6.1 Introduction

In March 2023, the United Nations General Assembly (UNGA) passed a resolution requesting the International Court of Justice (ICJ) to provide an advisory opinion on states’ international legal obligations in relation to climate change, including their obligations in relation to both current and future generations (UNGA 2023). This resolution was the culmination of a long civil society campaign spearheaded by youth climate justice activists, Vanuatu, and the Pacific Island countries (Wewerinke-Singh & Hinge Salili Reference Wewerinke-Singh and Hinge Salili2020; Pacific Island Students Fighting Climate Change 2024). The inclusion of obligations towards future generations in this mandate reflects, inter alia, the concern of Pacific Island countries about climate change threats – to their lives now, and the lives of their future generations – as well as their capacity to maintain their culture into the future. This chapter assesses the potential for this ICJ advisory opinion to act as an effective proxy for future generations in the climate change context. We offer recommendations on how our proposed normative framework could feed into an ICJ advisory opinion to further international, intragenerational, and intergenerational justice, without undermining the legitimacy of the Court. Whether or not the ICJ advisory opinion is decided in the direction proposed here, the arguments we set out will remain relevant for other climate litigation processes.

This chapter rests on the normative assumptions laid out earlier in this book, including the definition of proxy representation, and assumptions about the role of international law and institutions, including the ICJ. In addition, the legitimacy criteria applicable to international institutions such as the ICJ (Chapter 4) are crucial, given the close link between legitimacy and effectiveness. We introduced the concept of future legitimacy as involving the idea of looking back at decisions to be taken by the ICJ now from the perspective of future generations, including a projected vantage point decades from now, when scientists predict many parts of the world will be increasingly negatively impacted by climate change (Section 4.3.2).

An important part of the context includes the role of ICJ advisory opinions. While such opinions of the ICJ are non-binding, they nevertheless have the potential to influence the direction of international law and are an important part of the international political process. Advisory opinions of the ICJ can feed into national litigation, as well as having an impact in raising awareness and global public opinion (Wewerinke-Singh, Aguon & Hunter Reference Wewerinke-Singh, Aguon, Hunter, Alogna, Bakker and Gauci2021: 404). Even if, from this perspective, an ICJ advisory opinion can be interpreted as a form of representation of future generations, there are also tensions that need to be discussed.

One is the tension between the ICJ’s role of providing clarity to states’ existing international legal obligations, grounded on state consent, and the role of the Court (often unstated) as an agent for further development of international law, to meet the requirements of international, intragenerational, and intergenerational justice. Given there is already an elaborate climate change treaty regime, an advisory opinion that is overly prescriptive in terms of individual states’ obligations would, on the one hand, run the risk of undermining this treaty regime and weakening the legitimacy of the Court (Bodansky Reference Bodansky2017, Reference Bodansky and O’Connor2022, Reference Bodansky2023). On the other hand, an advisory opinion that is overly timid could be seen as missing a valuable opportunity to exert pressure on states to take action on climate change and provide crucial impetus for strengthening the existing treaty regime, in a context where the future of humanity – of which the international legal system is a part – is threatened. This chapter argues that the ICJ Advisory Opinion on Climate Change could further clarify existing international environmental law principles in ways which could strengthen the climate regime by providing greater specificity to states’ international legal obligations.

A further tension running through the discussion in this chapter can be characterised through the differences between inter and intragenerational and national and international (or transnational) justice. As mentioned in Chapter 2 (Section 2.5), the UN climate regime contains evidence of both an international and intergenerational justice framing. The former is embodied in the principle of ‘common but differentiated responsibilities’, which emphasises the duty of the developed countries to take the lead in addressing climate change. But the climate regime also contains evidence of intergenerational justice framings, with references to future generations contained, for example, in the ‘Principles’ section of the United Nations Framework Convention on Climate Change (UNFCCC) (1992), where Article 3 delineates principles to be considered in interpreting the UNFCCC and subsequent agreements, including the Paris Agreement (2015). The ICJ, in its advisory opinion will need to tread carefully in terms of these different dimensions of justice. On the one hand, if the Court overly focuses on obligations towards future generations, this could be perceived by developing states as exhibiting a bias towards the developed countries which have tended in the UN negotiations to emphasise the reduction of greenhouse gas emissions (mitigation), rather than adapting to climate change impacts (adaptation) (Humphreys Reference Humphreys2022: 1067). On the other hand, the intergenerational justice dimension is an explicit part of the question put to the Court and a crucial part of the rationale to address climate change. The normative framing of this book emphasises the importance of all three key axes of justice – intragenerational, intergenerational, and international – with the focus on future generations.

This chapter is structured as follows. First, we explain how the ICJ advisory opinion came about and what it could achieve (Section 6.2). We then address how the proxy representation functions (elaborated in Chapter 3) apply in relation to the ICJ advisory opinion and recall our normative framework of justice, solidarity, and vulnerability (Section 6.3). We then analyse the references to future generations in the mandate of the advisory opinion (Section 6.4). The normative framework is then applied to inform proposals as to how the Court should interpret the principle of harm prevention (Section 6.5), and fallback options, involving an indirect obligation of future generations (Section 6.6). We then examine how the Court could approach the principle of intergenerational equity (Section 6.7); analyse the potential of amicus curiae briefs, and expert witnesses, to act as proxy representatives of future generations (Section 6.8); and assess the advisory opinion in terms of the criteria of legitimacy and effectiveness (Section 6.9); before drawing some conclusions (Section 6.10).

