Lex Specialis

The Court began its analysis of lex specialis by noting that the principle is used when several rules could be relevant, to determine which of the rules prevail or whether they all “simply coexist” (para. 166). It cited the International Law Commission (ILC) for the proposition that, for lex specialis to apply, “it is not enough that the same subject matter is dealt with by two provisions; there must be some actual inconsistency between them, or else a discernible intention that one provision is to exclude the other” (para. 167, quoting ILC Articles on State Responsibility, Art. 55, commentary, para. 4).

With respect to the primary obligations of states regarding climate change, the Court stated that it could not find any inconsistency between the climate treaties and any other potentially relevant rules of international law. It pointed out that the preambles to the UNFCCC and the Paris Agreement refer to other norms of international law, indicating that the parties to the treaties “have thereby recognized that climate change constitutes a problem for whose solution other rules and principles also play a role” (para. 168). Nor did the Court discern any intention of the parties to displace other norms. By itself, the mere fact that the climate treaties were carefully negotiated was not evidence of an intention to exclude other sources of law (para. 170). The ICJ stated that “States parties to the climate change treaties were aware of their normative context and could have expressed a possible intention to displace other rules and principles had they so wished” (id.). They did not do so.

With respect to the secondary obligations regarding state responsibility, the Court similarly rejected the arguments made by the large emitters that the provisions of the Paris Agreement on “loss and damage” and complianceFootnote ¹¹ and the provision in the UNFCCC on dispute resolutionFootnote ¹² excluded the application of other rules of international law (para. 418). It noted that several states, when they joined the climate treaties, had made formal declarations “affirming the continued application of the customary international law rules on State responsibility,” which “support the interpretation that the climate change treaties do not derogate from or displace general international law of State responsibility” (para. 419).

Primary Obligations of States

The Court described five sources of international law as providing “the most directly relevant applicable law” in respect of climate change: (1) climate treaties; (2) customary international law; (3) other international environmental agreements; (4) the law of the sea; and (5) international human rights law (para. 172).Footnote ¹³

First, the Court construed the climate treaties. The Court noted that the UNFCCC and the Kyoto Protocol contain various legally binding obligations (e.g., paras. 202, 205, 211, 214, 221), but it stressed that the submissions of most of the participants in the proceedings had centered on the Paris Agreement, as the most recent and the most comprehensive climate treaty (para. 223).

Within the Paris Agreement, the Court focused on states’ obligations to mitigate climate change, which, it said, are “principally reflected” in Article 4(2) of the agreement.Footnote ¹⁴ Article 4(2) requires each of the parties to prepare, communicate, and maintain nationally determined contributions (NDCs), but it does not say exactly what the content of the NDCs should be (para. 234). On that basis, the major emitters generally argued that the treaty leaves the content entirely to the discretion of the parties (paras. 237, 239).

The Court rejected this argument. It noted that Article 4(3) of the Paris Agreement states that each party’s NDC “will represent a progression beyond the Party’s then current [NDC] and reflect its highest possible ambition, reflecting its common but differentiated responsibilities and respective capabilities, in light of different national circumstances” (para. 240). The Court read the word “will” in this provision as prescriptive, not merely hortatory, meaning that the NDCs of a party “must become more demanding over time” (paras. 240, 241). And it stated that the level of ambition to be reflected in a party’s NDCs must be capable of making “an adequate contribution” to the achievement of the parties’ collective temperature goal (para. 242).

What is that goal? Article 2 of the Paris Agreement sets a target of “holding the increase in the global average temperature to well below 2^oC above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5^o.”Footnote ¹⁵ However, the Court pointed to statements in 2021 and 2023 by the Conference of the Parties (COP) of the UNFCCC serving as the Meeting of the Parties (CMA) to the Paris Agreement, which “resolve[d] to pursue efforts to limit the temperature increase to 1.5^oC” (para. 224, emphasis removed).Footnote ¹⁶ Citing the rule of customary international law that subsequent agreements between the parties regarding a treaty must be taken into account in interpreting the treaty,Footnote ¹⁷ the Court determined that as a result of these statements, the 1.5^o target “has become the scientifically based consensus target under the Paris Agreement” (id.). Therefore, the parties to Paris are obligated to ensure that, “when taken together,” their NDCs “are capable of achieving the temperature goal of limiting global warming to 1.5^oC above pre-industrial levels” (para. 245).

