Wayfinders for Climate Change Action: The ICJ, ITLOS, and IACtHR Advisory Opinions on Climate Change

Abstract

Three international courts – the International Court of Justice (ICJ), the International Tribunal for the Law of the Sea (ITLOS), and the Inter-American Court of Human Rights (IACtHR) – have recently issued landmark advisory opinions regarding States’ responsibility to address climate change. The opinions show a high level of convergence, albeit with some notable differences. In essence, they find that, under various sources, international law imposes stringent yet differentiated mitigation obligations on States to address climate change, both individually and collectively. The courts aligned on the core tenets of the due diligence standard to meet these obligations, including on the objective to limit global warming to 1.5 °C and the need to rely on science, precaution, and differentiation. While the ICJ found a duty to regulate the emissions of private actors, the IACtHR went further in defining specific actions that States must take to regulate such actors. The opinions will likely affect future domestic and international regulation and litigation.

‘Today the world needs wayfinders – those who can guide us towards a path that protects our homes, upholds our rights and preserves our dignity’

(Mr Vishal Prasad on behalf of the Pacific Island Students Fighting Climate Change in the ICJ advisory proceedings on climate change, 13 December 2024)¹

1 Introduction

Three international tribunals (collectively: the ‘courts’) have recently issued landmark advisory opinions regarding States’ responsibility to address climate change: (i) the International Tribunal for the Law of the Sea (ITLOS) on 21 May 2024;² (ii) the Inter-American Court of Human Rights (IACtHR) on 29 May 2025;³ and (iii) the International Court of Justice (ICJ) on 23 July 2025.⁴ Although non-binding, these opinions carry significant weight and are expected to influence future climate change regulation – at both international and domestic levels – and climate litigation against States and companies.

The purpose of this article is to synthesize and analyze the courts’ core mitigation-related findings, identifying similarities and differences, and exploring their implications for future climate action.⁵ Despite differences in the questions put to each court, the legal instruments they addressed, the participants before them, and each court’s legal tradition, the resulting opinions show significant convergence, with some notable divergences. In essence, the courts concluded that international law imposes stringent yet differentiated mitigation obligations on all States to address climate change, both individually and collectively. In this note, we show how the courts have woven together various legal sources to create a coherent tapestry of complementary mitigation-related obligations.⁶

Given space constraints, we do not summarize the courts’ findings on issues besides mitigation, leaving out, therefore, the findings on adaptation and the responsibility of States for breaches of such obligations. For present purposes, it suffices to note that the ICJ confirmed that States may be held internationally responsible if they fail to meet their mitigation (or other) international obligations, whether for past or present conduct. In that event, a State may be required to provide compensation for injury caused to other States. Owing to the erga omnes character of the relevant customary international law obligations – particularly the duty to prevent significant transboundary harm (prevention obligation) – as they relate to ‘global common goods’ (like the climate system), any State may bring a claim of violation against another State.⁷ Hence, if a State fails to meet the mitigation obligations outlined in this article, it may incur serious legal consequences.

We first summarize the courts’ factual findings, which form the foundation for the legal findings and which a State must take into account to meet its mitigation obligations (Sect. 2). We then discuss the applicable law and the guiding principles on mitigation (Sect. 3), before addressing the core legal findings in relation to mitigation (Sect. 4). To conclude, we briefly outline the implications of the opinions for future rulemaking and litigation on climate change, as well as for other areas of international law (Sect. 5).

2 Factual Findings

In developing their factual findings, all three courts relied heavily on the work of the Intergovernmental Panel on Climate Change (IPCC), which they uniformly considered the ‘best available science’.⁸ The courts emphasized the rigorous review process through which IPCC reports are produced and noted that most participants relied on these reports.⁹ The courts’ heavy reliance on IPCC reports underscores the importance of safeguarding – and, where necessary, strengthening – the scientific rigor and legitimacy of the IPCC.¹⁰

The courts’ core factual findings include the following:

• Causes: The courts concluded that human activity has caused global warming. Specifically, the ICJ explained that the increase in the atmospheric concentration of GHGs leading to climate change is ‘primarily due to human activities’, as a result of (i) activities releasing GHG emissions, ‘including by the burning of fossil fuels’; and (ii) activities weakening or destroying carbon reservoirs and sinks, such as forests and oceans.¹¹

• Harms resulting from climate change: The courts consistently underscored the far-reaching adverse impacts of climate change on the climate system and, hence, the Earth’s biospheres and, ultimately, humanity. The ICJ explained that climate change poses an ‘urgent and existential threat’, with ‘severe and far-reaching’ consequences for natural ecosystems and human populations.¹² The ICJ cited General Assembly (GA) resolutions characterizing climate change as a ‘common concern of mankind’ and an ‘unprecedented challenge of civilizational proportions’ and concluded that it concerns ‘an existential problem of planetary proportions that imperils all forms of life and the very health of our planet’.¹³ ITLOS recorded that ‘climate change represents an existential threat’, and highlighted the numerous adverse impacts of climate change on the marine environment.¹⁴ The IACtHR described a current ‘climate emergency’, with ‘an unprecedented risk to both individuals and natural systems’, ‘caused by various anthropogenic activities, which incrementally affect and severely threaten humanity ‒ particularly those in situations of heightened vulnerability’.¹⁵

• Harms rise with the temperature: The courts also agreed that the consequences of climate change – in terms of adverse impacts and related loss and damage – ‘will escalate with every increment of global warming’,¹⁶ and are ‘higher for global warming of 1.5 °C than at present, and even higher at 2 °C’.¹⁷ Further, ‘[w]arming of 1.5 °C is not considered “safe” for most nations, communities, ecosystems and sectors and poses significant risks to natural and human systems.’¹⁸

• Different contributions: The courts also found that the GHG emissions from human activities vary across (i) regions and States, and (ii) activities. Specifically, the ICJ explained that ‘historical contributions of GHG emissions vary substantially across regions, and that differences remain today’, with least developed countries (LDCs) and small island developing States (SIDS) ‘having much lower per capita emissions of GHGs than the global average’.¹⁹ Likewise, the IACtHR explained that the contribution of States and regions is ‘markedly unequal’.²⁰ The ICJ noted that certain human activities have primarily caused the increase in GHG concentration, namely, the combustion of fossil fuels, followed by land-related activities (agriculture, deforestation), industrial processes, and waste management.²¹ In addition to making similar sector-specific findings,²² the IACtHR stated that 90 companies (the so-called ‘carbon majors’) are responsible for the majority of GHG emissions.²³

• Inequitable situation: ITLOS explained that developing countries vulnerable to the adverse effects of climate change face an ‘inequitable situation’: ‘Although they contribute less to anthropogenic GHG emissions, such States suffer more severely from their effects on the marine environment’.²⁴ The IACtHR explained, in broader terms, that ‘those who have contributed least to the climate crisis are those who have the fewest resources to confront its most devastating consequences and, therefore, it is they who suffer these consequences most intensely’.²⁵ The IACtHR, therefore, concluded that ‘[i]nequality, both between States and among their population, must be regarded as a key factor in understanding the climate crisis’.²⁶ The ICJ recognized that ‘certain States, in particular small island developing States, have faced and are likely to face greater levels of climate change-related harm owing to their geographical circumstances and level of development.’²⁷

