Abstract
The ICJ Advisory Opinion on Climate Change (July 23, 2025) addresses key topics of international climate change law: The obligation of progression under the Paris Agreement, the right to a clean, healthy and sustainable environment and the issue of state responsibility for climate harm. This article points to the debate on these issues prior to the Advisory Opinion, summarizes the Court's approach and contextualizes its findings. While the Court did not reinvent international climate change law, the Opinion is a landmark case. Because the ICJ solved important conflicts of interpretation, the Opinion will serve as the new point of departure for future debates on climate change law.
Keywords
Introduction
A key difference between international law and domestic law is the comparably limited jurisdiction of international courts and tribunals. Only if states provide their consent to a particular proceeding, there will be a judgment in the end. This means that conflicts about the interpretation of certain norms of international law may remain unsolved. If there is no decision by an international court, a state may claim that only its specific interpretation of a particular treaty provision or its understanding of customary international law reflects the state of the law. Divergent interpretations of the law may compete with each other.
Advisory Opinions by the International Court of Justice (ICJ) may limit the space for such competing interpretations. Even though formally non-binding, such decisions have a high legal authority. The statements of the ICJ, as the principal judicial organ of the United Nations, will likely affect future discussions on the law. Thus, they have the potential to solve conflicts of interpretation.
With this in mind, this contribution will assess the highly anticipated Advisory Opinion on the Obligations of States in respect of Climate Change of 23 July 2025. 1 We will focus on the obligation of progression under the Paris Agreement on Climate Change (II.), the right to a clean, healthy and sustainable environment (III.) and the issue of state responsibility (IV.). We will show that at the very least, the ICJ solved some issues of interpretation, providing a new point of departure for future discussions on climate change law.
Obligation of progression under the Paris agreement
Under the Paris Agreement, state parties have the undisputed obligation to submit nationally determined contributions (NDCs), which contain targets on the reduction of greenhouse gas (GHG) emissions (Art. 4 para. 1, 2 Paris Agreement). The legal scope of Art. 4 para. 3 Paris Agreement, however, has been one of the more controversial issues of international climate change law. Article 4 para. 3 Paris Agreement states: ‘Each Party's successive nationally determined contribution will represent a progression beyond the Party's then current nationally determined contribution and reflect its highest possible ambition, reflecting its common but differentiated responsibilities and respective capabilities, in the light of different national circumstances.’
Enhancing Successive NDCs
There were generally three possible readings of this paragraph: One that understood the term ‘will’ in Art. 4 para. 3 Paris Agreement as indicating mere non-binding soft law. 2 Another, suggesting that the word ‘will’ in this context ‘indicates an expectation, not an obligation.’ 3 According to this reading, parties to the Paris Agreement should generally strive to enhance their NDCs, but downward adjustments would be permissible if they are not made arbitrarily. 4 The third reading understood Art. 4 para. 3 Paris Agreement to contain an actual obligation of progression. In this understanding, each NDC functions as the floor for further NDCs, which must objectively exceed the ambitions of their predecessors. 5
The proceedings leading up to the Advisory Opinion showed already a strong tendency amongst stakeholders to prefer the latter understanding of Art. 4 para. 3 Paris Agreement. During the proceedings, a significant number of actors confirmed that Art. 4 para. 3 Paris Agreement contains a legally binding obligation to substantially and gradually develop their GHG emission goals in their successive NDCs. 6
Interestingly, the ICJ did not pick up this emerging consensus in its Advisory Opinion. Instead, the ICJ set out to interpret the norm ‘in good faith, on the basis of the ordinary meaning of the terms in Article 4, in their context and in light of the object and purpose of the Paris Agreement’ 7 , reproducing the standard of treaty interpretation laid down in Art. 31 para. 1 Vienna Convention on the Law of Treaties (VCLT) in textbook fashion. At the outset of its interpretation, the Court clarifies that Art. 4 para 3 ‘is not to be read as merely hortatory’. 8 Rather, the term ‘will’ would be used prescriptively ‘reflecting the expectation’ of a gradual progression. 9 While this sounds as if the ICJ embraced the second view mentioned above, the Court went even a step further: The wording ‘[e]ach Party's successive nationally determined contribution will represent a progression’ would mean that a state's NDCs ‘must become more demanding over time’. 10
With this, the Court solved an important conflict of interpretation. Of the three possible readings of the provision only one remained. Each parties’ NDC is the baseline for their subsequent ones. Parties to the Paris Agreement are under an obligation to enhance their ambitions.
