Due Diligence,Obligations to Cooperate and to Regulate Private Actors: Insights from Three Climate Change Advisory Opinions

Abstract

This article analyses three noteworthy aspects of the 2024–2025 Advisory Opinions (AOs) on climate change from 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). Primarily, the article considers what the three AOs suggest about the variable, fact-dependent content of the various obligations to exercise due diligence falling upon States. It is argued that while the content of due diligence obligations varies depending upon various context-specific factors, including the capabilities of the State concerned, this does not lead to unmanageable indeterminacy nor provide excuses for climate inaction based on national circumstances. Rather, these international courts and tribunals (ICTs) have provided significant guidance on objectively reasonable approaches to implementing the relevant obligations and have emphasised that applying due diligence standards is a matter for objective determination. Secondly, the article considers what the AOs suggest about the continuing nature of States’ obligations to cooperate, which are not exhausted by specific climate-related treaties or processes. Third, the article demonstrates that all three AOs recognise and elaborate upon a duty of States to regulate effectively the activities of private, non-State actors that cause climate change.

Keywords

international courts and tribunals climate change common but differentiated responsibilities and respective capabilities due diligence obligations to cooperate obligations to regulate private actors

Introduction

During 2024–2025, in a remarkable set of judicial developments, three wide-ranging AOs concerning climate change were delivered by three prominent ICTs: the ICJ,¹ the ITLOS,² and the IACtHR.³ This article focuses on three noteworthy aspects of this recent trio of AOs. The bulk of this article analyses what the three AOs suggest about the variable, fact-dependent content of the obligations to exercise due diligence falling upon States. It is argued that while the content of obligations to exercise due diligence varies depending upon a variety of context-specific factors, including the circumstances and capabilities of the State concerned, this does not result in unmanageable indeterminacy nor provide excuses for climate inaction based on national circumstances. Instead, these ICTs have moved to contain the potentially subjective considerations introduced into legal reasoning, have provided guidance on what objectively reasonable approaches to implementing the various obligations look like, and have emphasised that applying due diligence obligations in a specific situation is a matter for objective determination.

This article addresses two further aspects of the AOs. First, it considers the ongoing or continuing nature of States’ obligations to cooperate for the protection of the environment, which are not exhausted by the conclusion of specific climate-related treaties or processes, and which sees the customary obligation to cooperate becoming a yardstick for determining whether existing treaty-based forms of climate cooperation remain adequate. Second, the article analyses the contribution of the three AOs regarding States’ obligations to regulate effectively the activities of private, non-State actors that cause climate change.

Due Diligence Obligations: Variable and Fact-Dependent Content

The claim this section develops is that while the three AOs have recognised that the application of obligations to exercise due diligence depends upon a variety of fact-dependent factors that may change over time, including the circumstances and capabilities of the State in question, ICTs have sought to contain the potentially subjective elements introduced into legal reasoning. Specifically, ICTs have identified markers of what objectively reasonable approaches to implementing the relevant obligations look like and have emphasised that applying due diligence obligations in a specific situation is a matter for objective determination, rather than falling within the discretion of the State concerned. It will be argued that in interpreting and applying fact-dependent obligations, such as obligations to exercise due diligence, ICTs navigate a tension between, on the one hand, taking account of context-dependent factors, such as the circumstances and capabilities of the State in question – which is required by the well-established principle of common but differentiated responsibilities and respective capabilities (CBDR&RC)⁴ – and, on the other, identifying a general standard of conduct that can provide normative guidance for all States.

The scholarly commentary on due diligence obligations has recognised that interpreting and applying such obligations in a specific situation requires taking account of a variety of fact-dependent considerations that are likely to change over time, including relevant scientific and technical information and the circumstances and capabilities of the State in question.⁵ For instance, Ollino notes that:

[f]actual criteria and the assessment of state conduct in light of the particular circumstances of the case shape the degree of diligence required of a state … These factual conditions contribute to the understanding of due diligence obligations as flexible duties, for what may be considered as diligent in a certain case may not be so under different facts.⁶

Ollino further notes that the level of a State's capabilities ‘is generally considered relevant for due diligence obligations related to protecting the environment’, in particular where the relevant primary rules refer to a State's capabilities or are ‘to be interpreted in light of the principle of “common but differentiated responsibilities”’.⁷ The variable or evolutive nature of due diligence obligations is also connected to the fact that obligations requiring States to exercise due diligence are typically of a continuing or ongoing nature,⁸ which means that what is sufficient to meet an obligation of due diligence at one point in time may not be sufficient at another point, when relevant circumstances have changed.

The following three subsections seek to substantiate the above arguments through an analysis of relevant aspects of the AOs rendered by the ICJ, the ITLOS and the IACtHR.

