Cooperating for climate measures as a duty under international law – Indian Blog of International Law

Ojaswini Gupta

Introduction

The International Court of Justice’s (ICJ) Advisory Opinion (AO) has established the duty to co-operate as a foundational element of the international response to climate change—grounded in both treaty and customary international law, and no longer considered a matter of policy discretion. This obligation is anchored in key climate agreements, chiefly the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement, with the Court clarifying that the duty to co-operate is not just a procedural aspiration, but a legal requirement demanding active, sustained, and good-faith engagement among States.

The Duty to Co-operate in Treaty Law

The AO affirms that the duty to co-operate is a binding treaty obligation under key multilateral environmental agreements, especially the UNFCCC and the Paris Agreement. The Preambular statement in the UNFCCC “calls for the widest possible co-operation by all countries” (para 63). The Court makes clear that States Parties to the UNFCCC have a legal duty to co-operate in fulfilling the Convention’s core objective of taking action to reduce greenhouse gas (GHG) emissions under the Convention, primarily UNFCCCC, preamble, Article 3, paragraph 5, Article 4, paragraph 7, and Articles 5, 6 and 9 (para 183).

The Paris Agreement strengthens this cooperative framework within specific areas: adaptation (Article 7), finance (Article 9), and technology (Article 10), institutionalized through mechanisms like the Green Climate Fund and the Technology Mechanism. The Court emphasizes that cooperation under these agreements must be ongoing, meaningful, and conducted in good faith—mere token gestures are insufficient, especially when other States’ ability to meet obligations depends on robust cooperation.

Further, the Court applied this binding duty to Article 197 of the UN Convention on the Law of the Sea (UNCLOS), requiring States to cooperate globally and regionally to protect the marine environment from climate impacts. This expands the duty beyond climate-specific treaties to overlapping environmental regimes with shared vulnerabilities and responsibilities.

These legal requirements are firmly rooted in the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC) (see para 13.2.1), which demands that co-operation reflect States’ historical emissions, economic capacities, and development needs. Developed countries, therefore, must not only act domestically but also facilitate action worldwide, particularly by supporting developing States with finance, knowledge, and technology. The ICJ situated co-operation as a practical mechanism bridging ambition and ability, ensuring climate action is both equitable and effective.

The Duty to Co-operate in Customary International Law

Beyond treaties, the ICJ also recognized the duty to co-operate as a binding element of customary international law (CIL). This means the obligation extends to all States, including those that are not parties to specific treaties. The Court specifies that this duty is not met by ad hoc or symbolic acts, but through persistent, good-faith engagement, procedural transparency, and substantive coordination.

The Court’s earlier decisions in Pulp Mills on the River Uruguay (2010) and Gabčíkovo-Nagymaros Project (1997) recognized co-operation as an essential element in the prevention of transboundary environmental harm. These cases established that States must not only refrain from causing significant harm, but also actively consult, notify, and exchange relevant information with potentially affected States.

The current AO extends this rationale to the global atmosphere, reflecting the universal and interconnected nature of the climate crisis. By locating the duty to co-operate within customary international law, the Court broadens its scope of application. Unlike treaty obligations, which only bind States parties, customary norms apply to all States, irrespective of ratification.  The inclusion of “good faith” as a legal standard further demands genuine, transparent, and constructive participation, not mere box-ticking.

Reflections from Separate and Concurring Opinions

Although the Court unanimously affirmed the duty to co-operate as binding under both treaty and customary international law, the separate and concurring opinions add crucial interpretive nuance.

In their joint declaration, Judges Charlesworth, Brant, Cleveland, and Aurescu emphasize that the duty to co-operate under customary international law exists independently from obligations under the climate change treaties, and that compliance with treaty obligations cannot be assumed to satisfy customary obligations by default (paras. 1–6, 12–13). They argue that the treaties and custom are complementary but distinct, with custom applying more broadly and often more stringently, particularly in relation to non-party States or gaps within treaty regimes (paras. 7–10). Thus, the duty to co-operate must be evaluated on its own legal footing, not merely through the lens of treaty compliance—a point that reinforces the legal autonomy and normative force of customary international law in climate governance.

Judge Cleveland highlighted that preserving carbon sinks like forests and oceans entails both substantive and procedural obligations of co-operation, particularly from more capable States to less-resourced ones (paras. 4–13), and extends this duty to include transparent assessment and mitigation of emissions from armed conflict and military activity (paras. 14–20). Judge Sebutinde similarly stressed that co-operation requires not just universal participation, but differentiated responsibilities and material support aligned with the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC), criticizing the Court for sidelining innovative, cooperative remedies proposed by vulnerable States, such as debt relief and technology transfers (paras. 9–12). Judge Tladi, in turn, faulted the Court for failing to follow through on the legal consequences of obligations it identified as erga omnes, noting the absence of discussion on duties of non-recognition, non-assistance, and co-operation (paras. 34–36), and warned that this selective treatment signals a deeper jurisprudential incoherence, especially when viewed against the Court’s earlier conflation of erga omnes and jus cogens norms (para. 37).

Together, these judicial reflections move the duty to co-operate beyond its abstract formulation in international law, shaping it into a multidimensional standard, capable of responding to the asymmetries and urgencies of the climate crisis.

The AO marks a pivotal shift in international climate law by elevating the duty to co-operate from a diplomatic ideal to a binding legal obligation grounded in both treaty and custom. It recognizes co-operation as the structural core of climate governance—procedural in its call for transparency and dialogue, and substantive in its demand for material support, capacity-building, and shared responsibility, particularly by more capable States. Framed by principles of differentiated responsibilities and global equity, the Court’s approach aligns with contemporary trends that view co-operation as dynamic and outcome-oriented. While judicial perspectives diverge on its implementation, there is consensus that co-operation must now serve as both a legal and moral compass for climate action—anchored in justice, driven by solidarity, and essential to a truly effective international climate regime.

(Ojaswini Gupta is a Master’s Candidate for LLM in Public International Law at University of Groningen, Netherlands.)