The Duty of Due Diligence
But does that mean that parties are free to choose how much they progress in every successive NDC? While Art. 4 para. 3 Paris Agreement demands that each successive NDC represents a party's ‘highest possible ambition’, this term is not defined in the Paris Agreement, as the ICJ points out. 11 For its subsequent interpretation, the Court takes again recourse to the context, object and purpose of the Paris Agreement. 12 Of particular importance to the Court here is Art. 3 Paris Agreement. This article states that parties will fulfil their obligations ‘with the view to achieving the purpose of this Agreement as set out in Article 2.’ 13 Art. 2 Paris Agreement, in turn, contains the famous 1.5°C target. 14 Based on these considerations, the Court turns to the concept of due diligence. 15
In an earlier part of the Opinion, the Court derived the standard of due diligence from the customary duty to prevent significant harm to the environment.
16
In this context, the Court highlighted that the threat of climate change differs from other threats to the environment and makes the identification of a general standard of conduct particularly important. In the words of the Court: ‘[C]umulative GHG emissions are the primary source of risks arising from anthropogenic climate change […]. All States contribute to that risk, albeit to significantly differing degrees, and all States are affected by the cumulative effects of GHG emissions, depending on their respective situations. Climate change therefore poses a quintessentially universal risk to all States. This risk is of a general and urgent character, requiring the identification of a corresponding general standard of conduct, to be applied subject to the principle of common but differentiated responsibilities and respective capabilities.’
17
Against this backdrop, the Court points out ‘that the standard of due diligence for preventing significant harm to the climate system is stringent’ 18 , indicating that there is not much room for a state's individual margin of appreciation.
This reasoning affects the arguments of the Court on the NDCs under the Paris Agreement. The Court sees the parties to the Paris Agreement ‘obliged to exercise due diligence and ensure that their NDCs fulfil their obligations under the Paris Agreement’. This means ‘when taken together, [the NDCs must be] capable of achieving the temperature goal of limiting global warming to 1.5°C above pre-industrial levels.’ 19 Essentially, the Court defines ‘highest possible ambition’ in Art. 4 para. 3 Paris Agreement to contain a general standard of due diligence for all parties collectively.
At the same time, Art. 4 para. 3 Paris Agreement contains an individual obligation of due diligence for each party on its own. The Court underlines that ‘because of the seriousness of the threat posed by climate change, the standard of due diligence to be applied in preparing the NDCs is stringent’. 20 Accordingly, ‘each party has to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realise the objectives of the Agreement.’ 21 The Court then points out that ‘doing its utmost’ varies for each party because of the principle of common but differentiated responsibilities. 22 This means that ‘the standard to be applied when assessing the NDCs of different parties will vary depending, inter alia, on historical contributions to cumulative GHG emissions, and the level of development and national circumstances of the party in question.’ 23
How should the Court's reasoning on due diligence be evaluated? In a Separate Opinion, Judge Yusuf suggested that the majority has followed an ‘excessively formalistic approach’ in the Opinion. 24 Nevertheless, this criticism fails to acknowledge the Court's significant legal pronouncements. The Court clarifies that ‘rather than being entirely discretionary as some participants argued, NDCs must satisfy certain standards under the Paris Agreement.’ 25 From now on, states can no longer claim to have a wide margin of appreciation when determining the contents of their NDCs. 26 Moreover, if states’ NDCs collectively fail to meet the 1.5°C target, this creates the assumption that not every state committed itself to its highest possible ambition in its NDC. In this scenario, the industrialised and developed countries in particular will have to prove that their NDCs meet the expectations set by the Advisory Opinion. The Court has thus not only settled some legal debates around NDCs. It even provides us with a benchmark against which every future NDC can and will be assessed. 27
The right to a clean, healthy and sustainable environment
