# Opening the doors. Legal consequences of breaching international climate obligations in the ICJ Advisory Opinion

Yann Kerbrat, Sandrine Maljean-dubois

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Yann Kerbrat, Sandrine Maljean-dubois. Opening the doors. Legal consequences of breaching international climate obligations in the ICJ Advisory Opinion. Review of European, Comparative & International Environmental Law, 2026, 10.1111/reel.70034. halshs-05485891

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# Opening the doors.
## Legal consequences of breaching international climate obligations in the ICJ Advisory Opinion

Yann Kerbrat*, Sandrine Maljean-Dubois**

### 1. INTRODUCTION

On 23 July 2025, in its advisory opinion on the obligations of States regarding climate change, the International Court of Justice addressed the controversial issue of the consequences of unlawful acts and State responsibility. On this matter, it went further than the International Tribunal for the Law of the Sea (ITLOS) in its 2024 Advisory Opinion on climate change, following the request of the Commission on Climate Change and International Law of Small Island States. In that previous case, as the question of responsibility had not been explicitly submitted to the ITLOS, the Tribunal had been able to avoid this delicate issue.<fnref n="1" /> In contrast, the International Court of Justice was explicitly asked about this particular point. Indeed, the Court was called upon by the United Nations General Assembly not only to clarify the content of the obligations of States, under international law, ‘to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases’ (question a), but also to specify the legal consequences of these obligations ‘for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment’ (question b). The issue of the legal consequences of breaching climate obligations was arguably the most sensitive part of the Opinion and was particularly feared by major historical and/or current emitters of greenhouse gases.

While the Court provided an audacious interpretation of primary obligations, it was – as could be expected – more cautious about the legal consequences of violations. It avoided certain questions on the grounds that they can only be properly examined in the context of a particular dispute or a more specific request for a legal opinion (2). However, the Opinion is quite progressive. It paves the way for States to be held liable (3) and could be a game changer, by helping to reshape climate negotiations and by providing new impetus for climate litigation (3, 4).

### 2. REFUSAL TO RULE IN CONCRETO

The Court’s response to question (b), regarding the legal consequences of breaches of primary obligations, is much shorter than its response to question (a) on primary norms. It only takes up 13 of the Opinion’s 133 pages. This is partly due to the large number of applicable conventional and customary rules that the Court thoroughly examined and clarified when

<footnote>* Email: yann.kerbrat@univ-paris1.fr</footnote>

<footnote>** Corresponding author</footnote>

<footnote>Email: s.maljean-dubois@univ-amu.fr</footnote>

<footnote n="1">¹ Request for an Advisory Opinion submitted by the Commission of Small Island States on Climate Change and International Law Advisory Opinion, 21 May 2024.</footnote>
answering question (a). But the shorter response to question (b) also reflects the Court's more succinct approach to this topic.

Indeed, the Court refused to address a number of questions on the grounds that they can only be examined in concreto, as part of a particular dispute or a more specific request for a legal opinion. According to the Court, in the context of this Advisory Opinion, its task was solely 'to identify, in a general manner, the legal framework under which the conduct of States can be assessed in order to determine whether a State, or a group of States, is responsible for a breach of its obligations pertaining to the protection of the climate system; whether a State, or group of States, can invoke the responsibility of another State or group of States in breach; and the remedies that are available to the injured State or States in case of such a breach.'<fnref n="2" /> In other words, the Court refused to establish the individual responsibility of a State (or group of States), which can only be determined on a case-by-case basis. It specified that its Opinion 'does not prejudge the merits of any future claims that may be brought in relation to the subject-matter of the present proceedings before courts or tribunals.'<fnref n="3" /> By providing only the 'general legal framework,' the Court avoided ruling on a wide range of highly sensitive and divisive issues arising between States. It is worth noting that the International Law Commission had deliberately avoided the issue of compensation in its work on the protection of the atmosphere.<fnref n="4" />

For example, the Court refused to address the temporal issue of when a State becomes bound by an obligation that can be held against it, although it did provide some guidance by discussing certain rules and principles (such as the principle of non-retroactivity, enshrined in Article 28 of the 1969 Vienna Convention on the Law of Treaties with regard to treaty obligations, and the emergence of relevant rules of customary international law) as well as factual issues such as 'questions of sufficient scientific understanding of the causes of climate change and its adverse effects in respect of obligations under general international law.'<fnref n="5" /> It also sidestepped questions that necessarily arise when considering responsibility in climate matters. Thus, it refused to engage in the discussion to which it had been invited by many of the participants (particularly with regard to the critical date): could the rules governing State responsibility for acts having a continuing character (ILC Articles on State responsibility, Article 14) and for composite acts (Article 15) apply? According to the Court, this is one of the issues that must be addressed on a case-by-case basis.<fnref n="6" /> It also refused to determine the responsibility of a State or group of States, either in general or in a particular case, because '[s]uch a determination may involve complex questions regarding, inter alia, causation and apportionment of responsibility for harm, and can be made only by an in concreto assessment that is beyond the scope of these proceedings.'<fnref n="7" /> The question of whether any specific harm or risk of harm to a State constitutes a relevant adverse effect of climate change is also one that must be assessed in concreto in each particular situation or case.<fnref n="8" /> Finally, the Court refused to determine 'any specific legal consequences with respect to particular injured States or groups of States,'<fnref n="9" /> because in its view the rules on responsibility in general international law do not differ depending on the category or status of the injured State — something that Small Island States have found regrettable.<fnref n="10" /> This particular point was also criticized by Judge Yusuf who deplored that the Court had engaged in an abstract examination of the law of State responsibility, which he deemed detached from the reality of the serious harm caused to the