ICJ, Due Diligence and Objective Determinations

The ICJ initially considered the concept of due diligence when outlining States’ customary obligation to prevent significant environmental harm, noting that:

The conduct required by due diligence has several elements. These elements include States taking, to the best of their ability, appropriate and, if necessary, precautionary measures, which take account of scientific and technological information, as well as relevant rules and international standards, and which vary depending on each State's respective capabilities. Other elements of the required conduct include undertaking risk assessments and notifying and consulting other States, as appropriate.⁹

Drawing upon its earlier case law, the ICJ noted that ‘[t]he determination of what is required by due diligence ultimately “calls for an assessment in concreto” of what is reasonable under the specific circumstances in which a State finds itself’.¹⁰ At this point the Court already addressed the tension between taking account of fact-specific elements that inform what a standard of due diligence requires in a particular situation, and identifying an objective standard of conduct, applicable to all States. Specifically, the Court noted that

[t]his does not exclude the identification of a required standard of conduct at a general level, depending on the overall character of the risk to the part of the environment in question. … Climate change therefore poses a quintessentially universal risk to all States. This risk is of a general and urgent character, requiring the identification of a corresponding general standard of conduct, to be applied subject to the principle of common but differentiated responsibilities and respective capabilities.¹¹

Later in the AO, where the ICJ discussed in greater detail the content of States’ obligation to exercise due diligence to prevent significant harm to the environment, the Court again navigated a tension between on the one hand, taking account of fact-specific elements, including the circumstances and capabilities of the State in question, and, on the other, identifying a general standard of conduct applicable to all states and subject to objective determination. The ICJ began this section of its AO by observing that ‘[d]ue diligence is a standard of conduct whose content in a specific situation derives from various elements, including the circumstances of the State concerned, and which may evolve over time’.¹² Given the need to identify a general standard of conduct, capable of objective determination, the ICJ noted that ‘[t]he following elements are particularly relevant when it comes to determining what due diligence requires from a State in a particular situation, including in the context of climate change’.¹³ Discussing the various elements that inform what due diligence requires of a State in a particular situation, the Court noted that

[d]ue diligence also requires States to actively pursue the scientific information necessary for them to assess the probability and seriousness of harm, in conformity with the common but differentiated responsibilities and respective capabilities principle. On the other hand, where a State lacks the capacity to access and properly act on relevant scientific information, including when a State lacks necessary resources, failure to take appropriate preventive measures may not constitute a lack of due diligence.¹⁴

Perhaps unsurprisingly, the ICJ reasoned that ‘[t]he standard of due diligence may also become more demanding in the light of new scientific or technological knowledge’ and ‘the availability of technological means to prevent or mitigate relevant harm influences what can reasonably be expected of a State’.¹⁵

Addressing the different capabilities of States, as a factor that informs the standard of due diligence, the ICJ reasoned that

the obligation of a State “to use all the means at its disposal” to prevent significant harm to the environment … implies that the capabilities of a State are a key factor, as reflected in the principle of common but differentiated responsibilities and respective capabilities, for the determination of the applicable standard of due diligence in a particular situation.¹⁶

Ultimately, on the relevance of the different capabilities of States, the ICJ held that:

While developed States, in the context of climate change, must take more demanding measures to prevent environmental harm and must satisfy a more demanding standard of conduct, the standard required in each case ultimately depends on the specific situation of each State, namely “all the means at its disposal” … The difference between the respective capabilities of States, as one of the factors which determines the diligence required, cannot therefore merely result from a distinction between developed and developing countries, but must also depend on their respective national circumstances. The multifactorial and evolutive character of the due diligence standard entails that, as States develop economically and their capacity increases, so too are the requirements of diligence heightened. Finally, the reference to available means and capabilities cannot justify undue delay or a general exemption from the obligation to exercise due diligence.¹⁷

In concluding its consideration of the different elements that inform what due diligence requires in any particular situation, the ICJ emphasised that:

Having considered certain elements of the due diligence standard … the Court notes that their proper application in a specific situation may be a complex operation due to the variable and evolving nature of the standard. However, the Court considers that the relevant elements, individually and in combination, provide guidance for the identification of an appropriate standard of conduct for different situations. The Court is therefore of the view that the question whether a risk of significant harm exists and whether or how a relevant element of the obligation to exercise due diligence to protect the environment applies in a particular situation should be determined objectively.¹⁸

This passage is noteworthy as the ICJ is addressing the variable or evolutive and fact-dependent nature of due diligence obligations and emphasising that whether a risk of significant harm exists, and how a relevant element of States’ duty to exercise due diligence to protect the environment applies in a particular situation, are matters to be determined objectively. Put differently, rather than obligations of due diligence remaining entirely within the discretion of the State concerned, as ITLOS has noted, ‘[t]he content of an obligation of due diligence should be determined objectively under the circumstances, taking into account relevant factors’.¹⁹ Thus, as Besson has argued, ‘the variable character of due diligence does not make it a “subjective” standard … [nor] a profoundly “indeterminate” standard’.²⁰