• Mitigation efforts: The courts agreed that existing mitigation efforts are insufficient to limit global warming to 1.5° C.²⁸ Relying on the IPCC, the ICJ explained that, even with mitigation measures consistent with a very low GHG emission scenario, global warming will more likely than not reach 1.5° C before 2040; and the best estimate for 2081–2100 ranges between 1.4° C (very low emission scenario) and 4.4° C (very high emission scenario).²⁹ In addressing the need to reduce emissions, the IACtHR pointed to the concept of ‘carbon budget’, which determines the maximum amount of emissions that are possible to stay within a particular temperature goal (like 1.5° C).³⁰

• Urgency: The courts all agreed on the urgency of scaling up mitigation efforts. ITLOS cited the IPCC findings that, to limit global warming to 1.5° C, ‘[d]eep, rapid, and sustained’ GHG emission reductions are needed.³¹ Citing the IPCC, the ICJ further explained that there is a ‘rapidly closing window of opportunity to secure a liveable and sustainable future for all’; and the choices and actions implemented between 2020 and 2030 ‘will have impacts now and for thousands of years’.³² The IACtHR stressed that delays mean ‘transferring an extraordinary responsibility to future generations, and increase the risk of suffering the negative effects of climate change, particularly for the most vulnerable.’³³

Finally, it is worth noting that the courts did not take a position on when States (and others) were, or should have been, aware of the risks that human activities posed for the climate system. The ICJ explained that this question becomes relevant only when the responsibility of States is at issue.³⁴

3 Applicable Law, Interpretive Approaches, and Guiding Principles

3.1 Applicable Law and Interpretive Approaches

The courts analyzed States’ climate law obligations under different sources of international law. ITLOS focused on obligations under the UN Convention on the Law of the Sea (UNCLOS),³⁵ while the IACtHR addressed obligations under the American Convention on Human Rights and the Protocol of San Salvador.³⁶ For the interpretation of these treaty obligations, both courts relied on various other international law instruments, including the climate change treaties.³⁷ The ICJ, finally, decided to focus on what it considered to be the ‘most directly relevant’³⁸ international law obligations on climate change, namely, the UN Charter, the climate change treaties (UNFCCC, Kyoto, and Paris), UNCLOS, certain other environmental treaties,³⁹ customary international law (prevention and co-operation), and the ‘core’ human rights treaties (ICESCR and ICCPR) and human rights recognized under customary international law.

The ICJ and ITLOS rejected the lex specialis argument, advocated – in different shapes and forms – by certain high-emitting States in both proceedings. The thrust of this argument was that the climate change treaties exhaustively define States’ international climate change obligations to the exclusion of additional obligations from other sources. The courts rejected the lex specialis argument because (i) there is no conflict between the climate change regimes and other relevant sources of international law; and (ii) there is no discernible intention of the parties to the climate change treaties to displace rules from other sources.⁴⁰ The courts, therefore, held that the climate change treaties do not displace other rules, such as the prevention obligation, UNCLOS, and human rights; and that compliance with the climate change treaties does not automatically result in compliance with these other rules.⁴¹

Rather than displacing other rules, a series of ‘coexisting obligations inform each other’.⁴² This position is consistent with the generally applicable principle that, ‘when several rules bear on a single issue, they should, to the extent possible, be interpreted so as to give rise to a single set of compatible obligations’.⁴³ By applying this principle, the ICJ essentially wove the different parts of international law together, resulting in a tapestry of complementary mitigation obligations on States under international law.

In so doing, and to the surprise of many participants, the ICJ gave real teeth to the Paris Agreement obligations.⁴⁴ The ICJ found that, when applied in this more demanding way, compliance with that Agreement ‘suggests’ compliance with the customary prevention obligation, although the two sources ‘establish independent obligations that do not necessarily overlap’.⁴⁵ The ICJ’s finding also means that States – like the United States – cannot avoid stringent climate obligations by withdrawing from the Paris Agreement, as they continue to be subject to very similar obligations under customary international law, such as the prevention obligation.⁴⁶

3.2 Guiding Principles

For the interpretation and application of the most directly relevant applicable law, the ICJ took into account the following ‘guiding principles’: (i) common but differentiated responsibilities and respective capabilities (CBDR-RC); (ii) sustainable development; (iii) equity and intergenerational equity; and (iv) the precautionary approach or principle.⁴⁷ The ICJ did not take into account the polluter pays principle. We first discuss the CBDR-RC principle (Sect. 3.2.1), followed by the other principles, including the polluter pays principle (Sect. 3.2.2). In discussing these principles, we also address how they were treated by ITLOS and the IACtHR.

3.2.1 CBDR-RC

The principle of CBDR-RC was first expressed in the Rio Declaration, which states, in Principle 7, that ‘[i]n view of the different contributions to global environmental degradation, States have common but differentiated responsibilities’. The Declaration continues that ‘[t]he developed countries acknowledge the responsibility that they bear in the international pursuit of sustainable development in view of the pressures their societies place on the global environment and of the technologies and financial resources they command.’⁴⁸

Under this principle, the responsibilities of States differ in light of (i) their different factual responsibility for global environmental degradation, and (ii) their different technological and financial capabilities to address such degradation. Since 1992, CBDR-RC has been expressed in many other international declarations and treaties.⁴⁹

The principle of CBDR-RC is expressly referenced in the climate change treaties. The UNFCCC identifies CBDR-RC as a principle guiding states’ actions ‘to achieve the objective of the Convention and to implement its provisions’ (Article 3.1).⁵⁰ The Paris Agreement adds that the Agreement will be implemented to reflect CBDR-RC (Article 2.2), adding the phrase ‘in the light of different national circumstances’.⁵¹

Before ITLOS and the ICJ, some high-emitting developed countries took the position that the CBDR-RC principle has no role to play outside the climate change treaties, particularly under UNCLOS, the customary prevention obligation, and human rights. Developing countries contested this position.⁵²

ITLOS placed some reliance on CBDR-RC. The Tribunal ruled that ‘[i]n the context of marine pollution from anthropogenic GHG emissions, States with greater means and capabilities must do more to reduce such emissions than States with less means and capabilities’.⁵³ In this context, ITLOS first explained that CBDR-RC is a ‘key principle’ of the UNFCCC which is also reflected in the Paris Agreement.⁵⁴ It then observed that, although UNCLOS does not refer to CBDR-RC, it ‘contains some elements common to this principle’.⁵⁵ Specifically, Article 194(1) stipulates that UNCLOS Parties must take measures to prevent, reduce, and control marine pollution using ‘the best practicable means at their disposal and in accordance with their capabilities’.⁵⁶ ITLOS relied on this phrase to conclude that the obligation ‘accommodate[s] the needs and interests of States with limited means and capabilities’.⁵⁷

