“The strong do what they can and the weak suffer what they must.” — Taken from the oral statement of Nauru, citing Thucydides, The History of the Peloponnesian War, book 5, ch. 89 (trans. R. Crawley, 1874).

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From Dec. 2-13, the International Court of Justice (ICJ) held oral hearings in its ongoing climate advisory opinion proceedings. Involvement from 97 states and 11 international organizations make these the ICJ’s biggest-ever proceedings, as well as the largest of the three climate advisory opinion proceedings pending before international courts during the course of 2024 (the International Tribunal for the Law of the Sea [ITLOS] having delivered its opinion in May, and the Inter-American Court of Human Rights [IACtHR’s] due to be delivered in 2025). Of the three, the ICJ’s proceedings are also the broadest in scope, and they have involved much civil society attention as well as a hearing with Intergovernmental Panel on Climate Change (IPCC) climate scientists. These proceedings (in which I was involved on behalf of the International Union for Conservation of Nature) stand to issue important, authoritative guidance about the nature and interaction of States’ obligations, building on and complementing the guidance provided by other international courts.

Background on the Proceedings

The request for an advisory opinion that lies at the heart of these proceedings began as a student-led initiative, supported by Vanuatu and a core group of States. These proceedings posed a carefully crafted question to the ICJ, seeking clarification of States’ climate-related obligations under overlapping areas of international law. This question, as posed by the U.N. General Assembly and co-sponsored by 132 countries, asks the Court to clarify States’ climate-related obligations under:

(a) the U.N. Charter;
(b) several U.N. human rights instruments;
(c) the international climate regime;
(d) the law of the sea; and
(e) customary international law.

The Court is also asked to clarify the issue of reparations, and more specifically the legal consequences of significant climate and environmental harm, both with respect to States – particularly vulnerable small-island developing States – and to peoples and individuals belonging to present and future generations.

Given the number of parties involved, these proceedings are voluminous. The written comments received by the Court (namely 91 written statements and 62 comments on these statements) are successively being made available online, with verbatim reports of the oral interventions also being made available along with a video livestream. The ICJ’s opinion is still to be written. However, a number of broader themes and questions have already emerged from the proceedings.

The Main Lines of Argumentation: Two Camps

Overall, States’ positions during their oral interventions can be roughly segmented into two groups. On the one hand, many countries – especially, but not only, developing and low-emitting ones – argued that the Paris Agreement does not represent the sum total of State obligations on climate change, and that the customary no-harm rule (the well-established obligation to prevent significant transboundary harm to the environment, accompanied by a duty of due diligence in minimizing corresponding risks) and human rights law create additional, further-reaching obligations. In doing so, they emphasized science-led approaches and their own vulnerability to climate-related impacts like storms, droughts, and sea-level rise, coupled with their limited contribution to the causes of climate change. This fed into emphasis of the Paris Agreement’s principle of “common but differentiated responsibilities and respective capabilities, in light of different national circumstances” (CBDR-RC), and the need for equitable, ambitious and differentiated action from States that factors in regard for levels of development and historical emissions. Many of these States also invoked different human rights impacts, emphasizing the right to a healthy environment and the rights of Indigenous peoples and vulnerable groups. These and other rights, they argued, should apply extraterritorially. They additionally invoked risks to their territory, statehood and culture and the right to self-determination, emphasizing enduring colonial legacies, development needs and the impact of debt burdens and unchecked corporate and investor power. They drew on international environmental law, foregrounding the polluter pays and precautionary principles. And they framed their interventions against their disappointment with international negotiations under the U.N. Framework Convention on Climate Change (UNFCCC) umbrella, emphasizing intragenerational equity and the need for adaptation finance, compensation for loss and damage, and adequate reparations.

By contrast, several high-emitting and developed States focused on contesting the existence of binding international legal obligations to mitigate their emissions or provide reparations. To do so, they repeatedly emphasized the importance of continuing political negotiations and the “carefully negotiated” but easily destabilized solutions thereby achieved so far. Various States argued that the international climate regime was self-contained and represented a lex specialis to all other invoked rules. This treaty, they argued, left States free to set the content of their NDCs and did not provide any guidance for apportioning remaining emissions between them. Concerning reparations, they contested the existence of differentiated obligations for developing and developed States and argued that complex causal chains prevented the attribution of responsibility to individual States. In addition, they advanced several layers of argument to hedge against the application of the no-harm rule. This meant arguing that these rules apply only to bilateral relationships among neighboring States – or, if they do apply to a global problem like climate change, then they have only started to apply recently, and therefore do not apply to historical emissions; lastly, they argued, compliance with the Paris Agreement automatically also means compliance with customary law. Additionally, some States also defended the extraction of fossil fuels – whether directly, like Kuwait; or indirectly, like the joint Nordic states, who emphasized the complexity of causal chains given “the combined requirements of life worldwide.”