Besides the above discussion of due diligence in the context of interpreting States’ customary obligation to prevent significant environmental harm, the ICJ also considered due diligence when interpreting States’ obligation to prepare, communicate, and maintain nationally determined contributions (NDCs) under the Paris Agreement, and in explaining why the content of NDCs is not entirely left to the discretion of Parties to the Paris Agreement.²¹ Specifically, the Court held that ‘the discretion of parties in the preparation of their NDCs is limited’ by a requirement to exercise due diligence and to ensure that their NDCs fulfil their obligations under the Paris Agreement, including the collective temperature goal.²² Against this backdrop, the ICJ recalled, citing earlier jurisprudence of ITLOS’ Seabed Disputes Chamber, that ‘the standard of due diligence varies depending on a range of factors … In the current context, because of the seriousness of the threat posed by climate change, the standard of due diligence to be applied in preparing the NDCs is stringent’.²³ Importantly, the ICJ held that

consistent with the varying character of due diligence and the principle of common but differentiated responsibilities and respective capabilities, the standard to be applied when assessing the NDCs of different parties will vary depending, inter alia, on historical contributions to cumulative GHG emissions, and the level of development and national circumstances of the party in question.²⁴

The ICJ also addressed due diligence in interpreting the obligation under the Paris Agreement for Parties to ‘“pursue domestic mitigation measures, with the aim of achieving the objectives of” their successive NDCs’.²⁵ Here the ICJ reasoned that

parties are required to act with due diligence in taking necessary measures to achieve the objectives set out in their successive NDCs. Thus, a party's compliance with its obligations to pursue domestic mitigation measures under Article 4, paragraph 2, is to be assessed on the basis of whether the party exercised due diligence in its efforts and in deploying appropriate means to take domestic mitigation measures.²⁶

Recalling again that ‘the standard of due diligence varies depending on the particular circumstances to which the standard applies’, the ICJ held that ‘the standard of due diligence attaching to the obligation to pursue domestic mitigation measures is stringent’, taking account of the best available science regarding the risks posed by climate change.²⁷ The ICJ also held that ‘the fulfilment of adaptation obligations of parties [to the Paris Agreement] is to be assessed against a standard of due diligence … parties must use their best efforts, in line with the best available science’.²⁸

This subsection has demonstrated that while the ICJ recognised that due diligence obligations have a variable content, which depends upon the particular factual circumstances in which they are applied and even the capabilities of the State in question, the Court also sought to contain the potentially subjective elements introduced into legal reasoning, and to identify a general standard of conduct, capable of guiding the conduct of all States and of objective determination.

ITLOS, Variable, Fact-Dependent Obligations and Objective Determinations

This subsection argues that the ITLOS, similarly to the ICJ, sought to contain subjective elements in its interpretation of the relevant obligations – often obligations requiring States to exercise due diligence – such that applying the relevant obligations remains a matter for objective determination, even if the obligations depend upon fact-specific considerations, including the capabilities of the State in question, which may change over time. In short, the ITLOS, in interpreting the relevant obligations, also sought to contain subjective elements and to identify markers of what an objectively reasonable approach to implementing the relevant obligations would look like. Accordingly, the recognition of various fact-dependent elements relevant to interpreting and applying the applicable obligations – often obligations requiring States to exercise due diligence – does not lead to unmanageable indeterminacy nor provide excuses for inaction based on national circumstances.

The first area where ITLOS sought to contain subjective elements was in its interpretation of the obligation under Article 194(1) of the United Nations Convention on the Law of the Sea (UNCLOS), which provides, insofar as relevant for present purposes, ‘States shall take … all measures consistent with this Convention that are necessary to prevent, reduce and control pollution of the marine environment from any source, using for this purpose the best practicable means at their disposal and in accordance with their capabilities’.²⁹ Here ITLOS noted that

[w]hile article 194, paragraph 1, of the Convention leaves it to each State to determine what measures are necessary to prevent, reduce and control marine pollution, this does not mean that such measures are whatever measures States deem necessary to that end. Rather, necessary measures should be determined objectively.³⁰

According to ITLOS

there are various factors States should consider in their objective assessment of necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions. It is evident that the science is particularly relevant in this regard. International rules and standards relating to climate change are another relevant factor. There are other factors that may be considered, such as available means and capabilities of the State concerned.³¹

Given the reference in Article 194(1) to ‘the best practicable means at their disposal and in accordance with their capabilities’, ITLOS accepted that ‘the scope and content of [the] necessary measures may vary depending on the means available to States and their capabilities, such as their scientific, technical, economic and financial capabilities’.³² While recognizing the flexibility that these aspects of Article 194(1) introduce, ITLOS emphasised that ‘the reference to available means and capabilities should not be used as an excuse to unduly postpone, or even be exempt from, the implementation of the obligation to take all necessary measures under article 194, paragraph 1’.³³ Overall, ITLOS considered that:

while the obligation under article 194, paragraph 1, of the Convention does not refer to the principle of common but differentiated responsibilities and respective capabilities as such, it contains some elements common to this principle. Thus, the scope of the measures under this provision, in particular those measures to reduce anthropogenic GHG emissions causing marine pollution, may differ between developed States and developing States. At the same time, it is not only for developed States to take action, even if they should “continue taking the lead”. All States must make mitigation efforts.³⁴

There is an obvious parallel here to the ICJ AO, which found that the principle of CBDR&RC was relevant for interpreting States’ obligations under the Paris Agreement and other environmental treaties and for determining the content of their obligations under customary international law,³⁵ and which emphasised that ‘the reference to available means and capabilities cannot justify undue delay or a general exemption from the obligation to exercise due diligence’.³⁶