The ICJ considered the question of states’ obligations regarding climate change from multiple angles, among them human rights. Considering that the General Assembly, in its question to the Court, explicitly mentioned the International Covenant on Civil and Political Rights, the International Covenant on Economic, Social and Cultural Rights, and the Universal Declaration of Human Rights, 28 this was not surprising. Nevertheless, the overall relevance of human rights to the question of states’ obligations regarding climate change was heavily debated during the proceedings, something the Court itself highlights in the Opinion. 29 This debate especially concerned the human right to a clean, healthy and sustainable environment, whose existence in customary international law was denied by some participants in the proceedings. 30
Context
To contextualise the debate, it is important to know that by 2024, many UN-member states recognised the right to a healthy environment in their constitutions, legislation and regional treaties. 31 While some celebrate this widespread recognition as the result of extensive action from civil society ‘highlight[ing] what coordinated collaboration can achieve’ 32 , this does not mean that the right is also part of customary international law. Apart from Art. 11 San Salvador Protocol to the American Convention on Human Rights, 33 Art. 24 of the African Charter on Human and Peoples’ Rights 34 and Art. 38 of the Arab Charter on Human Rights, there are currently no other international human rights treaties recognizing this right. 35 Instead, on the international plane, states usually recognise the human right to a clean, healthy and sustainable environment indirectly, either as a precondition for the enjoyment of other human rights or in non-binding declarations, most of the time in the context of the United Nations. 36 Furthermore, due to this decentralised implementation, the actual scope of this right is not entirely clear. 37 Emblematic of this situation is General Assembly Resolution 76/300, adopted in 2022. While this resolution recognises the right to a clean, healthy and sustainable environment as a human right 38 and was accepted with 161 votes in favour, 8 abstentions and no votes against, 39 various states suggested in the General Assembly debate, that they currently do not understand the right to be part of customary international law, independently of their support of the resolution. 40 Some of these states reiterated this view in the proceedings leading up to the Advisory Opinion. 41 In the Advisory Opinion, the ICJ thus had to deal with two main questions: Does a human right to a clean, healthy and sustainable environment exist under customary international law? And what scope does it have?
Human Rights and Climate Change
The Court tackled these questions by first turning to a general analysis of the impact of climate change on human rights. The starting point for the Court's analysis is the recognition that states are generally under an obligation to ensure the enjoyment of human rights, which is also the case regarding climate change.
42
However, the Court goes far beyond that by explicitly stating: ‘The environment is the foundation for human life, upon which the health and well-being of both present and future generations depend (see Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, I.C.J. Reports 1996 (I), p. 241, para. 29). The Court thus considers that the protection of the environment is a precondition for the enjoyment of human rights, whose promotion is one of the purposes of the United Nations as set out in Article 1, paragraph 3, of the Charter.’
43
This short paragraph sets the tone for the Court's analysis. After a reference to current scientific research, proving a link between GHG emissions and the vulnerability of human populations, 44 the Court gives a non-exhaustive list of human rights it sees threatened by climate change. 45 This list includes the right to life, 46 the right to health, 47 the right to an adequate standard of living (including access to food, water and housing) 48 and the right to privacy, family and home 49 . The Court also points out that women, children and indigenous peoples are at a particular risk of being impaired in the enjoyment of their human rights by the effects of climate change. 50
This part of the judgement is an important prelude to the Court's reasoning on the right to a clean, healthy and sustainable environment. The Court treats the environment as the basis for the enjoyment of most – if not all – other human rights. Although this insight is not new, 51 we now have an authoritative statement by the ICJ on this issue.
The Court's Cautious Approach: Human Right to a Clean, Healthy and Sustainable Environment
The Court then starts its actual analysis on the right to a clean, healthy and sustainable environment with a short look into the history of international climate instruments.
52
The ICJ points out that the General Assembly recognised the intimate relationship between the environment and human beings as early as 1968.
53
On this basis, the Court concludes ‘that the effective enjoyment of a number of human rights cannot be fully realised if those who hold them are unable to live in a clean, healthy and sustainable environment.’