The IACtHR fully integrated the principle of CBDR-RC into its assessment. The IACtHR listed CBDR-RC among the ‘fundamental principles and obligations in the context of the climate emergency’ and differentiated mitigation obligations based on current and historical emissions, capabilities, and actual circumstances.⁵⁸ Finally, the ICJ labelled CBDR-RC a ‘cardinal principle of the climate change treaty framework’ and ‘a manifestation of the principle of equity’.⁵⁹ Whilst debated before, the ICJ explicitly confirmed that CBDR-RC ‘acknowledges […] the historical responsibility of certain States’, and on the other hand, ‘that the measures which can be expected from all States with respect to addressing climate change are not the same’.⁶⁰ The ICJ also clarified other important aspects of CBDR-RC:

• CBDR-RC reflects the need to ‘distribute equitably’ the burdens of the obligations to address climate change based on differences in: (i) responsibilities, that is ‘historical and current contributions to cumulative GHG emissions’;⁶¹ and (ii) ‘capabilities and national circumstances, including their economic and social development’.⁶²

• CBDR-RC places States on a ‘spectrum’, and does not merely draw a distinction between developed and developing countries.⁶³ On one end are ‘the most developed States’ which have contributed significantly to GHG emissions since the Industrial Revolution and have the resources and technical capacity to implement wide-ranging emission reductions.⁶⁴ On the other end are ‘those least developed states’ that have contributed only minimally and have limited capacity.⁶⁵ Without naming any States, the ICJ explained that there are countries ‘[i]n between’ the two ends, which ‘now contribute significantly to global GHG emissions and possess the capacity to engage in meaningful mitigation and adaptation efforts, as well as other States with significant resources and technical capabilities to contribute to addressing global climate change’.⁶⁶

• Given that all States are located somewhere on a spectrum, CBDR-RC does not exempt any State from climate obligations. Indeed, the ICJ confirmed that all States – even the smallest in terms of responsibility and capabilities – are subject to international mitigation obligations.⁶⁷

• CBDR-RC is a dynamic concept. With responsibilities and capabilities changing over time, so do the implications of CBDR-RC.⁶⁸ The implications of CBDR-RC for a State at any given time, therefore, depend ‘on an assessment of the [State’s] current circumstances’.⁶⁹

• CBDR-RC is relevant beyond the climate change treaties, including for the interpretation of treaties that do not reference CBDR-RC (such as UNCLOS); and beyond treaty law, for customary international law, including the prevention obligation, relating to the environment.⁷⁰

3.2.2 Other Guiding Principles

The ICJ concluded that the principle of sustainable development also guides the interpretation of applicable law, given its ‘continuous and uncontested universal recognition’ (e.g., in the climate treaties, the Rio Declaration, and UNGA resolutions).⁷¹ By reference to its prior case law, the ICJ defined sustainable development as the ‘need to reconcile economic development with protection of the environment’.⁷² This definition identifies only two of the three pillars on which the principle of sustainable development is generally understood to rest, omitting the third pillar of social development.⁷³ The IACtHR relied more strongly on sustainable development – and the SDGs more generally – to interpret States’ climate change obligations under human rights law, also emphasizing the social development pillar thereof.⁷⁴ According to the IACtHR, ‘the main obligation imposed on States to ensure the progressive realization of the human rights that are threatened or violated by climate change consists in promoting a transition towards on [sic] sustainable development.’⁷⁵

The ICJ also identified ‘equity’ and ‘intergenerational equity’ as relevant guiding principles.⁷⁶ The ICJ stressed that, like the other principles, these principles play a role infra legem (i.e., to interpret applicable law), not extra legem (i.e., to displace applicable law).⁷⁷ The ICJ explained that ‘equity’ could be used by a court to choose among possible interpretations the one that is ‘closest to the requirements of justice’.⁷⁸ Intergenerational equity is a manifestation of equity, expressing ‘the idea that present generations are trustees of humanity tasked with preserving dignified living conditions and transmitting them to future generations’.⁷⁹ In other words, when assessing possible interpretations in light of justice, the implications for future generations must be taken into account. The IACtHR also found that a State’s climate change obligations must be established on the basis of considerations of justice, such as those arising, among others, from the principle of intergenerational equity, which is ‘reinforced’ by the principle of intragenerational equity and CBDR-RC.⁸⁰

Next, like ITLOS and the IACtHR,⁸¹ the ICJ identified the ‘precautionary approach or principle’, as a relevant principle.⁸² In the words of the IACtHR, ‘the lack of full scientific certainty cannot be used as a reason for postponing the prevention of climate change and reducing its adverse effects’.⁸³ Precaution is relevant not only for the interpretation of the climate change treaties,⁸⁴ but also for the interpretation of other international climate obligations, such as those under UNCLOS, the prevention obligation, and human rights.⁸⁵

Finally, the ICJ dismissed the relevance of the polluter pays principle, despite its explicit expression in the Rio Declaration.⁸⁶ The ICJ did so because the principle is not reflected in the climate change treaties. At the same time, the ICJ pointed out that its finding does ‘not preclude the possibility that forms of strict liability for hazardous acts and other kinds of acts that are not wrongful under international law are developing’.⁸⁷ While some participants had argued otherwise, the ICJ ruled that it did not need to opine on the legal consequences of such acts in its opinion.⁸⁸ While ITLOS also did not rely on the polluter pays principle, the IACtHR considered it a ‘fundamental principle […]’ in the context of the climate emergency.⁸⁹ For the IACtHR, the polluter pays principle implies that States should impose stricter climate action obligations on high GHG-emitting companies.⁹⁰

4 Mitigation Obligations

In each proceeding, the three courts identified mitigation obligations flowing from the applicable law. Although each interpreted different sources of law, all three courts identified substantively very similar obligations requiring States to take effective action to reduce GHG emissions based on due diligence. We group their findings in Sect. 4.1. We thereafter discuss the duty to co-operate, including on mitigation action (Sect. 4.2). We finally discuss additional mitigation obligations identified by the courts under the applicable law (Sect. 4.3).