6.2 Origins and Functions of an Advisory Opinion on Climate Change

The United Nations General Assembly resolution establishing the request for an advisory opinion of the ICJ of March 2023 (UNGA 2023) was the culmination of a campaign led by Vanuatu and the Pacific Island countries (Stephens Reference Stephens2020), combined with a civil society campaign (Wewerinke-Singh & Hinge Salili Reference Wewerinke-Singh and Hinge Salili2020: 681 et seq.). This campaign had its origins in an initiative taken by law students at the University of the South Pacific in mid-2019, which became a regional and global campaign (Wewerinke-Singh, Garg & Agarwalla Reference Wewerinke-Singh, Garg and Agarwalla2023: 14; Seneviratne Reference Seneviratne2023). Pacific Island countries were deeply concerned at the failure of the UN climate treaty process to deliver sufficiently strong action. For these countries, climate change constitutes an existential threat, as they face-ever increasing inundation from rising sea levels and more frequent and extreme weather events.Footnote ¹

In terms of a litigation strategy involving the ICJ, an advisory opinion is the most desirable route, as it avoids many of the difficult issues relating to causation and attribution of harm to individual states in the climate change context (Bodansky Reference Bodansky2017; Sands Reference Sands2016: 23 et seq.). Under Article 34(1) of the Statute of the International Court of Justice (ICJ Statute) (1945), the Court limits standing in contentious proceedings to states. While it is possible for states to bring claims on behalf of their citizens and future generations, arguably most states will continue to be reluctant to bring a case against high greenhouse gas emitting states due to concerns about potential negative impacts on their trade and investment – given the powerful fossil fuel-related vested interests in many states (Influence Map 2020). For this reason, an ICJ advisory opinion is seen as the most desirable strategy for states seeking to strengthen action on climate change.

The ICJ Advisory Opinion on Climate Change could perform a range of functions. First, it could increase pressure on states to ratchet up the level of their ambition in their Nationally Determined Contributions (NDCs) under the Paris Agreement (2015) (Bodansky Reference Bodansky2017). Second, it could clarify customary international law principles that may be invoked in many ways in both international and national legal processes. Such clarification could bolster the bargaining power of states vulnerable to climate change in the ongoing international climate negotiations (Wewerinke-Singh & Hinge Salili Reference Wewerinke-Singh and Hinge Salili2020: 688). Finally, it could increase the legitimacy of climate science (Sands Reference Sands2016: 29; see also the contrary view in Bodansky Reference Bodansky2017). In this chapter, we do not address all these functions, but hone in on the issue of how the ICJ advisory opinion could act as vehicle for proxy representation of future generations, assessed in terms of our proposed effectiveness and legitimacy criteria (Chapter 4). This involves the second and third functions listed earlier; namely, clarifying the relevant rules of customary international law, and bolstering the legitimacy of climate science.

6.3 Proxy Representation and Its Normative Implications

As we explained in Chapter 2, proxy representation involves the representation of interests or persons who cannot speak for themselves. Current international institutions and rules manifest bias against the interests of future generations. This bias is evident in the climate change regime, which contains inadequate rules for future generations in terms of their stringency and bindingness. The failure of the Paris Agreement to include an effective enforcement regime, combined with developed countries’ failure to contribute promised climate finance and the weak implementation of individual states’ mitigation commitments (as contained in their NDCs), are further weaknesses (Stankovic, Hovi & Skodvin Reference Stankovic, Hovi and Skodvin2023; Torstad Reference Tørstad2020).

Proxy representation is justified as an expression of the relation between current and future relations. The recognition of this relation and its political implications can be described as intergenerational solidarity. Proxy representation of future generations is justified as a means for addressing bias towards current generations within political and legal institutions, using a yardstick of intergenerational justice defined as requiring, as a minimum, the protection of the human rights of future generations necessary to lead a decent life (see Section 3.4.1). This normative benchmark for intergenerational justice is used as a springboard for further clarifying the international legal principle of intergenerational equity by linking it to human rights obligations (see later). This approach finds resonance in the recent European Court of Human Rights (ECtHR) 2024 decision in Verein Klimaseniorinnen v Switzerland (hereafter, Verein Seniorinnen); central to this decision was the connection between human rights protection and intergenerational justice (Nolan Reference Nolan2024). The Court emphasised that the ‘damaging effects of climate change raise an issue of intergenerational burden sharing’ (Verein Seniorinnen 2024: para 410), noting that ‘future generations are likely to bear an increasingly severe burden of present failures and emissions to combat climate change’ while having ‘no possibility of participating in relevant current decision-making processes’ (para 420). This was used by the Court, together with state’s obligations under the UNFCCC Article 3 obligation to protect the climate system for the benefit of present and future generations (Verein Seniorinnen 2024: para 420), as the basis for deciding that the European Convention on Human Rights (ECHR) (1950) required certain minimal requirements in terms of contracting states’ mitigation efforts (para 520).

Further elements of our normative framework involve the protection of the vulnerable (Section 3.4.3), combined with solidarity principles which emphasise the commonality of interest between current and future generations (Section 3.4.2). Our pragmatist methodology involves pointing out that these normative principles are already embedded in the international legal order in a range of UN instruments and the ECHR (Section 2.4) and formally endorsed by the international community. Given existing proxy-style mechanisms of representation in the international legal order for various vulnerable groups (Chapter 5), the proxy representation of future generations in the climate context is an incremental, rather than a radical, step in the process of international legal reform.

We use Rehfeld’s audience-based concept of proxy representation, in which endorsement by the relevant audience of a proxy is a defining feature (Section 2.2). Proxy representation as a legal concept, as we have seen, involves authorisation by law to take on proxy functions (Section 2.2). We have explained that proxy representation in the international legal context can have a range of functions: representational, compliance, and norm entrepreneurial (Section 2.5). We define representation of future generations as having direct and indirect dimensions. Under this definition, indirect representation includes the situation where an international law rule embodies the interests of future generations (Section 2.4). Thus, the ICJ applying or interpreting a rule of international law which embodies or reflects the interests of future generations would fall within this definition, with the ICJ facilitating a form of proxy representation (Sections 2.4 and 2.5). We have also observed that discourses can capture the interests of future generations. Before applying these concepts to the ICJ advisory opinion, it is important to examine closely the questions put to the Court by the UN General Assembly.

If the ICJ is to facilitate a form of proxy representation, this would require it to recognise the outlined normative implications of representation more strongly than it has to date and to implement them both substantively in decisions and in the development of new mechanisms. In the following discussion, we show which conclusions can be drawn for the ICJ from the three normative heuristics as the basis for representation of future generations.