54
The Court then turns to contemporary international law and notes that several regional human rights instruments, as well as national legislation and regional and national courts, recognise (albeit in slightly different terms) the right to a clean, healthy and sustainable environment.
55
These regional developments have culminated in the global recognition of this right by the General Assembly in 2022.
56
Based on all these considerations, the Court delivers its conclusion which shall be quoted here in full: ‘Based on all of the above, the Court is of the view that a clean, healthy and sustainable environment is a precondition for the enjoyment of many human rights, such as the right to life, the right to health and the right to an adequate standard of living, including access to water, food and housing. The right to a clean, healthy and sustainable environment results from the interdependence between human rights and the protection of the environment. Consequently, in so far as States parties to human rights treaties are required to guarantee the effective enjoyment of such rights, it is difficult to see how these obligations can be fulfilled without at the same time ensuring the protection of the right to a clean, healthy and sustainable environment as a human right. The human right to a clean, healthy and sustainable environment is therefore inherent in the enjoyment of other human rights. The Court thus concludes that, under international law, the human right to a clean, healthy and sustainable environment is essential for the enjoyment of other human rights.’
57
It is noteworthy that the Court stops short of recognizing the human right to a clean, healthy and sustainable environment as customary international law. Instead, it limits itself to pointing out the essential significance of this right for the enjoyment of other human rights, leaving its legal status open. In the same vein, the Court remains vague regarding the scope of this right. The Court focuses on its interrelation with other human rights but does not go into much detail. In this regard, the Court just generally points to the fact that states’ obligations regarding human rights are informed by their obligations under climate change treaties, other environmental treaties and customary international law. 58 The Court does not discuss whether and how the human right to a clean, healthy and sustainable environment may go beyond existing legal obligations of states.
All in all, the Court chose a rather cautious approach on the human right to a clean, healthy and sustainable environment. 59 In their separate opinions and declarations, some judges expressed their conviction that the right is part of customary international law. 60 Judge Charlesworth even outlined some of its procedural and substantive aspects. 61 This is an indication that there was some debate among the judges on the status of this right.
What will be the future impact of this part of the Advisory Opinion? It is telling that some judges come to different conclusions in that regard. Whereas Judge Charlesworth writes that ‘the application of human rights law in the context of climate change has important procedural implications, through broadening access to dispute settlement procedures and granting standing to rights holders who would otherwise not be able to enforce climate change obligations on the international level’ 62 , Judge Nolte warns that ‘[i]f [the Advisory Opinion] is understood as encouraging litigation to compensate for a lack of political will on the part of the community of States to protect against climate change, it may turn out to be counterproductive.’ 63
In any event, by giving the right to a clean, healthy and sustainable environment such a prominent place in the Advisory Opinion, the Court has undoubtedly added to the overall significance of this right. The Court has made it clear how closely this right is linked to other (human) rights. Also, the Court has explicitly pointed to the deep link between human rights law and international environmental law, which had not been universally accepted before the Advisory Opinion. 64 Given the relevance of this right in national and regional legal systems today, 65 it is unlikely that future human rights litigation relating to climate change will not be based on the conclusions set out in the Advisory Opinion of the ICJ. 66
State responsibility
State responsibility for climate damage is perhaps the topic that was awaited with the greatest anticipation. Question (b) of the advisory proceeding mainly concerned the legal principles that guide such responsibility. The General Assembly had asked: ‘What are the legal consequences under [the obligations identified in question (a)] for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment…?’. We will focus on three themes of the Advisory Opinion: the applicability of the customary rules on state responsibility, attribution and establishing causality.
