On 23 July 2025, the International Court of Justice (the “Court”) issued its highly anticipated advisory opinion on Obligations of States in respect of Climate Change.  In a lengthy, unanimous opinion, the Court clarified States’ obligations under international law “to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases,” and addressed the legal consequences of violations of those obligations.  Opinion, para. 40.  Significantly, the Court also stated that the consequences of climate change “underscore [its] urgent and existential threat.”  Opinion, para. 73.

The Court’s advisory opinions are not binding, but they carry significant legal weight.  With this opinion, the Court has delineated States’ obligations with respect to climate change under existing international law, including—significantly—a wider-ranging due diligence obligation.  The Court has also confirmed that failure to comply with those obligations engages a State’s responsibility.  At the same time, the Court has opened the door for a new class of potential claims to be filed against States.  While there are jurisdictional challenges that limit the likelihood of a significant increase in the number of climate-change cases before the Court, it remains to be seen whether this new legal risk will meaningfully affect State climate change mitigation policies, including the regulation of private actors.

The Opinion extensively focused on the scope and content of States’ obligations under existing international instruments, including the climate change treaties—such as the United Nations Framework Convention on Climate Change (“UNFCC”) and Paris Agreement—as well as under customary international law.  Key aspects of the Opinion include:

• Binding Obligations Under the Climate Change Treaties: The climate-change treaties establish binding obligations for State parties.  This includes an obligation, under the Paris Agreement, to “prepare, communicate and maintain successive and progressive nationally determined contributions which, inter alia, when taken together, are capable of achieving the temperature goal of limiting global warming to 1.5°C above pre-industrial levels.”  Opinion, para. 457(f).

• Due Diligence: States have a “stringent” due diligence obligation under customary international law (“CIL”) “to prevent significant harm to the climate system,” not only through “the adoption of appropriate rules and measures, but also a certain level of vigilance in their enforcement and the exercise of administrative control.”  Opinion, paras. 138, 409.  The Court also confirmed that “due diligence” includes the “obligation of States to regulate the activities of private actors.”  Opinion, para. 428.

The Opinion is less well-developed on issues of State responsibility but, significantly, held that a violation of any of the obligations identified constitutes an internationally wrongful act entailing State responsibility, which may include an obligation of full reparation to injured States.  Opinion, paras. 444–445.

The issue of causation of harm had been heavily argued by participating States.  On this issue, the Opinion’s key findings include:

• Causation of damage is “not a requirement for the determination of responsibility as such” (state responsibility requires only a showing of an “internationally wrongful act and its attribution to a State, whether the act causes harm or not.”)  Opinion, para. 433.  Rather, causation “plays a role in determining reparation.”  Id.

• In the climate change context, causation involves “two distinct elements”: (i) whether a specific climate event can be attributed to anthropogenic climate change; and (ii) whether damage resulting from such an event may be attributed to a specific State or group of States.  Opinion, para. 437.

• The Court explained that the first element of this two-part test “may be addressed by recourse to science,” while the second element must be established in concreto.  Opinion, para. 437.  In respect of the second element, an applicant must still establish “a sufficiently direct and certain causal nexus” between the alleged wrongful act or omission and alleged harm.  Id.

The Court also acknowledged that “the causal link between the wrongful actions or omissions of a State and the harm arising from climate change is more tenuous than in the case of local sources of pollution,” but concluded that “this does not mean that the identification of a causal link is impossible in the climate change context.” Opinion, para. 438 (emphasis added).  This conclusion may be a signal that the Court would be willing to apply a lower standard of proof in this specific context.

The Court also held that certain obligations, including the obligation “to prevent significant transboundary harm” under CIL are owed to every State and any State may invoke another State’s responsibility for a breach of that obligation—even if they are not directly harmed. Opinion, paras. 439–443.  Similarly, the Court held that the State obligations in the UNFCC and Paris Agreement apply to all parties to those treaties and that any party may invoke state responsibility for violations of those obligations.  Opinion, para. 440.  While these findings are significant, any applicant would still need to demonstrate the Court has jurisdiction, whether based on the specific treaty invoked, a special agreement, or acceptance of the Court’s compulsory jurisdiction.  The Court’s Opinion may be found here and its official summary here.