<footnote n="2">² Advisory Opinion, para. 406.</footnote>

<footnote n="3">³ Advisory Opinion, para. 106.</footnote>

<footnote n="4">⁴ See the ILC's explanations on the scope of its work on the [protection of the] atmosphere: https://legal.un.org/ilc/summaries/8_8.shtml (accessed on 12 September 2025).</footnote>

<footnote n="5">⁵ Advisory Opinion, para. 97.</footnote>

<footnote n="6">⁶ Advisory Opinion, para. 423.</footnote>

<footnote n="7">⁷ Advisory Opinion, paras. 108 and 406.</footnote>

<footnote n="8">⁸ Advisory Opinion, para. 278.</footnote>

<footnote n="9">⁹ Advisory Opinion, para. 109.</footnote>

<footnote n="10">¹⁰ See also the Separate Opinion of Judge Yussuf.</footnote>
climate system and the harm suffered by those most vulnerable, like Small Island Developing States and least developed countries.<fnref n="11" /> Although this is to some extent true, the Advisory Opinion still represents significant progress and could have a substantial impact. Even if it does not go as far as many States would have liked, it leaves the door wide open to State responsibility and reparations (3), and therefore to litigation on these issues (4).

### 3. OPENING THE DOOR TO STATE RESPONSIBILITY

#### 3.1. GENERAL INTERNATIONAL LAW ON STATE RESPONSIBILITY APPLIES IN CLIMATE MATTERS

The Court first rejected the argument of several participants, primarily from northern and oil-producing countries, that international climate law constitutes a *lex specialis*, and would therefore exclude the application of general rules governing State responsibility. In the Court's view, neither the Warsaw Mechanism on Loss and Damage under Article 8 of the Paris Agreement, nor the mechanism under Article 15 of the Paris Agreement for the implementation and promotion of compliance, 'were [...] designed as a special regime to address disputes relating to the breach of the provisions of the Paris Agreement.'<fnref n="12" /> As a result, it stated 'that the rules to be applied by this Court or any potential arbitral tribunal in the event of a breach of the provisions of the climate change treaties are the general rules on State responsibility under customary international law,'<fnref n="13" /> as broadly set out in the 2001 ILC Articles on State responsibility. The Court found no evidence to suggest that the States intended to derogate from this principle.

#### 3.2 STATE RESPONSIBILITY IN CLIMATE MATTERS

The Court emphasised that climate change is 'a highly complex and multifaceted phenomenon involving possible responsibilities for multiple States over long periods of time.'<fnref n="14" /> Indeed, 'concentrations of GHG emissions are not produced by a single activity or group of activities identifiable or associated with a certain State or States.'<fnref n="15" /> During the proceedings, many northern States argued that these specificities of climate change would make it difficult or even impossible to establish the responsibility of a particular State in relation to climate issues. The Court took the opposite view and stated that the conditions for State liability may be met. Having found that an unlawful act can easily be established and attributed to a State in this area (3.2.1), it then underlined that the responsibility of a State that has failed to fulfil its climate obligations could be widely invoked (3.2.2). Referring to its previous case law, it went on to suggest that the responsible State could be required to pay substantial compensation to the injured States (3.2.3).

##### 3.2.1. In climate matters, an unlawful act can be established and attributed to a State or a group of States

*Identifying a wrongful act in climate matters

<footnote n="11">¹¹ Ibid.</footnote>

<footnote n="12">¹² Advisory Opinion, para. 417.</footnote>

<footnote n="13">¹³ Advisory Opinion, para. 417.</footnote>

<footnote n="14">¹⁴ Advisory Opinion, para. 421.</footnote>

<footnote n="15">¹⁵ Ibid.</footnote>
In its Opinion, the Court very clearly stated that '[i]n the absence of special rules to the contrary, the responsibility of a party may be engaged under the rules on State responsibility if there is any breach of the obligations identified in question (a).'<fnref n="16" /> This applies to treaty obligations and to customary obligations, such as the obligation to prevent harm: 'a State that does not exercise due diligence in the performance of its primary obligation to prevent significant harm to the environment, including to the climate system, commits an internationally wrongful act *entailing its responsibility*'.<fnref n="17" />

The Court then pointed out two specific situations in which a State may incur liability: a State may be responsible for the behaviour of its own bodies if it fails to take adequate measures to protect the climate system against GHG emissions, and a State may also be responsible as a result of the conduct of other actors if it has failed in its (own) duty of care by 'not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.'<fnref n="18" />

Regarding the first situation, the Court provided an important example: 'failure 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.'<fnref n="19" /> The Court thus tackled the issue of fossil fuels head on, an issue which has long been the 'elephant in the room' of climate negotiations. However, the Court erred on the side of caution, leaving room for case-by-case analysis by using the term 'may constitute'. Even though some judges regret that it did not take a firmer stance on the primary obligations of States,<fnref n="20" /> this cautiousness is consistent with the fact that the main mitigation obligations are due diligence obligations, the fulfilment of which must take into account the capacity of each State. Therefore, the conduct expected of a developing country that is highly dependent on such fuels cannot be placed on the same footing as that of a developed country with the resources to develop carbon-free energies.

