From Carbon Sovereignty to Trusteeship of the Climate Commons

Treated as a corollary of industrial development, the freedom to emit CO₂ and other greenhouse gases within a State’s territory was seen as both a prerequisite of economic progress and an expression of sovereignty. The International Court of Justice (ICJ)’s Advisory Opinion marks a shift: States must protect the climate system, putting their carbon sovereignty under an international-law duty of justification (1). The Court’s interpretation reinforces a public-order conception of international law guided by common interests (2). The Opinion articulates a vision of sovereignty as trusteeship of the climate commons that emphasises collective environmental responsibility and solidarity (3). It advances a normative counterpoint to interest-driven, short-term international relations, whose robustness will be tested when normative commitments collide (4).

1. Prevention and Cooperation: Legal Constraints on Carbon Sovereignty

The ICJ limits States’ carbon sovereignty through internal obligations of prevention (a) and external obligations of cooperation (b), and characterises the relevant obligations as erga omnes (partes) [440]. This places legal constraints on States’ exercise of territorial sovereignty when formulating and implementing climate policies and aligns with ITLOS’s Climate Change Advisory Opinion which held that the sovereign right in Article 193 UNCLOS “to exploit their natural resources pursuant to their environmental policies” is subject to the duty to protect and preserve the marine environment (ITLOS, paras 187, 380) [344].

a) Public Responsibility Beyond Market Logic

The Court affirms that States must mobilise public-law instruments to advance collective climate-mitigation efforts. It understands the climate system as a global commons [308, 440], whose safeguarding is a preventive duty incumbent on States. This conception orients the protective purpose of international environmental law toward a common ecological interest. Specific obligations to mitigate CO₂ emissions, and the general duty to prevent significant harm to the environment, arise under the applicable climate treaties and customary international law [116 and esp. 230 et seq.; 132 and 272 et seq.; see Voigt and Brunnée]. The Court narrows the discretion of States under the Paris framework [237], requiring them to justify their conduct against normative standards (cf. Voigt), particularly the principles of due diligence and common but differentiated responsibilities and respective capabilities (CBDR-RC), in light of national circumstances. Similarly, the customary due diligence duty to prevent significant harm requires effective regulation of public and private conduct [282; cf. Pulp Mills on the River Uruguay (Argentina v. Uruguay), para. 197; van Asselt], including fossil-fuel production and consumption, licensing and subsidies. Failure to do so may constitute an internationally wrongful act [427]. One might read this as support for an ecologically informed approach to market governance that prioritizes long-term climate protection over short-term interests.

b) A Duty to Cooperate for the Protection of the Environment

The ICJ further reinforces cooperation as a justiciable duty and situates it within a broader conception of community. Recognising climate change as a “common concern of humankind”, it characterises cooperation as “a pressing need and a legal obligation” [308]. While the existence, character, and content of a customary obligation to cooperate had previously remained unclear (cf. Boisson de Chazournes/Rudall 2020, 105, 113; Brunnée/Hey 2019, 46; Wolfrum 2000, 425–429; see also International Law Commission (ILC), Report of 67^th session (2015), para. 12), the Court now confirms such a duty for the protection of the environment [140; referencing the ITLOS Advisory Opinion; cf. El Mahmoud]. According to the Court, this customary obligation reinforces the Paris Agreement’s cooperation duties [261]. Although States retain discretion as to the means of cooperation under treaty or customary law, they must satisfy due-diligence and good-faith standards [262, 306]. In substance, the customary duty to cooperate extends beyond procedural requirements and beyond obligations regarding finance, technology transfer, and capacity-building as set out in the Paris Agreement [260 et seq.]; it requires that “States develop, maintain and implement a collective climate policy that is based on an equitable distribution of burdens and in accordance with the principle of common but differentiated responsibilities and respective capabilities” [306]. Cooperation emerges as an essential complement to the duty to prevent significant environmental harm, since effective prevention requires coordinated action [141, cf. Pulp Mills, para. 77]. This action must be geared to preventing significant climate harm and to fostering solidarity among States [305–306; 364]. Additionally, the duty to cooperate serves as a legal benchmark for assessing whether further collective action—particularly treaty-based obligations—must be undertaken [307].

2. Common Interests Guiding Interpretation

The restriction of carbon sovereignty through international legal obligations reflects an interpretive approach that conceives international environmental law as a public order grounded in common interests. The Court distils the content of the common interest from the most pertinent sources of international environmental and human-rights law relevant to the protection of the climate system. It engages in harmonious interpretation, reinforcing and consolidating the common interest within a coherent body of climate obligations through systemic integration: treaty and customary law inform each other [313–314] and customary rules guide – and may even reinforce – the interpretation of treaty obligations [261, 313].

With respect to interpreting customary international law, the Court relies not only on bottom-up State consent but proceeds deductively from rules and principles that express a common interest (cf. Talmon 2015; Worster 2024). It extends the duty to prevent significant harm from the transboundary context to the global phenomenon of climate change [134]. Although some States rejected this extension, pointing to the diffuse nature of climate change [133], the Court notes that it has applied this duty to areas beyond national jurisdiction—and thus to global environmental concerns—in earlier pronouncements [134, citing Advisory Opinion on Nuclear Weapons, para. 29]. It proceeds deductively from a broad prevention principle that protects long-term environmental and not merely present State interests, and by analogy to areas beyond jurisdiction.

Since treaty obligations are interpreted in light of these customary duties, this results in stricter treaty obligations. This is evident in the Court’s reading of Article 4 (2) and (3) Paris Agreement: It treats the clause that successive NDCs “will” represent a progression prescriptively, explaining that this interpretation is informed by the customary duty to prevent significant environmental harm, which requires States to exercise due diligence, including when setting NDCs [241; see also 133, 139]. Similarly, the Court adopts a strict reading of “highest possible ambition” [242 et seq.; 136 et seq.] and subjects Parties to stringent due-diligence requirements in implementing domestic mitigation measures [254].