6.4 The Mandate of the Advisory Opinion and Future Generations

At this point, it is essential to consider with some precision the scope of the mandate for the advisory opinion.Footnote ² The questions put to the ICJ by the UN General Assembly require the Court to determine:

In addition, the ICJ is asked:

Part (a) involves analysis by the Court of the obligations of states to protect the climate system ‘for States and for present and future generations’. One option is for the Court to refuse to answer this question on the grounds that it is too general (Mayer Reference Mayer2023: 48, 58, 78). However, refusing to answer this part of the question would undermine the Court’s legitimacy. Furthermore, this question does not require assessment of states’ obligations directly towards future generations. And the way in which the question is framed also indicates that the interests of future generations, as well as people currently living, should be factored into the analysis of states’ obligations in relation to protection of the climate system.

At first blush, the wording of part (a) of the mandate seems to break away from an anthropocentric approach, by asking the Court to examine states’ obligations to ensure protection of the climate system. These obligations are to be assessed in terms of protecting this system for ‘present and future generations’ – which has an anthropocentric flavour, implying that it is the benefit of the climate system for present and future generations – presumably ‘of human beings’ – which is to be the benchmark. Given the dependency of human beings on a functioning climatic system, however, this distinction might, in fact, be less significant than it appears at first glance.

Part (b)(ii) requests the Court to assess ‘the legal consequences under 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 with respect to’, inter alia, ‘Peoples and individuals of the present and future generations affected by the adverse effects of climate change’. This requires the Court to consider what secondary obligations flow from states breaching their due diligence duty to avoid causing significant harm to the climate system, including to both ‘[p]eoples and individuals of the present and future generations affected by the adverse effects of climate change’ (emphasis added). These secondary obligations could include, for example, obligations of restitution, including compensation (Dupuy & Viñuales Reference Dupuy and Viñuales2018: 315–17).

As there is no definition of ‘future generations’ in the UN General Assembly’s request, it will be interesting to see whether the Court defines this concept. The Court could avoid defining future generations. This may have some appeal from a pragmatic perspective, given the issue’s complexityFootnote ³ (and see later). But not defining future generations would entail the court failing to accord sufficient weight to this part of the mandate, including Part (b). For the sake of clarity, it would be helpful if the Court defined ‘future generations’ as ‘all persons born in the future, globally’. This would have the distinct advantage of keeping this concept separate from a more expansive definition which also includes younger persons alive now. As noted in the Introduction to this book (Section 2.1), young persons and future generations have overlapping, but not necessarily the same, interests. Defining future generations as persons born in the future carries the risk that young people alive now (who presently cannot vote) could end up at a representational disadvantage vis-à-vis future generations, if the latter were to benefit from representational institutions that exclude young people alive now. However, young people alive now are already clearly defined as children by the United Nations Convention on the Rights of the Child (CRC) (1989: art 1). It would be helpful to keep these categories distinct; therefore, considerations of legal certainty point in favour of the narrower definition of future generations as persons born in the future. The European Court of Human Rights (ECtHR) in its decision in Verein Klimaseniorinnen (2024: para 419) designated future generations as people born in the future, rather than including existing rights holders, such as children (Nolan Reference Nolan2024).

The proposed definition of future generations could nevertheless give rise to some thorny issues. One concern is that this category is too vague because the composition of this group is constantly changing, with persons born every second (Humphreys Reference Humphreys2022: 1066). Yet, other international legal concepts such as ‘children’ also include constantly changing members (CRC 1989: art. 1). A further concern is that the ICJ, by defining future generations as ‘persons born in the future’, would necessarily imply that they possess human rights, which is impossible if they are not yet born. The Court could, however, avoid this issue by focusing on the duties of states to take preventive action to ensure protection of the human rights of future generations upon being born. This would reflect the insight that the acquisition of human rights by persons upon being born is sufficient to ground normative duties towards future generations (Vanderheiden Reference Vanderheiden2008: 137).

In answering these questions, the Court will need to assess the substance of international legal rules applicable in the climate context. To the extent that these rules embody the interests of future generations, they may constitute a form of indirect proxy representation of future generations within the definition we use in this book. To the extent that these rules fail to ensure that future generations will be able to enjoy the human rights necessary for a decent life (our touchstone for intergenerational justice), the Court should use the normative framing set out in this book – including the concept of intergenerational justice – as a guide for interpreting and further developing the rules of international law to meet this objective. The Court in this scenario would be acting as a vehicle for proxy representation of future generations. More specifically, this would entail the Court taking on a proxy representative norm entrepreneurial role in promoting the rules necessary to meet this yardstick. Importantly, however, there are legitimacy constraints in the Court taking on this role, which we discuss later.

To the extent that the existing international legal rules applicable to climate change meet the requirements of intergenerational justice, the Court may play a proxy compliance function in that its advisory opinion, while strictly non-binding, can provide the basis for increased pressure on states to meet their relevant international legal obligations.

A further possibility is that the Court itself may purport to act as a proxy or guardian for future generations. Justice Weeramantry has argued for this in several ICJ judgements, including in the 1974 Nuclear Tests case (ICJ 1974: 341). This proposal, however, faces the difficulty that it involves the Court taking on a role which is not specified either in its Statute or in the treaties it is applying. Reflecting this reality, the other judges of the ICJ have not taken up Justice Weeramantry’s approach – with the exception of Justice Cançado Trindade in the 2014 Whaling in Antarctica (hereafter Whaling case), who came close to this view in urging the Court, in his Separate Opinion, to pronounce ‘upon a system of collective regulation of the environment for the benefit of future generations’ (Whaling case 2024: 381), noting the embodiment of intergenerational equity in a raft of international environmental treaties (140–44). Before considering further how the ICJ can utilise the concept of proxy representation, we turn to apply the normative framework set out above to the principle of harm prevention.