Having found that the obligation under UNCLOS Article 194(1) ‘requires States to act with “due diligence” in taking necessary measures to prevent, reduce and control marine pollution’,³⁷ ITLOS observed that:

In the words of the Seabed Disputes Chamber in the Area Advisory Opinion, due diligence is a “variable concept” … It is difficult to describe due diligence in general terms, as the standard of due diligence varies depending on the particular circumstances to which an obligation of due diligence applies. There are several factors to be considered in this regard. They include scientific and technological information, relevant international rules and standards, the risk of harm and the urgency involved. The standard of due diligence may change over time, given that those factors constantly evolve.³⁸

In light of the high risk posed by the problem of climate change, according to the best available science, ITLOS held that ‘the standard of due diligence States must exercise in relation to marine pollution from anthropogenic GHG emissions needs to be stringent’.³⁹ Nevertheless, ITLOS accepted that:

its implementation may vary according to States’ capabilities and available resources. Such implementation requires a State with greater capabilities and sufficient resources to do more than a State not so well placed. Nonetheless, implementing the obligation of due diligence requires even the latter State to do whatever it can in accordance with its capabilities and available resources to prevent, reduce and control marine pollution from anthropogenic GHG emissions.⁴⁰

Ultimately, concluding the analysis in relation to UNCLOS art 194(1), ITLOS held that

States Parties to the Convention have the specific obligations to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions … Such measures should be determined objectively, taking into account, inter alia, the best available science and relevant international rules and standards contained in climate change treaties such as the UNFCCC and the Paris Agreement, in particular the global temperature goal of limiting the temperature increase to 1.5°C above pre-industrial levels and the timeline for emission pathways to achieve that goal. The scope and content of necessary measures may vary in accordance with the means available to States Parties and their capabilities.⁴¹

ITLOS also discussed the variable nature of due diligence obligations, and sought to contain the subjective elements introduced, in its interpretation of other applicable obligations under Part XII of UNCLOS. For example, in relation to Article 194(2) of UNCLOS – an obligation to prevent transboundary harm – ITLOS held that ‘this obligation is an obligation of due diligence, and its implementation may vary in relation to several factors, including the capabilities of each State’⁴² and the risks of harm involved in the relevant activities.⁴³ Ultimately, ITLOS emphasized that:

an obligation of due diligence should not be understood as an obligation which depends largely on the discretion of a State or necessarily requires a lesser degree of effort to achieve the intended result. The content of an obligation of due diligence should be determined objectively under the circumstances, taking into account relevant factors.⁴⁴

Another example of ITLOS recognising the relevance of context-specific considerations, and trying to contain the discretion and subjectivity introduced into legal reasoning, is in relation to the interpretation of the obligation under Article 194(5) of UNCLOS. Interpreting this provision, ITLOS found that:

[t]hese measures are context-specific and call for objectively reasonable approaches to be taken on the basis of the best available science. Their implementation depends on the relevant domestic legal system and allows for the exercise of discretion. However, States do not have absolute discretion with respect to the action that is required. As stated by the Seabed Disputes Chamber in the Area Advisory Opinion, a “State must take into account, objectively, the relevant options in a manner that is reasonable, relevant and conducive to the benefit of mankind as a whole. It must act in good faith, especially when its action is likely to affect prejudicially the interests of mankind as a whole” (Responsibilities and obligations of States with respect to activities in the Area, Advisory Opinion, 1 February 2011, ITLOS Reports 2011, p. 10, at p. 71, para. 230). Although the Seabed Disputes Chamber addressed the specific obligations of sponsoring States under article 4, paragraph 4, of Annex III to the Convention, the Tribunal finds that the views it expressed are also applicable to measures taken to protect and preserve the marine environment in relation to the impacts of climate change and ocean acidification.⁴⁵

The key claim this subsection has developed is that while the ITLOS recognised that the application of the relevant obligations depended upon various fact-specific considerations that may change over time, including the circumstances and capabilities of the State in question, it nevertheless sought to contain the flexibilities recognised and to identify markers of what an objectively reasonable approach to implementing the relevant obligations would look like. In short, similarly to the ICJ, the ITLOS although recognising the variable and fact-dependent nature of the applicable obligations – often obligations requiring States to exercise due diligence – sought to avoid such obligations leading to unmanageable indeterminacy or providing excuses for inaction based on national circumstances.

IACtHR on Due Diligence and States’ Discretion in Addressing Climate Change

The IACtHR, in its Advisory Opinion AO-32/25, in interpreting States’ obligations to prevent human rights violations resulting from environmental damage, navigated a tension between, on the one hand, taking account of fact-dependent considerations that may vary over time, including the circumstances of the State concerned, and, on the other, providing guidance on a generally applicable standard of conduct. As we will see, this tension often arose in interpreting obligations that require States to exercise due diligence.

