On Wednesday, July 23, the International Court of Justice (ICJ) in the Hague issued its long-awaited advisory opinion on climate change, thereby concluding the biggest-ever proceedings before the “World Court.” This landmark opinion, while not legally binding in its own right, carries major persuasive legal weight. Given the legal and moral authority of the Court, the quality and scope of its reasoning and the scale of participation by States and international organizations in the proceedings, this opinion is sure to inform international litigation, U.N. Conference of Party negotiations (in the context of the U.N. Framework Convention on Climate Change (UNFCCC) and the Paris Agreement), and domestic procedures going forward. This article analyzes some of the key points among those the Court covered in its 140-page opinion.
Context of the Proceedings
This opinion stems from mobilization that began as a student-led initiative, supported by Vanuatu and a core group of States. Based on their proposals, the U.N. General Assembly issued a request to the ICJ to clarify States’ climate-related obligations under several overlapping areas of international law, including the U.N. Charter, international human rights law, the international climate treaties, the law of the sea, and customary international law (Question A). It also asked the Court to clarify the issue of reparations for significant climate and environmental harm inflicted in breach of these obligations (Question B).
On July 23, at a public hearing held in the Hague, ICJ President Judge Yuji Iwasawa announced the Court’s opinion, which was subsequently published online along with the separate opinions and declarations of several judges. Notably, although several judges indicated that they would have favored further-reaching findings (for instance on debt relief or intersectionality), and additional clarity on the human right to a healthy environment, the Court adopted its opinion unanimously, making it the fifth of the ICJ’s total 29 total advisory opinions issued in the last eighty years to be adopted with the votes of all judges in favor.
What Was at Stake?
During the course of their written and oral submissions, the States involved in the proceedings made a wide range of arguments about the intensity and nature of their international obligations around climate change. While many States – led by Vanuatu and other climate-vulnerable small island states – argued for robust recognition of State obligations in the face of climate change, others sought to minimize their international obligations. For example, the United States and Kuwait argued that the content of States’ Nationally Determined Contributions (NDCs) cannot be reviewed against any substantive international standards. The U.S. also objected to the idea of differentiated obligations for developed and developing States. China, Russia and others argued that the climate treaties (the UNFCCC, the Paris Agreement, and the Kyoto Protocol) are a specialized regime, or lex specialis, superseding other obligations. As concerns customary law, the Nordic Countries and others argued that a global problem like climate change was beyond the reach of customary rules against transboundary environmental harm.
These and other key issues were addressed in the advisory opinion. The stakes were high: if the Court had applied a restrictive interpretation of the international climate regime, and found that States’ obligations do not go beyond those that they have voluntarily opted into under that regime, this would have deeply set back climate justice movements and undermined recent developments in addressing the human rights impacts of climate change or the implications for marine pollution made by other courts. Instead, the ICJ’s opinion built on these judicial findings to establish that States have an overlapping set of obligations to cooperate and to combat, adapt to, and provide reparation for climate change.
Establishing the Obligations of States (Question A)
The ICJ’s analysis began by addressing Question A, concerning the international legal obligations of States. In particular, the ICJ (1) rejected the lex specialis argument that climate obligations stem solely from the international climate regime; (2) confirmed the binding nature of key obligations in the international climate treaties; (3) confirmed the applicability of obligations under customary international law; (4) built on International Tribunal of the Law of the Sea (ITLOS) findings regarding climate change; and (5) held that fundamental human rights treaties are applicable to the context of climate change.
(1) Rejecting the Lex Specialis Argument
The first key point of the ICJ’s reasoning, and a cornerstone of the opinion, was the rejection of the lex specialis argument. Different iterations of this argument were made by States, all converging around the idea that States’ international climate obligations are regulated solely by the international climate regime (e.g. the UNFCCC and the Paris Agreement), as the more specific legal framework. This approach would mean excluding further-reaching obligations under other treaties or customary international law from reaching climate issues. The Court clearly and emphatically rejected this argument, finding that the lex specialis rule applies only where there is an inconsistency between regimes, or a clear intention to displace more general rules with narrower, more specific ones. Neither condition was satisfied here, meaning that the climate treaties do not represent the sum of all climate obligations: they apply alongside and in addition to human rights law, environmental law, and customary law.