Certainly not all contributions aligned precisely with these two camps: the written and oral contributions were rich and varied. For example, the State of Palestine emphasized the responsibility of States for the climate-related impacts of armed conflict and occupation. Some European States, like Latvia, Liechtenstein, Albania and Slovenia expressed their support for the arguments of developing countries and underscored these States’ vulnerability. Colombia made a detailed case for reparations. Barbados discussed the declining insurability and thus recoverability of climate-related harms in vulnerable areas. The Netherlands encouraged regard for climate-induced displacement and shared its floor time with a youth representative. The Cook Islands noted the racially disparate impacts of climate change. Brazil noted that it had devised a methodology to quantify historical responsibility. Chile made a richly cross-referenced argument for extraterritorial human rights obligations. Ghana and Sierra Leone called for debt cancellation. Slovenia focused its contribution exclusively on the right to a healthy environment. And several States also called for attention to gendered impacts and the rights of future generations.

The above does not provide a complete summary of these complex proceedings. However, it sets the stage for looking more closely at several crucial issues at stake before the Court.

Central Points of Contention

The relationship between international climate law and other regimes

A central point of contention in these proceedings concerned the interplay of the various regimes involved – or, more specifically, the question of whether the Paris Agreement either supersedes or complements States’ obligations under human rights law, customary international law, and the law of the sea. In essence, some States argued that other obligations should be levelled down until they require nothing beyond the Paris Agreement’s binding obligations. For example, Germany sought to limit the scope and content of human rights law by arguing that “States fulfilling the Paris Agreement thereby simultaneously fulfil their human rights obligations.” Similar arguments were made about the customary no-harm and due diligence rules. Australia, the United Arab Emirates, the United States of America and the United Kingdom made various arguments to the effect that these rules either did not apply to greenhouse gas emissions, or did not require anything beyond the obligations in the Paris Agreement. A similar argument was also made (Saudi Arabia) concerning reparations, to the effect that the customary rules codified in the Draft Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA) had been displaced by the Paris Agreement’s provisions on loss and damage.

Most States objected to these interpretations, arguing that the Paris Agreement alone did not represent the full extent of States’ obligations. They referred variously to the fact that no such intention was expressed at the time of adoption, that the Paris Agreement does not contain secondary rules describing the consequences of a violation, and that human rights feature in its preamble. States also drew on the principle of harmonious interpretation or systemic integration under Article 31(3)(c) of the Vienna Convention on the Law of Treaties, arguing that it requires not levelling-down, but a mutual strengthening of applicable obligations. Mexico and Costa Rica further clarified that harmonization means acknowledging the limitations of the Paris Agreement, and creating a coordinated response that fills gaps in international climate law and creates accountability.

In rejecting the idea that the Paris Agreement constitutes the sum of States’ climate obligations, many States referred to the ITLOS’s climate opinion, which rebuffed the argument that the international climate regime constitutes a lex specialis to the law of the sea. There, the ITLOS noted that it “does not consider that the Paris Agreement modifies or limits the obligation under the [UNCLOS].” This argument was reiterated by Bangladesh, which strongly objected to a lex specialis argument, arguing that the application of this rule requires a conflict between different norms, which was not present here. Albania similarly encouraged the Court “to reject decisively the self-serving contention that key human rights treaties need not be considered.” At the same time, a small minority of States, specifically Russia and China, challenged the ITLOS opinion.

The existence and application of customary international law

These proceedings draw on several rules of customary international law, especially the well-established obligation to prevent significant harm to the environment (the “no-harm rule”), which is accompanied by a duty of due diligence in minimizing corresponding risks.

Some developed States disavowed the existence of the customary “no harm” rule. Belize neatly summarized the six main arguments against a customary obligation to prevent climate-related harm: first, “that greenhouse gas emissions are somehow the wrong sort of harm and thus fall outside the prevention obligation”; second, that the content of the due diligence obligation is not known; third, that it does not apply because the Paris Agreement is a lex specialis; fourth, that custom is displaced through the principle of harmonious interpretation; fifth, that compliance with the international climate regime satisfies all other obligations; and sixth, that the UNFCCC and Paris Agreement establish a self-contained regime, leaving no room for the application of other parallel rules.

These arguments were contested throughout the proceedings. However, some States, for example the United States, objected strongly to the arguments based on customary law made in the case – likely laying the groundwork for a persistent objection argument. Others, like France and Germany, questioned the temporal starting point of customary obligations. Germany and Switzerland argued that any customary obligations could not predate the first IPCC report, which was published in 1990; this argument was repeated several times during the proceedings to contest responsibility for historical emissions.

The interpretation of the Paris Agreement

Several States also put forward their own interpretations of the Paris Agreement. In its intervention, the United Kingdom described the argument of China, Kuwait, the United States, and Saudi Arabia in this regard as seeking to “empty that Agreement of meaningful content.” The UK’s view was that these States’ arguments sought to erode cornerstones of the Paris Agreement – rejecting the CBRD-RC principle, disputing the Article 4(2) obligation to pursue mitigation measures, and snubbing the principles of progression and highest possible ambition in mitigation measures. Some, like China, emphasized a textual temperature goal ranging from 1.5°C to 2°C instead of the 1.5°C goal currently prioritized by COP outcome decisions and IPCC reports.

