ICJ Advisory Opinion on Climate Change: The Variable and Evolutive Nature of Due Diligence Obligations

This post focuses on the concept of due diligence in the International Court of Justice (ICJ)’s Advisory Opinion in Obligations of States in respect of Climate Change (for earlier posts analysing other aspects of the Advisory Opinion see here, here, here and here). Specifically, this post analyses the ICJ’s reasoning regarding due diligence as ‘a variable concept’ (International Tribunal for the Law of the Sea (ITLOS), Responsibilities and obligations of States with respect to activities in the Area, Advisory Opinion 2011, para 117), ‘whose content in a specific situation derives from various [fact-specific] elements, including the circumstances of the State concerned, and which may evolve over time’ (ICJ Advisory Opinion, paras 280 and 246). What this means is that interpreting and applying obligations of due diligence in a specific situation requires taking account of a variety of fact-specific considerations that are likely to change over time, including relevant scientific and technical information and the circumstances and capabilities of the State in question (Viñuales 2020, 125–26; see also International Law Commission, Draft Articles on Prevention of Transboundary Harm from Hazardous Activities, 2001, commentary to Art. 3 paras 11, 13, 17). Similarly, Ollino 2022 observes 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’ (Ollino 2022 108–109, see also 167–68, 175). 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”’ (Ollino 2022 181). The variable or evolutive character 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 (see Ollino 2022 107; Viñuales 2020, 113–14, 122), which means that, as already touched on, 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.

In interpreting obligations of due diligence in the Advisory Opinion, a tension the ICJ navigated was between, on the one hand, providing guidance on a general standard of conduct applicable to all States, given the universal risk posed by climate change, and on the other, taking account of ‘the principle of common but differentiated responsibilities and respective capabilities’, the latter being well established in the climate change treaties and in international environmental law more generally (ICJ Advisory Opinion paras 137, 148 and 151). This post argues that the ICJ was largely successful in navigating this tension, such that obligations of due diligence, as interpreted by the Advisory Opinion, provide reasonably clear guidance to States on what is required of them. While the content of due diligence obligations, as interpreted by the Court, does vary depending on various fact-specific considerations, including the circumstances and capabilities of the State in question and even its historical contribution to the problem of climate change, it will be argued that the standard of conduct required does not become unworkably indeterminate nor justify climate inaction due to national circumstances. In short, the references to States’ circumstances and capabilities in the Advisory Opinion do ‘not provide a carte blanche for [States] evading their international obligations’ (Lanovoy 2024, 1049). As we will see, the ICJ was careful to emphasise that, notwithstanding ‘the variable and evolving nature of the [due diligence] standard’, the application of the different elements of the obligation to exercise due diligence to protect the environment ‘in a particular situation should be determined objectively’ (ICJ Advisory Opinion, para 300). In a nutshell, as Besson 2023 argues, ‘the variable character of due diligence does not make it a “subjective” standard … [nor] a profoundly “indeterminate” standard’ (Besson 2023, 119).

Due Diligence as ‘a Variable Concept’

The Court initially considered the concept of due diligence in discussing States’ customary obligation to prevent significant harm to the environment (para 135), observing that ‘[t]he 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’ (para 136). Citing its earlier case law, the Court 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’ (para 137). Understandably the Court then emphasised 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’ (para 137). At this point the Court noted that:

‘[c]limate 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’ (para 137, emphasis added).

The Court concluded this section of the Advisory Opinion by holding that ‘[u]nder these circumstances, the Court recognizes that the standard of due diligence for preventing significant harm to the climate system is stringent’ (para 138).

The Court further addressed the concept of due diligence in the part of the Advisory Opinion where it found that the discretion of Parties to the Paris Agreement to prepare Nationally Determined Contributions (NDCs) is limited by several factors (see Advisory Opinion para 245, and generally paras 237–245). At this point the Court recalled ‘that the standard of due diligence varies depending on a range of factors’, citing paragraph 117 of ITLOS’ 2011 Advisory Opinion (ICJ Advisory Opinion, para 246, see also para 254). In that paragraph, ITLOS noted that:

‘“due diligence” is a variable concept. It may change over time as measures considered sufficiently diligent at a certain moment may become not diligent enough in light, for instance, of new scientific or technological knowledge. It may also change in relation to the risks involved in the activity.’ (ITLOS, Responsibilities and obligations of States with respect to activities in the Area, Advisory Opinion 2011, para 117).