(2) Acknowledging Binding Obligations Under the International Climate Regime
Turning next to the international climate regime, the Court’s opinion established the relevant obligations and general principles set forth by that regime (i.e. common but differentiated responsibilities and respective capabilities (CBDR-RC), the precautionary principle, sustainable development, equity and intergenerational equity). It clarified that the UNFCCC and the Paris Agreement create binding obligations regarding mitigation, adaptation, and cooperation. It also clarified the Paris Agreement’s temperature limit, which is set out in Art. 2(1)(a) of the treaty. That provision aims at limiting global warming to “well below 2°C” while “pursuing efforts to limit the temperature increase to 1.5°C.” The Court held that the 1.5°C threshold has now become the parties’ agreed primary temperature goal, superseding the 2°C threshold (para. 224).
This threshold, the Court further held, informs States’ mitigation obligations. Some parties to the proceedings had argued that the content of States’ NDCs is discretionary, meaning that it cannot be subjected to substantive evaluation. After examining the Paris Agreement’s requirements that NDCs be progressive and reflect each State’s “highest possible ambition,” however, the Court held that States’ discretion is limited. All States’ NDCs, when taken together, must make it possible to achieve the 1.5°C temperature limit set out by the Paris Agreement (para. 249). States must also act with due diligence in taking measures to achieve their NDCs.
(3) Applying a Stringent Customary Standard of Due Diligence
Turning to customary international law, the ICJ established that the obligation to prevent significant transboundary environmental harm (the already established and accepted customary rule of “no harm”) applies to climate change, concurrently with the climate treaties. Some States had argued that this rule applies only to localized cross-border harm between neighboring States, and not a global problem like climate change. However, the Court found that the climate system is an integral part of the environment, and must be protected for present and future generations. As a result, the Court opined, States are under a stringent standard of due diligence to prevent significant harm to the climate system, including by taking science-informed precautionary measures and conducting risk assessments and environmental impact assessments. The measures required vary depending on a State’s capabilities: the principle of CBDR-RC tailors obligations to States’ resources and levels of development and ensures an equitable response to climate change.
The Court also noted that States have a customary duty to cooperate for the protection of the environment (para. 301). It also expressly emphasized that customary obligations “are the same for all States and exist independently regardless of whether a State is a party to the climate change treaties” (para. 315). This means that States who denounce the Paris Agreement, such as the United States, do not enter a legal vacuum, but continue to be bound under international law by customary obligations to prevent climate change-related harm and cooperate with other States.
(4) Confirming (and Going Beyond) the ITLOS Advisory Opinion
Along with considering the relevant obligations under various environmental treaties (the Ozone Layer Convention, its Montreal Protocol, the Biodiversity Convention, and the Desertification Convention), the Court engaged with the obligations entailed by the U.N. Convention on the Law of the Sea (UNCLOS). In doing so, it affirmed the 2024 ITLOS advisory opinion on climate change, recognizing that greenhouse gas (GHG) emissions constitute marine pollution and trigger State obligations to protect and preserve the marine environment.
Going beyond the ITLOS opinion (and with gratitude to Pranav Ganesan for highlighting this), the Court also engaged with the loss of land and territorial sea due to sea-level rise. In doing so, it resolved long-standing questions within international law about what happens when rising sea levels impact a State’s maritime or land borders. The Court held that States will not lose the maritime zones belonging to their territory if sea levels rise, and that even the complete loss of land territory through sea level rise does not necessarily mean that a State ceases to exist. This feeds into discussions about the possibility of virtual States, but also relates to climate-induced displacement and the migration-related obligations of States.
