Signs of Crystallization in Climate Law: Recent Judicial and Institutional Developments

By Reza Maddahi, Doctoral Researcher

The End of Constructive Ambiguity

For much of the past three decades, international climate law operated under a form of constructive ambiguity, with treaty language calibrated to maintain broad participation even at the cost of precision and enforceability. Between 2023 and 2025, however, this dynamic began to shift. A series of notable advisory opinions and institutional developments contributed to a clearer articulation of states’ duties, prompting a gradual movement from political discretion toward more structured expectations under existing legal frameworks. This period -sometimes described as a phase of “crystallization” – saw elements of the Paris Agreement’s normative structure increasingly interpreted through the lens of customary international law and the obligations associated with the prevention of significant harm. This blog discusses three core components of this evolution: the contribution of the International Tribunal for the Law of the Sea (ITLOS) in interpreting states’ duties under the law of the sea, the International Court of Justice (ICJ) clarification of state responsibility in the context of climate change, and the operationalization of the Fund for Responding to Loss and Damage as an institutional mechanism for addressing climate impacts.

I. ITLOS: Recognizing the Ocean’s Vulnerability

In May 2024, the ITLOS issued a unanimous advisory opinion examining the legal implications of greenhouse gas (GHG) emissions for marine governance. The request for this opinion came from the Commission of Small Island States on Climate Change and International Law (COSIS), and it addressed the question: can anthropogenic GHGs be considered “pollution” under the United Nations Convention on the Law of the Sea (UNCLOS)?

The Tribunal concluded that human-caused GHG emissions fall within the scope of “pollution of the marine environment” as defined in Article 1.1.4 of UNCLOS. This interpretation encompasses both emissions directly entering the ocean and atmospheric emissions that contribute to ocean warming and acidification. By framing GHGs in this way, the Tribunal emphasized that states have obligations under Part XII of UNCLOS to take measures to prevent, reduce and control such pollution.

A notable aspect of the advisory opinion was its discussion of the Paris Agreement. Some high-emitting states had previously argued that fulfilling their Nationally Determined Contributions (NDCs) under the Paris regime satisfied their legal obligations regarding marine pollution. The Tribunal clarified that, while the Paris Agreement provides the primary political framework for climate action, compliance with it alone does not automatically fulfill the due diligence obligations under UNCLOS (paragraphs 223 & 224).

This creates a situation in which a state could meet its voluntary Paris commitments yet still be expected to take additional measures under UNCLOS to prevent harm to the marine environment. The opinion highlights the complementary nature of climate agreements and UNCLOS obligations, signalling that voluntary climate pledges may need to be supplemented by further measures to address marine impacts.

The ITLOS Advisory Opinion of May 2024 is significant both legally and normatively within the framework of international climate and ocean law. While advisory opinions are non-binding, they carry substantial authoritative weight, providing guidance on the interpretation and application of UNCLOS in the context of climate change. This opinion clarifies that anthropogenic GHG emissions can constitute “pollution of the marine environment” and underscores that compliance with the Paris Agreement, though politically important, does not relieve states of their due diligence obligations under UNCLOS. By articulating these obligations, ITLOS contributes to the development of customary international law and strengthens the legal discourse around state responsibility for transboundary environmental harm, particularly in relation to vulnerable marine ecosystems. Consequently, the opinion serves as a reference point for states, international institutions, and courts, shaping the evolving architecture of climate governance and highlighting the interplay between voluntary climate commitments and binding maritime obligations.

II. The ICJ Advisory Opinion: Clarifying States’ Obligations in Respect of Climate Change

The momentum generated by ITLOS culminated in July 2025, when the ICJ delivered its advisory opinion on the “Obligations of States in Respect of Climate Change.” This opinion, requested by the UN General Assembly, has been widely recognized as a landmark legal statement clarifying states’ obligations in respect of climate change.

The ICJ moved beyond treaty law to confirm that climate protection is an obligation erga omnes (i.e. owed to the international community as a whole) (paragraph 440). The Court found that the duty to prevent “significant harm” to the climate system is now a rule of customary international law, binding on all states regardless of their ratification status of specific treaties (paragraph 409).