Furthermore, although the Court did not explicitly mention the phase-out of fossil fuels, this is what clearly emerges when this passage is interpreted in conjunction with other parts of the Opinion, especially the fact that mitigation is based on 'reducing GHG emissions through measures such as transitioning away from fossil fuels,'<fnref n="21" /> the need to limit the increase in temperatures to 1.5 °C and the reference to the conclusions of the Global Stocktake Report, according to which 'limiting global warming to 1.5°C with no or limited overshoot requires deep, rapid and sustained reductions in global greenhouse gas emissions of 43 per cent by 2030 and 60 per cent by 2035 relative to the 2019 level, and reaching net zero carbon dioxide emissions by 2050.'<fnref n="22" /> But as Judges Bhandari and Cleveland pointed out, many governments continue to support, finance and develop fossil fuel production, which is completely at odds with the Court's findings. By 2030, countries are planning to produce more than double the amount of fossil fuels that would be compatible with keeping global warming to 1.5°C.<fnref n="23" /> The Court's Opinion contradicts this plan. States embracing the 'drill baby drill' motto of the current President of the United States of America could be held liable. Interpreted in this way, the

<footnote n="16">¹⁶ Advisory Opinion, para. 408.</footnote>

<footnote n="17">¹⁷ Advisory Opinion, para. 409 (our emphasis).</footnote>

<footnote n="18">¹⁸ Advisory Opinion, para. 428.</footnote>

<footnote n="19">¹⁹ Advisory Opinion, para. 427.</footnote>

<footnote n="20">²⁰ Joint Declaration of Judges Bhandari and Cleveland.</footnote>

<footnote n="21">²¹ Advisory Opinion, para. 85.</footnote>

<footnote n="22">²² Advisory Opinion, para. 243.</footnote>

<footnote n="23">²³ Joint Declaration of Judges Bhandari and Cleveland, quoting the Stockholm Environment Institute, Climate Analytics, E3G, International Institute for Sustainable Development and the United Nations Environment Programme, *The Production Gap: Phasing down or phasing up? Top fossil fuel producers plan even more extraction despite climate promises*, 2023, p. 4, p. 11.</footnote>
Opinion goes far beyond the mere commitment of a 'transition away from fossil fuels in energy systems' made by States at the 2023 COP 28 in Dubai.<fnref n="24" />

• Attributing an unlawful act in climate matters

The Court’s Opinion also offered several important clarifications on the issue of the
attribution to a State of an internationally wrongful act. Its contribution in this regard was
expected, as it had been suggested during the proceedings that it would be impossible to
attribute climate disturbances to a specific State. Many participants had emphasised that climate
change is the result of cumulative acts, making it difficult, if not impossible, to determine the
contribution of a particular State to climate change, especially since they are largely attributable
to non-State actors, namely private companies and individuals who simply consume fossil fuels
and whose actions cannot be attributed to a State. The Court rejected this argument by providing
a welcome threefold clarification.

To start with, it stated that when it comes to State liability, ‘the question of attributing
the conduct of private actors to a State does not arise.’<fnref n="25" /> Indeed, as far as environmental damage
caused by private actors is concerned, the only question that arises at the stage of attribution of
an unlawful act is whether the State has exercised the due diligence to prevent the emissions of
private individuals from causing damage to the climate system, in particular by adopting proper
regulatory and legislative measures. Here the Court specified an important point: ‘the
internationally wrongful act in question is not the emission of GHGs per se, but the breach of
conventional and customary obligations [...] pertaining to the protection of the climate system
from significant harm resulting from anthropogenic emissions of such gases.’<fnref n="26" />

Furthermore, the Court rejected the argument that the cumulative nature of the acts
contributing to climate change would be an obstacle to the responsibility of States and to its
application. It began by highlighting that ‘while climate change is caused by cumulative GHG
emissions, it is scientifically possible to determine each State’s total contribution to global
emissions, taking into account both historical and current emissions’ based on information
provided by States since the UNFCCC came into force, and on scientific studies – in particular
those of the IPCC.<fnref n="27" /> The Court then pointed out that, while it ‘acknowledges that the fact that
multiple States have contributed to climate change may indeed increase the difficulty of
determining whether and to what extent an individual State’s breach of an obligation identified
in question (a) has caused significant harm to the climate system,’ it ‘considers that, in principle,
the rules on State responsibility under customary international law are capable of addressing a
situation in which there exists a plurality of injured or responsible States.’<fnref n="28" /> The Court quoted
its judgment in the case of *Armed Activities on the Territory of the Congo (Democratic Republic
of the Congo v. Uganda)* ‘where it observed that “in certain situations in which multiple causes
attributable to two or more actors have resulted in injury ... responsibility for part of such injury
should [be] allocated among [the] actors”.’<fnref n="29" /> According to the Court, this observation is
applicable in the context of climate change.<fnref n="30" /> This is a key clarification, as it makes it possible
to seek compensation from a particular State ‘without invoking the responsibility of all States
that may be responsible.’<fnref n="31" />

<footnote n="24">²⁴ Decision 1/CMA. 5, *First Global Stocktake*, §28 d), doc. FCCC/PA/CMA/2023/16/Add.1.</footnote>

<footnote n="25">²⁵ Advisory Opinion, para. 428.</footnote>

<footnote n="26">²⁶ Advisory Opinion, para. 427.</footnote>

<footnote n="27">²⁷ Advisory Opinion, para. 429.</footnote>

<footnote n="28">²⁸ Advisory Opinion, para. 429.</footnote>

<footnote n="29">²⁹ *Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda)*, I.C.J. Reports 2022 (I), pp. 49-50, para. 98.</footnote>

<footnote n="30">³⁰ Advisory Opinion, para. 430.</footnote>

<footnote n="31">³¹ Advisory Opinion, para. 430.</footnote>
Lastly, the Court addressed the argument made by several States, in particular by the Democratic Republic of the Congo in its written statement, echoed by various other African States in their written observations, that in cases of damage caused by several States, each State may be required to compensate its share of the damage if that share can be calculated, or the entire damage if such a calculation is not possible. The Court accepted that customary law on international responsibility in relation to situations involving multiple responsible States or multiple injured States applies in the case of climate change.<fnref n="32" /> For the first time, the Court referred to Article 47 of the 2001 ILC Articles, stating that ‘where several States are responsible for the same internationally wrongful act, the responsibility of each State may be invoked in relation to that act’.<fnref n="33" /> For the Court, ‘each injured State may separately invoke the responsibility of every State which has committed an internationally wrongful act resulting in damage to the climate system [...]. And where several States are responsible for the same internationally wrongful act, the responsibility of each State may be invoked in relation to that act.’<fnref n="34" /> However, the Court found, quoting its judgment on reparations in the case of *Armed Activities on the Territory of the Congo (DRC v. Uganda)*, that, in the event of a damage caused by several States, it is necessary to allocate to each actor ‘responsibility for part of such injury’.<fnref n="35" /> Thus, it did not accept the position of the DRC regarding the possibility of attributing all damages to a single State.