4.1 The Due Diligence Standard for Taking Mitigation Action

4.1.1 The Core Tenets of Due Diligence

The three courts identified obligations on States to take mitigation action based on due diligence.⁹¹ ITLOS did so under Articles 192 and 194 of UNCLOS, the IACtHR under the applicable human rights treaties, and the ICJ under the prevention obligation, Article 4 of the Paris Agreement, UNCLOS, and human rights law.⁹² Due diligence is not an obligation in itself, but rather the standard of conduct required of States to fulfill these treaty- or custom-based obligations.⁹³ We bring together the courts’ findings on the due diligence standard to mitigate emissions under the different sources of law. As Voigt explains, as a result of the courts’ interpretation, due diligence has now ‘emerged as a potent and powerful standard against which to assess compliance with international obligations’.⁹⁴

The courts agreed on key aspects of the due diligence standard in relation to mitigation action:

• Stringent: The courts agreed that the standard of due diligence for preventing significant harm to the climate system is ‘stringent’,⁹⁵ requiring a ‘heightened degree of vigilance and prevention’⁹⁶ with a State required to do ‘its utmost’⁹⁷ to prevent harm. This is because of the severe and widespread risks arising from climate change, which the ICJ referred to as ‘quintessentially universal’.⁹⁸

• Universal: Each State is subject to the obligation to take mitigation action under the prevention obligation. In line with the position taken in other climate cases, the courts rejected the so-called ‘drop in the ocean’ argument. The ICJ explained that a State is subject to the prevention obligation, even if its emissions are insignificant when considered in isolation.⁹⁹ The ICJ stressed that each State contributes to the risk through its emissions, and those emissions must be evaluated together with emissions from other States.¹⁰⁰ Doing so shows that accumulated GHG emissions are ‘causing significant harm to the climate system’, triggering the application of the prevention obligation to each State individually.¹⁰¹ For States subject to treaty-based due diligence-based obligations (such as the Paris Agreement, UNCLOS or the human rights treaties), these obligations apply to all parties to the treaty in question.¹⁰²

• Differentiated: As explained above, the courts accepted that, while all States are subject to due diligence obligations, the conduct required is more demanding for some States as compared to others.¹⁰³ Under the prevention obligation, the ICJ noted that ‘States contribute, in markedly different ways’ to the significant harm to the climate system (with States responsible under this obligation for harm resulting from activities undertaken within their jurisdiction or control).¹⁰⁴ In assessing the conduct required under the due diligence standard, the ICJ then discussed differentiation in the context of different capabilities, noting that CBDR-RC ‘must be taken into account’.¹⁰⁵ Under the climate change treaties, the ICJ explained that CBDR-RC recognizes that the conduct required of States may differ depending on parties’ ‘economic situations, their historic contribution to anthropogenic GHG emissions and their capabilities to adapt to and mitigate the adverse impacts of climate change’.¹⁰⁶ ITLOS differentiated obligations based on capabilities under the relevant UNCLOS provisions.¹⁰⁷ The IACtHR specified that each State should set a mitigation target based on factors linked to considerations of justice, including CBDR-RC and intra- and intergenerational equity: (i) its current and historical emissions; (ii) its capacity to adopt mitigation measures; and (iii) its actual circumstances.¹⁰⁸

• Science-based in light of the 1.5 °C temperature goal: The courts agreed that, in light of the best available science, mitigation action must be taken with a view to limiting atmospheric warming to 1.5 °C.¹⁰⁹ The IACtHR stated, even more ambitiously, that the 1.5 °C target is ‘a minimum starting point’ – not ‘the finishing line’ – for a State to determine its mitigation action, because significant harm already occurs at 1.5 °C. ¹¹⁰ The courts further agreed that, in determining appropriate mitigation action, States must rely on the best available science.¹¹¹ As a corollary, the ICJ explained that States must ‘actively pursue’ scientific information (taking into account CBDR-RC),¹¹² and ‘pursue technical co-operation and knowledge-sharing initiatives’ with other States.¹¹³ New scientific information (including from the IPCC) may, as a result, make the conduct required under the due diligence standard ‘more demanding’.¹¹⁴ With regard to mitigation action based on new technologies, the ICJ warned that ‘when technologies pose further risks, States are expected to use them with prudence and caution’.¹¹⁵

• Precaution-based: The courts also agreed that, in light of the precautionary approach, the absence of scientific certainty is no excuse to postpone mitigation action.¹¹⁶ The IACtHR, in particular, held that the precautionary principle is a ‘fundamental’ facet of the prevention obligation.¹¹⁷ It means, according to the IACtHR, that States are obliged to act ‘when they are, or should be, aware of the possibility that the acts or omissions of their agents or of private individuals may create a risk of severe and irreversible damage, within or outside their territory, even when they lack absolute certainty in this regard’.¹¹⁸

• Taking into account relevant international rules and standards: To act with the required level of due diligence, the courts agreed that a State must take into account relevant international rules and standards.¹¹⁹ The ICJ clarified that this includes binding and non-binding norms.¹²⁰ These could include, for example, relevant rules and standards under customary international law and treaties, but they may also be reflected in ‘certain [COP decisions] and in recommended technical norms and practices, as appropriate’.¹²¹

• With appropriate rules, combined with monitoring and enforcement: The courts agreed that appropriate mitigation action entails not only the adoption of appropriate rules, but also enforcement and monitoring.¹²² A State must take action that includes ‘regulatory mitigation mechanisms that are designed to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system.’¹²³ These ‘rules and measures must regulate the conduct of public and private operators within the States’ jurisdiction or control and be accompanied by effective enforcement and monitoring mechanisms to ensure their implementation.’¹²⁴ As further explained below, the IACtHR went further in specifying the types of measures which States must adopt.¹²⁵ ITLOS also identified enforcement obligations under UNCLOS.¹²⁶

• With environmental impact assessments (EIAs): Under the prevention obligation, the duty to exercise due diligence requires States to undertake EIAs for the purpose of preventing significant risk to the climate system.¹²⁷ While EIAs could be conducted at a general level (covering different forms of activities), the ICJ also considered it important for all States to conduct ‘EIAs with respect to particularly significant proposed individual activities’, including ‘for the purpose of assessing their possible downstream effects.’¹²⁸ The ICJ thus seemed to agree with other courts that individual EIAs should take into account a proposed project’s downstream emissions, which could also occur in other jurisdictions.¹²⁹

• With notification and consultation: The ICJ found that, under the prevention obligation, the notification and consultation of other States is ‘particularly warranted’ when an activity within the State’s jurisdiction or control significantly affects collective efforts to address harm to the climate system.¹³⁰ The ICJ noted that this could arise in case of ‘policy changes in relation to the exploitation of resources linked to GHG emissions’ or ‘with respect to information that is necessary for meaningful co-operation among States to address the adverse effects resulting from GHG emissions’.¹³¹

4.1.2 Regulating Private Actors

We explained above that the courts agreed on the core tenets of the due diligence standard: a State must do its utmost to reduce GHG emissions in order to keep atmospheric warming to 1.5 °C, and must do so based on science, precaution, and differentiation. The courts also agreed that this obligation necessarily includes a duty for a State to take regulatory action to address the emissions of private actors, such as companies and individuals, with the IACtHR taking the most demanding approach among the three courts.