Already in its discussion of States’ obligation to prevent severe or irreversible environmental damage, the Court noted ‘[e]ven if the measures that should be adopted to comply with the obligation of prevention vary according to the right that is sought to protect and the circumstances of each State Party, the Court has established certain minimum obligations in relation to the prevention of human rights violations resulting from environmental damage’.⁴⁶ Citing its earlier case law, the IACtHR held that:

the standard of due diligence must be appropriate and proportionate to the level of risk of environmental damage. Thus, the Court concurs with the International Tribunal for the Law of the Sea in that due diligence is a variable concept that depends on the specific circumstances, available scientific and technological information, relevant international rules and standards, the risk of harm, and the urgency involved.⁴⁷

The Court held that given existing information in relation to the climate emergency,

the extreme gravity of climate impacts and … the urgency of effective measures to avoid irreparable harm to the individual … States must act with enhanced due diligence to comply with the obligation of prevention arising from the obligation to guarantee the rights protected by the American Convention in the context of the climate emergency.⁴⁸

Importantly for present purposes, the IACtHR recognised that ‘[t]he content of the standard of due diligence will depend on the specific risks to each of the human rights that are threatened in the context of the climate emergency’, and ‘[t]he way in which enhanced due diligence is put in practice in each case will depend on the specific risk faced, the measures required to protect the threatened rights, and the possible situation of vulnerability of the victims’.⁴⁹ While these factors all emphasise the variable and fact-dependent content of due diligence obligations, the Court then set out a list of actions that ‘in general, enhanced due diligence requires’ of State, namely:

(i) identification and thorough, detailed and in-depth assessment of the risks; (ii) adoption of proactive and ambitious preventive measures to avoid the worst climate scenarios; (iii) utilization of the best available science in the design and implementation of climate actions (infra paras. 486–487); (iv) integration of the human rights perspective into the formulation, implementation and monitoring of all policies and measures related to climate change to ensure that they do not create new vulnerabilities or exacerbate preexisting ones (infra paras. 341, 342 and 388); (v) permanent and adequate monitoring of the effects and impacts of the adopted measures (infra paras. 349, 354, 355, 366, 389, 528, 560, 578 and 579); (vi) strict compliance with the obligations arising from procedural rights (infra paras. 471–587.), in particular, access to information, participation, and access to justice; (vii) transparency and accountability in relation to State climate action; (viii) appropriate regulation and supervision of corporate due diligence (infra paras. 345–351), and (xi) enhanced international cooperation, particularly regarding technology transfer, financing, and capacity-building.⁵⁰

In the following paragraph, the Court returned to the point that the measures required of States by a due diligence standard may vary over time, based on a range of relevant fact-specific considerations, noting:

The necessary measures to comply with this standard may vary over time, for example, based on scientific discoveries, new technologies, or the identification of new forms of risk. The scale and cost of those measures are also determined by the principle of common but differentiated responsibility, as well as by the particular circumstances of each State. However, the existence of this obligation does not depend on States’ level of development; in other words, the obligation of prevention applies equally to developed and developing States, without prejudice to the details relating to the obligation to cooperate and the principle of common but differentiated responsibilities that will be referred to subsequently.⁵¹

What the above discussion suggests is that the IACtHR, like the ITLOS and the ICJ, was navigating a balance between, on the one hand, acknowledging the variable and fact-dependent content of due diligence obligations, including given the different circumstances and capabilities of States and the principle of CBDR&RC, and, on the other, identifying a general standard of conduct for all States, whose application is subject to objective determination.

There are additional aspects of the IACtHR's AO that also suggest that the Court was navigating a balance between identifying objective requirements that States’ approaches to addressing climate change must comply with and recognising the discretion retained by States and the potential for measures to vary depending upon the differing circumstances and capabilities of States. For example, in the context of discussing States’ obligations to define a mitigation target for GHG emissions, the Court held that:

in addition to taking into account the best available science, the mitigation target should be established based on considerations of justice, such as those emanating from the principles of common but differentiated responsibilities and intra- and intergenerational equity … Consequently, the scale of each State's mitigation should be determined based on: (i) its current and historical cumulative contribution to climate change; (ii) its capacity to contribute to mitigation measures and, finally, (iii) its actual circumstances.⁵²

The Court further held that ‘[t]he mitigation target should also be based on: (ii) the capabilities of each State. This condition indicates that, in addition to the current and historical contribution to climate change, each State should establish its mitigation target based on the resources it possesses’.⁵³ Here the Court outlined various factors to guide how the capabilities of States should impact mitigation targets.⁵⁴ Additionally, according to the Court, ‘[i]t is also necessary to take into consideration: (iii) the circumstances of each State’ and the Court identified a variety of relevant circumstances.⁵⁵ While many of these factors highlight the variable and fact-dependent nature of States’ obligations, the IACtHR was also clearly identifying criteria that are intended to guide what States’ mitigation measures should look like.⁵⁶

Turning to States’ obligation to adopt domestic measures to achieve mitigation targets, the Inter-American Court again navigated a balance between recognising the discretion retained by States and setting out criteria that are intended to guide domestic mitigation measures. Specifically, the Court held that while

each State should define its own strategy to achieve the mitigation target it has established (supra para. 327) and adopt binding measures to comply with it (supra paras. 328–332). However, bearing in mind, the foregoing (supra paras. 333–334), both the definition and the implementation of this strategy should be carried out in keeping with the standard of enhanced due diligence. According to this standard, the strategy should meet certain procedural and substantive requirements. The former arise from the content of procedural rights; the latter focus on the coherence and effectiveness of the necessary conditions, measures, time frames and resources to implement the relevant strategy.⁵⁷