6.5 Normative Evaluation of the Principle of Harm Prevention

The UNGA resolution establishing the advisory opinion makes specific reference to ‘the duty of due diligence’ and ‘the principle of prevention of significant harm to the environment’ (UNGA 2023: chapeau and first operative paragraph), requiring the Court to examine states’ international legal obligations to prevent harm to the global climate system, with reference to the impact this has on future generations (UNGA 2023: first operative paragraph).Footnote ⁴

The principle of harm prevention is a rule of customary international law, according to which states have the responsibility to ensure that activities within their jurisdiction or control do not cause damage to the environment of other states or of areas beyond the limits of national jurisdiction. The principle has been affirmed by the ICJ as part of customary international law in the 1996 Legality of Nuclear Weapons case and the 1997 Gabčíkovo-Nagymaros Project case (para 140).Footnote ⁵ The International Law of the Sea Tribunal (ITLOS) in its 2024 Request for an Advisory Opinion Submitted by the Commission of Small Island States on Climate Change and International Law (ITLOS Advisory Opinion on Climate Change) highlighted that Article 192 of the UN Convention on the Law of the Sea (UNCLOS) (1992) reflected the customary international law principle of harm prevention, entailing a due diligence obligation on states applicable to marine pollution caused by anthropogenic greenhouse gas emissions (ITLOS Advisory Opinion on Climate Change 2024: para 250).Footnote ⁶

Currently, there is uncertainty as to whether the principle of harm prevention involves an obligation of due diligence or is, rather, a rule of result.Footnote ⁷ The latter approach would foreclose application of the principle of harm prevention in relation to foreseeable harm likely to occur to future generations resulting from climate change, as proof of harm to future generations prior to its actually occurring could never be brought to a tribunal. The normative principles of this book could be invoked to argue that the ICJ should come down on the side of the ‘due diligence’ interpretation of the principle of harm prevention. Given conflicting authorities in this area and the duty of the Court to promote justice – extending to intergenerational justice – the Court should conclude that the principle of harm prevention establishes obligations on states in the climate change context to prevent foreseeable harm to future generations. This would be supported by the elements of the normative framework involving: (i) giving equal weight to current and future generations and (ii) the intergenerational justice requirement that future generations enjoy the human rights necessary to lead a decent life. Limiting the harm avoidance principle to cases where damage has already occurred – thus entailing the exclusion of future generations’ interests – would run counter to these elements of the normative framework. This interpretation of the harm avoidance principle would also be consistent with an emphasis on the duty of states to proactively prevent damage to the environment and with the UN General Assembly resolution establishing the advisory opinion, which, as noted earlier, refers to the ‘principle of prevention of significant harm to the environment’ (UNGA 2023: first operative paragraph).

In this section, we make proposals as to how the ICJ could, in its advisory opinion, clarify the principle of harm prevention in the climate context in ways based on the normative principles outlined in this book. First, we argue that the principle is a rule of due diligence rather than a rule of result. Second, we argue that the scope of the principle of harm prevention should be extended to include harm to future generations. Third, we argue that the prevention of harm principle in the climate change context should be interpreted as requiring a threshold of harm linked to certain elements of the Paris Agreement (2015). In each of these arguments, the traditional sources of international law applied by the ICJ are applied in conjunction with the normative principles of this book. Before turning to these various elements, we introduce the principle of harm prevention.

An advisory opinion could also helpfully clarify that the principle of harm prevention extends to imposing an obligation on states to prevent foreseeable harm to future generations. Support for this conclusion can be found in three key sources. First, the Court could build on some of its previous pronouncements on sustainable development. Second, the Court could rely on the principle of intergenerational equity. Third, various elements of the normative framework set out in this book could be relied upon as entailing a duty on states to avoid harm to the human rights of future generations (Lawrence & Köhler Reference Lawrence and Köhler2018). Fourth, extension to future generations could also be grounded on the idea that this is a vital interest which the international community needs to protect. The ‘future generations’ referred to here would comprise the ‘future generations of all states’, reflecting the scientific reality that the harms caused by a failure to mitigate greenhouse gas emissions occur globally.

First, extension of the principle of prevention to include harm to future generations could be based on the ICJ’s recognition of the principles of sustainable development and intergenerational equity.Footnote ⁸ The ICJ in the 1997 Gabčíkovo-Nagymaros case considered a dispute between Hungary and Slovakia relating to the construction of a hydroelectric facility by Hungary which impacted the environment of downstream Slovakia (Gabčíkovo-Nagymaros Project ICJ 1997). The majority judgement referred to ‘new scientific insights and to a growing awareness of the risks for humankind – for present and future generations – of pursuit of interventions in nature’ (Gabčíkovo-Nagymaros Project ICJ 1997: para 140). The judgement went on to recognise the development of ‘new norms’ which need ‘to be taken into consideration … not only when states contemplate new activities but also when continuing with activities begun in the past. This need to reconcile economic development with protection of the environment is aptly expressed in the concept of sustainable development’ (ICJ 1997: para 140). While the passage leaves unclear whether these new norms are binding, the sustainable development principle was integral to the Court’s order instructing the parties to negotiate a solution to the volume of water issue, balancing environmental and economic considerations (ICJ 1997: para 140).

Justice Cançado Trindade, in his strong dissent in the 2010 ICJ Pulp Mills case, concludes that intergenerational equity ‘forms part of conventional wisdom of International Environmental Law’ reflected in numerous treaties (Pulp Mills Case ICJ 2010: para 122) and that sustainable development is a general principle of international environmental law (para 183). He describes the principle of intergenerational equity as being linked to a concept of conservation involving a cultural manifestation of the integration of human beings with nature, in turn, linked to an obligation to other generations past and future (Pulp Mills Case ICJ 2010: para 114). Justice Cançado Trindade links the preventive character of environmental protection to other areas of international law including human rights, which he notes extend obligations into the future to ‘potential or prospective victims’ (ICJ 2010: para 116). The integration of the due diligence harm prevention principle with human rights benchmarks argued for in this chapter (see later) fits well with this approach.

First, Justice Cançado Trindade emphasises ‘solidarity in time’ between current and future generations as manifest in a number of decisions of the Inter-American Court of Human Rights (IACtHR)Footnote ⁹ which gave effect to the particular relationship of Indigenous people to their land, which includes a spiritual element essential for transmitting their cultural legacy to future generations (ICJ 2010: paras 128–31). This resonates with the solidarity element of the normative framework of this book.

