obligations under these frameworks are not merely aspirational, but are legally enforceable, with states being ultimately subject to legal consequences if they fail to comply with their climate obligations.

Drawing on principles of due diligence and common but differentiated responsibility and respective capabilities (CBDR-RC), with additional reference to human rights and sustainable development, the Court established that States are held responsible for ensuring their compliance with the duty to prevent harm to the environment and the duty to cooperate for environmental protection. The Court’s application of these general environmental principles to the issue of climate change is a significant development in international environmental law that will have implications for how states approach the task of limiting greenhouse gas (GHG) emissions. The opinion constitutes a turning point in climate governance that demonstrates the UN’s resolve to prevent the exacerbation of climate change.

Climate Change Comes to the ICJ

The question of state obligations regarding environmental concerns has been at the forefront of international discussions on the matter since the 1972 United Nations Conference on Human Environment hosted in Stockholm. The issue of reconciling state sovereignty and development rights with obligations to avoid causing environmental harm has historically proved to be a major challenge. This point of contention has caused divisions to emerge in the General Assembly, particularly between developed states and developing states, with further fragmentation between larger GHG emitters and smaller emitters. The lack of consensus on how existing legal frameworks might be interpreted and applied to the climate system resulted in the need for clarification from the ICJ.

The request for an advisory opinion was an initiative started in 2021 by the state of Vanuatu, where a youth group called Pacific Islands Students Fighting Climate Change pressured their government to take a more assertive approach to calling for climate justice in the international community. After years of lobbying fellow UN Member states, Vanuatu’s initiative was adopted in a General Assembly resolution in March 2023. Resolution 77/276 sought the ICJ’s opinion on the following questions:

(a) What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases?

(b) What are the legal consequences under these obligations for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment, with respect to: (i) States, (ii) Peoples and individuals?

The Court’s Interpretation of International Law

The ICJ referenced a wide range of existing legal provisions – both environmental and general – to inform and contextualize its opinion. Perhaps the most fascinating component of the opinion was the Court’s review of customary international law. The Court held that customary law, which recognizes duties to prevent significant harm to the environment and to cooperate for the protection of the environment, directly applies to the climate system. Moreover, the ‘duty to prevent’ is not limited to direct cross-border harm, but rather applies to global environmental impacts on the climate system. Notably, the Court opined that these two customary environmental duties apply to all States regardless of whether or not they are parties to climate change treaties.

The opinion urged a robust approach to attending to these duties. Referencing its judgement on Argentina v. Uruguay, the Court reaffirmed that a state is “obliged to use all the means at its disposal in order to avoid activities which take place in its territory, or in any area under its jurisdiction, causing significant damage to the environment of another State.”

The Court determined that compliance with climate obligations is not a matter of state discretion, but is rather a matter of due diligence, such that states are required to take action to limit their impacts on the climate system. States must take all available precautionary measures within their respective capabilities, including the use of risk assessments and consultations with other states where appropriate, so as to prevent environmental harm. Linking customary norms of due diligence with conventional norms laid out in climate treaties like the Paris Agreement, the Court relied upon the principle of systemic integration.

With customary rules and conventional rules converging on the matter of climate obligations, the Court’s opinion suggested that non-party states can still be compelled to follow treaty provisions due to their customary obligations. For instance, if a non-party state cooperates with climate change treaties in a manner that comports with the conduct required by those treaties, it can be considered to fulfil its customary obligations. Yet if a non-party state does not cooperate in this manner, according to the court’s opinion, it has the “full burden of demonstrating that its policies and practices are in conformity with its customary obligations.”

Ultimately, the court rejected the arguments put forth by several states, which suggested that climate change treaties constitute lex specialis. States including the U.S., Japan, and Russia, have used this principle to assert that climate change treaties constitute a special category of law, such that customary rules in international law cannot be applied to matters covered by those laws. According to the International Law Commission, in order for lex specialis to take precedence and thereby override general law, there must be an inconsistency between legal provisions, or a discernible intention that one provision was designed to exclude the others. The Court did not find either of these conditions to be satisfied and rejected the proposition that lex specialis applies.

Implications for the Climate Conversation

With 91 countries issuing written statements and 97 countries participating in the proceedings, this was the largest case ever seen by the ICJ. The vast number of countries issuing statements vividly demonstrates the relevance attributed to the case in the eyes of UN member states.

The Court’s answers to the questions posed by General Assembly Resolution 77/ 276 indicate a clear direction for the development of international law pertaining to climate change. The opinion affirms that environmental law can be used as a tool to create robust obligations for climate action. It emphasises the legal repercussions of states’ failure to take adequate measures, which can include failure to pursue Nationally Determined Contributions under the Paris Agreement. According to the opinion, if states breach their legal responsibility to fulfill climate obligations, this may constitute a wrongful act under the law. Depending on the circumstances, states may be required to offer guarantees of non-repetition and make reparations. This potential for legal enforcement could help to incentivize states to invigorate their efforts to reduce GHG emissions.

While ICJ advisory opinions are not binding, they carry significant legal weight in the international community. ICJ opinions can contribute to the formation of opinio juris, a crucial element for establishing customary international law.  Further, the opinion can be referenced by national courts in climate litigation, empowering courts to demand more accountability.

The Court’s evaluation of international law in the context of climate obligations offers new legal standards for conceptualising the breadth and depth of state responsibilities with regard to the environment. By drawing from a diverse array of specific international conventions and general customary norms, the Court produced an opinion that carefully integrates the UN’s commitments to environmental protection, human rights, equity, and sustainable development, while applying these principles to the issue of climate change, and in turn, creating an impetus for strict adherence to climate obligations.

Image by Lybil BER via Wikimedia Commons