On Wednesday, 23 July 2025, the International Court of Justice (ICJ) issued a long-awaited Advisory Opinion (AO) regarding the obligations of States in respect of climate change. The ICJ, which is the United Nations' highest court, can issue legal advice called Advisory Opinions (AOs). While AOs are not legally binding like the court's actual judgments in regular cases, they still carry significant weight. They come from the world's top court, so AOs strongly influence how international law is understood and used, giving them significant “political pull”.
Three years ago, the Republic of Vanuatu led an initiative demanding the UN General Assembly to request an AO from the ICJ elaborating on States’ obligations concerning climate change, resulting in Resolution 77/276. The request posed two major questions to the court. First, it asked for the “obligations of States under international law to ensure the protection of the climate system and other parts of the environment”. Second, it inquired about the legal consequences for breaching those obligations.
While climate litigation in national jurisdictions has been a growing field in the past decades, until recently there were no cases regarding climate change at international courts. This is changing as the ICJ follows the publication of two other Advisory Opinions by international courts: The International Tribunal for the Law of the Sea and the Inter-American Court of Human Rights. A fourth request for an AO is currently pending at the African Court of Human and Peoples’ Rights.
Key takeaways from the Advisory Opinion – A call to action for climate?
The advisory proceeding was the biggest in the history of the ICJ, drawing an unprecedented 91 written statements from countries and organizations. So, what is in the more than 100-page document that judges put forward? In the unanimous decision, the ICJ expressed clearly that States have a legal obligation under international law to act on climate change. Not only are States obliged to act, but their actions must also be of the highest ambition. Besides confirming this legal obligation for ambitious action on the climate crisis, the court affirmed that the goal of limiting global warming to 1.5 °C, as enshrined in the Paris Agreement, is legally binding. It puts an emphasis on the duty to cooperate, referring to financial assistance, technology transfer and capacity-building as possible forms of cooperation, pointing out that climate action is a joint task. Furthermore, the opinion paves the way for reparations for breaching obligations to prevent harm to the climate system. The ICJ also acknowledged that human rights are inseparable from climate matters.
Clarifying the relationship between climate treaties and other international law
After intense debates, the AO is providing clarity in regards to the relation between the UN climate treaties – the United Nations Framework Convention on Climate Change, the Kyoto Protocol and the Paris Agreement – and other international law, including customary rules of international law. The court opines that the rule of lex specialis does not apply here. The legal interpretation principle of lex specialis means that a specific legal norm overrides more general norms. However, this is only the case if the general and the specific norm govern the same factual matter in inconsistent or conflicting ways. The court found no such conflict between climate treaties and other international law. In non-legal words, the three climate treaties are not the single legal source defining the obligations States have regarding climate crisis. Rather, they are complemented by customary international law, its principles and other international treaties that are relevant for the climate, such as the UN Convention on the Law of the Sea.
With this the ICJ provides two important clarifications: First, legal obligations to protect the climate apply, no matter whether a specific State is currently a Party to a climate treaty or not. This is because these obligations do not solely derive from the treaty, but they are also anchored in other international law. Second, this means that States are not limited to UN climate treaties when seeking redress for climate harm. Such argumentation could give new momentum to addressing loss and damage, including seeking pledges to the Fund for Responding to Loss and Damage.
What does the Advisory Opinion mean for COP 30 and the long-term legal momentum for climate action?
The Opinion is an important signal ahead of COP 30 to strengthen the rule-based order and international law to address climate change. COP 30 is expected to deliver strengthened climate plans in the form of Nationally Determined Contributions – the NDCs 3.0. Here, the ICJ provides a powerful legal foundation for demanding more ambition. First, it clarifies that States need to act with due diligence. This means that simply formulating an NDC is insufficient; countries genuinely need to strive to achieve and implement their commitments. While countries have discretion in determining how they cut emissions, the ICJ confirms that NDCs are subjected to scrutiny in terms of their robustness. Second, the ICJ formulates that it is a legal duty to present NDCs in line with the 1.5 °C temperature goal. Third, it defines the failure to take appropriate action regarding climate change as legally wrongful acts, including actions such as investing in fossil fuel production, granting fossil fuel exploration licenses or the provision of fossil fuel subsidies. All strengthens the scope and relevance of NDCs also as a legal instrument and further enhance climate policy. The ICJ Advisory Opinion transforms the climate crisis from primarily a political or ethical challenge into a clear legal obligation for States. It provides a robust legal foundation that will accelerate climate action, strengthen legal challenges in international and domestic courts and shape the future of international climate law.