Applying the Frame of State Responsibility
At the start, the ICJ held that the climate law treaty framework does not block the application of the general rules on state responsibility. 67 Based on its interpretation of the UNFCCC, the Kyoto Protocol and the Paris Agreement, the ICJ concluded that there was no evidence of a discernible intention to render the state responsibility framework inapplicable in the context of climate change. 68 Neither the procedural mechanisms on ‘loss and damage’ and ‘compliance’ in the Paris Agreement, nor the dispute settlement mechanism under the three climate law treaties would point in that direction. 69 Consequently, responsibility for the violation of obligations under climate change law would need to be assessed in light of ‘the well-established rules on State responsibility under customary international law’. 70
Prior to the Advisory Opinion, this was a controversial issue. Some scholars highlighted that using state responsibility to address climate change issues was particularly tricky. 71 Others referred to existing competing views on whether the legal regime on climate change law is lex specialis. 72 During the proceedings, a (small) number of large emitter states confronted the ICJ with the non-applicability of state responsibility on this basis. 73 Against this background, Judge Sebutinde took the view that the Court should not have addressed the issue of state responsibility for climate change damage in the first place, for reasons of judicial economy. 74
The ICJ, however, did not take that path. Instead, the ICJ used its authority as the principal judicial organ of the United Nations to solve a highly relevant conflict of interpretation concerning climate change law and rejected the lex specialis argument. From now on, it will be regarded as settled law that state responsibility for wrongful acts can be invoked in the context of climate change law. This will be a highly relevant baseline for future discussions. 75
Facilitated Attribution (except for Historical Emissions)
As one key issue of attribution, 76 the ICJ dealt with the acts of private companies as the primary emitters of greenhouse gases. As a general rule in public international law, Art. 8 Draft Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA) is regarded to reflect customary law attributing private acts on the basis of the state's ‘control’ over such acts. 77 Also, Art. 8 had been discussed as a standard in the literature on climate change responsibility. 78
Interestingly, the ICJ did not see the need to address the issue, whether a state exercises ‘control’ over the private actor in the sense of Art. 8 ARSIWA. Instead, the Court considered a state's failure to take appropriate protective action, such as licensing fossil fuel exploration or providing fossil fuel subsidies, to be a potential international wrongful act. 79 Moreover, the failure to exercise ‘due diligence by not taking the necessary regulatory and legislative measures’ against private actors within its jurisdiction could constitute such an act. 80
Thus, given that the primary obligation for states in the context of climate change is a due diligence obligation, insufficient regulation of emissions in itself may be the basis for attribution. As the ICJ emphasised, because of its focus on ‘a failure to exercise regulatory due diligence’, the issue of attributing the conduct of private actors does not arise. 81 This will likely make the attribution of conduct by private actors relatively easy to prove, at least when compared to the ‘effective control test’ under Article 8 ARSIWA. 82 As some observers highlight, the Advisory Opinion may thus have a role to play in addressing and affecting private emitters rather directly. 83
Moreover, the ICJ addressed the issue of historical emissions – a key topic of climate justice debates. 84 Historical emissions are the emissions that can be attributed to a particular state since the beginning of the Industrial Revolution in the mid-nineteenth century. The ICJ generally points out that each state's contribution to global greenhouse gas emissions, including historical emissions, can be assessed on the basis of science. 85
Nevertheless, the Court by no means determined that such a scientific attribution will have an impact on the legal assessment. 86 As the Court highlights, the wrongful act is ‘not the historical emissions in and of themselves but actions or omissions causing significant harm to the climate system in breach of a state's international obligations’. 87 This brings the doctrine of intertemporal law to the field of climate change law. 88 As Article 13 of the ARSIWA states, a state is in breach of an international obligation only if ‘the State is bound by the obligation in question at the time the act occurs’. Applying this to climate change, one may wonder whether granting licenses for fossil exploration in the 1960s or 1970s can be considered a wrongful act. The UNFCCC, as the first climate change convention, only entered into force in 1994, and the customary duty to prevent greenhouse gas emissions only seems to have emerged relatively recently as well. Against this background, Judge Nolte concluded that ‘only a limited amount of all anthropogenic GHG emissions since industrialisation has been caused by wrongful acts’. 89 All in all, it seems rather difficult to legally attribute historical emissions to a state in light of the ICJ's decision – at least if one takes the principle of intertemporal law seriously.