Second, it could be argued that the principle of harm prevention must, in the climate context, be interpreted consistently with the principle of intergenerational equity, which should be interpreted by the ICJ as requiring states to take mitigation action to ensure that future generations enjoy human rights necessary to lead a decent life (see Section 6.7.3). The principle of intergenerational equity is a vital element of the principle of sustainable development which, as we have seen, has been recognised by the ICJ, and is also integrated into the UN climate regime (see Section 6.7). Indeed, extension of the principle of harm prevention to include an obligation to avoid harm to future generations could be seen as required by sustainable development. While sustainable development and intergenerational equity are indeterminate concepts in terms of specifying the exact weight to be accorded to future vis-à-vis current generations, excluding harm occurring to future generations entirely from the scope of the harm prevention rule would conflict with any minimal content of intergenerational equity.

Third, elements of the normative framework set out in this book would also support an extension of the principle of harm prevention to future generations. Extension of the principle of harm prevention to future generations could be seen as a vital expression of intergenerational solidarity and flowing from a recognition of the vulnerability of future generations. In addition, intergenerational justice grounded on a concept of equality – with the interests of future generations being given equal consideration to the interests of contemporaries. Excluding harm to future generations from the scope of the principle of harm prevention would cut across this normative requirement. Intergenerational justice is defined as entailing an obligation to ensure that future generations enjoy the human rights necessary to lead a decent life. Extension of the principle of harm prevention to future generations is required by this concept of intergenerational justice as, absent such an extension, international law would be permitting harm to occur to future generations.

Fourth, extension of the principle of harm prevention to future generations could be grounded on the idea that the interests of future generations are vital interests which the international legal order needs to protect. This is recognised in the UN General Assembly resolution establishing the mandate for the advisory opinion, which states that ‘climate change is an unprecedented challenge of civilisational proportions and that the well-being of present and future generations of humankind depends on our immediate and urgent response to it’ (UNGA 2023). This resolution, adopted by consensus, but strictly non-binding, would provide a basis for the Court to interpret the principles of international environmental law in a progressive manner which gives sufficient weight to the interests of future generations. Just as the principle of harm prevention was extended by the ICJ to include harm to areas outside national jurisdiction and the global commons, reflecting the value of the environment of these areas to the entire international community, extension of the principle of harm prevention to protect all future generations reflects an equally important value.

While, to date, the ICJ has not had the opportunity to proclaim in the climate change context on erga omnes based obligations owed to the whole international community (Tams Reference Tams2005), the approach argued for here would mesh well with a step in this direction (Bodansky, Brunnée & Rajamani Reference Bodansky, Brunnée and Rajamani2017: 51). The International Law Commission (ILC) has previously suggested that not preventing massive environmental harm to areas beyond national jurisdiction could constitute breach of erga omnes obligations, while in its recent reports relating to international law applicable to climate change, states pushed back against a suggestion that there were international legal obligations owed erga omnes in relation to the global atmosphere (United Nations Department for General Assembly and Conference Management 2021: 31). It has been pointed out that human beings’ enjoyment of the right to life is dependent on preservation of the global climate system; it would therefore make sense for the ICJ to pronounce that preservation of the global climate system is an obligation owed erga omnes to the international community as a whole, given the crucial importance of this to the international community as a whole (Duvic-Paoli & Gervasi Reference Duvic-Paoli and Gervasi2022: 232). The ICJ could, in making such a pronouncement, rely on various elements of the global climate regime, including the reference in the UNFCCC Preamble to ‘change in the Earth’s climate and its adverse effects are common concern of humankind’ (UNFCCC 1992: first preambular paragraph).

Defining future generations as ‘all persons born in the future globally’, when combined with an extension of the principle of harm prevention in the climate context to include a responsibility on each state to avoid harm to the future generations of all states, would have complex ramifications.Footnote ¹⁰ One troubling ramification could be that this would entail placing responsibility on developing countries to prevent harm to the future generations of developed countries.

To address this concern, the Court could, first, clarify that, in the context of climate change, the principle of harm prevention needs to be interpreted in a manner which considers the principle of ‘common but differentiated responsibilities’ contained in the global climate regime (Rajamani Reference Rajamani2016). As already mentioned, the interpretation of this principle is contentious, also within the philosophical discourse (Edenhofer et al. Reference Edenhofer, Wallacher, Lotze-Campen, Reder, Knopf and Müller2012: 312ff.). However, the Court could avoid this controversy by specifying that the principle of harm prevention in the climate change context must be interpreted with reference to both responsibility for climate emissions and capacity.

Second, the Court could point out that the principle of harm prevention has always entailed a due diligence duty on a state to take reasonable measures to prevent harm, considering its capacity, rather than a duty (of result) to stop any harm occurring (ILC 2001: 155). Third, given that the sources of greenhouse gas emissions in any one state contribute (in varying proportions) to climate change globally, it makes no sense to limit the principle of harm prevention to harm to the future generations of citizens of a particular state bearing that responsibility. Such an interpretation would be consistent with recent jurisprudence indicating that states’ obligations in relation to the protection of human rights extend extraterritorially to include climate-related harms on citizens in other states, where there was a causal link to the harm and activities within the control of the source state.Footnote ¹¹ It would also mesh with the justice framework, with the principle of harm prevention modified to maximise both intragenerational and intergenerational justice. While each state would be responsible for its share of emissions, this would still leave thorny issues of attribution and causation (Nedeski, Sparks & Hernandez Reference Nedeski, Sparks and Hernández2023: 312, 324–25).

The principle of harm prevention is vague in terms of the substantive threshold required to be met; the ICJ could make a distinctive contribution by clarifying this in the climate context. The advisory opinion could clarify the threshold of the principle of harm prevention in the climate context by drawing on certain elements of the Paris Agreement (2015) in a manner which supplemented, and did not undermine, the UN climate regime.

