Towards a Constitutionalization of International Climate Law?

[Bin Zhao holds a PhD in international law]

Climate litigation has moved from national courts to international benches. On 9 April 2024, the European Court of Human Rights (ECtHR) spoke first. On 21 May 2024, the International Tribunal for the Law of the Sea (ITLOS) followed. A year later, on 3 July 2025, the Inter-American Court of Human Rights (IACtHR) added its voice. Only weeks afterwards, the International Court of Justice (ICJ) delivered a wide-ranging Climate Advisory Opinion. These rulings have undoubtedly made significant contributions to the fight against climate change by clarifying states’ obligations to protect the climate.

However, doubts arise about their overall effectiveness. Based on distinct legal mandates and delivered by separate judicial bodies, they inevitably reflect fragmentation, functional particularity, and a high degree of autonomy. Does this patchwork strengthen international climate governance, or deepen the fragmentation of international climate law? How do we construct a more coordinated and integrated order of international climate law capable of maximizing its effect in practice by mitigating greenhouse gas emissions and strengthening adaptation to climate impacts?

This post argues that international climate rulings, particularly the ICJ’s climate change Advisory Opinion (AO), are not merely advancing the legalization of climate protection obligations but are, in fact, contributing to constitutionalizing international climate law. Not a written “climate constitution”, but rather a gradual assembly of constitutional features that form the coherence, unity, and ultimately the de facto authority of the regime. The analysis draws on theory of the constitutionalization of international law (see, for example, The Constitutionalization of International Law and Constitutional Fragments: Societal Constitutionalism and Globalization) and the systems theory (see Law as a Social System). It claims that the constitutionalization of international climate law is three-dimensional: the international climate legal system’s reflexivity activates the constitutional process, which generates fundamental structures that, in turn, realize constitutional functions.

The Constitutional Process of the International Climate Law: Reflexivity

In the view of systems theory, reflexivity concerns the system’s capacity to engage in self-learning and self-adjustment to adapt and evolve in response to external irritations from its environment. It is this reflexivity that generates the distinction between ordinary law and constitutional law. In the context of international climate law, reflexivity appears in two interlinked ways.

First, through the cross-fertilization among international courts. International climate opinions listen, cite, incorporate and reinforce each other. For example, in interpreting the UNCLOS, the ICJ ascribed great weight to ITLOS’ finding that greenhouse gas emissions constitute “pollution of the marine environment” (ICJ Opinion, paras. 336–354). With respect to the adverse effects of climate change on the enjoyment of human rights, the ICJ took extensive account of the views and reports of United Nations human rights bodies and expert organizations (paras 377-384), the IACtHR advisory opinion AO-32/25 (para 385), and the ECtHR’s Verein KlimaSeniorinnen Schweiz and Others v. Switzerland (para 385 ).

This is reflexivity in action. Each advisory opinion carries its own limits and functional specialization; for instance, the IACtHR in human rights law, ITLOS in the law of the sea, and the ICJ in general public international law. Yet their resonance generates complementarity, avoids conflicting conclusions, and gradually builds coherence within the international climate legal order. Over time, the repeated recognition and cross-fertilization among advisory opinions gradually sediment into shared baselines, which transcend any single treaty or forum, thus increasingly constituting a constitutional character: general, cross-regime, and integrative.

Second, reflexivity extends to the systemic integration of international climate norms. In addressing the question of lex specialis, the ICJ refers to “harmonious interpretation”: “when several rules bear on a single issue, they should, to the extent possible, be interpreted so as to give rise to a single set of compatible obligations” (para 165; see also Article 31(3)(c) of the VCLT; see also the Oil Platforms case, at para 41; Maritime Delimitation in the Indian Ocean (Somalia v. Kenya) at para 89). It thus rejects the view that climate change treaties displace other rules of international law (para 170).

The same approach applies to intra-climate treaty relations. The UNFCCC, the Kyoto Protocol, and the Paris Agreement are understood as mutually supportive; should conflicts arise, they are to be resolved by the law of treaties, particularly through the principles of “harmonious interpretation” and “systemic integration” (para 195). 