Because climate change poses such a serious threat, the Court stated that the obligation of the parties is of “stringent” due diligence, meaning that “each party has to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realize the objectives of the Agreement” (para. 246). Finally, the parties must make their best efforts to achieve the content of their NDCs, including by taking effective measures regarding private actors (paras. 253, 270).

Second, the ICJ addressed customary international law. Here, it focused on two obligations: the duty to prevent significant harm to the environment, and the duty of cooperation for the protection of the environment (para. 131). The Court stated that several other principles raised by participants, including sustainable development, common but differentiated responsibilities and respective capabilities (CBDR-RC), equity, and intergenerational equity, should guide interpretation of the climate (and other) treaties, as well as the relevant rules of customary international law, but the Court did not describe the principles themselves as norms of customary international law (paras. 147, 151, 154, 157, 178–82).Footnote ¹⁸

The ICJ stated that the duty to prevent significant environmental harm applies not only to instances of direct cross-border harm, but also to the climate system as a whole (para. 134). Here, too, the standard is of “stringent” due diligence, given the special circumstances of climate change (para. 138). Each state must use all the means at its disposal to avoid activities in its territory or jurisdiction causing significant damage to the environment (para. 281). In the context of climate change, the Court stated, the appropriate rules and measures include “regulatory mitigation mechanisms that are designed to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system,” and adaptation measures that reduce the risk of significant harm (para. 282). “These rules and measures must regulate the conduct of public and private operators within the States’ jurisdiction or control and be accompanied by effective enforcement and monitoring mechanisms to ensure their implementation” (id.).

The Court acknowledged that states vary greatly in their contribution to climate change and in their capability to address it, and stated that in determining the appropriate measures to be adopted by a state, the principle of CBDR-RC must be taken into account (paras. 279, 290). The specific standard of due diligence for each state therefore depends on the specific situation of that state: “as States develop economically and their capacity increases, so too are the requirements of diligence heightened” (para. 292).

The Court stated that the duty of states to cooperate for the protection of the environment is also part of customary international law (para. 142), and that the duty takes on a special importance in relation to the need to reach a collective global temperature goal (para. 305). The Court stated that the duty to co-operate is founded on “the recognition of the interdependence of States,” and that it requires states “to continuously develop, maintain and implement a collective climate policy that is based on an equitable distribution of burdens and in accordance with the [CBDR-RC] principle” (para. 306).

With respect to the relationship between the climate treaties and the rules of customary international law, the Court stated that “at the present stage, compliance in full and in good faith by a State with the climate change treaties, as interpreted by the Court, suggests that this State substantially complies with the general customary duties to prevent significant environmental harm and to co-operate” (para. 314). However, it immediately added that “[t]his does not mean … that the customary obligations would be fulfilled simply by States complying with their obligations under the climate change treaties” (id.). This language may seem slightly self-contradictory, but its thrust seems to be to warn against the assumption that the obligations under the climate treaties and those under customary law are completely identical. In the Court’s view, the two sources of law “inform each other,” but nevertheless “establish independent obligations that do not necessarily overlap” (id.).

This means, among other things, that non-parties to the Paris Agreement (or any other climate agreement), such as the United States, are still subject to obligations under customary international law. The Court stated that the customary obligations of non-party states “find expression, at least in part, in the general practice of States which is in accordance with their obligations under the climate change treaties” as interpreted by the Court (para. 315). Therefore, a non-party that cooperates with other states in a way that is equivalent to acting as a state party may be considered to fulfill its customary obligations; conversely, a non-party state that does not so cooperate has “the full burden of demonstrating that its policies and practices are in conformity with its customary obligations” (id.).

Third, the Court briefly reviewed international environmental treaties. It stated that the obligations in the ozone treaties, the Biodiversity Convention, and the Desertification Convention also contribute to ensuring the protection of the climate system (paras. 324, 330, 334).

