CUSTOMARY INTERNATIONAL LAW AND THE DUTY TO PREVENT HARM – Indian Blog of International Law

Dr. Sudipta Roy Choudhwry

Introduction

In its Advisory Opinion on the Obligations of States in Respect of Climate Change (AO/Opinion), delivered on 23 July 2025, the International Court of Justice (ICJ/Court) clarified the binding legal duty to prevent significant transboundary environmental harm. This duty is now applicable as a climate obligation, sourced in customary international law (para 132). This blog article analyzes how the jurisprudence on the “duty to prevent significant transboundary harm” developed as a customary international law and how this duty is applicable to the climate crisis.

Duty to Prevent Significant Environmental Harm as an Erga Omnes Obligation

The Court affirmed that States are bound by general international law to act both individually and collectively to prevent environmental harm stemming from activities under their jurisdiction or control, even where such harm is cumulative and contributes to global phenomena such as climate change (paras. 314–315, 341–347). This obligation is not a matter of policy preference but a binding legal requirement. The Court explicitly described the obligation to prevent as erga omnes, meaning that it is owed to the international community as a whole and is not dependent on reciprocity (paras. 439–443). In (para 440), the Court affirmed that “obligations pertaining to the protection of the climate system and other parts of the environment from anthropogenic greenhouse gas emissions … are obligations erga omnes,” underscoring the climate system’s character as a shared global commons. This recognition anchors the duty to prevent significant environmental harm within the framework of collective responsibility owed to the international community as a whole. The Court further linked this duty to treaty-based commitments under the Paris Agreement, noting that states must ensure their climate action is “progressive and transparent” through the preparation, communication, and maintenance of Nationally Determined Contributions (NDCs) (para 240). Crucially, the Court confirmed that this duty to prevent significant harm applies in full to the context of climate change, thereby integrating customary international law obligations with specific treaty-based climate commitments (para 457).

In his Separate Declaration, Judge Dire Tladi underlined that the erga omnes character of the duty is “central to its universality.” He observed that “each State is under an obligation to take measures to prevent environmental harm irrespective of whether other States are taking similar measures” (para 34). He stressed that the failure to regulate harmful greenhouse gas emissions may constitute a breach of international law even before concrete harm materializes. His formulation reinforces that the duty to prevent is forward-looking and operates autonomously for each state.

The Court held that obligations “pertaining to the protection of the climate system and other parts of the environment from anthropogenic greenhouse gas emissions, in particular the obligation to prevent significant transboundary harm under customary international law,” are erga omnes obligations (para 440). Obligations under the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement were identified as erga omnes partes since they protect the essential interest of all States in safeguarding the climate system, which benefits the international community as a whole (para 440). This means that all States or all state parties have an interest in compliance and may invoke the responsibility of other States for nonfulfillment. While such invocation may be diplomatic, the Court opened the possibility of contentious proceedings. It clarified that a noninjured state cannot claim reparation for itself (para 443). For the first time, the ICJ explicitly recognized the legal standing (jus standi) of all States regarding erga omnes obligations under customary law, citing Article 48(1)(b) of the ILC’s Articles on Responsibility of States for Internationally Wrongful Acts (ICJ Advisory Opinion on Climate Change, para 442).

This position is consistent with the International Tribunal for the Law of the Sea (ITLOS) Advisory Opinion on Climate Change (2024), where the Tribunal found that anthropogenic greenhouse gas emissions constitute “pollution of the marine environment” under UNCLOS Part XII (paras. 179–188, 197, 203–207). ITLOS confirmed that States must exercise due diligence to prevent, reduce, and control such pollution, guided by the best available science and relevant international climate agreements. It also held that States with greater resources have heightened obligations (paras. 236–243, 269–275).

The ICJ’s reasoning also draws on its earlier jurisprudence. In the Pulp Mills on the River Uruguay (Argentina v. Uruguay) Judgment (2010), the Court found that the duty to prevent significant transboundary environmental harm is a general international law norm and that due diligence requires both appropriate measures and active vigilance and monitoring. The Court held that a breach requires either actual harm or a sufficiently serious risk (paras. 103–105, 203–205). In Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v Nicaragua) (2015), the Court reaffirmed that the obligation to prevent significant transboundary harm is part of customary international law and that preventive review is required even for seemingly minor activities (paras 104–105). In the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion (1996), the Court affirmed that States must ensure that activities within their jurisdiction respect the environment of other States or of areas beyond national control (para 29). In the Gabčíkovo-Nagymaros Project (Hungary v. Slovakia) Judgment (1997), the Court noted that vigilance and prevention are required because of the often irreversible character of environmental damage (para 140).