### 3.2.2 An extensive right to invoke State responsibility in climate matters

The Court also set out important clarifications regarding the invocation of responsibility. It clearly stated what many legal scholars had already acknowledged: States’ obligations relating to climate protection – at least some of them – are erga omnes obligations because ‘all States have a common interest in the protection of global environmental commons like the atmosphere and the high seas.’ They all have an ‘essential interest [...] in the safeguarding of the climate system, which benefits the international community as a whole.’<fnref n="36" />

As a result, any State, even one not injured by the wrongful act, has the right to invoke the responsibility of any other State that has committed a violation. In accordance with Article 48 of the 2001 ILC Articles, to which the Court referred expressly for the first time,<fnref n="37" /> any State, even if it has not been injured, can request the cessation of the wrongful act and/or enforce the obligation of reparation towards the harmed State(s). In particular, ‘all States parties have a legal interest in the protection of the main mitigation obligations set forth in the climate change treaties and may invoke the responsibility of other States for failing to fulfil them.’<fnref n="38" /> For this reason (and for another reason that will be explained in subsection 4 below), this recognition of the erga omnes nature of climate obligations is a crucial step. However, its scope is quite unclear.

Does it apply to all the obligations identified in the Court’s answer to question (a)? At first glance, one might be tempted to think so, as it is stated that it applies to ‘States’ obligations pertaining to the protection of the climate system and other parts of the environment from anthropogenic GHG emissions, in particular the obligation to prevent significant transboundary harm under customary international law.’ As a result, not only the principle of prevention, but also all the other obligations under general international law mentioned in the answer to

<footnote n="32">³² Advisory Opinion, para. 431.</footnote>

<footnote n="33">³³ Advisory Opinion, para. 431.</footnote>

<footnote n="34">³⁴ Ibid.</footnote>

<footnote n="35">³⁵ *Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda)*, I.C.J. Reports 2022 (I), pp. 49-50, para. 98.</footnote>

<footnote n="36">³⁶ Advisory Opinion, para. 440.</footnote>

<footnote n="37">³⁷ Advisory Opinion, para. 442.</footnote>

<footnote n="38">³⁸ Advisory Opinion, para. 441.</footnote>
question (a) would appear to be *erga omnes*. Thus, the violation of the obligation to cooperate could, for instance, be invoked by any State.

However, with regard to treaty obligations, the opinion is not so straightforward. The Court acknowledged that climate change is ‘a common concern of humankind’ (UNFCCC, 1st paragraph of the preamble; Paris Agreement, 11th paragraph of the preamble) that requires ‘a global response’ (Paris Agreement, Article 2). The climate treaties seek to protect the essential interest of all States in the safeguarding of the climate system, which benefits the international community as a whole. As such, the Court found that the obligations of States under these treaties are obligations *erga omnes partes*.<fnref n="39" /> This affirmation is potentially very important, as it could mean that not only mitigation obligations, but also adaptation obligations or obligations to assist developing countries and the most vulnerable States, could be considered *erga omnes partes* obligations, in particular financial and technological obligations as interpreted by the Court in its response to question (a). This could have significant practical consequences and create an incentive for future negotiations.

However, a few lines later, the Court introduced an element of uncertainty: ‘As a result, all States parties have a legal interest in the protection of the *main mitigation obligations* set forth in the climate change treaties and may invoke the responsibility of other States for failing to fulfil them.’<fnref n="40" /> Does the *erga omnes partes* nature therefore apply only to mitigation obligations, and furthermore, only to the ‘main’ ones? In any case, the Court’s position on this point is unclear. This issue must be clarified in the future. In our view, limiting the *erga omnes* nature to mitigation obligations alone would be too restrictive. However, extending it to all obligations under the UNFCCC and the Paris Agreement would be difficult to justify. This qualification should be limited solely to non-bilateralisable obligations under these treaties, precisely because only these obligations are intended to protect a common interest. Therefore, it cannot be asserted that all obligations arising from these treaties are *erga omnes partes*; each obligation must be examined individually.

Lastly, the Court provided important clarifications on the consequences of
responsibility.

3.2.3 The consequences of State responsibility and the reparation of climate damage

The consequences of the responsibility of a State depend on the nature of the breached obligation (whether it is substantive, procedural, continuing, etc.), but also ‘on the specific breach in question and on the nature of the particular harm.’<fnref n="41" /> They must be determined *in concreto*. All the same, they can potentially include ‘the *entire panoply of legal consequences* provided for under the law of State responsibility. These include obligations of cessation and non-repetition, which are consequences that apply irrespective of the existence of harm, as well as the consequences requiring full reparation, including restitution, compensation and/or satisfaction.’<fnref n="42" />

The Court did not address all the consequences of State responsibility identified in the ILC articles, but focused solely on a few of them, with particular emphasis on the reparation of damage caused by climate change.

• The consequences of State responsibility addressed by the Court

<footnote n="39">³⁹ Advisory Opinion, para. 440.</footnote>

<footnote n="40">⁴⁰ Advisory Opinion, para. 441 (our emphasis).</footnote>

<footnote n="41">⁴¹ Advisory Opinion, para. 445.</footnote>

<footnote n="42">⁴² Advisory Opinion, para. 445 (our emphasis).</footnote>
The Court offered insights on three consequences of responsibility in climate matters.