Fourth, the Court looked at the law of the sea. It gave “great weight” to the analysis of the International Tribunal for the Law of the Sea (ITLOS), which had issued its own advisory opinion on climate change in May 2024 (para. 338).Footnote ¹⁹ The ICJ agreed with ITLOS that Part XII of the UN Convention on the Law of the Sea (UNCLOS),Footnote ²⁰ which concerns the protection and preservation of the marine environment, applies to anthropogenic GHG emissions (para. 340). The ICJ also agreed that Article 192 of UNCLOS, which requires its parties to protect and preserve the marine environment, “requires States parties to take measures ‘as far-reaching and efficacious as possible’ … ‘to prevent or reduce the deleterious effects of climate change and ocean acidification on the marine environment’” (para. 343).Footnote ²¹

The Court noted that participants voiced strong concerns about the implications of sea level rise resulting from climate change, which the Intergovernmental Panel on Climate Change has described as unavoidable (paras. 355–56). Many states argued that sea level rise should not affect their existing baselines, maritime delimitations, and statehood—indeed, that even “the complete submergence of their territory should not deprive them of their maritime entitlements” (para. 355). Although the Court opined in passing that sea level rise “is not without consequences” for the right of self-determination, it focused on a narrower issue: it stated that after the maritime zones measured from a state’s baseline have been established and publicized in accordance with UNCLOS, the Convention does not require the parties to update them, including in the context of sea level rise caused by climate change (paras. 357, 359, 362).Footnote ²²

Several small island states requested the Court to state that even if sea level rise caused the complete loss of a state’s territory and the displacement of its population, “a strong presumption in favour of continued statehood should apply” (para. 363). The Court stopped short of providing such a clear statement; instead, it said only that “once a State is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood” (id.). However, it did go on to say that the duty of international cooperation is particularly relevant in the context of sea level rise, and requires states “to work together with a view to achieving equitable solutions, taking into account the rights of affected States and those of their populations” (para. 365).

Fifth, the ICJ examined the relationship of human rights law and climate change. In accord with many human rights bodies, the Court stated that the adverse effects of climate change, including sea level rise, drought, desertification, and natural disasters, may significantly impair the enjoyment of human rights, including the rights to life, health, an adequate standard of living, and privacy, family, and home (paras. 376–81). It stated that climate change may also impair the enjoyment of the rights of women, children, and Indigenous peoples (paras. 382–84). States must take the “necessary measures” to protect the climate system and other parts of the environment, because the climate system and the environment as a whole are necessary for the full enjoyment of human rights (para. 403).

Of particular note, the Court strongly endorsed the human right to a clean, healthy, and sustainable environment as part of international law. The ICJ pointed out that the right has been recognized in several regional instruments, enshrined in the constitutions or domestic legislation of more than one hundred states, and recognized by the General Assembly in 2022 (paras. 390–92). It stated that the right results from the interdependence between human rights and environmental protection, and that it is “difficult to see” how states can fulfill their obligations under human rights law without ensuring the protection of the right to a clean, healthy, and sustainable environment (para. 393). As a result, the right is both “inherent in” and “essential for” the enjoyment of other human rights (id.).

The Court stated that international human rights law, climate and other relevant treaties, and customary international law all inform one another. States must therefore take their human rights obligations into account when implementing their obligations under climate and other treaties and customary international law, and vice versa (para. 404).

Secondary Obligations of States

The second question presented by the General Assembly concerned the legal consequences for states where they have caused significant harm to the climate system. Notably, the ICJ did not consider whether states might be liable for climate harm even in the absence of a violation of international law (para. 405). Instead, it stated that any breach of the obligations described in its answer to the first question could engage the responsibility of the state committing the breach (id.). For example, “a State that does not exercise due diligence in the performance of its primary obligation [under customary international law] to prevent significant harm to the environment, including to the climate system, commits an internationally wrongful act entailing its responsibility” (para. 409).

