By Cymie Payne
Professor, Rutgers University, Dept Human Ecology and Law School; ELI Fellow; Chair, Ocean Law Specialist Group, World Commission on Environmental Law, International Union for Conservation of Nature (IUCN)
The ICJ is the third influential international court to have weighed in with an advisory opinion on national climate change obligations, each offering its own perspective but in broad agreement. The July 23, 2025 opinion of the International Court of Justice (ICJ), sometimes called “the World Court” will be considered by many to be the most significant.
Cooperation is a central theme in the ICJ opinion. However, the opinion also offers new opportunities for contentious legal cases between states affected by climate change harms and states whose greenhouse gas (GHG) emissions have contributed the most to the problem by asserting that attribution is sufficiently advanced to allow determinations of causation and clarifying that a state can be in breach of its obligation to take measures to mitigate GHG emissions from private actors, specifically the fossil fuel industry.
Another important finding in this opinion addresses sea level rise’s effect on the stability of maritime zones under the UN Convention on the Law of the Sea (UNCLOS), and the related question of whether countries that lose their entire land territory can continue as sovereign states. The Court’s analysis of UNCLOS concludes with its determination that rising sea levels do not necessarily require moving the terrestrial baselines from which maritime boundaries are measured, and that once a State is established, the disappearance of one of its constituent elements (territory) would not necessarily entail the loss of its statehood, consistent with the recent report of the International Law Commission.
The UN General Assembly (UNGA) request asked the ICJ to consider the two chief human rights Covenants (ICCPR and ICESCR), the Universal Declaration of Human Rights, and the Convention on the Rights of the Child. Through its analysis of these and other human rights instruments, the Court concluded that the right to a clean, healthy and sustainable environment is essential to the enjoyment of other fundamental human rights, such as the right to life and the right to health. To reach this conclusion, it turned to the IPCC reports on the vulnerability of human populations to climate change, reinforcing the link between the science and the law.
The UNGA asked the ICJ to provide an advisory opinion on two issues: the obligations of States to ensure the protection of the climate system, and the legal consequences when States have caused significant harm to the climate system with respect to vulnerable states, especially small island states, and to present and future generations harmed by climate change.
The Obligations
The ICJ found that sovereign states have legal duties under both treaties they are party to and customary international law, and that some these require conformity to a particular result while others are “obligations of conduct”. In international law, obligations of conduct are evaluated according to a due diligence standard, which is variable depending on the level of risk present. In the case of GHG emissions, the risk is existential, and the obligation is therefore for states to do their utmost.
The ICJ states that the climate change treaties—UN Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, and Paris Agreement—are the principal legal instruments regulating the international response to climate change. The Court explains that the UNFCCC sets out objectives and general principles which are translated by Kyoto and Paris into specific interrelated obligations. They require countries that are parties to adopt mitigation and adaptation measures, and to cooperate: obligations of conduct. They also require developed states to report on their GHG sources and sinks, and control measures: obligations of result. The Paris Agreement sets the important goal of holding the temperature increase to 1.5°C, and it further requires parties to prepare, publish and maintain and account for their Nationally Determined Contributions, a procedural obligation of result. The content of the NDCs, the Court said, is not discretionary or self-defined, but is subject to the highest standard of due diligence, parties must do their utmost to collectively maintain a safe climate. The Court found that cooperation required under the Paris Agreement includes financial assistance, technology transfer and capacity-building.
Furthermore, parties to other international environmental agreements have obligations to protect the climate system and other parts of the environment from anthropogenic GHGs under those treaties.
For all states, whether or not they are parties to these treaties, customary international law also requires them to prevent significant harm to the environment by acting with due diligence, using all means at their disposal “to prevent activities carried out within their jurisdiction or control from causing significant harm to the climate system and other parts of the environment, in accordance with their common but differentiated responsibilities and respective capabilities;” and cooperating in good faith on sustained and continuous measures to prevent harm.
The Legal Consequences
A surprising turn in this opinion is its explicit application to the fossil fuel industry: state conduct in fossil fuel production and consumption, including granting of fossil fuel exploration licenses and subsidies, may constitute an internationally wrongful act attributable to the state. A state that fails to regulate private actors by adopting measures to limit their emissions will be in breach of its obligations.
The ICJ found that the rules of state responsibility apply: thus, a state must cease the wrongful behavior, assure that it will not be repeated, and provide reparation. The controversial question of loss and damage is referred to the discussion of state responsibility and reparations under that well-established customary international law doctrine.
In this part of the opinion, the Court was careful to underscore that a determination of responsibility would depend on the specific facts of a case. For example, the complaining government would have to provide evidence that a given event or trend that caused harm is attributable to climate change as well as to a specific state or states. In the ICJ’s view, “it is scientifically possible to determine each State’s total contribution to global emissions,” and “factual questions arising in the context of attribution and apportionment of responsibility are to be resolved on a case-by-case basis.” The Court found that climate change obligations are owed both to the international community as a whole (erga omnes) and to the other parties of particular treaties (erga omnes partes), offering a wide range of states that can invoke a breach of the obligation.
Implications
The authoritative legal interpretations provided by advisory opinions can be highly influential in guiding states’ behavior. Although they are only rarely legally binding, they indicate how the judges on that court would be likely to rule in a contentious case that raised the same issues. A strength of the ICJ is that its 15 judges are both globally representative and incorporate the major powers, including, for example, a U.S. judge and a Chinese judge.
Reactions to the ICJ opinion in the United States have ranged from optimism to outright skepticism. The United States has a complex relationship to international law in general and international courts in particular. The United States participates in cases at the ICJ, including in this advisory opinion. Judge Sarah Cleveland was nominated to the ICJ by the United States; there has always been a judge nominated by the United States on the Court. The United States is a party to the UNFCCC, and it has been a party at two different times to the Paris Agreement; most recently, President Trump withdrew the United States; and the United States signed but never became a party to both the Kyoto Protocol and the UN Convention on the Law of the Sea, two of the treaties analyzed in this opinion.
At this particular historical moment, when the use of power dominates U.S. domestic and foreign policy, it is well to remember that most of the nearly 200 other countries—more than half of whom submitted their views for the Court to consider—continue to cooperate on shared long-term interests and continue to seek to sustain the rule of law, and that many of them will reference this opinion in national legislation and litigation and in international negotiations.
Moreover, the ICJ’s unanimous opinion is consistent with the International Tribunal for the Law of the Sea (2024) opinion and the Inter-American Court of Human Rights opinion, although each court is reviewing somewhat different questions under different legal instruments. Blogs on the ITLOS opinion are here and on the IACtHR are here. In addition, other regional and national courts have spoken to these and similar questions.
The ICJ opinion concludes with a reflection on the role of the international judiciary—“important but ultimately limited”. This is a fair assessment. This advisory opinion (and the judges’ separate opinions and declarations) will also warrant close reading to evaluate the international law interpretations that are raised by climate change but that apply to other global issues.