The Court explained that the temperature goals of the Paris Agreement help define the content of states’ due diligence obligations under customary international law, including the duty to prevent significant harm and to protect human rights affected by climate change (paragraphs 179–182, 196–199). It also stressed that states must rely on the “best available science” when designing and implementing their climate measures, referring specifically to the IPCC’s assessment reports, such as those outlining pathways compatible with limiting warming to 1.5°C (paragraphs 162–165, 208–210). The Court further indicated that climate policies may fail to meet states’ legal obligations when they are not aligned with the best available science and fall short of the temperature goals set out in the Paris Agreement (paragraphs 200-204).

The Court clarified the consequences that follow from a breach of states’ climate-related obligations under customary international law. It confirmed that any internationally wrongful act triggers the general law of state responsibility, including the duties to cease the wrongful conduct and to make reparation for the injury caused (paragraphs 254–259). The Court reiterated that reparation may take the established forms of restitution, compensation, and satisfaction, depending on the nature of the harm and the feasibility of remedial action. By situating climate-related harm within the established framework of state responsibility, the Court strengthened the legal basis for claims commonly discussed under the rubric of “loss and damage,” signalling that such harms may give rise to enforceable obligations rather than purely political or voluntary support mechanisms.

III. The Loss and Damage Fund: Institutionalizing Liability

Parallel to the recent judicial developments, the political architecture for addressing climate-related harms advanced significantly with the establishment and early operationalization of the Fund for Responding to Loss and Damage (FRLD). Following the agreement at COP28 to operationalize the Fund, its Board moved throughout 2024 and 2025 to establish the necessary institutional foundations. In a decision with notable geopolitical implications, the Board selected the Philippines as the host of the Fund’s governing body, reflecting a broader shift towards greater leadership by climate-vulnerable countries in global climate governance. The appointment of Ibrahima Cheikh Diong as the inaugural Executive Director marked another step toward building an independent secretariat capable of administering grants and other support modalities.

Implementation, however, was accompanied by significant institutional debate. A central point of contention concerned the arrangement under which the World Bank would serve as the interim host institution. Developing countries and civil society organizations expressed persistent concerns that the Bank’s lending-oriented operational culture, along with the governance dynamics associated with its shareholder structure, could compromise the Fund’s independence or limit accessibility for the most vulnerable states.

To mitigate these risks, the hosting agreement established the Fund as a Financial Intermediary Fund (FIF) for an initial four-year period, incorporating safeguards to preserve Board authority over funding decisions, eligibility criteria and operational policies. While these measures represented a compromise acceptable to most parties, the Fund’s long-term effectiveness remains uncertain, particularly given the continuing gap between current pledges and the scale of finance required, widely assessed to reach into the hundreds of billions of dollars annually for climate-vulnerable nations.

At COP30, Parties adopted the 2025 Report of the FRLD and provided formal “guidance” to the Fund, including the launch of the first call for funding requests under the so-called “Barbados Implementation Modalities” (BIM) for 2025–2026. The COP30 decision links the FRLD more explicitly to the broader climate-finance architecture, by tying future resource mobilization for loss and damage to the newly agreed global climate-finance goals (notably the “New Collective Quantified Goal”).

Conclusion

Taken together, the developments of 2024 and 2025 represent a notable phase in the maturation of international climate law. Through their advisory opinions, ITLOS and ICJ clarified existing legal obligations rather than creating new ones, strengthening the interpretation of states’ duties under UNCLOS, customary international law and human rights frameworks. These opinions reduced some of the interpretive ambiguity that had previously allowed states broad latitude in designing climate measures, particularly by emphasizing due diligence, the relevance of the best available science, and the need to prevent significant transboundary harm. In parallel, the operationalization of the Loss and Damage Fund marked a concrete institutional advancement, offering an initial (though still evolving) mechanism for addressing climate-related impacts and supporting vulnerable countries. While these developments do not eliminate the discretionary space inherent in international climate governance, they collectively signal a shift toward clearer expectations of conduct and greater scrutiny of state practice, reinforcing an emerging trajectory of accountability within the international legal system.