Most notably, the Court notes that “international human rights law, the climate change treaties and other relevant environmental treaties, as well as the relevant obligations under customary international law, inform each other” (para 405). It adds that states must take each set of obligations into account when implementing the others (para 404; paras 335, 354). In effect, this ruling confirms that climate-related norms dispersed across different branches of law collectively “radiate” the existence of a broad field of international climate law.

Together, the ICJ’s approach builds a normative order that is mutually supportive, coherent, and systemically integrated. This marks the emergence of a mode of constitutional interpretation of international climate law.

The Constitutional Structure of International Climate Law: Values, Rights, Hierarchy

The theory of the constitutionalization of international law invites us to move beyond a state-centric conception of constitutionalism, and to draw analogies with the constitutional elements embedded in domestic constitutions. For instance, UNCLOS is often regarded as the “constitution for the oceans”, establishing a foundational framework of rules, institutions, and enforcement mechanisms within which specialized regimes, such as the BBNJ Agreement, are developed. Likewise, once  the constitutional elements become stabilized in the context of climate change, they collectively form the structural foundations for the constitutionalization of international climate law. Three aspects are particularly salient.

First, guiding principles as constitutional values. The ICJ treats certain key principles as guiding principles for interpreting climate rules: common but differentiated responsibilities and respective capabilities, the precautionary approach or principle, sustainable development, equity, and intergenerational equity (para 178). These principles permeate climate instruments and shape treaty interpretation, serving as constitutional values, which Judge Xue calls the “cornerstones” of the climate treaty regime (separate opinion, para 20).

The IACtHR goes a step further, declaring that the progressive crystallization of some fundamental principles in international environmental law reveals the creation of a peremptory prohibition regarding “anthropogenic conducts with irreversible impacts on the vital equilibrium of the planetary ecosystem” (AO-32/25, para 290). As peremptory norms “reflect and protect fundamental values of the international community” (ILC Draft Conclusions on Jus Cogens, Conclusion 2), attributing jus cogens status to the fundamental principles of international climate law would serve to consolidate and safeguard their constitutional value.

Second, embedding rights at the core. Domestic constitutions protect fundamental rights. At the international level, the ICJ likewise recognizes the right to a clean, healthy, and sustainable environment as “inherent in” the rights to life, health, and an adequate standard of living (para 393). It affirms that climate change may significantly impair the enjoyment of certain human rights (paras 372–386) and stresses that full enjoyment of human rights cannot be ensured without protecting the climate system and other parts of the environment (para 403). In this regard, the Court constitutionalizes climate protection by embedding it at the core of human rights realization.

Third, the emergence of a normative hierarchy, reflected in four aspects:

(1) The recognition of foundational instrument. The ICJ sees the UNFCCC as “the foundational treaty to address climate change”, setting a general framework for anthropogenic emissions (para 191). The Kyoto Protocol and the Paris Agreement, in particular the 1.5°C temperature target, clarify and expand on the UNFCCC obligations, advancing its “ultimate objective”: stabilizing greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system (paras 194–195).

(2) The emergence of jus cogens norms. The IACtHR has held that the obligation not to cause irreversible damage to the climate and the environment to constitute a jus cogens norm (paras 287–294). According to the ILC Draft Conclusions, peremptory norms are “universally applicable and are hierarchically superior to other rules of international law” (Conclusion 2). As the IACtHR holds, recognizing such a peremptory norm can “give effect to existing norms more integrally” (para 293). Elevating climate protection to jus cogens reshapes the normative hierarchy in international law by placing it at the apex of the international law.

(3) The review of domestic regulations at the international level. International climate law’s constitutional framework also extends into domestic law by placing limits on domestic climate measures. For instance, the ICJ interprets Article 4(2) of the Paris Agreement, as entailing a stringent due diligence requirement into domestic implementation: to achieve their nationally determined contributions (NDCs), states must adopt domestic mitigation measures that may include a national system (legislation, administration, enforcement) and “adequate vigilance” to ensure it function effectively (paras 250–254). This is not classic constitutional review, but it operates as a constitutional benchmark for evaluating the “constitutionality” of domestic climate action.

