The ICJ’s Advisory Opinion on Climate Change

“An existential threat” – this is how the International Court of Justice (ICJ) characterized climate change in its long-awaited advisory opinion on the obligations of States with respect to climate change. In the most significant development in international climate law since the adoption of the Paris Agreement, the ICJ outlined numerous obligations that could significantly shape the contours of international environmental law and global climate governance.

The ICJ’s message is clear: climate obligations are not aspirational – they are legal, substantive, and enforceable. Drawing from an array of legal sources and rejecting arguments based on the lex specialis principle aimed at limiting the rules applicable to climate governance, the ICJ affirmed that States have binding obligations to prevent significant environmental harm, cooperate internationally, and uphold fundamental rights in the face of escalating climate risks. These duties extend to all states, and the climate system must be protected for present and future generations.

Among the most consequential aspects of the opinion is the Court’s articulation of a stringent due diligence standard. Based on the scientific consensus as established in the work of the Intergovernmental Panel on Climate Change (IPCC) – understood as “the best available science on the causes, nature and consequences of climate change” (para. 74) – states must act urgently. This includes not only setting and updating robust national climate plans under the Paris Agreement but also regulating private actors and providing support to more vulnerable nations. Inaction, or failure to act decisively, may constitute an internationally wrongful act – triggering consequences under the law of state responsibility.

Strikingly, the ICJ adopted the entire operative part of the opinion unanimously – a rare feat that underscores the global consensus around the legal obligations at stake. With climate litigation surging globally, the Court has laid down a clear and united marker: climate obligations are real, and the law has a role to play.

There is no question that the ICJ’s advisory opinion will shape climate governance in the years to come. The specific ways in which it will influence policy and practice will undoubtedly become the focus of intense debate across academia, courts, bureaucracies, and civil society in the months and years ahead.  But before that, the advisory opinion must be analyzed and discussed in its various dimensions. For this reason, the Sabin Center for Climate Change Law’s Climate Law Blog and Verfassungsblog are launching a joint blog symposium, inviting leading experts to analyze selected aspects of the advisory opinion. This introductory blog post briefly outlines the background of the advisory opinion and the ICJ’s most important findings, setting the stage for the discussion ahead.

From Vanuatu to The Hague

Requested by the United Nations General Assembly following a campaign initiated by law students at the University of South Pacific, then led by Vanuatu and supported by over 130 states, the opinion marks the first time the ICJ has authoritatively interpreted the legal duties of States in relation to climate change mitigation, adaptation, and transboundary harm (for background, see here and here).

Prior to issuing its advisory opinion, the ICJ received over a hundred  written and oral submissions, which revealed both unprecedented global engagement with the legal dimensions of the climate crisis and deep divisions over how international law should respond (see here, here, and here). Nearly 100 States and multiple international organizations participated in the ICJ’s proceedings, offering sharply contrasting views on the applicable legal framework, the content and reach of States’ obligations, and the legal consequences of climate harm.

The submissions reflected competing legal paradigms: one that views international law as a living tool for advancing climate justice and accountability, and another that emphasizes legal caution, treaty limits, and political sensitivities. The ICJ’s opinion was expected to navigate these tensions carefully – seeking legal clarity while acknowledging the complexity of the climate crisis and its evolving legal terrain. Instead, the ICJ’s responses went far beyond this careful approach, delivering a bold and far-reaching opinion, which is poised to influence climate policy well into the future.

Scope of State Obligations and Legal Framework Clarified

The ICJ began by noting that it had jurisdiction to hear the case and that there were no compelling reasons to decline to answer the questions submitted by the General Assembly. It also reviewed the relevant scientific literature, drawing heavily from the work of the IPCC. Thereafter, the ICJ addressed the question of the “most directly relevant applicable law” that governs the questions (paras. 113 ff). The ICJ identified a comprehensive legal framework that includes the Charter of the United Nations, climate change treaties (such as the United Nations Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, and Paris Agreement), the United Nations Convention on the Law of the Sea (UNCLOS), and other environmental treaties like the Ozone Layer Convention, the Montreal Protocol, the Kigali Amendment to the Montreal Protocol, the Convention on Biological Diversity, and the Convention to Combat Desertification. Customary international law duties also come into play, especially the duty to prevent significant harm to the environment (paras. 132-139) and the duty to cooperate for environmental protection (paras. 140-142). Notably, the ICJ found that international human rights law is part of the most directly relevant applicable law, including rights to life, health, housing, food, and a clean, healthy, and sustainable environment. Other principles, such as the principles of sustainable development, common but differentiated responsibilities and respective capabilities (CBDR-RC), equity, intergenerational equity, and the precautionary principle are also applicable (para. 161).