The ICJ and ITLOS found that most GHG emissions are released by private actors.¹³² The ICJ, therefore, concluded that States must deploy ‘appropriate means to take domestic mitigation measures, including in relation to activities carried out by private actors’ that are subject to their jurisdiction or control.¹³³ Specifically, it added, States must regulate ‘both consumption and production activities’,¹³⁴ covering ‘the conduct of public and private operators within the States’ jurisdiction or control’.¹³⁵

The ICJ thus clarified that a State must regulate emissions by private actors, although it did not specify how a State must do so (nor how the burden of emission reduction should be shared among private actors). The choice of climate mitigation measures (e.g., carbon pricing, product or process standards, labeling, reporting, corporate due diligence, anti-greenwashing rules, and/or subsidies for green products and technologies) is, therefore, left to the discretion of each State, provided that the collection of measures chosen is sufficiently effective. ITLOS seemed to agree on the need to regulate emissions by private actors and likewise left the choice of instrument to each UNCLOS Party.¹³⁶

The IACtHR, however, made more specific findings.¹³⁷ It stressed more strongly the (differentiated) contributions by, and duties on, private actors regarding climate change. The Court is ‘convinced’ that private actors have an ‘essential role’ to play in addressing the climate emergency.¹³⁸ Referring to the work by the UN Working Group on Business and Human Rights, the Court held that companies too ‘have obligations and responsibilities’ with regard to climate change.¹³⁹ This implies, it said, that ‘States must establish such obligations’ on companies in their domestic regulatory regime ‘and ensure full compliance’.¹⁴⁰ The Court further specified certain climate mitigation measures which States ‘must’ establish:¹⁴¹

• Emission reduction obligations: (i) ‘urge all business enterprises domiciled or operating in their territory and jurisdiction to take effective measure to combat change and its related impacts on human rights’; (ii) ‘require business enterprises to take measures to reduce such emissions, and to address their contribution to the climate and to climate mitigation targets, throughout their operations’. The Court did not further specify which type of emission reduction obligations a State must impose (e.g., carbon pricing);¹⁴²

• Disclosure obligations: ‘require public and private businesses to disclose in an accessible way the [GHG] emissions along their value chain’;¹⁴³

• Due diligence obligations: ‘[e]nact legislation that requires business enterprises to conduct human rights and environmental due diligence to identify and address human rights and environmental impacts, including climate change-related impacts, across the entire value chain’.¹⁴⁴ The Court specified that ‘[t]he purpose is to identify, prevent, mitigate and, when applicable, remedy the possible adverse impacts of business activities on the environment or on human rights, pursuant to the United Nations Guiding Principles on Business and Human Rights and the most recent developments in international and comparative law’;¹⁴⁵ and

• Anti-greenwashing obligations: ‘adopt a range of regulations to discourage greenwashing and undue corporate influence in the political and regulatory domains in this regard, and to support the actions of human rights defenders’.¹⁴⁶

The IACtHR further specified that States should regulate some companies more than others ‘based on the actual and historical’ GHG emissions, with more stringent obligations on companies (including parent companies) generating higher GHG emissions.¹⁴⁷ These obligations may relate, among others, to business operating conditions, tax burden, and contributions to adaptation and to loss and damage.¹⁴⁸ The differences in obligations among companies ‘should be aimed at putting into practice the polluter pays principle and ensuring the effectiveness of domestic mitigation measures’.¹⁴⁹ This ‘should necessarily include measures to advance in progressive reduction of GHG emissions’ from high-emitting activities, such as ‘from fossil fuels, agriculture, livestock, deforestation, and other land use.’¹⁵⁰ Finally, States are obliged to ‘strictly supervise and monitor public and private activities that generate GHG emissions’, including, ‘at the very least’, ‘exploration, extraction, transportation and processing of fossil fuels, cement manufacture, agro-industrial activities, and other inputs used in those activities’.¹⁵¹

4.1.3 Regulating Fossil Fuels

Another pertinent question before the courts, in particular before the IACtHR and the ICJ, was whether States are obliged in international law to regulate fossil fuel-related activities.¹⁵²

The IACtHR responded affirmatively. States’ mitigation strategies ‘should necessarily include measures to advance in progressive reduction of GHG emissions from fossil fuels’ and must ‘strictly supervise and monitor […] exploration, extraction, transportation and processing of fossil fuels.’¹⁵³

For its part, the ICJ found that States ‘may’ violate international law by failing to take action to address fossil fuel-related activities.¹⁵⁴

During the ICJ’s written and oral proceedings, participants made limited submissions on whether international law obliges countries to address specifically fossil fuel-related activities, such as production, consumption, licensing, and subsidies. This may have prompted Judge Cleveland from the United States to ask an explicit question on this issue at the close of the oral proceedings.¹⁵⁵ The responses from participants were very diverse – ranging from an argument that States must phase out fossil fuels and fossil fuel subsidies (often while taking into account CBDR-RC) to denying the existence of any such obligations.¹⁵⁶

In its ruling, the ICJ itself did not explicitly address Judge Cleveland’s question. However, it made findings that are specifically relevant to the question.

First, the ICJ found that ‘most GHGs come from fossil fuel combustion’.¹⁵⁷

Second, as explained above, the ICJ confirms that States must undertake an EIA with respect to particularly significant GHG-emitting activities, which will include many fossil fuel projects, including their downstream emissions, and must notify and consult other States when doing so.

Third, the ICJ found that States ‘may’ violate their mitigation-related obligations if they fail ‘to take appropriate action to protect the climate system from GHG emissions ‒ including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies’.¹⁵⁸ To recall, it found that a State’s mitigation-related obligations require that a State do its utmost to reduce emissions to hold atmospheric warming to 1.5 °C, based on science and precaution, and taking into account CBDR-RC.¹⁵⁹ With the use of the word ‘may’, the ICJ captures the due diligence nature of this mitigation-related obligation, and the fact that the determination of whether a violation has occurred must be made on a case-by-case basis for each State: that is, whether a particular State has met the ‘stringent’¹⁶⁰ due diligence standard must be assessed in light of all relevant circumstances, including the State’s mitigation action with regard to fossil fuel and other GHG-emitting activities within the State’s jurisdiction or control and CBDR-RC. The ICJ added that a violation may ‘give rise to the entire panoply’ of legal consequences under the law of State responsibility, including a requirement ‘to revoke all administrative, legislative and other measures that constitute an internationally wrongful act of that State’.¹⁶¹

In their concurring joint declaration, Judges Bhandari and Cleveland ‘firmly agree[d]’ with the ICJ’s statements on fossil fuel activities, and held that States’ mitigation obligations ‘necessarily include State conduct related to fossil fuel production, consumption and infrastructure’.¹⁶² They argued that the Court could have been ‘more forceful’ in addressing these activities in light of the IPCC’s findings on fossil fuels.¹⁶³ Arguing that ‘global production of fossil fuels is on a collision course with the scientific consensus path forward for combating climate change’, the two judges concluded that, ‘as the [ICJ] recognizes’, States must ‘take full responsibility for’, and ‘aggressively redress’, ‘the contributions of production, licensing and subsidies of fossil fuels’, also taking into account CBDR-RC.¹⁶⁴

4.2 The Duty to Co-operate

Co-operation is a ‘paramount principle’, and a duty on States, that forms part of customary international law.¹⁶⁵ Co-operation is also ‘intrinsically linked’ to the customary prevention obligation, and is a ‘central obligation’ in the climate change and other relevant treaties (like UNCLOS), and governed by the principle of good faith.¹⁶⁶ The courts therefore agreed that, as well as taking individual action, States must take collective action to address emissions.¹⁶⁷ The courts clarified that the duty to co-operate is not merely procedural but also requires finance and technology transfers.