(5) Applying International Human Rights Law
In addition to climate treaties, customary law, and environmental treaties, the Court applied international human rights law. It held that the core U.N. human rights treaties apply in this context, given that a clean, healthy, and sustainable environment is a precondition for the enjoyment of human rights. The Court recognized that the adverse effects of climate change may significantly impair the effective enjoyment of the human rights to health, an adequate standard of living, access to food, water and housing, privacy, and the rights of women, children, and Indigenous peoples. It also engaged with the right to a clean, healthy and sustainable environment, recognizing that such a right emerges from the interdependence of human rights and the environment, and that it is “inherent in the enjoyment of other human rights” (para. 393).
Reparation for Breaching Climate-Related Obligations (Question B)
The ICJ made landmark findings regarding reparations, including on (1) causation and shared responsibility; (2) fossil fuel exploitation and the regulation of private actors; and (3) the status of certain climate obligations as obligations erga omnes partes.
(1) Causation and Shared Responsibility
Turning to reparation, the Court declared that customary international rules about responsibility for wrongful acts apply in the context of climate change. On this basis, the Court made several important findings, establishing that the plurality of both responsible and injured States makes reparation more difficult, but certainly not impossible. What is required is the presence of an internationally wrongful act attributable to a State, and the Court solidified the meaning of these requirements in more detail.
Both as concerns attribution of responsibility and proof of causation, several States made “drop in the bucket” arguments aimed at avoiding responsibility by pointing to the contributions of other States. Rejecting these arguments, the Court found that the customary rules on State responsibility can address situations concerning a plurality of injured or responsible States (para. 430). In determining whether a breach of climate obligations was causally linked to damage suffered by other States, the Court set out a two-part test that examines (1) whether a given impact can be attributed to climate change, based on climate science, and (2) whether damage caused by climate change can be attributed to a particular State or group of States in the concrete case (para. 437).
(2) Responsibility for Fossil Fuels and Regulation of Private Actors
In determining what kinds of breaches of international obligations (wrongful acts) incur State responsibility, the Court engaged with two contentious topics: fossil fuel exploitation and the behavior of private actors. In both regards, the Court recognized the possibility of State responsibility. On fossil fuels, it held that if States fail to protect the climate system from GHG emissions — including by continuing fossil fuel production and consumption, or by granting fossil fuel exploration licenses or subsidies — they may be committing an internationally wrongful act. The Court also held that States may incur international responsibility where they fail to take regulatory or legislative measures to limit GHG emissions by private actors in their jurisdictions.
(3) Climate Obligations as Obligations Erga Omnes Partes
Lastly, and importantly, the Court recognized that obligations under climate treaties and the no-harm rule under customary international law are obligations erga omnes partes, meaning that all States parties have a legal interest in the protection of the treaties’ main mitigation obligations. This issue was hotly debated during the proceedings. In its opinion, the Court found that any State has a legal interest in avoiding climate change, and can accordingly invoke the responsibility of other States for violating their legal obligations pertaining to climate (para. 440). This opens the door for proceedings by any State, including non-injured States, seeking cessation of the wrongful act, guarantees of non-repetition, and reparation in the interest of injured States (para. 443).
Conclusion
The reasoning of the ICJ’s inaugural climate opinion is sure to attract extensive analysis and further activity over the course of the coming months, including discussion of the questions left open – such as, for example, the legal status of the right to a healthy environment, or the implications for international investment law or the UNFCCC loss and damage framework. However, already at this early stage, the opinion can be expected to have far-reaching implications for how States approach climate change on the international and domestic stage. The opinion opens the door to litigation between States in international fora, including the ICJ and the ITLOS, and climate-vulnerable States can be expected to rely on the opinion to shift power dynamics within the upcoming COP30 meeting in Brazil.
With this unanimous opinion, the ICJ has taken a clear stand in favor of a coherent and equitable international legal response to climate change. In doing so, it has clarified that States’ climate action does not exist in an international legal vacuum, but rather that States must take proactive, cooperative, science-informed measures to avoid, adapt to and make reparation for global warming. This does not hinge solely on whether they ratify relevant climate treaties, such as the Paris Agreement, but also stems from separate but complementary obligations under customary international law, human rights law, environmental law, and the law of the sea.
– Corina Heri, Published courtesy of Just Security.