The Court held that ‘mitigation strategies should necessarily include measures to advance in progressive reduction of GHG emissions from fossil fuels, agriculture, livestock, deforestation, and other land use, as well as to eliminate SLCP emissions as quickly as possible’.⁵⁸

Yet another instance where the IACtHR navigated a tension between fact-specific and variable elements on the one hand, and identifying a general standard of conduct on the other, was in its discussion of States’ obligation to establish adaptation plans. Here the Court held that:

as part of their duty to guarantee human rights affected by the climate emergency, States are required to define and regularly update their national adaptation plans. This obligation to define and update such plans is of immediate enforceability. However, the implementation of the measures contained therein is, by its nature, progressive and must be adapted to national capacities, scientific advances, and evolving circumstances.⁵⁹

While setting out a wide variety of criteria that States’ adaptation plans must comply with,⁶⁰ the Court also acknowledged that ‘[g]iven that adaptation measures must respond to the particular needs of each State, their definition and updating, as well as the prioritisation and phasing of their adoption, must be determined by the competent national authorities’.⁶¹ Overall, this subsection has argued that in a broadly comparable manner to the ICJ and the ITLOS, the IACtHR also navigated a tension between acknowledging the variable and fact-dependent nature of the obligations at stake, many of which require States to exercise due diligence, and outlining general criteria that are intended to guide, and to provide a framework for evaluating, States’ responses to climate change.

More generally, this section of the article has argued that looking across the three existing AOs on climate change, ICTs have navigated a tension between recognising the variable and fact-specific nature of the relevant obligations – many of which require States to exercise due diligence – and setting out a general standard of conduct that can guide the conduct of all States. Therefore, it appears that due diligence obligations, as interpreted in the three AOs, do not become unworkably indeterminate nor end up providing excuses for climate inaction based on national circumstances and capabilities.⁶² Rather, ICTs have identified factors to guide what objectively reasonable approaches to implementing the relevant obligations look like and have emphasised that applying a standard of due diligence in a specific situation is a matter for objective determination. Overall, as Voigt has contended, one of the most important contributions of the three AOs is to identify ‘a comprehensive set of factors to determine due diligence’.⁶³ The identification and elaboration of such factors means that while ‘[d]ue diligence is a variable and evolving concept … it does not escape determination and concretization’.⁶⁴ Thus, as Voigt argues, ‘[d]ue diligence … has now emerged as a potent and powerful standard against which to assess compliance with international obligations. There is no hiding behind discretion and sovereign entitlements anymore.’⁶⁵

Obligations to Cooperate: Ongoing or Continuing Nature

The claim this section develops is that the AOs have highlighted that States have obligations to cooperate in relation to climate change that are of an ongoing or continuing nature, meaning that the conclusion of specific climate-related treaties or processes (e.g., the UN Framework Convention on Climate Change (UNFCCC) or the Paris Agreement) does not exhaust such obligations once and for all. Rather, States have an obligation to make ongoing efforts to cooperate, and the customary duty to cooperate for the protection of the environment may serve as a yardstick for assessing whether existing forms of treaty-based climate cooperation remain adequate.⁶⁶

For example, the ITLOS held that the obligation to cooperate under Article 197 of UNCLOS

is of a continuing nature. It requires States to make an ongoing effort to formulate and elaborate rules, standards and recommended practices and procedures. The adoption of a particular treaty, such as the UNFCCC or the Paris Agreement, does not discharge a State from its obligation to cooperate, as the obligation requires an ongoing effort on the part of States in the development of new or revised regulatory instruments, in particular in light of the evolution of scientific knowledge.⁶⁷

More generally, the ITLOS concluded that:

articles 197, 200 and 201, read together with articles 194 and 192 of the Convention, impose specific obligations on States Parties to cooperate, directly or through competent international organizations, continuously, meaningfully and in good faith in order to prevent, reduce and control marine pollution from anthropogenic GHG emissions.⁶⁸

The ICJ's AO goes into greater detail on the obligation to cooperate, engaging with States’ customary obligation to cooperate for the protection of the environment, in addition to treaty-based obligations to cooperate (e.g., under the UNCLOS).⁶⁹ Specifically, the ICJ held that:

[t]he duty to co-operate requires sustained and continuous forms of co-operation, of which treaties and their coordinated forms of implementation are a principal expression. While States are not required to conclude treaties, they are required to make good faith efforts to arrive at appropriate forms- of collective action. In the field of climate change, this requires agreement on forms of bona fide co-operation, such as those contained in the Paris Agreement. However, this does not mean that States discharge their duty to co-operate only by the conclusion and fulfilment of treaties.⁷⁰

Interestingly, the ICJ drew upon the customary duty to cooperate to protect the environment as a yardstick for assessing whether existing treaty-based forms of climate cooperation remain adequate. Specifically, the ICJ held that:

[t]he customary duties of States to co-operate and to prevent significant harm to the environment constitute a legal standard for determining whether any existing forms of co-operation, including treaties and their implementation, still serve their purpose and whether further collective action must be undertaken, including the establishment of further treaty-based obligations.⁷¹

