On July 23, 2025, the International Court of Justice (ICJ) issued a landmark advisory opinion, adopted unanimously, on the international legal obligations of states concerning climate change. This long-anticipated decision, requested by the United Nations General Assembly (UNGA) in 2023, and led by a coalition of Pacific island nations spearheaded by Vanuatu, marks a pivotal moment in the legal framing of climate responsibility. For the first time, the ICJ has clarified how existing international law, spanning environmental treaties, customary norms, and human rights obligations, applies to the climate crisis. The opinion brings legal precision to what has long been seen as a domain of political negotiation and moral appeal.

Key Findings  

The Court affirmed that states have legal obligations to prevent, reduce, and redress the adverse effects of climate change. These obligations arise not only under specific climate treaties such as the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement, but also under customary international law and international human rights law.

Importantly, the ICJ emphasized the principle of “due diligence,” requiring states to take all reasonable measures to prevent significant transboundary environmental harm, particularly in light of the best available scientific knowledge—such as the findings of the Intergovernmental Panel on Climate Change (IPCC). The Court also reaffirmed the principle of “common but differentiated responsibilities and respective capabilities (CBDR-RC),” acknowledging historical emissions and varying capacities among countries. 

Furthermore, the Court linked climate obligations with the protection of fundamental human rights, especially the rights to life, health, food, and a healthy environment, thus reinforcing the emerging intersection between climate and human rights law.

Legal Debates

While the ICJ’s advisory opinion brings important clarity to the legal foundations of climate responsibility, it also raises a series of deeper legal debates that merit closer examination.

First, the Court did not definitively establish when states’ international legal obligations related to climate change began to take effect, nor when breaches may have occurred. Clarifying the temporal scope of responsibility, whether grounded in treaties, customary law, or general principles, remains a complex and contested issue, especially for claims involving historical emissions.

Second, the opinion affirms the applicability of the “no-harm principle” in customary international law to climate change but stops short of precisely defining the extent of state duties under this principle or its relationship to treaty obligations under the UNFCCC and Paris Agreement. Whether treaty norms codify, supplement, or override customary rules remains unresolved.

Third, while the Court recognized general obligations under the UN climate regime—such as mitigation and international cooperation—it offered limited analysis of key treaty obligations like climate finance, adaptation, and technology transfer. This cautious approach leaves room for future legal interpretation but also prompts debate over the sufficiency of the Court’s articulation of state duties.

Fourth, the methodology the Court employed to identify and interpret customary international law, particularly in relation to the law of the sea and human rights law, is of great legal interest. While the opinion cross-references the UN Convention on the Law of the Sea (UNCLOS) and various human rights treaties, it provides limited analysis of how specific climate-related norms have crystallized through general and consistent state practice accompanied by opinio juris.  

Fifth, in addressing state responsibility and legal consequences, the Court acknowledged the potential for breaches to incur responsibility but refrained from elaborating on remedial measures such as reparation, compensation, or guarantees of non-repetition. This reserved treatment reflects judicial prudence but also signals that further doctrinal development is necessary.

These ongoing legal debates underscore the evolving nature of international climate law and the complex interpretive challenges it presents. While the ICJ’s opinion lays important groundwork, it simultaneously highlights the pressing need for continued legal scholarship, strategic litigation, and international negotiation to further clarify the contours of state responsibility in the face of a warming planet.

Political Significance

While the Court’s reasoning prompts important legal debates, its implications extend beyond jurisprudence to the political and normative dimensions of global climate governance. Although advisory opinions are not legally binding, they hold considerable legal authority and moral weight—especially when issued by the principal judicial organ of the United Nations. This particular opinion may serve as a persuasive interpretive tool in shaping the trajectory of future climate litigation, informing treaty negotiations, and guiding national climate policy.

For developing countries and climate-vulnerable small island states, the opinion marks a diplomatic and legal milestone. It bolsters their calls for climate justice by reaffirming that states contributing to climate change are accountable under international law—not just broadly, but through specific obligations such as prevention, mitigation, and reparation.

For developed countries, particularly major emitters, the opinion intensifies pressure to translate climate pledges into enforceable legal commitments. It also signals to domestic courts and civil society actors that failure to act—or inadequate action—on climate change may now amount to a breach of international legal obligations. This could further encourage strategic climate litigation aimed at holding governments and corporations accountable for their contributions to global warming.

Moreover, the opinion reinforces the integration of climate obligations within the broader framework of international law—including human rights law, environmental law, and the law of state responsibility. In doing so, it contributes to greater legal coherence in an otherwise fragmented system of climate governance.

Practical Obstacles to Implementation

While the advisory opinion has great potential, it also faces significant practical and political challenges. 

First, as a non-binding instrument, its legal weight depends on how states, courts, and international bodies choose to interpret and apply it. Deep divergences persist in how countries understand concepts such as “equity,” “fairness,” and “historical responsibility,” making it difficult to translate the opinion’s principles into consistent global action.

Second, the Court exercised judicial restraint by avoiding prescriptive emission targets or a clear hierarchy of obligations among states. This cautious approach leaves unresolved ongoing disputes over the level of ambition required and the fairness of burden-sharing. It also sidesteps the complex task of aligning climate obligations with other international legal regimes, such as trade, investment, and energy frameworks, where conflicts may arise.

Finally, the opinion offers no enforcement mechanism. Even where obligations are clearly articulated, the current international legal system lacks effective tools to ensure compliance. This gap underscores the crucial role national courts, regional bodies, and political coalitions must play in turning legal interpretation into tangible climate action.

Toward a Climate Rule of Law

The ICJ’s advisory opinion on climate change represents a decisive step toward the legal codification of climate responsibility. It marks a shift from voluntary climate commitments to normative legal expectations grounded in international law. By framing climate change as not only a scientific and political challenge but also a legal one, the Court has paved the way for a more structured and just approach to global climate governance.

For countries in the Global South, the opinion provides a robust tool to pursue climate justice and demand accountability. For all states, it serves as a reminder that climate inaction is not only a policy failure, it may also constitute a legal violation. As the world faces escalating climate risks, this opinion lays a foundation for building a climate rule of law where responsibility is shared, obligations are clear, and justice is no longer optional.

The ICJ’s opinion also arrives amid a growing global wave of climate litigation, where courts are increasingly called upon to adjudicate climate-related harms, clarify legal duties, and compel action. From youth-led lawsuits to claims rooted in constitutional and human rights law, litigation has become a strategic tool for shaping policy and enforcing accountability. With the Court now affirming that states have legal obligations tied to climate change, the groundwork is laid for even more assertive legal strategies to address climate injustice and to ensure that governments do not merely promise action—but deliver it.

This issue’s Flagship Analysis was written by Nong Hong, Executive Director at ICAS on August 5, 2025.