First, it pointed out that a violation of international law does not exempt the author of such act from its duty to fulfil its obligations. The rule is well known and codified in Article 29 of the ILC Articles. However, the Court does not merely reiterate this rule, it gives two important examples of its application. The first one is the ‘duty to preserve and improve the absorption capacity of reservoirs and sinks’ which must be respected ‘notwithstanding any breaches of those obligations under the climate change treaties.’<fnref n="43" /> The second one is consistent with current domestic and international case law: ‘in the case of a State party setting an inadequate Nationally Determined Contribution (NDC) under Article 4 of the Paris Agreement, a competent court or tribunal could order that State to perform its obligations by adopting an NDC consistent with its obligations under the Paris Agreement.’<fnref n="44" /> Thus, it reinforced the movement initiated by domestic courts, which for several years have been willing to condemn a State on the grounds that its GHG reduction plan is insufficient to achieve the objectives of climate treaties.

In addition to that, the ICJ underlined that the responsible State is under the obligation to cease the unlawful act if it is a continuing act, and added, in line with its previous case law, that '[i]n this context [...] the obligation to put an end to the wrongful act may require a State to revoke all administrative, legislative and other measures that constitute an internationally wrongful act of that State.'<fnref n="45" />

Finally, the Court reiterated the established rule according to which in the event of damage, the responsible State is under an obligation to make full reparation for the damage caused by the internationally wrongful act and that it should ‘wipe out all the consequences of the illegal act and reestablish the situation which would, in all probability, have existed if that act had not been committed.’<fnref n="46" /> In this regard, the ICJ offered some important insights regarding the reparation for damage caused by climate change.

### The reparation of damage caused par climate change

The ICJ’s clarifications on damage reparation focus on causation, the forms of reparation and compensation for damage.

Regarding causation, the Court recalled that, in contemporary international law, ‘causation of damage is not a requirement for the determination of responsibility as such.’<fnref n="47" /> The existence of an internationally wrongful act attributable to a State is sufficient to trigger the responsibility of that State. But causation plays an essential role in reparation, which presupposes the existence of damage caused by the wrongful act suffered by one (or more) injured State(s) or one (or more) injured individual(s). Contrary to the arguments put forward by some participants in the proceedings, this causal nexus cannot be presumed. In accordance with the Court’s previous case law, ‘sufficiently direct and certain’ causation between the wrongful act and the damage must be established.<fnref n="48" /> Nevertheless, referring to its 2022 judgment on reparations in the *DRC v. Uganda* case, the ICJ recalled that the absence of absolute certainty does not prevent the Court from finding a causal link between the violation of international law and the damage. Indeed, the criteria ‘may vary depending on the primary rule violated and the

<footnote n="43">⁴³ Advisory Opinion, para. 446.</footnote>

<footnote n="44">⁴⁴ Ibid.</footnote>

<footnote n="45">⁴⁵ Advisory Opinion, para. 447.</footnote>

<footnote n="46">⁴⁶ *Factory at Chorzów* case, 1928, P.C.I.J., Series A, No. 17, p. 47.</footnote>

<footnote n="47">⁴⁷ Advisory Opinion, para. 443.</footnote>

<footnote n="48">⁴⁸ *Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua)*, Compensation, Judgment, *I.C.J. Reports 2018* (I), p. 26, para. 32; *Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda)*, Reparations, Judgment, *I.C.J. Reports 2022* (I), p. 48, para. 93.</footnote>
nature and extent of the injury.’<fnref n="49" /> Thus, such criteria is ‘flexible enough to address the challenges arising in respect of the phenomenon of climate change.’<fnref n="50" />

In climate matters, two causal nexuses must be established. First, it must be shown that ‘a given climatic event or trend can be attributed to anthropogenic climate change,’ which ‘in many cases’ will not pose a difficulty, as science clearly establishes this nexus. Second, it is necessary to determine ‘to what extent damage caused by climate change can be attributed to a particular State or group of States.’<fnref n="51" /> 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, this does not mean that the identification of a causal link is impossible in the climate change context; it merely means that the causal link must be established in each case through an *in concreto* assessment while taking into account the aforementioned elements outlined by the Court.’<fnref n="52" />

Regarding forms of reparation, the Court reiterated that according to its established case law, compensation must be full ('wipe out all the consequences of the illegal act and reestablish the situation which would, in all probability, have existed if that act had not been committed')<fnref n="53" />, ant that “'full reparation'” can be achieved by restitution, compensation, satisfaction or a combination thereof.’<fnref n="54" /> At first glance, restitution may appear difficult, if not impossible, given that environmental damage is generally irreversible. The Court acknowledged this difficulty but overcame it, stating in particular that ‘in the circumstances of climate change caused by emissions of GHGs, restitution may take the form of reconstructing damaged or destroyed infrastructure, and restoring ecosystems and biodiversity.’<fnref n="55" /> Some judges have criticised the Court’s timidity with regard to restitution. Judge Bandhari, for example, wrote that the Court should have gone further and specified that restitution could consist of ‘the continued recognition by all States of the maritime entitlements and sovereign rights of States adversely affected by sea-level rise, including in cases where their territory becomes submerged or otherwise compromised.’<fnref n="56" /> However, given that the Court affirmed the stability of maritime delineations as a primary norm in its answer to question (a),<fnref n="57" /> such recognition was unnecessary. Nevertheless, the ICJ’s clarifications are important and should be welcomed, even if such restitution measures will be difficult to implement in practice. Indeed, it is unlikely that States affected by exceptional climate events would allow the main emitters to enter their territory to carry out such restoration measures. It is more likely that both sides would prefer reparation in the form of compensation. However, the examples provided by the ICJ will then serve as a benchmark: when restitution is not possible or is not desired by the injured State, compensation must be calculated taking into account the cost of rebuilding infrastructure and restoring biodiversity. In this respect, the Opinion aligns with the ruling on compensation in the case of *Costa Rica v. Nicaragua*, which recognised compensation for purely ecological damage.<fnref n="58" /> This is confirmed by the Court two paragraphs later :it ‘has previously had occasion to confirm that environmental damage is compensable under international law, and that