(4) Clarification of secondary international rules and remedies. Finally, the ICJ confirms that breaches of climate obligations entail state responsibility, and the legal consequences include cessation, non-repetition, and reparation (para 457). That closes the loop: constitutional values and hierarchy are backed by state responsibility rules.

The Constitutional Functions of International Climate Law

The constitutionalization of international climate law fulfills two primary functions. 

First, the constitutive function. While advisory opinions are not formally binding, it is widely acknowledged that an advisory opinion “entails an authoritative statement of international law” which carries “no less weight and authority” than contentious judgments, since it is rendered “with the same rigour and scrutiny by the ‘principal judicial organ’ of the United Nations” (Mauritius/Maldives, paras 202–203). In the absence of an “international legislator”, international advisory opinions can contribute to constitutionalizing an emerging, structural international climate legal order that integrates climate change treaties with other rules of international law. Judge Nolte observed, the Opinion may “reinforce the commitment of States to tackling climate change through legally framed forms of political and administrative co-operation.” Such a climate legal order demarcates the boundary between law and politics, securing an autonomous legal space in which the legal system can operate independently and resist direct political interference. At the same time, it exerts a counter-effect on politics: as the ICJ explained, it hopes its Opinion “will allow the law to inform and guide social and political action to address the ongoing climate crisis” (para. 456).

This is not a mere aggregation of scattered norms of international law. It is a systemic integration in the sense that international climate change norms are interpreted and applied in light of other relevant rules of international law, as required by Article 31(3)(c) of the VCLT, thereby ensuring coherence, reducing normative conflicts, filling normative gaps and fostering complementarity across regimes. As a result, it produces regime-optimal outcomes, creating a constitutional framework in which the coordinated order delivers systemic benefits, making the whole greater than the sum of its parts.

Second, the constraining function. Constitutionalization also serves to limit power. A climate regime must avoid unlimited encroachment into other domains such as economic, political, or social systems. Constitutionalization acknowledges the cross-cutting nature of international climate law, but the challenge lies in striking a constitutional balance – ensuring its legitimate integration into adjacent regimes of international law, while at the same time requiring self-limitation to prevent overreach.The obvious vehicle is the principle of sustainable development. 

Here the jurisprudence remains underdeveloped. The ICJ spends only two paragraphs (147, 181) to this principle, stating that it concerns the “need to reconcile economic development with protection of the environment.” That is correct, but quite terse. Judge Xue criticizes the Court has not given due weight to the principle (Separate Opinion, para 1). She stresses that “mitigation and adaptation measures must be done in an integral manner, taking into account economic, social, environmental and human rights considerations together, in the context of sustainable development and poverty eradication” (para 2).

As one of the aforementioned constitutional values, sustainable development should operate as a self-limiting device. It balances climate protection and development. This constraining function is under-recognized by the ICJ. But constitutionalization without constraint risks backlash. It is to be hoped that future advisory opinions or litigation will address this dimension more thoroughly.

Final Remarks

If we set aside the imagination of a single constitutional text, we are witnessing the emergence of a constitutionalized order within the field of international climate law. Although some scholars have regarded the UNFCCC as functioning like UNCLOS and serving as a constitution for climate law, the constitutionalization of international climate law transcends this single treaty and more closely resembles the English constitution: unwritten and constituted by a plurality of legal sources. This constitutionalization is an ongoing process. It should be adaptive to changing political and social contexts, particularly in light of the urgency of the climate crisis, where quicker responsiveness is imperative. 

Climate constitutionalization forms part of the broader constitutionalization of international law. It addresses the fragmentation of international law by establishing normative hierarchies, promoting systemic integration, and reinforcing the unity of the international legal order. But it also brings something new. Judicialization, especially advisory jurisdiction, has become its principal driving force. Additionally, the complexity and urgency of the climate crisis have compelled the constitutionalization process to advance at a rapid pace.

While international law, as the ICJ reminds us, plays an “important but ultimately limited role” in resolving the climate crisis (para. 456), it is we – the people – who play an equally important role of unlocking its fullest potential within those limits. And perhaps, the path lies in constitutionalization.