The ICJ emphasized that co-operation is particularly important given that the climate system is ‘a resource shared by all States’ ‒ a significant finding in itself – and climate change is ‘an issue of common concern’.¹⁶⁸ Hence, co-operation is ‘not a matter of choice for States but a pressing need and a legal obligation’.¹⁶⁹ The duty to co-operate is an obligation of conduct.¹⁷⁰ This duty applies to all States, though the obligation varies among States, ‘first and foremost’ in light of CBDR-RC.¹⁷¹

The ICJ stressed that ‘the specific character of climate change requires States to take individual measures in co-operation with other States’.¹⁷² Specifically, ‘[w]hile States have obligations to make individual contributions to collective efforts under the [prevention obligation], the interpretation and fulfilment of their substantive obligations under that duty must also take account of the situation of other States and, as far as possible, be fulfilled in co-operation with other States’.¹⁷³ States must ‘make good faith efforts to arrive at appropriate forms- [sic] of collective action’.¹⁷⁴ At the same time, the ICJ also stressed that the duty to co-operate is not discharged ‘only by the conclusion and fulfilment of treaties’.¹⁷⁵

The ICJ further stressed that the duty to co-operate takes on ‘special importance’ in keeping global warming below 1.5 °C.¹⁷⁶ Specifically, States ‘must co-operate to achieve concrete emission reduction targets or a methodology for determining contributions of individual States’.¹⁷⁷ This may involve a methodology for the sharing of the remaining carbon budget (RCB) available to keep global warming below 1.5 °C. The collective climate policy must be ‘based on an equitable distribution of burdens and in accordance with [CBDR-RC]’.¹⁷⁸ The ICJ also confirmed that co-operation requires the transfer of finance or technology,¹⁷⁹ and mentioned specific facets of co-operation, including financial assistance, and technology transfer and capacity building under the climate change treaties.¹⁸⁰ The ICJ clarified that ‘good faith co-operation’ in this context entails taking into account COP decisions pertaining to financial assistance, technology transfer and capacity building.¹⁸¹ The ICJ elaborated on treaty-based co-operation obligations under the Paris Agreement¹⁸² (and the UNFCCC¹⁸³), which are ‘reinforce[d]’ by the customary duty to co-operate.¹⁸⁴

Like the ICJ, the IACtHR stressed that the duty to co-operate is based on the principles of fairness and CBDR-RC,¹⁸⁵ specifying in detail what this duty entails, which includes inter alia: ‘(i) financial and economic aid to the least developed countries to contribute to a just transition; (ii) technical and scientific co-operation involving communication and common enjoyment of the benefits of progress; (iii) implementation of mitigation, adaptation and reparation actions that can benefit other States; and (iv) establishment of international forums and formulation of collaborative international policies’.¹⁸⁶

In addition, the IACtHR drew particular attention to the obligation of States to co-operate in ‘the promotion of an open and advantageous international economic system that leads to sustainable development, particularly for developing countries, thereby allowing them to better address climate change.’¹⁸⁷

ITLOS, finally, elaborated on obligations to co-operate under UNCLOS.¹⁸⁸ It opined that the obligation is ‘ongoing’, and co-operation must be ‘meaningful’ and undertaken in ‘good faith’ at global or regional level.¹⁸⁹ The duty is one of conduct, with compliance measured in ‘efforts’ and not ‘outcome’.¹⁹⁰ The Tribunal clarified that Parties must co-operate (i) to formulate international rules on an ongoing basis and based on scientific evidence; (ii) to promote studies, undertake scientific research, and exchange information/data, including on mitigation; and (iii) to establish scientific criteria on the basis of which international rules can be developed.¹⁹¹ In addition, States must ‘promote programmes of scientific, educational, technical and other assistance to developing States’¹⁹² for the protection of the marine environment, and the prevention, reduction, and control of marine pollution, with as ‘main recipients’ ‘those developing and least developed States that are most directly and severely affected by the effects of such emissions on the marine environment’.¹⁹³ Finally, States must also ‘take, through the international organizations of which they are members, the measures necessary to put into effect preferential treatment for developing States’ through funding, technical assistance, and pertinent services from international organizations.¹⁹⁴

4.3 Additional Mitigation Obligations

In this section, we provide a brief overview of the courts’ findings on additional mitigation obligations. We discuss findings on the obligations under the climate change treaties (drawing mainly on the ICJ), UNCLOS (drawing mainly on ITLOS) and human rights (drawing mainly on the IACtHR). We note that the ICJ also discussed obligations under other treaties.¹⁹⁵

4.3.1 Climate Change Treaties

The ICJ discussed, in turn, States’ mitigation (and other) obligations under the UNFCCC, the Kyoto Protocol, and the Paris Agreement. We focus on the ICJ’s findings on the Paris Agreement, as this ‘is the most recent legally binding universal instrument addressing the issue of climate change’ and it pursues (and is interpreted in light of) the UNFCCC’s objective and principles.¹⁹⁶

Significantly, in interpreting the climate treaties, the ICJ gave weight to decisions of the COP, the ‘supreme body’ for review of the climate treaties.¹⁹⁷ According to the ICJ, the COP decisions carry ‘legal effects’ in certain circumstances: (i) COP decisions can be ‘legally binding’ when the treaty so provides (e.g., under Article 4.8 of the Paris Agreement), in which case they establish obligations on the Parties; and (ii) COP decisions may constitute ‘subsequent agreements’ under Article 31.3(a) of the Vienna Convention on the Law of Treaties (VCLT) when they express an ‘agreement in substance between the parties’ relating to the interpretation of the underlying treaty, in which case they are taken into account in interpreting the treaty.¹⁹⁸ In addition, COP decisions can assist in identifying customary international law when they reflect State practice and indicate that States regard themselves as bound by a norm (opinio juris).¹⁹⁹ Finally, as explained above, COP decisions should be taken into account to meet the due diligence standard under the prevention obligation and under the duty to co-operate in good faith.²⁰⁰

The ICJ relied on COP decisions, as subsequent agreements, to support the important conclusion that 1.5 °C has become the ‘scientifically based consensus target’ or ‘primary temperature goal’ under the Paris Agreement, despite the explicit language in Article 2.1(a) (that is, well below 2 °C, with efforts to limit warming to 1.5 °C).²⁰¹ In his Declaration, Judge Tladi agrees with characterizing 1.5 °C as the ‘primary’ temperature goal, but does so with a more robust interpretative foundation, relying on elements besides the COP decisions.²⁰²

Turning to the specific mitigation obligations under Article 4.2 of the Paris Agreement, the ICJ gave what can be called normative ‘teeth’ to the obligations on Parties to (i) ‘prepare, communicate, and maintain’ Nationally Determined Contributions (NDCs); and, thereafter, (ii) implement the NDCs. These are not simply formalistic exercises in which States have unfettered discretion – States must meet minimum legal benchmarks.

Specifically, the ICJ held that the obligation to ‘prepare, communicate and maintain’²⁰³ is not met simply by formally preparing, communicating, and maintaining an NDC.²⁰⁴ This follows from a good faith interpretation of Article 4, consistent with the VCLT rules.