These discussions are not mapped comprehensively here, but it was clear that the principle of CBRD-RC was of great concern to developed States, for example Switzerland, which argued that this principle should be dynamic and not focused on historic emissions. Going even further, the United States argued that CBDR-RC is not a legal principle, and that it does not imply a differentiation of commitments between developed and developing countries. This, as Singapore noted, runs contrary to the long pedigree of this principle across different treaties.

Human rights, especially the right to a healthy environment

As noted above, several developed and high-emitting States argued that human rights law could not apply to the impacts of their greenhouse gas emissions, whether historical or current. The representative of Namibia described these arguments as “grossly insensitive and self-serving,” as well as incorrect as a matter of law. In doing so, he drew on the 2024 KlimaSeniorinnen judgment of the European Court of Human Rights (ECtHR), which was cited several dozen times in the course of the proceedings and which confirmed that compliance with the Paris Agreement does not exhaust State obligations in the context of climate change.

Much like the no-harm rule, human rights law may prove central to the Court’s opinion. The proceedings relate to the rights enshrined in different U.N. instruments, including rights to food, water, life, education, health, and culture, among others. It could also acknowledge the rights of future generations. And, as Singapore noted, it could draw on the U.N. Convention on the Rights of the Child, which, although not part of the question asked to the Court, enjoys near-universal ratification – much like the Paris Agreement. Furthermore, many States referred to the role of the human right to a clean, healthy and sustainable environment. Several States argued that this right had crystallized into a norm of customary international law, emphasized its central importance for protecting other human rights or, in the case of Portugal, called for its regional recognition in the Council of Europe. Meanwhile, the United States and Russia rejected the existence of such a right under international human rights law, while Canada argued that it had no internationally agreed content despite its non-binding recognition by the U.N. General Assembly in 2022, and Germany argued that the right was purely non-binding.

A Preliminary Assessment

This hearing concludes “the year of climate in international courts” and puts the existential risks facing climate-vulnerable small-island developing States on the world stage. Some of the counterarguments – e.g. contesting the applicability of the no-harm and due diligence principles and the ARSIWA, minimizing the obligations in the Paris Agreement and the UNCLOS, and seeking to disapply human rights law – received immediate attention. States that contested the application of these rules provided cascading argumentation, hedging against various possible outcomes and emphasizing the primacy of the Paris Agreement while narrowly interpreting the binding obligations contained therein.

Strikingly, these arguments appear to come from a minority of the parties appearing before the Court. Notably few States contested the existence and applicability of core legal principles, such as the no-harm rule, due diligence and CBDR-RC, or the human rights impacts of climate change. Developed and high-emitting States focused instead on arguments intended to shift responsibility elsewhere, with a few – e.g. the United States – seeming to pave the way for “persistent objector”-style arguments. Overall, in other words, the bulk of disagreement concerned the interpretation of the relevant principles, not their applicability, and the apportioning of reductions and reparations obligations. Parties also made extensive references to the climate case-law of international, regional, and domestic courts and tribunals, once again underscoring the intensive inter-regime judicial dialogue that characterizes climate cases.

Admittedly, the questions raised before the ICJ are far from resolved. Some of the concepts invoked, like CBDR-RC, are the result of protracted and delicate negotiations, and apportioning shares of historical and current responsibility for climate change remains particularly thorny. At the same time, the existing international climate regime has not succeeded in resolving these issues or avoiding very real climate-related harms and risks that trigger the applicability of other legal regimes. From the perspective of human rights law, the idea that the Paris Agreement precludes the application of all other international law has already been rejected by the ECtHR, among others. Indeed, this argument is flawed not only because the Paris Agreement references human rights in its preamble, and not only because there is no necessary conflict between the two, but also because (I would argue) human rights law is not a general order that recedes behind the more specialized Paris Agreement. In fact, shifting the focus to the impacts at stake, and to safeguarding human lives, livelihoods, and well-being, renders human rights law the more specialized order.

Ultimately, the fact that these are advisory opinion proceedings may prove particularly relevant. Because of the nature of these proceedings, the Court need not find any individual State responsible for the impacts of its greenhouse gas emissions, or calculate how much it owes in terms of reparations. Instead, it is asked to clarify legal obligations and principles – and the argumentation of the majority of States has shown that this is both feasible and urgently necessary.

In addition, while the arguments discussed in these proceedings reflect several entrenched debates certainly familiar to international climate negotiators, it is worth noting that these proceedings are different from a COP-style negotiation in several important ways. Not only have all participating parties been heard by the Court on equal footing, but this has taken place in a transparent way: written and oral pleadings have been made publicly available. This makes it possible to analyze States’ climate-related positions in an unprecedented way. And importantly, these proceedings are not a negotiation – they are in the hands of a judicial decision-maker with the ability to provide authoritative answers on the broadest range of international legal questions concerning climate change to come to a court thus far.

IMAGE: Representatives from small Pacific island states gather outside the International Court Of Justice on December 2, 2024 in The Hague, Netherlands. More than 100 nations and organizations are seeking an advisory opinion from the top UN court on what countries are legally obligated to do to fight climate change and help vulnerable nations mitigate its impact. (Photo by Michel Porro/Getty Images)