As Bodansky (Reference Bodansky2017: 20) argues, the ICJ could helpfully elaborate obligations of states ‘to ensure that their greenhouse gas emissions do not cause serious damage to other states’, which could exert pressure on countries to improve their implementation of the Paris Agreement in line with the effectiveness criteria set out above.

Indeed, the Court could go even further by deciding that due diligence in the climate change context entailed an obligation on each state to put in place:

The connection to the principle of harm prevention entailed in this obligation is that, without these targets and supporting measures, each state is arguably not taking action to meet its responsibility to prevent harm to the environment of other states or in areas of national jurisdiction ‘for present and future generations’ (para (a) mandate) but is complicit in such harm.Footnote ¹³

The proposal sketched above would find justification in the normative framework set out in this book, in that, absent the benchmark specifying the content of the due diligence obligation, the interests of future generations would be negatively impacted by continuing greenhouse gas emissions. This would violate the equality principle which underpins intergenerational justice, as it would entail future generations being ascribed less value than people currently living, and would also be contrary to the principle of intergenerational solidarity (Section 3.4.2).

By referring to the ‘principle of common but differentiated responsibilities’ and both ‘historic emissions’ and ‘capacity’, the ICJ would recognise the greater responsibility of developed states to take action on climate change, while avoiding taking sides in the ongoing conflict between developing countries, on the one hand, who emphasise historic emissions as the basis for the responsibility of developed countries to take the lead in reducing emissions, and, on the other hand, the US and many developed countries, which argue that mitigation and adaptation action more appropriately should rest on the ‘capacity to pay principle’ (Vanderheiden Reference Vanderheiden2008: 184; Shams Reference Shams2023). Put simply, the ICJ would need to retain the ambiguous concept of ‘differentiation’ in the Paris Agreement.

A further concern is that the principle of harm prevention interpreted in this manner would be problematic from a developing country perspective: it would mean that developing countries owed duties to their own future generations that arise from impacts to which they have contributed very little. However, the proposal above makes clear that responsibility is proportionate to the state’s historic emissions and capacity.Footnote ¹⁴

The approach suggested here could be opposed on the basis that the climate regime is a lex specialis and therefore takes priority over more general customary international law obligations (Zahar Reference Zahar2014: 230). However, there is no inconsistency between the general customary international law obligations referred to above and the provisions of the Paris Agreement (Duvic-Paoli & Gervasi Reference Duvic-Paoli and Gervasi2022: 229).

Nevertheless, the ICJ could have concerns that extending the prevention of harm principle to include an obligation to future generations would be beyond the scope of the Court’s mandate. It is to this issue that we now turn.Footnote ¹⁵

6.6 Fallback Options: An Indirect Obligation to Future Generations

The extension of the principle of harm prevention to include a duty to prevent harm to future generations could be argued to be beyond the scope of the advisory opinion mandate because, as pointed out above, the Court is not being asked whether states have a direct obligation to future generations. While the extension would be in the spirit of the mandate, critics might point out that the General Assembly could have specifically asked – if that was its intention – what obligations under international law were owed to future generations.

To address this concern, some fallback options might prove attractive to the Court. One such option would be to specify the duty of states to protect the climate system with reference to a threshold defined in terms of potential harm to future generations. Thus, the ICJ could specify that states must put in place laws and policies sufficient to ensure protection of the climate system and other parts of the environment at a level which ensures that current and future generations, who rely on this climate system, can enjoy human rights to lead a decent life. This would be consistent with the mandate and would avoid the Court examining whether states have a direct obligation towards future generations. However, this approach would still require the Court to define the threshold, which would inevitably lead us back into the complex definitional issues set out above.

In summary, the normative framework would provide a basis for the ICJ interpreting the principle of harm prevention as a rule of due diligence, extending to harm to future generations, and entailing certain substantive obligations on each state of sufficient stringency to ensure that it takes action to ensure that the overarching 1.5°C/2°C Paris Agreement target is met, in line with its historic responsibility/capacity and the principle of common but differentiated responsibility.

A further important principle of international environmental law which the Court will have to consider in its decision is the principle of intergenerational equity, to which we now turn.

6.7 Intergenerational Equity

6.8 Proxy Representation and Procedural Reform: NGO/Expert Amicus Briefs

Implementing proxy representation could provide a vital mechanism to maximise the likelihood of the ICJ interpreting the international law rules to meet the criteria of justice (including intergenerational justice), democratic legitimacy, and effectiveness. This would involve the ICJ interpreting its procedural rules in a way that facilitates proxy representation of future generations.

Proxy representation involves representation of interests or persons who cannot speak for themselves (Lawrence & Köhler Reference Lawrence and Köhler2018: 654). We have seen that international climate law is seriously biased against the interests of future generations (see Introduction). Proxy representation is justified as a means for redressing this bias; it is also justified as increasing the likelihood of attaining intergenerational justice (Lawrence Reference Lawrence2022: 558–61). Proxy representation can also be justified by democratic values which have been accepted by consensus globally in a range of UN instruments endorsed by the international community (see Section 3.2). As we have pointed out (Section 2.3), representation of future generations may be direct or indirect: ‘direct representation’ occurs where a representative explicitly claims to act on behalf of future generations; ‘indirect representation’ occurs where the representative does not purport to act on behalf of future generations but, rather, highlights their interests (Lawrence Reference Lawrence2022: 553). While not expressed in the language of proxy representation, the ECtHR decision in Verein Seniorinnen (2024: para 489) to allow standing to particular associations purporting to represent vulnerable groups – on the basis that, even in democracies, these groups do not have a voice – was underpinned by democratic values similar to those argued for here, with the Court explicitly noting the representational disadvantage of future generations (Verein Seniorinnen 2024: para 484; Letsas Reference Letsas2024).

How do these abstract concepts map onto the advisory opinion? One option is that the Court itself purports to act as a proxy or guardian for future generations. Justice Weeramantry has argued for this in several ICJ judgements, including in the Nuclear Tests Case (ICJ 1974: 341). However, as mentioned earlier, this proposal faces the difficulty that it involves the Court taking on a role which is not specified either in the ICJ Statute or in the treaties it is applying. Reflecting this reality, other judges of the ICJ have not taken up this approach (with, as with have seen, the exception of Justice Cançado Trindade, who came close to this view in the Whaling Case).