The Court specifically addressed issues of attribution, causation, and remedy. With respect to attribution, it emphasized that the wrongful act giving rise to state responsibility would not be the emission of GHGs per se, or the conduct of private (non-state) actors, but rather the breach by a state of any of the conventional or customary legal obligations identified previously in its opinion. So, for example, “[f]ailure of a State to take appropriate action to protect the climate system from GHG 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). With respect to private actors, the issue would normally not be attribution of their conduct to the state, but whether the state had failed to exercise regulatory due diligence over them (para. 428).

As for causation, the ICJ acknowledged that the fact that multiple states have contributed to climate change may well increase the difficulty of determining whether and how far an individual state’s breach has caused significant harm, but noted that the customary international rules on state responsibility contemplate the possibility of allocation of responsibility among two or more actors that have caused injury (para. 430).

With respect to causation more generally, the Court distinguished between whether a given event or trend can be attributed to climate change, on the one hand, and the extent to which damage caused by climate change can be attributed to a particular state or states. It stated that the first issue may be addressed by science, while the second “must be established in concreto in respect of specific claims” (para. 437). It opined that “while the causal link between the wrongful actions or omissions of a State and the harm arising from climate change is more tenuous than in the case of local sources of pollution,” that does not mean that identification of the causal link is impossible, and it reaffirmed the applicability of the law of state responsibility in that respect (para. 438).

The Court stated that the obligations of states to protect the climate system are obligations erga omnes (para. 440). When such obligations arise under the climate treaties, all parties to the treaty may invoke responsibility; when obligations arise under customary international law, any state may do so (para. 442).

Finally, with respect to remedies, the Court stated that breaches of states’ obligations with respect to climate change may give rise to “the entire panoply” of legal consequences, including obligations of cessation and non-repetition (which apply regardless of harm), and obligations of reparation, including restitution, compensation, and satisfaction (para. 445). However, the Court said that it could not in these proceedings specify what consequences are entailed by such breaches, “since such consequences depend on the specific breach in question and on the nature of the particular harm” (id.).

* * * *

The advisory opinion covers so many issues, and addresses a topic of such vital importance, that it will undoubtedly receive attention from governments, advocates, and scholars for many years to come. Arising as it did from frustration with the glacial pace of negotiation and implementation of climate treaties, the opinion will play a vital role in campaigns seeking to change climate law and policy, as well as in litigation before international tribunals and domestic courts.Footnote ²³

Three of its interpretations seem especially likely to be repeated and amplified in these forums.

First, the Court adopted the strongest feasible reading of the Paris Agreement. It stated that despite the circumscribed and arguably ambiguous language in Article 4, the NDCs have real legal effect: states must ensure that their NDCs incorporate their highest possible ambition, and they must do all that they can to implement them. Moreover, states are only in compliance if their collective efforts suffice to meet not just the Article 2 target, but the Article 2 target as subsequently interpreted by the parties, which the Court said means holding the increase in global average temperature to no more than 1.5 degrees. This goal may seem unrealistic in light of rapidly increasing global temperatures, but it is in line with a position repeatedly endorsed by the parties themselves. In this respect and throughout its interpretation of the Paris Agreement, the Court held the parties to the strongest possible interpretation of their own statements, even if those statements were not seen by all of the parties as legally binding at the time they were made.Footnote ²⁴

Second, the Court effectively construed customary international law to impose substantive requirements on all states roughly equivalent to those set out in the Paris Agreement. This means, among other things, that the United States (and other states) cannot avoid the “stringent” duty of due diligence by withdrawing from Paris.

Third, the Court reinforced the position—already strongly affirmed by international human rights bodies—that states have obligations under human rights law to protect the climate system (and the environment generally) in order to protect the human rights that depend upon it. The Court’s recognition that the right to a clean, healthy, and sustainable environment is an integral part of international law will further accelerate the already-rapid development of environmental human rights law and bolster rights-based climate litigation.Footnote ²⁵

In short, the Court set out a comprehensive framework for understanding and analyzing states’ primary and secondary responsibilities under international law with respect to climate change. Despite the breadth of its opinion, the Court was able to reach strong, even ambitious interpretations of the relevant international norms without sacrificing legal rigor. That must be considered a significant achievement.