<footnote n="49">⁴⁹ Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda), Reparations, Judgment, I.C.J. Reports 2022 (I), p. 48, para. 93. Advisory Opinion, para. 436. On the flexibility of the Court on the causation issue, see Y. Kerbrat, ‘La réparation dans la pratique de la Cour internationale de Justice,’ *Revue belge de droit international*, vol. 2025/1, pp. 19-40.</footnote>

<footnote n="50">⁵⁰ Advisory Opinion, para. 436.</footnote>

<footnote n="51">⁵¹ Advisory Opinion, para. 437.</footnote>

<footnote n="52">⁵² Advisory Opinion, para. 438.</footnote>

<footnote n="53">⁵³ Factory at Chorzów case, 1928, P.C.I.J., Series A, No. 17, p. 47.</footnote>

<footnote n="54">⁵⁴ Advisory Opinion, para. 450, citing the judgment on the merits in the *Factory at Chorzów* case (1928, P.C.I.J., Series A, No. 17, p. 47).</footnote>

<footnote n="55">⁵⁵ Advisory Opinion, para. 451.</footnote>

<footnote n="56">⁵⁶ Separate Opinion, para. 7.</footnote>

<footnote n="57">⁵⁷ Advisory Opinion, paras. 355-360.</footnote>

<footnote n="58">⁵⁸ Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua), Compensation, Judgment, I.C.J. Reports 2018 (I), pp. 28-29, paragraphs 41-43.</footnote>
compensation will be due for both damage caused to the environment, “in and of itself” —
which may include “indemnification for the impairment or loss of environmental goods and
services in the period prior to recovery” — and expenses incurred by injured States as a
consequence of such damage.’<sup>59</sup>

Finally, the Opinion's main contribution on reparation is its stance on compensation. The Court rejected the argument put forward by several States during the proceedings that, in climate matters, the uncertainties surrounding the exact extent of the damage caused by climate change are such that it would be difficult, if not impossible, to award compensation. The Court acknowledged that ‘in the climate change context, reparations in the form of compensation may be difficult to calculate, as there is usually a degree of uncertainty with respect to the exact extent of the damage caused.’<sup>60</sup> But it rejected the arguments of these States. Referring to its 2022 judgment on compensation in the case of *DRC v. Uganda*, it stated that ‘where there is uncertainty as to the exact extent of the harm caused, compensation in the form of a lump sum, within the range of possibilities indicated by the evidence and taking into account considerations of fairness, may be awarded on an exceptional basis.’<sup>61</sup> In practical terms, this allows compensation to be awarded as a last resort for damage caused by climate change, despite the uncertainties surrounding the extent of the damage and the role played by climate change in such damage. In such a context, in line with the precedent set in 2022, the Court could be satisfied with fragmentary evidence. It could rely on science and environmental economics to assess the probability that climate change caused a given amount of damage, and the extent to which it contributed to it. It would then be able to determine the amount of compensation to be awarded, taking into account equitable considerations. This acts as ‘a safety valve,’ preventing the absence of precise proof from defeating reparation altogether, but without exempting claimants from the need to thoroughly document and prove their claims.<sup>62</sup>

4. NEW IMPETUS FOR CLIMATE LITIGATION?

With this Opinion, the Court positions itself as a key player of global governance.<fnref n="63" /> It speaks not only to the UN and to States, but also to judges all around the world, confirming their role in advancing climate reparations and designing innovative remedies for climate justice. The Opinion comes at a time when there has been an explosion in the number of what are known as ‘climate’ lawsuits against States and companies worldwide. Over 3,000 climate-related cases have been filed in nearly 60 countries around the world.<fnref n="64" /> Extending and even reinforcing in some respects other international decisions and the opinions of the ITLOS and the Inter-American Court of Human Rights, the Court’s interpretation, because it carries persuasive authority, could have a domino effect on future disputes, which is likely to vary depending on whether such disputes are between States, transnational or domestic.

<footnote n="59">⁵⁹ Advisory Opinion, para. 451.</footnote>

<footnote n="60">⁶⁰ Advisory Opinion, para. 454.</footnote>

<footnote n="61">⁶¹ *Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda)*, Reparations, Judgment, 9 February 2022, *I.C.J. Reports* 2022, p. 52, para. 106; Advisory opinion, para. 454.</footnote>

<footnote n="62">⁶² *Khalid Adnan*, ‘Compensation after the Climate Advisory: Reading the ICJ’s Remedies Jurisprudence,’ *EJIL: Talk!* , September 5, 2025.</footnote>

<footnote n="63">⁶³ L. Kotzé et al., ‘Courts, Climate Litigation and the Evolution of Earth System Law,’ *Global policy*, 15.1 (2024), pp. 5-22.</footnote>

<footnote n="64">⁶⁴ J. Setzer, C. Higham, *Global trends in climate change litigation: 2025 snapshot*, LSE, 25 June 2025, https://www.lse.ac.uk/granthaminstitute/publication/global-trends-in-climate-change-litigation-2025-snapshot/, (accessed on 3 September 2025). UNEP, *Climate Change in the courtroom: Trends, impacts, and emerging lessons*, 2025, https://www.unep.org/news-and-stories/press-release/over-3000-climate-litigation-cases-are-reshaping-global-climate (accessed on 7 October 2025).</footnote>
4.1. *New avenues for interstate litigation*