No Legal Silos: The ICJ on Lex Specialis

The ICJ rejected the argument made by some states – including the United States – that climate change treaties constitute lex specialis and therefore render other rules of international law inapplicable (paras. 162-171). Lex specialis is a principle of legal interpretation used to determine whether a more specific rule takes precedence over a more general one, or whether the two rules apply concurrently. The ICJ relied on the work of the International Law Commission, which states that mere overlap in subject matter does not automatically trigger the application of lex specialis. For the principle to apply, there must be either a real inconsistency between two legal provisions or clear evidence that one was intended to exclude the other. The ICJ found no inconsistency between climate treaties, such as the UNFCCC and the Paris Agreement, and other relevant rules and principles of international law. In fact, both treaties explicitly refer to other legal frameworks in their preambles, indicating that they are intended to operate within, rather than outside, the broader international legal system (paras. 168-170).

Due Diligence Over Discretion

Among the most consequential aspects of the opinion is the Court’s articulation of a stringent due diligence standard. In light of the scientific consensus, states must act urgently. This includes not only setting and updating robust national climate plans under the Paris Agreement but also regulating private actors and providing support to more vulnerable nations. Inaction, or failure to act decisively, may constitute an internationally wrongful act, triggering consequences under the law of state responsibility.

The ICJ found that the discretion of parties to the Paris Agreement in preparing their Nationally Determined Contributions (NDCs) is limited (paras. 237-249). Under the Paris Agreement, each party is required to submit NDCs outlining efforts to reduce greenhouse gas emissions and adapt to the impacts of climate change. Some States argued that NDCs fall entirely within the discretion of each State party. The ICJ disagreed. It held that parties are under an obligation to exercise due diligence when preparing their NDCs, ensuring that, when taken together, they achieve the 1.5°C temperature goal  (para. 245).

Customary International Law and the Climate System

The ICJ held that the customary duty to prevent significant harm to the environment applies fully to the climate system. Crucially, this duty applies to all States, including those that are not parties to climate change treaties. As a vital part of the global environment, the climate system must be protected for present and future generations (para. 273). The risk of significant environmental damage must be assessed based on the likelihood and magnitude of potential harm. Furthermore, the ICJ recognized that significant harm can also arise from the cumulative impacts of multiple activities – both by states and non-state actors (para. 276).

The standard of conduct required is due diligence – a flexible, context-sensitive obligation shaped by a range of legal and scientific considerations. The ICJ identified several key elements that define how due diligence must be understood in the climate context (paras. 281-299). These include: (i) the adoption of appropriate legal and regulatory measures, such as effective policies aimed at achieving deep, rapid, and sustained reductions in greenhouse gas emissions; (ii) the availability and assessment of scientific and technological information, which states are expected to  acquire and analyze actively; and (iii) the relevance of both binding and non-binding norms, including decisions by the Conferences of the Parties (COPs) to climate treaties and recognized technical standards and best practices.

In addition, the ICJ stressed: (iv) the CBDR-RC principle, noting that states with greater resources and governance capacity are expected to exercise a higher standard of care, though all states must act within the limits of their capabilities; (v) the necessity of taking preventive action even amid scientific uncertainty, which should not be used as a pretext for delay or inaction; (vi) the requirement that states undertake thorough risk and impact assessments for proposed activities within their jurisdiction that may contribute to climate harm, based on the best available science; and (vii) the obligation to notify and consult in good faith with other states when activities may create a risk of significant transboundary harm or interfere with collective climate efforts.

The ICJ also reaffirmed the customary duty to cooperate, emphasizing that international collaboration is essential when addressing a global commons like the climate system (paras. 301-302).

While states maintain discretion in how they regulate greenhouse gas emissions, this does not exempt them from legal accountability (para. 306). Discretion must be exercised in good faith and in accordance with the required level of due diligence.

Protection of the Marine Environment, Sea Level Rise, and Statehood

The ICJ identified UNCLOS as one of the instruments most directly relevant to the questions submitted to it by the General Assembly. The Court recalled the advisory opinion rendered by the International Tribunal for the Law of the Sea (ITLOS) last year (see here), noting that it would “ascribe great weight to the interpretation adopted by the Tribunal” (para. 338). In this light, the ICJ reaffirmed that anthropogenic greenhouse gas emissions fall within the definition of marine pollution under Article 1, paragraph 1, subparagraph 4, of UNCLOS, which means that Part XII of UNCLOS on the protection of the marine environment is applicable to climate governance (paras. 339-340).