Collectively, these decisions establish a coherent legal framework in which the duty to prevent significant environmental harm, including through regulation of greenhouse gas emissions, is an erga omnes obligation binding on all states. States are therefore under a positive obligation to adopt all appropriate measures, such as regulating fossil fuel activities, enforcing mitigation standards, conducting environmental assessments, and promoting renewable energy alternatives to avert foreseeable harm. Failure to meet this due diligence standard, even in the absence of immediate damage, may constitute an internationally wrongful act. This convergence of judicial authority confirms that climate change is a defining context for the evolution of environmental preventive obligations under international law.

Environmental Impact Assessment, Due Diligence, and the Duty to Prevent Harm

The Court reaffirmed that Environmental Impact Assessment (EIA) is a procedural cornerstone of the customary duty of due diligence to prevent significant harm. Building on Pulp Mills on the River Uruguay (paras 101–105, 203–205) and Certain Activities Carried Out by Nicaragua in the Border Area (paras 104–105), the Court confirmed that an EIA is required whenever there is a risk of serious transboundary harm, and extends this requirement to encompass climate change, biodiversity degradation, and human rights impacts arising from anthropogenic greenhouse gas emissions (paras 240, 440, 457).

In this Opinion, the EIA obligation is no longer confined to project-specific, localised risks, but now covers activities and policies with cumulative or indirect effects, including those that contribute to global atmospheric change. States must integrate climate harms, ecosystem and biodiversity (EBV) harms, and human rights harms into their assessments before authorising activities. This expansion means that EIAs should quantify cumulative greenhouse gas emissions and assess their projected climate impacts, evaluate biodiversity and ecosystem risks including the integrity of carbon sinks and reservoirs, and identify and address human rights impacts, particularly to life, health, water, food, housing, and cultural rights, where climate effects pose foreseeable threats. EIAs must also rely on best available science and be periodically updated to ensure the continuing adequacy of mitigation and adaptation measures.

The Opinion’s integration of human rights, environmental, and climate harm within a single prevention framework underscores that failure to conduct a climate-integrated EIA may constitute a breach of the due diligence standard. This is consistent with the ITLOS Advisory Opinion on Climate Change, which recognised greenhouse gas emissions as marine pollution triggering strict prevention duties under UNCLOS (paras 179–188, 197, 203–207, 236–243, 269–275).

The Cleveland–Bhandari Joint Declaration and Its Relevance

The Joint Declaration of Judges Sarah Cleveland and Dalveer Bhandari, appended to the Advisory Opinion, adds substantive clarity to how the duty to prevent harm should operate in practice. It emphasises that effective prevention demands EIAs that assess the full fossil fuel value chain, cumulative emissions, downstream effects, and impacts on carbon sinks. The Declaration also requires that EIAs explicitly integrate human rights assessments where climate change poses foreseeable risks to fundamental rights.

Judge Cleveland’s separate remarks stressed the legal necessity of safeguarding and restoring carbon sinks and reservoirs as part of mitigation obligations. Taken together, these views sharpen the Court’s general guidance by setting out practical benchmarks for lawful EIAs in the climate era: they must be lifecycle-based, biodiversity-sensitive, rights-oriented, transparent, and regularly updated. By doing so, the Joint Declaration links international legal standards directly to domestic regulatory duties, signaling that States which fail to adopt enhanced EIA methodologies risk violating the customary duty to prevent significant harm as now understood in the climate context.

Conclusion

The Opinion decisively reframes the duty to prevent significant transboundary environmental harm as a customary obligation that applies squarely to climate change and to greenhouse gas emissions, and it situates Environmental Impact Assessment at the procedural core of due diligence (paras. 240, 314–315, 341–347, 440, 457). Read with the Court’s earlier rulings in Pulp Mills and Certain Activities (see Pulp Mills, paras. 101–105, 203–205; Certain Activities, paras. 104–105), and with ITLOS’s parallel finding that anthropogenic emissions constitute marine pollution (ITLOS AO, paras. 179–188, 197, 203–207), a coherent preventive regime emerges. The Opinion’s recognition of erga omnes obligations and of the jus standi of all States under customary law, together with the Court’s citation of Article 48(1)(b) of the ILC Articles on State Responsibility (para 442), opens collective avenues for accountability while preserving the limits on reparation for non-injured States (para 443). Judicial interventions, notably Judge Dire Tladi’s separate declaration and the Joint Declaration of Judges Cleveland and Bhandari, sharpen the duty’s anticipatory and science driven content, emphasizing the need to assess cumulative and value chain effects, carbon sinks, and human rights implications in EIA practice. For practitioners and policymakers in India and beyond, the practical lesson is unambiguous: treaty compliance is necessary but not sufficient; States must operationalize the prevention duty through robust, transparent, and periodically updated EIA procedures, targeted regulation of high emission activities, and sustained monitoring informed by the best available science. In so doing States will better align domestic decision making with their international obligations and reduce the risk of internationally wrongful acts for failures of prevention.

(Dr. Sudipta Roy Choudhwry is an Assistant Professor at the Amity Law School, AUUP, Noida.)