Later in its AO, the ICJ, like the ITLOS, held that the duty to cooperate under Article 197 of UNCLOS is an obligation:

of a continuing nature and it requires States parties, inter alia, to make ongoing efforts to formulate and elaborate rules, standards and recommended practices and procedures. The adoption of treaties for the protection of the climate system, such as the UNFCCC or the Paris Agreement, does not release States parties from this requirement under UNCLOS … Moreover, the Court agrees with ITLOS that Article 197 does not exhaust the obligation to co-operate under Section 2 of Part XII of UNCLOS. In the view of the Court, States are also required to co-operate under Articles 200 and 201 of the Convention.⁷²

The upshot of the ITLOS’ and the ICJ's reasoning is that obligations to cooperate to protect the environment, whether under customary international law, or under various provisions of Part XII of UNCLOS, are complementary rather than mutually exclusive, and, importantly, are not exhausted by the conclusion of specific climate-related treaties (e.g., the UNFCCC or the Paris Agreement). Instead, fulfilment of such obligations to cooperate requires ongoing efforts by States, and the customary duty to cooperate for the protection of the environment provides a yardstick for determining whether existing treaty-based forms of cooperation in relation to climate change remain adequate.

Conduct of Private Actors: Obligations of States to Regulate

This article now turns to States’ obligations to regulate effectively the conduct of private actors in relation to climate change. Specifically, it considers what the three AOs suggest about States’ obligations, whether under the Paris Agreement, other treaties (e.g., UNCLOS or human rights treaties), or under customary international law, to regulate the activities of private actors that cause climate change.

Already in considering the material scope of the questions posed, the ICJ found that ‘[t]he Court's inquiry must therefore have a broad material scope encompassing States’ obligations concerning all actions or omissions of States, and of non-State actors within their jurisdiction or effective control’ that adversely affect the climate system through GHG emissions.⁷³ The Court then addressed the obligation to regulate private actors in considering States’ obligation under Article 4(2), second sentence, of the Paris Agreement to ‘pursue domestic mitigation measures, with the aim of achieving the objectives of’ their successive nationally determined contributions (NDCs). The ICJ reasoned that ‘the obligation to pursue domestic mitigation measures is an obligation of conduct’ and is subject to a standard of due diligence.⁷⁴ Importantly, the Court held that ‘a party's compliance with its obligations to pursue domestic mitigation measures under Article 4, paragraph 2, is to be assessed on the basis of whether the party exercised due diligence in its efforts and in deploying appropriate means to take domestic mitigation measures, including in relation to activities carried out by private actors’.⁷⁵

The ICJ further considered States’ obligation to regulate private actors in the context of elaborating upon the content of States’ customary obligation to prevent significant harm to the environment.⁷⁶ Specifically, the ICJ held that in relation to the adoption of appropriate rules and measures, in order to fulfil the customary obligation to exercise due diligence, such ‘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’.⁷⁷

The ICJ made further remarks on the obligation to regulate private actors in the part of the AO that considered attribution issues in the context of determining State responsibility in the context of climate change. While ‘[s]ome participants … argued that the conduct of private actors resulting in emissions of GHGs is not attributable to States’, the Court found this argument inapposite:

In relation to private actors, the Court observes that the obligations it has identified … include the obligation of States to regulate the activities of private actors as a matter of due diligence. Therefore, attribution in this context involves attaching to a State its own actions or omissions that constitute a failure to exercise regulatory due diligence. In such circumstances, the question of attributing the conduct of private actors to a State does not arise. The legal standard to assess compliance with the obligation to regulate, as well as the nature of the actions or omissions that lead to attribution, has been set out by the Court in several cases … Thus, a State may be responsible where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.⁷⁸

In its AO, the ITLOS considered the obligation to regulate private actors, in the context of discussing the obligation of States under UNCLOS Article 194(1) to ‘act with “due diligence” in taking necessary measures to prevent, reduce and control marine pollution’.⁷⁹ Specifically, ITLOS reasoned that ‘[t]his obligation of due diligence is particularly relevant in a situation in which the activities in question are mostly carried out by private persons or entities. The obligation to regulate marine pollution from anthropogenic GHG emissions is a primary example in this respect.’⁸⁰ ITLOS also found that States’ obligation under Article 194(2) of UNCLOS, in relation to transboundary pollution, covers ‘activities carried out by both public and private actors’.⁸¹ Interpreting the obligation to protect and preserve the marine environment under Article 192 of the UNCLOS, the ITLOS found that this obligation ‘requires States to ensure that non-State actors under their jurisdiction or control comply with such measures. The obligation of the State, in this instance, is one of due diligence.’⁸² Additionally, the ITLOS held that in relation to the obligation to conduct environmental impact assessments under Article 206 of the UNCLOS, which refers to ‘planned activities under … [a State's] jurisdiction or control’, ‘[t]he activities under assessment comprise both those planned by private entities and those planned by States’.⁸³