Furthermore, the Court expressly invokes the common interest in climate protection—and the treaty notion of the common concern of humankind—to characterise the relevant obligations to protect the climate system as erga omnes (partes) [440]. On this basis, any State may invoke responsibility irrespective of injury [441]. This approach differs from earlier reasoning that grounded erga omnes status in the universal or quasi-universal character of treaties or in the jus cogens nature of the obligations (cf. Barcelona Traction, para. 34). With this approach, the Court recognizes the broader “commons” dimension of international environmental law, acknowledging the inherent common interest in protecting shared environmental systems (cf. Brunnée/Hey 2019, 52).

3. Sovereignty as Trusteeship: Responsibility and Solidarity in International Environmental Law

Against this background, the Advisory Opinion forms part of a shift in international environmental law from managing “neighbourly relations” to pursuing long-term community interests (cf. Birnie/Boyle/Redgwell 2021, 40; Wolfrum para. 40) and thus, from a private-law to a public-law paradigm in which state sovereignty is oriented not to individual or aggregate States’ interests but to the interest of the international community. Conceptually, the Court transposes the notion that present generations act as trustees of humanity [156] to States, which must give due regard to interests of future generations when exercising sovereign rights affecting the climate system [157]. In parallel, States act as trustees of the international community. The trusteeship notion is reflected in the common concern concept which the Court invokes alongside intergenerational equity to buttress duties of cooperation and prevention [155, 308, 440, Special Rapporteur Murase for the ILC on the “protection of the atmosphere” called common concern and the obligations of prevention and cooperation the “trinity for the protection of the atmosphere”, Second Report, para. 78]. The Opinion emphasises collective responsibility, accountability and solidarity, recasting the freedom underpinning sovereignty as contingent on the common goal of climate protection. Accordingly, States must subject their individual interests to the common interest in climate protection by self-limitation, so that all States have equal opportunities in exercising CO₂-related freedoms. Fulfilling this common interest is not only a moral imperative, but also a prerequisite for States’ compliance with their human rights obligations and a yardstick for assessing the lawfulness of the exercise of territorial sovereignty.

Assuming that emitting CO₂ “appropriates” the atmosphere’s absorptive capacity (cf. Moellendorf, 106), this conception of sovereignty aligns with traditions of common-ownership thinking. It moves beyond Grotius’s view of the high seas as inexhaustible res communis not subject to appropriation (Mare liberum), from which scholarship derived open access and first-come-first-served principles, enabling unrestricted exploitation (cf. Schrijver/Prislan, esp. 180–181). Grotius also held that God bestowed the Earth in common and that private ownership arose later by convention; access to the original commons had to be preserved in necessity, and harm to commonly used goods avoided (De iure belli ac pacis 1625, II.2 II.1, IV.2–4, XI). Vattel likewise stressed that the Earth was given for human subsistence (The Law of Nations 1758, § 81). The theological idea of divinely granted common ownership has been recast in modern philosophy in terms of human rights: each person holds equal claims to access and use natural resources such as the atmosphere as a CO₂ sink (Risse 2012). Building on these ideas, some argue that territorial sovereignty must be complemented by other-regarding duties (Benvenisti 2013, 310–311), or that sovereign rights over territory and resources are legitimate only within a collective, justificatory legal framework that secures equal opportunities to exercise such exclusionary rights (cf. Ypi 2012, building on Kant). Others transpose Rawls’s “just savings” principle to justify fiduciary duties of States to preserve natural resources for future generations (Brown-Weiss 1984) and combine it with common-ownership thinking about Earth and atmosphere (Moellendorf 2011; Wood 2014).

It is submitted that, on this reading, territorial sovereignty should be regarded as implicitly conditioned by the requirements of common use, warranting joint regulation. While the Court does not ground climate-protection duties in extraterritorial human rights law [402; cf. Keller], it develops a legal framework of positive State obligations to prevent harm to the climate commons. Beyond mere remediation, it advances a model of proactive prevention and cooperation attuned to the inter- and intragenerational power asymmetries inherent in territorial rights. By applying CBDR-RC across the relevant obligations, the Opinion treats States not as “black boxes” but as representatives of populations requiring special protection or bearing particular responsibilities. Accordingly, the Opinion conceives sovereign territorial rights not as an idealised extension of natural rights of appropriation, but as politically constituted and bound by a duty of justification.

4. From Vision to Practice

The Advisory Opinion advances a normative vision of global community and cooperative pursuit of shared interests despite short-term political or economic incentives. It substantiates this vision with duties of cooperation and prevention that address structural obstacles to effective global climate protection. The duty of prevention requires States to adopt a long-term perspective and regulate emissions by public and private actors. Regulatory measures can remedy market failures, especially where prices fail to internalise the social costs of emissions. Reinforced cooperative obligations may counteract collective-action problems of the climate commons. The recognition of customary obligations prevents States from evading international law obligations on climate protection simply by withdrawing from the Paris Agreement (cf. also Art. 41(2) VCLT; Rauber). The risk of legal accountability for non-compliance acts as an incentive to cooperate.

Nevertheless, the legal effectiveness of international environmental law will be tested specifically in its interaction with international economic law [cf. 173; Declaration of Judge Cleveland, para. 21, see also Bernstorff/Venzke). International environmental law’s transformative force will depend on the extent to which its objectives and State obligations will be integrated into the legal instruments of global trade, finance and investment, will prevail in case of normative conflict, and will materialise in state responsibility. This is where the legal framework will demonstrate if it can act as a catalyst (cf. Bernstein 2018) and guide the transformation of the global economy.