Nevertheless, it is important to note two weaknesses, one internal and one external to the Court’s opinion.

The internal weakness is that the opinion pays almost no attention to a critical issue: the allocation of individual responsibility to states in the virtually certain event that their collective efforts fall short of the 1.5 degree goal.Footnote ²⁶ The ICJ repeatedly stated that its mandate was not to assign specific responsibility to specific states, leaving such tasks to future in concreto proceedings.Footnote ²⁷ But it did little to clarify how these future proceedings should assign individual responsibility for collective failure.

Instead, the Court stated only that the standards to be applied in assessing individual NDCs will depend on, among other things, the state’s historical contributions, its level of development, and its “national circumstances” (para. 247). It stopped far short of indicating how these factors should affect the determination of individual state responsibility or, conversely, of the remedies that might be claimed by states that are, in the words of the General Assembly resolution, specially affected by or vulnerable to the effects of climate change.Footnote ²⁸ Nor did the Court examine underlying systemic issues such as the relationship of historic GHG emissions to colonialism and racism and their continuing effects.Footnote ²⁹

Perhaps the Court felt that it addressed the standard of individual state responsibility sufficiently by emphasizing that each state must “do its utmost” to ensure that its NDCs reflect its “highest possible ambition” (para. 246). One reading of the opinion is that the most relevant metric, in the Court’s view, of whether a state is meeting its responsibilities under international law is simply whether the state is doing all that it can.

The weakness external to the advisory opinion is obvious: states are not bound to follow it. As a result, whether the Court’s interpretations are adopted depends on a mix of legal and policy issues that vary enormously from country to country. Although the ICJ has no direct control over the bindingness of its opinions, the judges do seem to have done all that they could to increase the normative weight of this one. Perhaps most remarkably, they issued it unanimously. The diversity of views that they had to reconcile to reach unanimity is illustrated by the fact that apart from the advisory opinion itself, the judges issued twelve separate opinions and declarations, only two of which received more than one vote.

The Court also took care to present its positions as representing a mainstream view of the applicable legal norms. For example, wherever possible, it explicitly aligned its positions with those already taken by other international bodies, including ITLOS and human rights bodies such as the Human Rights Committee. In interpreting the climate treaties, it purported to base its analysis on well-accepted tools of treaty interpretation, even if it sometimes employed those tools to reach results that not all of the states—especially the major emitters—are likely to endorse.Footnote ³⁰

Similarly, with respect to customary international law, the ICJ focused on the two most obviously relevant norms: the duty to avoid significant environmental harm and the duty of international cooperation. It avoided describing other norms as part of customary law in ways that some of the participants undoubtedly would have supported but that might have been perceived as controversial by others. For example, the Court did not characterize CBDR-RC and other principles of international environmental law as elements of customary law, instead describing them as guiding principles. Nor did it describe the human right to a healthy environment as a customary international norm, despite being invited by some states and encouraged by some members of the Court to do so.Footnote ³¹

In short, the Court situated its interpretations within the range of positions taken by the majority of the participating states, albeit sometimes at the more progressive end of that range. However, it bears repeating that this majority was composed largely of states that are relatively more vulnerable and emit lower levels of GHGs; when the Court rejected arguments, they were often those taken by the major emitters, especially the large emitters in the Global North.Footnote ³² As a result, it may be expected that the advisory opinion will have the least influence where it is needed most. It is hard to imagine that the opinion will have any direct effect on many of the largest emitters, especially the United States, whose current administration is seeking, in its words, to drive “a dagger through the heart of climate-change religion.”Footnote ³³

In an eloquent final paragraph, the Court recognized that international law has “an important but ultimately limited role” in solving this “existential problem of planetary proportions” (para. 456). The Court said that a solution “requires human will and wisdom—at the individual, social and political levels—to change our habits, comforts and current way of life in order to secure a future for ourselves and those who are yet to come” (id.). So far, states—especially the major emitters—have not met the moment in the ways that the ICJ has now said that they must. Whether they find the will and wisdom to do so is perhaps the most consequential issue of our time.