The ICJ advisory opinion clarifying States legal obligations on Climate Change

In an unprecedented move, the International Court of Justice (ICJ) has affirmed that human rights, including the right to a clean, healthy and sustainable environment form the foundation of states’ obligations to address climate change.

The advisory opinion was given on 23 July 2025 after the General Assembly of the United Nations requested it to the ICJ in a resolution 77/276 adopted on 29 March 2023. The General Assembly’s questions to the ICJ were:

(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 for States and for present and future generations?

(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, including, in particular, small island developing States?

(ii)   Peoples and individuals of the present and future generations?

In response to question a) the International Court of Justice addressed the legal obligations of states to protect the climate system under international law:

• Source of Law:

The Court relied on the entire corpus of international law as mandated by the UNGA, identified obligations under main environmental law treaties and relevant customary law.

• Obligations under Climate Treaties:

The Court identified States parties’ obligations to ensure the protection of the climate system comprises other obligations under the UNFCC, the Kyoto Protocol and the Paris Agreement such as:

-        The obligation to contribute to mitigation and adaptation under the standard of due diligence,

-       The obligation to prepare and pursue measures to achieve the temperature goal of limiting global warming to 1.5°C above pre-industrial levels under the Paris Agreement,

-       States’ obligations to cooperate, with principal modalities including financial assistance, technology transfer, and capacity building.

• Obligations under customary international law:

The Court found that customary international law (CIL) sets forth obligations for States to ensure the protection of the climate system and other parts of the environment from CO2 emissions. The Court bases this obligation on the customary duty to prevent significant harm to the environment but also on the principle of common but differentiated responsibilities. The Court clarified that treaty obligations and CIL retain independent legal force but should be interpreted coherently. Climate treaties and CIL inform each other’s content.

 Thus, States not parties to climate agreements may still be bound by CIL obligations shaped by the treaty regime and bear the burden of proving compliance.

• The recognition of Human rights

The Court recognizes the right to a clean, healthy, and sustainable environment as indispensable for the enjoyment of other human rights. Climate change significantly threatens rights to life, health, food, water, housing, particularly impacting vulnerable groups.

In response to question b) the ICJ clarified the legal consequences for States breaching their climate obligations, providing key insights on attribution, causation and the scope of State responsibility under international law.

• States’ responsibilities under Customary International Law:

The Court rejected the lex specialis argument and stated that the climate change treaties do not exclude the application of other rules of international law and affirmed that the general rules of state responsibility under CIL govern.

• Attribution of wrongful acts follows established International Law:

Acts or omissions of any state organ are attributable to the State. Failure to act – such as neglecting legislative measures - constitutes a breach of obligations. The Court also observed that States may be responsible where it has failed to exercise due diligence to limit quantity of emissions caused by private actors under its regulation. The wrongful act lies not in the GHG emissions per se but in a State’s failure to exercise due diligence to protect the climate system.  

• Specification for Causation

The Court recognized the challenges in linking State conduct to climate harm but reaffirmed that the required legal standard of “a sufficient direct and certain causal nexus” between the wrongful action and the damage is flexible enough for cumulative and transboundary impacts.

Therefore, to hold a State responsible, another States must follow two steps:

-       Establishing whether a climatic phenomenon is attributable to anthropogenic climate change (established by a general scientific assessment.

-       Determining the extent to which damage is attributable to the conduct of one or several States (a legal assessment).

The cumulative contributions of multiple States do not preclude individual accountability. Each injured State may invoke responsibility against one or several responsible States.

• Erga Omnes Parties Obligations

Another important take away is that since all States have a common interest in the protection of global environmental commons (like the atmosphere and the high seas), the Court considers that States’ obligations to protect the climate system and other parts of the environment from anthropogenic greenhouse gas emissions are obligations erga omnes, it applies to all States. Treaty-based obligations also have erga omnes partes character, giving all States parties a legal interest in compliance with core mitigation commitments.

• Legal consequences

Responsible States must:

-       Cease unlawful conduct and adopt measures to prevent recurrence.

-       Provide assurance and guarantees of non-repetition where appropriate.

-       Use all means available to reduce GHG emissions effectively.

-       While restitution (restoring the status quo ante) is often impractical for environmental harm, it may include ecosystem restoration where feasible. Otherwise, States may owe compensation, though quantifying climate-related damages remains complex.

During the proceedings in front of the ICJ, 91 written statements and 62 written comments were filed by States and international organizations and transmitted to the Court. It was the highest level of participation in the history of the Court.

The Green Rights Coalition had the opportunity to transmit an Amicus curiae to the Court which was signed by young volunteers from more than 34 different countries.

In this Amicus curiae, the Green Rights Coalition invited the Court to recognize the legal value of environmental rights, including the rights of present and future generations and the right to a healthy environment and to base States’ climate obligations on these human rights.

The advisory opinion, through its landmark recognition of environmental rights, echoes the arguments advanced in the Green Rights Coalition’s Amicus curiae and positions, alongside the ECHR and IACHR in embedding human rights within global climate governance.

This opinion is a milestone in clarifying states’ obligations and embedding environmental human rights at the heart of climate justice.”

As the Court stated, international law, while essential, is only one part of a broader response to the climate crisis. Addressing this unprecedented challenge requires global cooperation and collective human resolve.