Erga omnes Obligations, Causation and the Necessity of a Case-by-Case Analysis
Responding to claims by a number of states, the ICJ addressed the controversial issue of the erga omnes character of certain obligations in climate change law. 90 The ICJ held that the customary obligation to prevent significant transboundary harm, as well as the obligations under the UNFCCC and the Paris Agreement, would be obligations erga omnes and erga omnes partes respectively. 91 The Court highlighted that all state parties to the climate change treaties could invoke the responsibility of other states for violations of their mitigation obligations. 92 In this context, the ICJ underscored that non-injured states could only claim cessation and non-repetition, while injured states could also claim reparation. 93 Thus, the ICJ clarified that there are certain erga omnes obligations in the field of environmental law. This means that if states fail to meet their obligations, every state is now in a position to single out the states it considers most responsible for that failure and invoke responsibility.
On causality, the ICJ reminds us that an obligation to provide reparation may arise if there is a causal link between a wrongful act of a state and the damage suffered by the injured state. 94 Building on previous case law, the ICJ regards as the standard of causation the existence of ‘a sufficiently direct and certain causal nexus between the wrongful act […] and the injury suffered by the [state]’. 95 While it would not be impossible to establish such a nexus, the ICJ emphasises that ‘the causal link must be established in each case through an in concreto assessment’. 96 In the Advisory Opinion, the ICJ refrains from delving into such a more concrete assessment.
With regard to compensation, the ICJ highlighted that, although the exact extent of the damage caused by GHG emissions could hardly be determined, ‘compensation in the form of a global sum, within the range of possibilities indicated by the evidence and taking into account equitable considerations, may be awarded on an exceptional basis’. 97 At the same time, the issue of climate change would be precisely the case in which compensation ‘may be difficult to calculate’. 98
Some judges (and scholars) 99 criticise the ICJ for remaining too abstract on these issues. According to Judge Yusuf, the Court ‘eluded the substance of the question’ posed to it by not addressing more concretely the obligations of the responsible states and injured states. 100 Judge Bhandari suggested that the Court should have recommended the establishment of a claims commission to the General Assembly ‘to address the potentially vast number of claims.’ 101 Vice-President Sebutinde argued that the ICJ should have been more specific about potential remedies such as ‘monetary compensation, reforestation, biodiversity recovery, coastal erosion prevention, disaster or debt relief, technological transfer and infrastructural rebuilding’. 102 However, a more thorough engagement with concrete obligations or pronouncements on policy recommendations to the General Assembly was not to be expected in the Advisory Opinion.
As the ICJ held, the Court was not tasked with establishing ‘the individual responsibility of a State or group of States for the damage caused to the climate system’. 103 Rather, the object of this Opinion is to provide a reply that sets out the general legal framework that governs the responsibility of states for their failure to comply with their obligations’. 104 While it is true that in other advisory opinions the ICJ made rather concrete statements in relation to particular states or entities, 105 the questions posed in this Advisory Opinion were rather abstract.
Conclusion: where to?
The ICJ opinion is a landmark case in climate change law. As explained, the ICJ resolved several conflicts of interpretation. Rather than viewing the Paris Agreement as containing only procedural obligations, the ICJ emphasised the existence of a legal due diligence obligation for each state to strive for the highest ambition. Rather than rejecting the link between human rights law and climate change law, the ICJ recognised the right to a clean, healthy and sustainable environment as applicable to climate change law. Instead of closing the door for climate change reparation claims, 106 the ICJ highlighted that state responsibility is the appropriate legal framework for addressing the complex issue of climate change damage. All in all, the Court has established a reliable and authoritative framework for future discussion.
It remains unclear where the discussion is heading in a time of crisis in international law. While some read the Advisory Opinion as an ‘invitation’ 107 or a ‘groundwork for a new phase of climate litigation’, 108 expectations should not be too high. Establishing causality in the context of climate change remains a difficult issue. There are also considerable hurdles to future proceedings before the ICJ, as (some) major emitters such as the US, China and India have not accepted the optional clause. Nevertheless, the opinion will at the very least provide a baseline for future political negotiations among states. Moreover, its statements on the law will have an impact on future climate litigation cases which will certainly arise.
Footnotes
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.