Mirage of Legality, Miracle of Legitimacy

The International Court of Justice’s (ICJ) long-awaited advisory opinion on climate change has already generated an avalanche of scholarly commentary. Some have hailed it as a “historic legal victory” for climate justice, a “landmark decision” which has significant positive impacts. Yet others are far less sanguine. Expectations were high: scholars such as Soenke Kreft and Maren Solmecke anticipated that the Court would articulate robust legal standards on state responsibility, while Sara K. Phillips and her four colleagues from the Stockholm Environment Institute emphasized clarifying the scope of obligations under the Paris Agreement, and Markus Gehring suggested that the Court could even lay the groundwork for new customary norms. However, some dissenting ICJ judges (e.g., Separate opinion of Judge Yusuf, Separate opinion of Judge Xue) criticized the opinion for avoiding bold pronouncements on new obligations or state responsibility and offering little substantive innovation. Similarly, other scholars including Mario Prost and Ramindu Perera have noted that the opinion remained conspicuously silent on several pressing issues. This divergence between judicial and academic perspectives underscores the continuing uncertainty surrounding the legal consequences of the Court’s findings.

Indeed, both strands of commentary have their merits. The celebratory voices emphasize that the advisory opinion fills a longstanding gap: it systematically maps the relevant sources of international law on climate change and delivers an authoritative statement on states’ obligations and responsibilities. In this sense, it is praised for establishing that such obligations do exist. By contrast, the critical voices focus less on the existence of obligations and more on their degree of precision. They argue that, while the Court has taken an important first step, it stopped short of clarifying certain rules or elaborating key principles. Yet, despite their differences, both sides ultimately converge on the same evaluative lens: they assess the opinion primarily in terms of legality, namely, the extent to which it compels or enables states to comply with climate-related rules.

This post argues, however, that this narrow focus risks obscuring the opinion’s true value: its ability to generate legitimacy, which can guide interpretation (e.g., of existing treaty obligations), build consensus (e.g., among states and institutions on shared normative standards), and gradually shape binding law (e.g., through consistent state practice and opinio juris).

The Limits of Legality-Based Criticism

Most critical reactions converge around one point: the ICJ did not do “enough” (Joint declaration of Judges Charlesworth, Brant, Cleveland and Aurescu, para. 13). The Court refrained from spelling out the concrete consequences of failing to meet climate obligations, it did not unambiguously attribute responsibility to specific states, and it declined to crystallize new customary rules. Actually, the Court only reaffirmed the general obligation of states to prevent significant harm to the climate system and to exercise due diligence in that regard (Climate change advisory opinion (CHAO), paras. 272-300, especially para. 280), and briefly outlined how treaty obligations, human rights law and customary international law may interact in interpreting these duties (CHAO, paras. 299-306). In short, it offered little that could be immediately operationalized in international litigation or treaty negotiations.

But this line of critique overlooks the function and constraints of the advisory procedure, a response that, I suggest, applies across the board, since each of these criticisms ultimately reduces to the charge that the Court’s explanation of rules is insufficiently forceful. The Court was not asked to resolve a bilateral dispute but to provide guidance of a general nature. Advisory opinions are not instruments of enforcement; they are tools of clarification and persuasion. This function is inherent in Article 65 of the ICJ Statute, which authorizes the Court to “give an advisory opinion on any legal question” at the request of authorized organs, but confers no binding effect on such opinions. The Court itself has consistently characterized its advisory function in these terms, describing it as providing “authoritative legal guidance” rather than creating obligations, for example in Interpretation of Peace Treaties with Bulgaria, Hungary and Romania (Advisory Opinion, ICJ Reports 1950 (first phase), pp. 71-72) and Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (Advisory Opinion, ICJ Reports 2004, paras. 47-48).To demand from them the creation of binding rules is to misunderstand their role in the framework of international law (Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1CJ Reports 1996, p. 266). This is not to deny that several judges, in their separate or dissenting opinions, voiced dissatisfaction with the Court’s restrained posture, arguing that it did not fully engage with the questions put to it by the General Assembly (e.g. Interpretation of Peace Treaties with Bulgaria, Hungary and Romania (Advisory Opinion), ICJ Reports 1950, Separate Opinion of Judge Alvarez , para. 4; Certain Expenses of the United Nations (Advisory Opinion), ICJ Reports 1962, Dissenting Opinion of Judge Sir Percy Spender, pp. 183-184). These critiques reflect a common tendency to measure advisory opinions solely by the concreteness of legal rules. To be sure, some opinions did spell out specific obligations, such as the Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (Advisory Opinion, ICJ Reports 2004, para. 136) and the Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Advisory Opinion, ICJ Reports 1951, p. 15), but the present request raises more diffuse questions, making such precision less feasible. While understandable, such a perspective overlooks the advisory function and the opinion’s contribution to providing authoritative interpretive guidance that helps build shared understanding and acceptance of climate-related obligations.