The Court’s Opinion could take loss and damage issues out of the climate regime and negotiations and into the judicial sphere. This means that these issues can now be framed not only in political terms (moral duties of cooperation and solidarity), but also (and mainly?) in legal terms (legal responsibility). At the very least, the UNFCCC negotiations should now be viewed as just one component of a wider legal battle regarding responsibility for climate damages.<fnref n="65" /> Indeed, at the interstate level, the content of the Opinion provides a solid basis for litigation, given its strict interpretation of States’ obligations and the various avenues it opens up with regard to the legal consequences of unlawful acts, in particular regarding damage compensation. By stating that the main mitigation obligations are *erga omnes*, the Court has also left the door wide open to proceedings seeking recognition by the Court of the responsibility of States that have failed to comply with their obligations. Indeed, the ICJ has linked standing to the *erga omnes* nature of an obligation. In the event of a breach of a such obligation, any State has standing to invoke responsibility of a State for alleged breaches and bring proceedings before the ICJ in order to determine the alleged non-compliance with obligations *erga omnes partes* and to put an end to it.<fnref n="66" /> Yet, only the injured State can ask for reparation.<fnref n="67" />

However, it is unlikely that there will be a tsunami of interstate litigation. Such proceedings face a number of difficulties. States wishing to bring a case before an international court may first encounter the obstacle of the principle of consent to international jurisdiction. They may have difficulty finding a competent court, unless defendant States agree to submit to jurisdiction once the dispute has arisen, which is highly unlikely. Indeed, the dispute settlement clause in Article 14 of the United Nations Framework Convention on Climate Change, which applies *mutatis mutandis* to the Paris Agreement (Article 24), has little binding force. In practice, very few States have agreed in advance to submit their disputes to the International Court of Justice or to an arbitral tribunal on this basis. Only the Netherlands has accepted the jurisdiction of the ICJ and recourse to arbitration, while the Solomon Islands and Tuvalu have accepted the compulsory arbitration of Article 14 § 2. Due to the reciprocity requirement, the clause could therefore only apply between the Netherlands and Tuvalu and the Solomon Islands, or between Tuvalu and the Solomon Islands. Thus, such a clause is unlikely to be of much help to a State wishing to start proceedings. Other avenues could certainly be explored. For example, countries such as Australia, Canada, Germany, Japan and Norway could be brought to the Court through prior acceptance of its compulsory jurisdiction under Article 36(2) of the Statute. There are also various arbitration clauses in relevant treaties, such as the United Nations Convention on the Law of the Sea (Part XV), the Montreal Protocol on Substances that Deplete the Ozone Layer (Article 14 in conjunction with Article 11 of the Vienna Convention), or the Convention on Biological Diversity (Article 27), or even more general treaties, such as those of friendship and good neighbourhood.<fnref n="68" />

Even if a court agrees to hear an interstate case, other obstacles may arise. The Court has confirmed the application of the general rules on State responsibility and determined that the issues relating to causality and attribution can be resolved. The same applies to the legal consequences, since the Court considers that the full range of these consequences may be used,

<footnote n="65">⁶⁵ A. Orford, ‘Reparations, climate change, and the background rules of international law’, *AJIL*, 2025, Vol. 119:3, p. 453.</footnote>

<footnote n="66">⁶⁶ See *Questions relating to the Obligation to Prosecute or Extradite (Belgium v. Senegal)*, Judgment, 20 July 2012, I.C.J. Reports 2012 (II), p. 449-450, paras. 68-70, and Application of the convention on the prevention and punishment of the crime of genocide (the Gambia v. Myanmar), judgment (prel. Obj.), 22 July 2022, I.C.J Reports 2022, p. 516 para. 108.</footnote>

<footnote n="67">⁶⁷ Advisory Opinion, para. 443, implicitly referring to Art. 48 of the ILC Articles.</footnote>

<footnote n="68">⁶⁸ S. Maljean-Dubois, ‘À quand un contentieux interétatique sur les changements climatiques ?,’ *Questions of International Law*, November 2021.</footnote>
including restitution and compensation. However, the nature of the potential disputes will be
the deciding factor, as these can vary greatly: violation of substantive or procedural obligations;
treaty-based or customary obligations; strategic litigation or compensation claim for damage
suffered. Regarding strategic litigation, it seems relatively easy to obtain recognition that a State
has failed in its due diligence and to receive reparation in the form of satisfaction. While the
legal consequence may be symbolic, such litigation can nevertheless play a significant role in
preventing future damage. It will be more difficult but not impossible to obtain compensation
for material damage (partial or total disappearance of territory, damage to ecosystems,
destruction of homes following a cyclone or mega-fire, drought causing food shortages or even
famine, etc.). The Court has opened the door to this possibility, and the science of attribution is
making rapid progress.<fnref n="69" />

4.2. Expanding horizons in transnational legal actions

At the transnational level, this Opinion could of course lead to new human rights
challenges before UN committees or regional human rights courts. In this respect, it fits into a
continuum of precedents. In addition to that, the Court's strict interpretation of States'
obligations to regulate and control greenhouse gas emissions, including those of private actors,
could create tensions with international investment law. A number of bilateral and multilateral
treaties protect foreign investors, who could take legal action against a State that tightens its
climate measures, thereby affecting the profitability of their investment or even calling it into
question (prohibitions on granting licences, tax reforms, etc.). This is especially the case with
first-generation investment treaties, as second-generation treaties already recognise the
prerogatives of States in this area. This is a point that the Court did not wish to address, taking
the view that international investment law was certainly relevant, but not the most directly
relevant.<fnref n="70" /> Therefore, it did not deal with potential conflicts between climate obligations and
international investment law. Such conflicts could nevertheless arise, raising issues of
inconsistency between States' obligations. This may lead States to fear having to pay significant
reparations and be tempted by a regulatory chill. If disputes were to arise, it is likely that the
Court's Opinion would support the position of the defendant States. The Opinion could also
prompt a review of treaties that are incompatible with the Court's interpretation of States'
commitments, facilitate the inclusion in new treaties of carve-out clauses preventing any action
against climate measures of this type, or even lead to the most polluting activities being subject
to a different regime.<fnref n="71" />