A further possibility is that the ICJ acts to facilitate such representation. This would mean that the ICJ interprets its Statute and Rules of Procedure in a manner which allows submissions by states, intergovernmental organisations (IGOs), non-governmental organisations (NGOs) and/or scientists on behalf of future generations, or to highlight their interests. By facilitating proxy representation, the Court could help increase the likelihood that, in its substantive decisions, it reflects the requirements of justice (including intergenerational justice), democratic legitimacy, and effectiveness. This is because proxy representation involves factoring in the distinctive interests of future generations.

One possibility for such submissions is amicus curiae briefs, which are a vehicle for advocacy in the public interest, allowing legal arguments to be aired that would otherwise not be heard (Chinkin & Mackenzie Reference Chinkin, Mackenzie, Boisson de Chazoures, Romano and Mackenzie2002: 136). While the ICJ Statute does not make any specific provision for amicus briefs, Article 66 provides that, when a request for an advisory opinion is received, the Court shall notify, in addition to states entitled to appear before it, any ‘international organisation … considered by the Court as likely able to furnish information on the question [being considered]’ and that such organisation is entitled to make both oral and written statements (ICJ Statute: art 66(2)). To date, this has been interpreted narrowly by the Court to allow IGOs to make submissions but to exclude NGOs amicus briefs in advisory proceedings of the ICJ (Sands & Mackenzie Reference Sands and Mackenzie2008: para 9). Continuing its established practice, the ICJ has generally only permitted states and IGOs (such as the World Meteorological Organisation) to make submissions in the advisory opinion process, although the IUCN (World Conservation Union) World Commission on Environmental Law, which brings together governments and civil society, has been allowed to make a submission.Footnote ²¹ Pursuant to the established practice of the Court, where NGOs make submissions, these will be placed in the library of the Peace Palace and made available for any state, intergovernmental organisational, or judge to consult, but will not form part of the official record of the case (Shelton Reference Shelton2007: 152).

There are strong reasons why the ICJ should reverse its narrow interpretation of Article 66 of its Statute. First, this provision was based on a similar provision in the predecessor court – the Permanent Court of International Justice (PCIJ), and ‘Advisory proceedings of the PCIJ support the view that the reference to “international organisations” was intended to include non-governmental organisations’ (Shelton Reference Shelton2007: 146).Footnote ²² Various elements of the normative framework of this book provide further grounds for an interpretation of Article 66 of the ICJ Statute to allow NGOs to submit amicus briefs on behalf of future generations in the advisory opinion. One such element is the value of promoting intergenerational justice through facilitating the representation of future generations by proxies, given that future generations will be affected by the advisory opinion. In addition, such proxy representation can highlight the distinctive interests of future generations and help ensure democratic legitimacy in the process.

Could a move in this direction backfire by enabling fossil fuel industry lobby groups to become involved in a way which undermined potential progress? The UN climate negotiations have in place workable mechanisms for regulating NGO and industry association input (Streck Reference Streck, Barnes and Long2021), which the ICJ could use as a model. A further practical option would be for the Court to limit submissions to NGOs possessing consultative status at the United Nations (Shelton Reference Shelton2007: 149).

A further option would be for the ICJ to use Article 50 of its Statute, which empowers the Court to entrust any organisation it may select ‘with the task of carrying out an enquiry or giving an expert opinion’ – this could be interpreted as allowing the Court to request an amicus brief from a particular NGO possessing relevant expertise (Shelton Reference Shelton2007: 151). States will no doubt be submitting scientific evidence as part of their submissions, as well as including scientists as part of their teams making oral submissions. In addition, the Court could rely on Article 50 to itself call on scientific or legal experts to provide evidence related to the case. Scientific experts making input to the case through one of these routes – to the extent that they are highlighting the impacts of climate change on future generations – could be considered proxy representatives of future generations, within the definition of indirect representation (Section 2.3). This could provide a crucial vehicle for ensuring that the court is sensitised to the distinctive interests of future generations.

The extent to which this would be effective is interconnected with the question of legitimacy; we now turn to these issues.

6.9 Legitimacy and Effectiveness

As we have already observed, the legitimacy of an international tribunal in a general sense is based on state consent to the treaty which establishes it (Section 4.2.2). Thus, at a general level the legitimacy of an ICJ advisory opinion rests on state consent to the UN General Assembly resolution establishing the mandate of the advisory opinion (Bodansky Reference Bodansky2023: 186). Article 38 of the ICJ Statute requires the Court to apply international law. However, it is well recognised that, in practice, the Court’s role goes well beyond this, by playing a role in clarifying such rules when they are unclear, and even further developing rules of international law (Section 4.2.2). We argue that, where the Court takes on this role, it should do so through pursuing an overarching objective of promoting justice and, by extension, intergenerational justice – interpreted to mean that, regardless of when and where people are born, they are entitled to enjoy the human rights necessary to lead a decent life.

We further argued (Section 4.2.2) that international tribunals, including the ICJ, ought to be democratically legitimate and that this can help bolster their effectiveness. Democratic legitimacy was defined, following Bogdandy and Venzke (Reference Bogdandy and Venzke2014), as the Court: (i) acting on behalf of the demos extended to include both current and future generations; (ii) ensuring impartiality and independence in terms of procedural fairness in relation to court processes; and (iii) responsiveness to the demos involving transparency, deliberation, and participation, including involvement of affected individuals in court processes.

Of course, while future generations cannot be directly involved in ICJ processes, they can be indirectly involved through the Court factoring in their interests when interpreting or further developing particular rules. In addition, proxy representation through expert and amicus curiae brief processes is possible (see above). The ECtHR in Verein Seniorinnen (2024) recognised that the obstacles faced by marginalised groups (elderly people, children, and future generations) in influencing climate-related decision-making in a democracy provided a strong rationale for the Court to allow associations, subject to certain conditions, to represent these interests in court processes (para 554) and relaxing somewhat the victim requirement for a plaintiff to demonstrate that their individual human rights have been violated (paras 499, 502). While not couched in the language of proxy representation, this approach is, in substance, strikingly similar to the approach we argue for here.