More fundamentally, legality itself rests on the principle of state consent. As repeatedly affirmed by the ICJ, international law derives its binding force from the will of states, expressed through their consent to be bound (The Case of the S.S. Lotus (France v. Turkey), Judgment, PCIJ Series A No 10, p. 18). In the domain of climate change, a paradigmatic global public good, consent is notoriously difficult to secure, being constantly shaped by geopolitical rivalries, divergent levels of economic development, and conflicting domestic priorities. This makes it exceedingly hard to delineate concrete state obligations solely within the existing substantive framework of treaties, customary law, and general principles.

Seen this way, the disappointment with the Court’s “cautious” approach reflects a narrow fixation on legality. While legality matters, it is not the only nor always the most important measure of an advisory opinion’s worth. To unlock the opinion’s significance, we need to shift the evaluative lens from legality to legitimacy.

Beyond Legality: The Advantages of a Legitimacy-Based Perspective

Legality and legitimacy are related but distinct dimensions of international law. Legality refers to the conformity of an act with established legal rules whether treaty provisions, customary norms, or general principles. By contrast, legitimacy speaks to the broader acceptability of an act within the international community: its consistency with shared values, fairness, and the perceived authority of the institution articulating it (Brunnée/Toope, Legitimacy and Legality in International Law

An Interactional Account). Legality answers the question: “Is this rule binding?”, while legitimacy addresses: “Is this rule persuasive and justifiable?” In the context of climate change, where consent-based legality is fragile and rules remain underdeveloped, legitimacy often carries equal if not greater weight in shaping normative expectations.

First, legitimacy provides orientation by grounding interpretation in fairness, procedural integrity, and shared understanding (Alexiou, The Revival of the ICJ’s Advisory Function: Enhanced or Contested Legitimacy?). Instead of asking whether a rule is formally binding, legitimacy asks whether its application reflects justifiable principles and broad international consensus. In this way, vague or fragmented treaty language can be situated within a framework that states recognize as both fair and credible.

The Court’s opinion illustrates this mechanism in several respects. On the level of substantive fairness, it emphasizes differentiated responsibilities, acknowledging that vulnerable states and developed countries bear distinct burdens (CHAO, pp. 148-151). On the level of procedural fairness, it highlights the system of nationally determined contributions under the Paris Agreement, which institutionalizes participation, transparency, and review (CHAO, paras. 77-78). Finally, with regard to shared understanding, the Court anchored its reasoning in the most widely accepted instruments of international law: the United Nations Framework Convention on Climate Change (UNFCCC) as the foundational treaty of the climate regime, the Paris Agreement as its operational core, and complementary agreements such as the Nagoya Protocol. It also referred to the UN Charter and core human rights treaties, thereby situating climate obligations within the broadest spectrum of international consensus. (CHAO, pp. 115, 143-145) Notably, the Court endorsed the 1.5℃ temperature goal as the expression of the maximum existing political and legal convergence (CHAO, para. 224). Taken together, these choices provide states and institutions with a coherent set of interpretive reference points that make existing obligations more intelligible and persuasive. This approach is made explicit in the advisory opinion (CHAO, para. 161), where the Court concludes that principles such as sustainable development, common but differentiated responsibilities, equity, intergenerational equity, and the precautionary principle are applicable as guiding principles for interpreting and applying the most directly relevant legal rules. Taken together, these choices provide states and institutions with a coherent set of interpretive reference points. While the Court did not create new binding obligations, it synthesized and clarified existing norms drawing on treaty obligations, human rights law, and customary principles in a manner that renders them more intelligible and persuasive. By organizing these sources into a structured framework, the opinion offers a novel tool for interpretation, enabling states and institutions to navigate complex legal obligations with greater confidence.