First, the content of an NDC ‘must satisfy certain standards’.²⁰⁵ To comply with Article 4, the ICJ identified the following minimum standards for an NDC:

• Reflect ‘its highest possible ambition’ (Article 4.3), and thus be ‘capable of making an adequate contribution’ to the 1.5 °C temperature goal, taking into account CBDR-RC (as confirmed by the need for developed countries to ‘continue taking the lead’, as provided by Article 4.4);²⁰⁶

• Become ‘more demanding’ over time, given the language in Article 4.3 that ‘[e]ach Party’s successive [NDC] will represent a progression’;²⁰⁷

• Be informed by ‘the outcomes of the global stocktake’ (Article 4.9), which shows that Parties are not yet collectively on track to meet the 1.5 °C temperature goal.²⁰⁸

As a result, the discretion of the Parties to prepare their NDC is ‘limited’, with a ‘stringent’ due diligence standard imposed on them.²⁰⁹

Second, under Article 4.2, second sentence, ‘Parties shall pursue domestic mitigation measures, with the aim of achieving the objectives of’ their successive NDCs, which entails a similarly stringent due diligence standard to proactively take domestic mitigation measures ‘that are reasonably capable of achieving the NDCs’.²¹⁰

In sum, as Judge Tladi succinctly explained, the ICJ ‘has not fallen into the trap of the unfettered discretion and has instead given a robust interpretation to the obligation to prepare and maintain NDCs’.²¹¹

As noted, there is significant overlap between these substantive obligations under the Paris Agreement and the prevention obligation. This means that core tenets of the due diligence standard, set out above, are relevant to Parties when preparing and implementing their NDC.²¹² It also means that if a Party prepares and implements an NDC that meets the obligations of the Paris Agreement and that Party meets the other obligations under the climate change treaties, a State will essentially be presumed to comply substantially with the prevention obligation.²¹³

4.3.2 UNCLOS

In discussing States’ obligations under UNCLOS, the ICJ ascribed ‘great weight’ to the ITLOS advisory opinion.²¹⁴ As a result, the ICJ and ITLOS opinions set out a closely aligned interpretation of the UNCLOS obligations with regard to climate change. We therefore focus on ITLOS.

ITLOS found that anthropogenic GHG emissions fall within the definition of marine pollution under UNCLOS, triggering the relevant obligations under the Convention.²¹⁵ ITLOS found that, under Articles 192 and 194, paras. 1 and 2, UNCLOS Parties are subject to stringent and differentiated due diligence obligations to reduce emissions.²¹⁶ The key elements of the Tribunal’s findings have been discussed above (Sect. 4.1 above).

To summarize features relating to the law of the sea, under Article 194(1), States are subject to a ‘stringent’ due diligence obligation to adopt measures that are ‘necessary’ to prevent, reduce and control marine pollution from anthropogenic GHG emissions, limiting global warming to 1.5 °C.²¹⁷ Necessary measures should be determined objectively, taking into account factors such as (i) science (on a precautionary basis); (ii) international rules and standards (e.g., climate change treaties, MARPOL, Chicago Convention, Montreal Protocol); and (iii) available means and capabilities.²¹⁸ The climate change treaties are, therefore, ‘relevant in interpreting and applying the Convention’, though UNCLOS obligations are not ‘satisfied simply by complying’ with the Paris Agreement.²¹⁹

The due diligence standard under Article 194(2) can be ‘even more stringent’ than under para. 1, because it imposes an obligation regarding transboundary harm.²²⁰ The Tribunal held that para. 2, which it said bears a ‘close resemblance’ to the customary prevention obligation, requires States to adopt necessary measures with regard to activities under their ‘jurisdiction or control’ (this is a ‘broad’ concept, which includes, for example, a State’s Exclusive Economic Zone (EEZ), continental shelf, and ships or aircraft registered in a State), in accordance with their available means and capabilities.²²¹

In addition, other UNCLOS provisions impose enforcement obligations with regard to specific sources of emissions, including from land-based activities, aircraft, and vessels.²²² Parties must, in particular, (i) adopt laws and regulations to reduce emissions (for land-based activities and aircraft, ‘taking into account’ internationally agreed rules and standards; for vessels, ‘at least have the same effect’ as that of ‘generally accepted’ international rules and standards);²²³ and (ii) enforce their laws and regulations.²²⁴

4.3.3 Human Rights

The connection between climate change and human rights is, by now, very well established and, indeed, the ICJ treated the human rights treaties as part of the ‘most directly relevant’ law for its opinion.²²⁵ The ICJ explained that the protection of the environment is a ‘precondition’ for the effective enjoyment of human rights.²²⁶ It held that the adverse effects of climate change ‘may significantly impair’ the enjoyment of multiple human rights, some of which protect individuals (e.g., right to life, right to health, right to an adequate standard of living, right to privacy, family and home) while others protect vulnerable groups (e.g., children, women, and indigenous groups).²²⁷

Participants disagreed on the existence of a right to a clean, healthy, and sustainable environment, though the majority argued in its favor.²²⁸ The ICJ explained that ‘the human right to a clean, healthy and sustainable environment is […] inherent in the enjoyment of other human rights’.²²⁹ In addition, although not crystal clear, the Court also seemed to acknowledge this right as self-standing under customary international law.²³⁰

On the territorial scope of human rights law, participants disagreed on whether States owe human rights obligations to people outside their territory. The ICJ treated this question cautiously. It confirmed its jurisprudence that, under certain treaties (e.g., the ICCPR), States owe human rights obligations when they have jurisdiction over people located outside their territory.²³¹ At the same time, the ICJ did not opine on when a State can be regarded as exercising its jurisdiction outside its territory because this depends on the terms of each treaty. However, it emphasized that ‘the scope of human rights treaty law and that of customary law are distinct.’²³²

The ICJ was relatively brief in its discussion of States’ obligations under human rights law. Given the adverse impact of climate change on human rights, it said that States ‘must’ take measures to protect the climate system, including necessary mitigation action (also in relation to private actor activities).²³³ It thereby wove together States’ human rights obligations with their obligations under other sources of international law;²³⁴ that is, as explained above (Sect. 3.1), the relevant obligations to tackle climate change under customary and treaty law ‘inform each other’.²³⁵ Hence, when implementing their obligations under other sources, States must take their human rights obligations into account. Equally, when implementing their human rights obligations, States must take their obligations under other sources into account.