We also introduced the concept of future legitimacy, which involves looking at an ICJ decision from the vantage point of future generations to see how it would be potentially responsive to the demos alive in the future. To have future legitimacy, an advisory opinion, needs to be considered from a vantage point decades from now, at a time when climate scientists predict that many parts of the Earth will be significantly and negatively impacted by climate change. Only through such an extension of legitimacy into the future can we capture the existential threat climate change poses to humanity – including the entire international community and its institutions. Legitimacy involves meeting the requirements of international and intra-generational justice, as well as intergenerational justice.

In considering the legitimacy of an ICJ advisory opinion, it is important to recognise that state consent remains a constraint. If the ICJ pushes too far in developing rules of international law in a manner which states perceive as exceeding its mandate, this will undermine its legitimacy. However, developing rules of international law by the Court, within certain constraints, is nevertheless possible and desirable.

Legitimacy is linked to effectiveness, which we define (Section 4.4) in terms of the Paris Agreement 1.5°/2°C targets, as well as the adaptation, climate resilience and sustainability, and finance elements set out in Article 2 of the Paris Agreement (2015). We assess the potential effectiveness of an advisory opinion against this yardstick.

While future generations cannot participate directly in ICJ processes, their interests can, nonetheless, can be factored into an ICJ advisory opinion by the Court interpreting relevant applicable rules in a manner which includes the interests of both people currently living and future generations. The interpretation of the principle of intergenerational equity in the climate context argued for above would help bolster the Court’s legitimacy. Giving the principle of intergenerational equity more substance and normative bite would meet the requirements of both traditional and future legitimacy. This, in turn, would enhance the likelihood of the ICJ advisory opinion being effective in terms of increasing the likely attainment of the Paris Agreement 1.5°C /2°C target, as well as its resilience and adaptation goals. While it must be recognised that the ICJ’s functions, as set out in its Statute, do not include addressing climate change in these terms, the role of the Court at a general level, as argued earlier, is to interpret international law rules in a manner which furthers justice, including intergenerational justice. To the extent that the Court succeeds or fails in this respect, it must be recognised that it is constrained by the quality (or lack of quality) of the rules of international law which it is interpreting.

We have seen in this chapter that the normative framework set out earlier in this book would provide a strong basis for an extension of the ‘principle of harm prevention’ rule to establish an obligation on states in the climate context to prevent harm to future generations. Such an approach could be criticised on the basis that it involves excessive judicial activism that undermines the legitimacy of the Court. In response, however, it could be pointed out that the ICJ has already shown a willingness to extend the principle of harm prevention by deciding in the Legality of Nuclear Weapons Advisory Opinion (ICJ 1996: 242) that the rule extends to environmental harm occurring outside areas of national jurisdiction.Footnote ²³

The effectiveness of an ICJ advisory opinion on climate change is difficult to judge, particularly given that advisory opinions are non-binding. However, ICJ advisory opinions are authoritative and capable of transforming international law and impacting behaviour at the grassroots level by being taken up by national courts or incorporated into national legislation. As we pointed out in Chapter 4, equity or fairness is an integral element of the climate regime; therefore, by implementing a concept of justice – extended to include intergenerational justice – the ICJ will enhance its legitimacy and, in turn, the effectiveness of an ICJ advisory opinion on climate change.

In this chapter, we have argued that the ICJ should take a reforming role in further fleshing out the principle of intergenerational equity. Whether this will filter through to behavioural change – in the form of modified state action (and action at the substate and nonstate level) and, in turn, a strengthening of the global climate regime – will depend on a whole range of political-economic factors that are difficult to predict. One of the uncertainties is the extent to which, over the coming years, young people (whose interests overlap with future generations) can use political activism to increase pressure on governments to take stronger action on climate change. This group is likely to welcome an ICJ advisory opinion that gives more substance to the principle of intergenerational equity.

6.10 Conclusion

The ICJ, in approaching its Advisory Opinion on Climate Change, faces several tensions. If the Court is overly tentative, it could miss an invaluable opportunity to further clarify important principles of international environmental law which could strengthen the global climate regime. On the other hand, if the Court goes too far in pushing reform, it may suffer in terms of legitimacy – and thereby effectiveness – in pronouncing that particular rules of international law exist where there is no basis in state consent for this.

We have argued that the ICJ should apply the normative framework proposed in this book to: (i) interpret the principle of harm prevention as a rule of due diligence, extending to future generations and entailing a threshold informed by elements of the Paris Agreement; (ii) give substance and bite to the principle of intergenerational equity by defining it in terms of requiring states to take climate action sufficient to ensure protection of the human rights of future generations necessary for them to lead a decent life; and (iii) interpret the Court’s rules of procedure to allow NGOs to make submissions on behalf of future generations and call scientific experts to inform the Court’s decision by highlighting the distinctive interests of future generations.

Crucial benefits would follow from taking these steps, including increased pressure on states to strengthen their NDCs; positive flow-on into national climate litigation – which, in turn, can positively interact with the international legal regime; and important ammunition for civil society pressure on governments to raise their levels of ambition on climate mitigation and adaptation efforts.

By taking the steps we advocate here, the ICJ would indirectly facilitate the proxy representation of future generations consistent with the role of the Court in promoting justice. This would also be consistent with the criteria of democratic legitimacy and future legitimacy which, as we have seen, include extending the demos to encompass future generations who will be impacted by international law-making occurring in the present and considering an ICJ advisory opinion adopted now from the perspective of future generations.

The approach argued for here would allow the ICJ to demonstrate impartiality, in terms of the interests of developing and developed countries, in that the Court could emphasise the common but differentiated responsibility of states in addressing climate change embedded in the Paris Agreement, while recognising differentiation in terms of responsibilities of individual or groups of states in accordance with historic emissions and capacity. This approach would help to increase pressure on states to implement and strengthen the Paris Agreement, while avoiding undermining the Agreement.