4.3. Reframing and supporting domestic litigation

Most climate litigation takes place at the national level. International law is already
heavily present in national climate lawsuits, albeit mostly indirectly.<fnref n="72" /> The Court’s Opinion
could strengthen the position of climate advocates in disputes against States and provide
convincing support for their arguments.<fnref n="73" /> We can also anticipate knock-on effects on cases

<footnote n="69">⁶⁹ See for instance I. Noy, D. Stone, T. Uher, ‘Extreme events impact attribution: A state of the art,’ *CelPress Review*, vol. 1, Issue 5, 24 May 2024 ; R.F. Stuart-Smith, F.E.L. Otto, A.I. Saad *et al.*, ‘Filling the evidentiary gap in climate litigation,’ *Nature. Climate Change*, 11, 2021, pp. 651-655.</footnote>

<footnote n="70">⁷⁰ Advisory Opinion, para. 173.</footnote>

<footnote n="71">⁷¹ J. Paine, E. Sheargold, ‘A Climate Change Carve-Out for Investment Treaties,’ *Journal of International Economic Law*, Volume 26, Issue 2, June 2023, pp. 285–304.</footnote>

<footnote n="72">⁷² S. Maljean-Dubois, ‘L’“éloge du flou” ? L’Accord de Paris devant les juges nationaux,’ in L. Arroyo Zapatero (ed.), *Cheminer avec Mireille Delmas-Marty. Mélanges ouverts en l’honneur de Mireille Delmas-Marty*, Mare et Martin, Paris, 2022, pp. 377-378.</footnote>

<footnote n="73">⁷³ See for instance the recent federal decision in the DINI ZE' LHO'IMGGIN case in Canada, https://decisions.fct-cf.gc.ca/fc-cf/decisions/en/item/529040/index.do (accessed on 13 October 2025).</footnote>
against companies. States have strict obligations with regard to both mitigation and adaptation. They must regulate and control activities carried out under their jurisdiction, as well as protect the population, including future generations. From this point of view, the content of the third series of national contributions to the Paris Agreement could be subject to very close scrutiny.

## 5. CONCLUSION

The Opinion of the International Court of Justice provided important insights on the content and application of States' climate obligations, thereby contributing to greater legal certainty in this area. The Opinion is rich, detailed, informative, and remarkably clear. In fact, it can be described as illuminating. Without a doubt, this progressive ruling constitutes a major milestone in the history of international climate law. The Court seems to have struck the right balance between boldness and caution, avoiding the pitfalls of platitudes and of a 'nuclear strike' that could have weakened its position and limited the influence of the Advisory Opinion.<fnref n="74" />

The gamble taken by Small Island States has paid off: in these circumstances, although not formally legally binding on States, the Opinion could positively influence climate policies at a time when many States – and as a result, businesses – are wavering somewhat on the issue. As some of the reasoning can be applied beyond climate issues, the Opinion could also inspire ambitions in other areas of cooperation related to other global threats (such as biodiversity and chemicals). Finally, it contributes in its own way to the formation of global ‘public awareness’. However, as the Court readily admits, the Opinion alone will not solve this existential ‘daunting, and self-inflicted’ problem.<fnref n="75" /> It obviously cannot offer a silver bullet to the global crisis we are currently experiencing.<fnref n="76" /> Beyond the courts, the effectiveness of the principles set forth will depend on the will of States, but also on the commitment of each and every one of us to take part in the joint effort to meet this immense challenge, which requires nothing less than, as the Court rightly says, a surge of will and wisdom, ‘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.’<fnref n="77" />

Yann Kerbrat is Professor of international law at the Sorbonne Law School (Université Paris 1, Panthéon-Sorbonne). He was the director of the Sorbonne Research Institute for International and European Law (IREDIES) for over seven years. His teaching and research focuses primarily on public international law and international environmental law. He has edited several books and numerous articles in these fields, in particular on international responsibility. He was a member of the French team in these advisory proceedings. The views expressed in this article are those of the author.

Sandrine Maljean-Dubois is a researcher at the CNRS and she teaches international environmental law at the Faculty of Law of Aix-Marseille University. She has edited several books and numerous articles in this field, focusing particularly on climate change and biodiversity regimes, the Anthropocene and planetary boundaries, and implementation and enforcement mechanisms. She is leading the interdisciplinary PROCLIMEX project on expertise in climate litigation (ANR-21-CE03-0011-01). She represented the Democratic

<footnote n="74">⁷⁴ D. Bodansky, ‘Advisory opinions on climate change: Some preliminary questions,’ *RECIEL*, 2023, vol. 32, p. 188.</footnote>

<footnote n="75">⁷⁵ Advisory Opinion, para. 456.</footnote>

<footnote n="76">⁷⁶ L. Kotzé et al., ‘Courts, climate litigation and the evolution of earth system law,’ *op. cit.*, p. 16: ‘Although courts are unlikely to offer a silver bullet for the planetary climate crisis (at least not in the short term).’</footnote>

<footnote n="77">⁷⁷ Advisory Opinion, para. 456.</footnote>
Republic of Congo in this case and appeared before the ICJ. The views expressed in this article are those of the author.