The ICJ found that States have both positive and negative obligations under UNCLOS: they must take active steps to protect and preserve the marine environment (Article 192) and avoid degrading it (paras. 342-343). This includes taking all necessary measures to prevent, reduce, and control marine pollution (Article 194), even if complete prevention is not immediately achievable. The ICJ emphasized a stringent standard of due diligence, requiring States to act based on the best available science and their capabilities (paras. 345-349). It also underscored States’ obligations to cooperate under Article 197 (paras. 350-351), to conduct environmental impact assessments when planned activities pose significant risks under Article 206 (paras. 352-353), and to support research and data sharing under Articles 200-201 (para. 351). The Court stressed that UNCLOS and other rules of international law “inform each other” and must be applied hand in hand (para. 354).

The ICJ also addressed concerns about sea level rise, particularly its impact on maritime territories and the statehood of small island States. It found that UNCLOS does not require states to revise established baselines or maritime boundaries due to physical changes like coastal recession (para. 362). Thus, existing maritime entitlements remain valid even as sea levels rise. More fundamentally, the ICJ found that “once a State is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood” (para. 363). In other words, according to the ICJ, the complete submergence of a territory does not necessarily negate a State’s legal status. This confirmation of the existence of a presumption of continuity of statehood will undoubtedly be welcomed by the States most immediately affected by sea level rises, but it is unfortunate that the Court did not elaborate further on this important finding, as deplored by Judge Peter Tomka and Judge Bogdan Aurescu.

International Human Rights Law and the Climate

The ICJ underscored the indivisibility of climate justice and human rights. It recognized that a stable climate is foundational to the enjoyment of numerous rights, including the rights to life, health, food, water, and housing (paras. 373ff). Climate vulnerable groups such as children, women, and indigenous peoples were given special attention by the Court (paras. 382ff), which also clarified that states have obligations under the principle of non-refoulement when there is a real risk of irreparable harm to life, citing the Human Rights Committee’s decision in Teitiota v. New Zealand (para. 378). Remarkably, the Court went on to describe a clean, healthy, and sustainable environment as “a precondition” for the enjoyment of human rights, and that the right to such an environment “results from the interdependence between human rights and the protection of the environment” and is “therefore inherent in the enjoyment of other human rights” (para. 393).

Reaffirming that human rights treaties may apply extraterritorially when a State exercises jurisdiction outside its borders (paras. 394ff), the ICJ once more stressed that international human rights law, climate change treaties, and environmental agreements are mutually reinforcing and “inform each other” (para. 404). Therefore, when implementing obligations under one body of law, states must consider and harmonize their responsibilities under the others.

State Responsibility and Reparations

While acknowledging the complexity of climate change in terms of its causality, temporal scope, attribution, and causation, the ICJ did not shy away from affirming that reparations are warranted when acts or omissions can be attributed to a state, giving rise to state responsibility. In particular, the ICJ affirmed that “[f]ailure of a State to take appropriate action to protect the climate system from [greenhouse gas] emissions – including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies – may constitute an internationally wrongful act which is attributable to that State.” (para. 427).

The ICJ further held that when a State fails to meet its international obligations related to climate change – whether under treaty law, customary international law, or human rights law – it incurs responsibility under the law of state responsibility (para. 444), triggering “a panoply of legal consequences” (para. 445). These include obligations of (i) cessation and non-repetition, (irrespective of harm) and (ii) full reparation, including restitution (returning things to the status quo ante, if possible), compensation (for financially assessable damage) and/or satisfaction (for moral or non-material harm). The Court also noted that breaches of states’ obligations do not affect the continued duty of the responsible state to perform the obligation breached.

The ICJ confirmed that these remedies can apply for wrongful acts caused by cumulative greenhouse gas emissions (para. 429) or failures to regulate private actors (para. 438), provided they constitute a breach of an international obligation. These obligations are erga omnes – owed to the international community as a whole (paras. 439ff). This means that any state may invoke responsibility, not just injured states.

A Clear Message

As contributors to this symposium will explore in the coming days, the opinion’s true legacy may lie not only in its legal conclusions, but in its power to influence political will, guide national courts, and support vulnerable communities seeking justice. Amid a global surge in climate-related lawsuits, the ICJ has sent a clear message: legal systems must reckon with climate duties, and action can no longer be deferred.