The IACtHR developed in much more detail (and with less caution) the human rights obligations of States under human rights treaties, including regarding vulnerable groups.²³⁶ It concluded that the obligation not to create irreversible damage to the climate and the environment has the character of jus cogens – a finding not made by the ICJ.²³⁷Jus cogens norms protect fundamental values of the international community and are therefore universally applicable and hierarchically superior to other rules of international law. To reach this conclusion, the Court did not apply the criteria for identifying a jus cogens norm as set out in the work of the International Law Commission (ILC) and which the ILC drew from the jus cogens definition in Article 53 of the VCLT.²³⁸ Rather, for the Court, the jus cogens nature of this right logically flows from the fact that the balance of conditions for a healthy life in the Earth’s ecosystems is a ‘requirement’ – or necessary condition (sine qua non) – for the enjoyment of fundamental rights already protected by international law as norms from which derogation is not permitted.²³⁹

The Court also concluded that Nature is a subject of rights (a finding not made by the ICJ), based on Nature’s ‘structural role in the vital balance of the conditions that make this planet inhabitable’.²⁴⁰ Hence, Nature is not exclusively an object of property or an exploitable resource.²⁴¹ With this understanding, the Court underlined that States are not only subject to a duty to refrain from causing significant environmental harm, but also have ‘the positive obligation to adopt measures to guarantee the protection, restoration and regeneration of ecosystems’.²⁴²

Unlike the ICJ, the Court also answered the question of when a person falls within the jurisdiction of a State for the purpose of triggering human rights obligations. The Court confirmed its jurisprudence that when activities within the State’s jurisdiction or control (e.g., emissions) have caused harm to individuals outside its territory (which could arise as a result of emissions), a State owes human rights obligations to those individuals.²⁴³ These victims in third countries are considered to be under the State’s jurisdiction.

Turning to specific human rights, the Court considered that the right to a healthy environment (which is among the rights protected by the American Convention) is ‘the main right affected by climate change’²⁴⁴ and that the ‘human right to a healthy climate’ is an ‘independent right’, derived from the right to a healthy environment.²⁴⁵ Consistent with the principles of CBDR-RC and intra- and inter-generational equity, the right to a healthy climate must protect the global climate system for ‘the benefit of humanity as a whole’, comprising present and future generations.²⁴⁶ It means that States ‘should ensure an equitable distribution of the burden of climate action and climate impacts, taking into account their contribution to the causes of climate change and their respective capabilities’.²⁴⁷ Thus, States should avoid disproportionate burdens on both future and present generations.²⁴⁸

In the context of the climate emergency, the right to a healthy environment, and to a healthy climate in particular, translates into specific substantive mitigation obligations.²⁴⁹ Some key elements of these obligations were discussed above (Sect. 4.1). The IACtHR also discussed obligations resulting from procedural rights, including the obligation on States to provide climate-related information (and to fight disinformation) and to ensure effective access to justice.²⁵⁰

5 Implications

The implications of the ICJ opinion were well captured by Mr. Vishal Prasad, a young man from Fiji, who spoke at the hearing on behalf of the Pacific Island Students Fighting Climate Change. As a student, Mr. Prasad had led the initiative of Vanuatu students to ‘[take] the world’s biggest problem to the world’s highest court’.²⁵¹ Mr. Prasad said to the Court that ‘international law must serve as a compass for justice and accountability’.²⁵² Through their advisory opinions, the three courts have laid the foundations for international law to fulfill this role.

The high level of substantive convergence across the three advisory opinions (including the lack of dissenting opinions) reinforces their authority. The opinions demonstrate unequivocally that States are bound by a dense web of complementary mitigation obligations under interrelated sources of international law, including customary international law and treaty law (e.g., climate treaties, UNCLOS, and human rights instruments). The high level of substantive alignment in the mitigation obligations across these sources means that States cannot evade these duties – if they are not subject to a treaty, they are subject to customary international law. Further, the courts’ consistent factual findings, drawing heavily on the IPCC’s work, confirm that States must significantly increase their mitigation efforts to meet their demanding mitigation obligations.

In ‘putting flesh on the bones’ of States’ mitigation obligations, the courts also align on: (i) the objective to be achieved – limiting global warming to 1.5 °C; (ii) the stringent due diligence conduct required of each State to achieve this objective – based on differentiation, science, and precaution and taking into account relevant international rules and standards – including the need for specific actions such as the regulation of private actors (covering production and consumption), EIAs, and effective monitoring and enforcement; and (iii) the duty of States to co-operate in achieving this objective, including through the provision of financing and technology.

Compared to the other courts, the IACtHR went further in defining the specific mitigation actions that States must take to regulate the conduct of private actors (with stricter requirements on high-emitting companies). While the ICJ and ITLOS concur with the obligation to effectively regulate the private sector, they do not specify the particular means by which States must do so.

To meet their international obligations, States therefore have to adopt and enforce ambitious mitigation action.²⁵³ The three opinions are thus likely to contribute to the current global trend toward ‒ and soften the recent backlash in some jurisdictions against ‒ more demanding mitigation action by governments, companies and consumers. States may take action through policies such as carbon pricing, emission standards and labeling, climate-related disclosures, mandatory climate transition plans, due diligence, anti-greenwashing, green subsidies and procurement.

The opinions will likely influence the ongoing advisory proceedings on climate change before the AfCHPR and lead to increased litigation. They are part of a trend to take climate change to the courts, both national and international, with cases against governments and companies. Some cases seek to establish climate-related duties regarding future emissions (e.g., on climate transition plans), while others aim at responsibility for past emissions. The opinions will contribute to these trends toward climate litigation, with their clear findings on climate science, the duties of States, and the need for the regulation of private actors.

At the same time, the opinions provide a strong impetus – and even duty – for States to co-operate regionally and internationally to address climate change. The ICJ opinion shows that the COP is a valuable forum to make further co-operative progress, as its decisions may carry legal weight (at the same time, some countries may be more cautious in reaching COP decisions as the legal implications are now clearer).²⁵⁴ Of particular importance will be the efforts of States to co-operate on a methodology to allocate the remaining carbon budget to keep global warming below 1.5 °C, taking into account CBDR-RC. However, a failure by States to find a co-operative solution does not excuse individual States from taking ambitious mitigation action to meet the temperature goal, taking CBDR-RC into account.

The opinions also offer instructive reasoning relevant to other environmental challenges under international law. They are, for example, particularly relevant for countries seeking to break the deadlock in the ongoing plastics treaty negotiations. The ICJ’s opinion demonstrates how carefully crafted treaty obligations ‒ combined with a balanced set of principles and objectives ‒ can have legal teeth. At the same time, the opinions suggest that States could already be subject to demanding obligations under several sources of international law (e.g., customary international law, UNCLOS, and human rights treaties) to tackle plastic pollution, a consideration that must be factored into the negotiation of new treaty rules to address plastic pollution.

The opinions will also have ripple effects beyond Multilateral Environmental Agreements (MEAs). For WTO discussions on trade-related climate measures and policy, the courts have highlighted the vital importance of genuine and continuing co-operation, and the ICJ has identified guiding principles for action – including sustainable development, CBDR-RC, equity, intergenerational equity, and the precautionary approach.²⁵⁵ The ICJ also placed fossil fuel subsidies in the spotlight as a potential violation of international law. This may energize WTO efforts to develop rules on these subsidies. The opinions could also encourage countries to join the Agreement on Climate Change, Trade and Sustainability (ACCTS) – the first international agreement with specific obligations to reduce fossil fuel subsidies – or to develop similar plurilateral initiatives.²⁵⁶