A Comparative Analysis of ITLOS and ICJ Advisory Opinions – Indian Blog of International Law

Meenakshi Priya

Small Island Developing States (SIDS) have sought advisory opinions on climate change from various international judicial bodies, resulting in three landmark rulings delivered within a year. The International Tribunal for the Law of the Sea (ITLOS) was the first to issue its Advisory Opinion on Climate Change on 21 May 2024, followed by the Inter-American Court of Human Rights (IACtHR), which delivered its Advisory Opinion on Climate Emergency and Human Rights on 3 July 2025 (In this analysis, I am not contrasting with IACtHR Advisory Opinion). Finally, the international Court of Justice (ICJ) delivered its long-awaited Advisory Opinion on Climate Change on 25 July 2025, on the initiative of SIDS through the United Nations General Assembly. Interestingly, all three advisory opinions were delivered unanimously, reflecting a strong consensus among international judicial bodies on the urgent legal responsibilities of states to prevent and mitigate climate change impacts.

ITLOS Advisory Opinion on Climate Change and International Law

In its ITLOS Advisory Opinion on climate change, the Tribunal significantly advanced its interpretation of States’ due diligence obligations under United Nations Convention on the Law of the Sea (UNCLOS), particularly regarding marine pollution arising from anthropogenic greenhouse gas emissions. The Tribunal established a due diligence standard as “stringent”, reflective of the high risk of serious and potentially irreversible harm posed by climate change and ocean acidification to the marine environment (para 243 of ITLOS AO). This sui generis interpretation goes beyond earlier formulations by explicitly linking stringency to the precautionary approach, making clear that States cannot rely on scientific uncertainty as a justification for inaction. They are required to take robust preventive and proactive measures even when scientific uncertainty persists regarding the extent and nature of future harm. (para 242 of ITLOS AO and see the commentary here)

ITLOS further clarified that while the stringent standard applies to all States, its implementation is context-dependent: States are having greater capabilities and resources are expected to be more cautious, whereas those with fewer resources may implement the standard within their means, but the fundamental obligation to effectively prevent, reduce, and control marine pollution remains (paras 225-229 of ITLOS AO). Moreover, the Tribunal emphasized that measures taken must be rooted in the best available science, and that obligations under international frameworks such as the Paris Agreement or UNFCCC are relevant but do not, on their own, satisfy the requirements of UNCLOS (para 243 of ITLOS AO).

States are thus required to establish and maintain comprehensive legal and administrative frameworks, going beyond mere enactment to ensure effective enforcement, monitoring, and control over all relevant activities under their jurisdiction. Where the risk of significant transboundary harm exists, the required level of due diligence may rise even further. Although ITLOS did not provide an exhaustive checklist of specific actions constituting “stringent” due diligence, it set clear legal benchmarks that States must take robust, science-based action, adjusting their efforts to their capacities, but always to prevent, protect, reduce, and control the marine environment from GHG-related harm.

ICJ Advisory Opinion on the Obligations of States with respect to Climate Change  

In its Advisory Opinion, the ICJ relied on multiple international conventions and customary law obligations that collectively shape the evolving framework of international environmental law and climate governance. The Court affirmed that States have binding, substantive, and enforceable obligations to prevent significant environmental harm, to cooperate internationally, and to uphold fundamental human rights in the context of escalating climate risks (paras 369-404 of ICJ AO).

Moreover, the ICJ also endorsed ITLOS’s interpretation that the due diligence obligation must meet a stringent standard. States must act urgently, guided by the “best available science”, such as IPCC findings (para 347 of ICJ AO). This encompasses not only the formulation and periodic enhancement of robust national climate plans but also includes the regulation of private actors, even in cases where the State is not a party to relevant climate change conventions, and the provision of support to vulnerable nations. The Court grounded its reasoning in customary international law to interpret and apply obligations under the climate change conventions. Significantly, the ICJ emphasized that inaction or insufficient action may amount to an internationally wrongful act, thereby invoking the law of State responsibility (para 409 of ICJ AO).

Further, the ICJ reaffirmed that due diligence is an obligation of conduct, meaning that States are required to act with a certain standard of care, rather than to guarantee a specific result. This position builds on earlier jurisprudence, including the Pulp Mill Case, Bosnia and Herzegovina Genocide Case, and is consistent with the approach taken in the ITLOS Advisory Opinion on Climate Change. The Court also acknowledged that the distinction between obligations of conduct and obligations of result is not absolute. Certain international obligations may embody features of both, depending on their context and formulation. Moreover, the Court clarified that neither type of obligation is inherently more burdensome; rather, they often coexist and serve complementary roles, pursuing the same overarching objectives through different legal modalities. (para 175 of ICJ AO)

The ICJ identified key elements that define the scope and content of States’ due diligence obligations in a context including climate change. The Court observed that States must adopt legal and regulatory frameworks to achieve deep, rapid, and sustained reductions in Greenhouse gas (GHG) emissions. States are expected to assess and utilize the best scientific and technological information, conduct risk and impact assessments, and take preventive action despite uncertainty. States must notify and consult if activities could cause transboundary harm. These obligations are influenced by both binding international agreements and relevant non-binding norms, including decisions made at climate treaty conferences and recognized best practices. The ICJ emphasised that the principle of Common but Differentiated Responsibilities and Respective Capabilities shapes the standard of care required: while each State’s efforts are tailored to its own resources and capacities, all States must act to the fullest extent possible. Finally, the ICJ reaffirmed the necessity of sustained international cooperation, including providing financial and technical support to vulnerable nations. (paras 280-300 of ICJ AO) 

Conclusion

ITLOS and ICJ both affirm that States have stringent due diligence obligations to take appropriate measures to address climate change impacts. ITLOS’s interpretation prioritizes the protection of the marine environment by applying UNCLOS obligations to GHGs. It defined “stringent” due diligence as a context-specific obligation, proportionate to each State’s capabilities, anchored in the precautionary principle. In contrast, the ICJ adopted a broader and more integrated approach, embedding climate-related due diligence in conventions and customary international law, including human rights. Together, these advisory opinions mark a significant step forward in shaping the future of international climate governance, offering a legal foundation for more robust and coordinated global action.

(Meenakshi Priya is a Research Scholar in International Law at South Asian University, New Delhi. Her research focuses on the protection and preservation of the marine environment, with a particular emphasis on the intersection of the Law of the Sea, Climate Change, and Environmental Principles.)