Unsurprisingly, the ICJ reasoned that ‘because of the seriousness of the threat posed by climate change, the standard of due diligence to be applied in preparing the NDCs is stringent’ (para 246, see also para 254). More interestingly, the Court found that while:

‘[t]he obligation to prepare and communicate NDCs capable of realizing the objectives of the Agreement applies to all parties to the Paris Agreement (see paragraph 234 above). However, 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.’ (para 247, emphasis added).

Factors Informing the Content of a Due Diligence Standard

The Court returned to the concept of due diligence in analysing States’ customary obligation to prevent significant harm to the environment, considering the different elements that inform ‘what due diligence requires from a State in a particular situation, including in the context of climate change’ (para 280ff). Perhaps unsurprisingly, the Court held that ‘[t]he standard of due diligence may also become more demanding in the light of new scientific or technological knowledge’ (para 284) and:

‘the availability of technological means to prevent or mitigate relevant harm influences what can reasonably be expected of a State … the cost of the relevant technologies may be a crucial factor in determining what can reasonably be expected of a State, depending on its capabilities’ (para 286).

Turning to the different capabilities of States, the Court 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’ (para 290, see also para 291).

Ultimately, on the different capabilities of States, the Court held that:

‘[w]hile 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.’ (para 292, emphasis added, see also para 226, Separate Opinion of Vice-President Sebutinde, para 10, and Joint Declaration of Judges Bhandari and Cleveland, paras 24, 27).

An Objective Assessment

The ICJ concluded this section of the Advisory Opinion by holding that:

‘[h]aving 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.’ (para 300).

Interestingly, ITLOS, in its 2024 Advisory Opinion concerning climate change, also emphasised that:

‘an obligation of due diligence should not be understood as an obligation which depends largely on the discretion of a State … The content of an obligation of due diligence should be determined objectively under the circumstances, taking into account relevant factors.’ (Request for Advisory Opinion submitted by the Commission of Small Island States on Climate Change and International Law, Advisory Opinion, 21 May 2024, para 257, emphasis added).

ITLOS emphasized this point after having outlined the variable, fact-specific, and evolutive nature of a due diligence standard (see ibid para 239) and having held that the implementation of a due diligence obligation ‘may vary according to States’ capabilities and available resources’ (ibid para 241, see similarly paras 249, 441(3)(c)).

Discussion and Conclusion

Obligations to exercise due diligence are familiar concepts in the environmental jurisprudence of the ICJ and of other international courts and tribunals. This post has focused on the variable and evolutive nature of obligations to exercise due diligence (see also Tanaka 2021, 92), highlighting how what is required by a standard of due diligence in any specific situation depends on a variety of fact-specific considerations that are likely to change over time, including the circumstances and capabilities of the State in question. At one level, this means that obligations to exercise due diligence (eg to prevent significant environmental harm) are ‘indeterminate, multifactorial and evolutive in nature’ (language borrowed from Declaration of Judge Nolte, para 7). However, as the ICJ was careful to emphasise, applying an obligation of due diligence in a particular situation, while depending upon a range of fact-specific considerations, is a matter to be determined objectively (ICJ Advisory Opinion, para 300; see also ITLOS, 2024 Climate Change Advisory Opinion, para 257). It is suggested that the ICJ’s Advisory Opinion, assessed in the round, provides sufficient guidance to States and other interpreters, such that standards of due diligence do not become unworkably indeterminate nor justify climate inaction due to national circumstances. Put differently, contrary to certain concerns that have been expressed, due diligence, as interpreted in the ICJ’s Advisory Opinion, does not become ‘“all things to all people”,  an inherently variable and flexible concept, highly contextual and fact specific, which could end up diluting rather than reinforcing states’ compliance with their international obligations’ (Lanovoy 2024, 1030–31).

Finally, it is worth recalling that, as other commentators have suggested, the variable or evolutive character of due diligence obligations also has important benefits as it enables legal rules to keep track with both factual changes (eg changes in relevant science or in technical capacity) and changes in the wider relevant normative environment (see Krieger and Peters 2020, 380–81; N. Matz-Lück and E. Van Doorn, ‘Due Diligence Obligations and the Protection of the Marine Environment’ (2017) 42 L’Observateur des Nations Unies 177, 190–95; Mbengue 2016, 286).