The IACtHR engaged in the most detailed discussion of the obligation to regulate the conduct of private actors in relation to climate change.⁸⁴ The IACtHR considered the obligation to regulate corporate conduct as a component of States’ obligation to regulate climate mitigation, in the context of considering climate mitigation obligations arising from the right to a healthy climate.⁸⁵ The Court was ‘convinced that business enterprises are called on to play an essential role in addressing the climate emergency’, and considered that ‘not only States, but also business enterprises “have obligations and responsibilities with respect to climate change, and [their] impacts […] on human rights”’.⁸⁶ The IACtHR then enunciated a variety of more specific obligations of States to regulate corporate conduct in relation to climate change, holding that:

States must: (i) urge all business enterprises domiciled or operating in their territory and jurisdiction to take effective measure to combat [climate] change and its related impacts on human rights; (ii) Enact 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; (iii) require public and private businesses to disclose in an accessible way the greenhouse gas emissions along their value chain; (iv) 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, and (v) 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.⁸⁷

Interestingly, somewhat analogously to the variable nature of due diligence obligations, the IACtHR held that ‘States should establish differentiated obligations with regard to climate action based on the actual and historical contribution of business enterprises to climate change, and impose stricter obligations on those whose activities are major sources of GHG emissions’.⁸⁸ As Peel has noted, ‘[i]n effect this transposes the international climate law principles of states’ “common but differentiated responsibilities” in addressing climate change and “polluter pays” to the context of corporate climate accountability’.⁸⁹

The IACtHR also recognised certain climate-related activities that States must supervise and monitor as a minimum standard, finding:

States are obliged to strictly supervise and monitor public and private activities that generate GHG emissions, as established in their mitigation strategy. Although the activities that are supervised and monitored vary from one State to another, it is the State's duty to supervise and monitor, at the very least: exploration, extraction, transportation and processing of fossil fuels, cement manufacture, agro-industrial activities, and other inputs used in those activities.⁹⁰

The IACtHR also identified obligations of States to require assessment of the climate impacts of projects ‘regardless of whether a project is implemented directly by the State or by private natural or legal persons’.⁹¹ According to the Court:

those projects or activities that pose a risk of generating significant GHG emissions should be subject to a climate impact assessment … the first duty of States in this regard is to identify, in keeping with its mitigation strategy, which projects and activities require the adoption of an environmental impact assessment that adequately examines their climate impact. This identification may be made through an initial study, or by domestic regulations establishing the activities subject to this assessment. In the case of such activities, the environmental impact assessment must compulsorily include a section that evaluates climate impact in a way that clearly differentiates this impact.⁹²

The Court then further elaborated on States’ obligations to regulate both the content and the process of environmental impact assessments, addressing numerous different requirements.⁹³ Finally, the IACtHR also identified obligations of States to require businesses to disclose a variety of information regarding the climate impacts of their activities.⁹⁴ Thus, all three AOs have made noteworthy contributions in identifying and fleshing out minimum standards regarding how States must regulate the activities of private, non-State actors that cause climate change. In short, the three AOs recognise an obligation of States to regulate effectively the activities of non-State actors that cause climate change and add substantial detail regarding what such effective regulation of the activities of non-State actors looks like.

Conclusion

This article has focused on three noteworthy aspects of the recent trio of AOs on climate change. Most importantly, the article has analysed what the three AOs teach us in relation to interpreting and applying variable fact-dependent obligations, especially obligations requiring States to exercise due diligence. While the content of due diligence obligations depends upon a variety of fact-specific matters that can evolve over time, including the circumstances and capabilities of the State in question, the three AOs have emphasised that applying the different elements of a due diligence standard is a matter for objective determination. Moreover, the ICTs have provided significant guidance on what objectively reasonable approaches to implementing the relevant obligations look like. Thus, if properly interpreted, obligations requiring States to exercise due diligence do not lead to unmanageable indeterminacy nor provide excuses for climate inaction based on national circumstances.

This article has foregrounded and analysed two other interesting aspects of the AOs. Firstly, the ongoing or continuing nature of States’ obligations to cooperate, which are complementary rather than mutually exclusive, and are not exhausted by the conclusion of particular climate-related treaties. This sees the customary duty to cooperate for the protection of the environment become a yardstick for determining whether existing treaty-based forms of climate cooperation remain adequate. Secondly, in relation to States’ obligations to regulate the activities of private actors that cause climate change, all three AOs identify and flesh out a duty to regulate effectively such conduct.

Footnotes

Acknowledgements

This article draws on and expands upon two prior blogposts by the author: (1) ‘ICJ Advisory Opinion on Climate Change: The Variable and Evolutive Nature of Due Diligence Obligations’, EJIL Talk! (Blog of the European Journal of International Law), August 21, 2025 https://www.ejiltalk.org/icj-advisory-opinion-on-climate-change-the-variable-and-evolutive-nature-of-due-diligence-obligations/; (2) ‘The ITLOS Advisory Opinion on Climate Change: Selected Issues of Treaty Interpretation’, EJIL Talk!, June 03, 2024 . The author thanks Devika Hovell for helpful comments on a draft of the first blog post and Bharat Desai for comments on a draft of this article.

ORCID iD

Joshua Paine

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

Declaration of Conflicting Interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.