Second, legitimacy has a transformative potential. While it does not itself create binding obligations, it can pave the way for legality (Brunnée/Toope, Legitimacy and Legality in International Law An Interactional Account) The Court’s reliance on principles such as common but differentiated responsibilities (CBDR), intergenerational equity, and the precautionary principle (CHAO, pp. 148-151, 155-158) exemplifies this dynamic. These notions once emerged as political declarations, like CBDR in the 1992 Rio Declaration, intergenerational equity in soft-law debates, and the 1.5°C target in Intergovernmental Panel on Climate Change reports, and by reaffirming them in its opinion, the Court lends them judicial authority. This authority can be taken up by states in treaty implementation, or by domestic and international courts when adjudicating climate-related claims, as already seen in the German Constitutional Court’s 2021 climate decision or the Inter-American Court of Human Rights’ Advisory Opinion OC-23/17.

Over time, such references may crystallize into customary norms or reshape treaty practice. The trajectory is cyclical: legitimacy → practice and common understanding → legality (Gu/Qian, Reconstruction of the Evaluation of Legitimate Expectations within Investor–State Dispute Settlement: A Vantage Point from Fuller’s Interactional Theory). This reflects a methodological approach in which norms gain binding force not instantly, but through repeated recognition and adoption by states and institutions. This process reinforces common understanding, which in turn stimulates further practice, eventually culminating in binding agreements or obligations. A case in point is the principle of common but differentiated responsibilities mentioned above, which first appeared in political declarations, most notably the 1992 Rio Declaration (Principle 7), and has gradually acquired legal force through its incorporation in international treaties, including the UNFCCC, the Kyoto Protocol, and the Paris Agreement. In this sense, the Court’s “soft” contribution may be more powerful than an unattainable leap toward immediate hard law.

From Legality to Legitimacy: The Court’s Generative Legacy

Seen through this lens, the ICJ’s advisory opinion is not a hollow exercise. Its legitimacy lies in how it translates broad normative aspirations into concrete interpretive guidance. The Court reaffirms states’ existing obligations under international environmental law, clarifies the relevance of principles such as equity and common but differentiated responsibilities, and integrates them into the framework of state responsibility. In doing so, it lends juridical coherence to what had long remained political discourse.

This is where the advisory opinion’s legitimacy takes shape, not in creating new rules, but in structuring how old ones are understood and invoked. By shaping interpretive horizons and reinforcing normative consensus, it sets processes in motion that may be more consequential than any single judgment. Indeed, a rigid insistence on immediate legality risks overlooking how international law actually evolves. Customary rules and treaty interpretations rarely emerge fully formed; they grow out of practices, debates, and authoritative statements that gradually converge. Advisory opinions are one of the forums where this convergence can begin.

The lesson is clear: the Court’s contribution should not be dismissed simply because it did not legislate. Its real value lies in equipping states, institutions, and civil society with a language of legitimacy that can be mobilized toward future legality. The ICJ’s advisory opinion on climate change may disappoint those who expected instant legal innovation. But to judge it solely by legality is to misjudge its purpose. Its significance lies in legitimacy – its ability to orient interpretation and prepare the ground for new legal consensus. Far from being an empty gesture, the opinion is a generative act that could shape the legal architecture of climate governance for years to come.

Acknowledgments
An earlier draft of this article was presented at the 2025 GCIS Summer Academy of International Law, held at Renmin University of China from 2 to 6 September 2025. The author is grateful for the valuable comments received from Professor Yang Liu, Professor Xu Qian, Dr. Doudou Huang, Dr. Zhenni Li, Dr. Yue Cao, Dr. Fan Yang, Dr. Xinchao Liu, LLM student Ziyue Li and other participants.