One of the most profound findings in the International Court of Justice’s (ICJ’s) climate change advisory opinion (AO) is that State obligations to mitigate climate change to a level that holds warming to the Paris Agreement’s 1.5oC threshold are spread out over the large canvas of international law, including United Nations (UN) climate treaties, particularly the Paris Agreement, UNCLOS, human rights treaties, other environmental treaties, and the customary international rule to prevent significant harm to the environment. While most of these treaties, and the customary no-harm rule, do not contain obligations to achieve a particular result, they establish obligations on states to act in a particular manner – or obligations of conduct – which are assessed against a standard of due diligence.
The ICJ AO, in interplay with the advisory opinions by the International Tribunal for the Law of the Sea (ITLOS) and the Inter-American Court for Human Rights (IACtHR), clarifies that the due diligence standard of conduct applies in an objective and stringent manner in the context of climate change (see here, here, and here). It also provides a detailed set of factors to determine the content of due diligence.
The ICJ found that some norms, previously thought not binding and falling under the unfettered discretion of States (e.g. the content of NDCs) are in fact binding obligations of conduct based on a due diligence standard, and their breach gives rise to state responsibility.
In this blog post, I address some pertinent issues regarding due diligence as addressed by the ICJ, as well as ITLOS and the IACtHR. In particular, I focus on the relationship between obligations of result and obligations of conduct, the nature of due diligence, factors to determine its content, and the legal consequences of not acting with the required diligence.
Relationship to obligations of result
The distinction between obligations of conduct and obligations of result is not always clear-cut. However, in general, the fulfillment of obligations of result can be measured against an achieved objective or outcome, while obligations of conduct require a State to act in a particular manner and are usually assessed against a standard of due diligence. The ICJ noted that one type of obligation is not more onerous than the other; the two often coexist and seek to achieve the same objectives through different means” (para. 175). This aligns with the emphasis by ITLOS that an obligation of due diligence does not require a lesser degree of effort to achieve the intended result, but that “[i]In many instances, an obligation of due diligence can be highly demanding” (ITLOS, para. 257). Importantly, an obligation of due diligence, though different in character, is as conducive to the achievement of a particular objective as an obligation of result.
The legal nature of due diligence: objective, non-discretionary and determinable
While long considered to be subjective, discretionary, and indeterminate, ICJ, ITLOS and the IACtHR clarified that this is not the case.
ITLOS noted the objective nature of due diligence (ITLOS, para. 257). The ICJ concurred that “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” (ICJ, para. 300).
It follows, therefore, that “an obligation of due diligence should not be understood as an obligation which depends largely on the discretion of a State” (ITLOS, para. 257).
Further, due diligence is not indeterminable. One of the most important aspects of the courts’ findings is a comprehensive set of factors to determine due diligence. ITLOS noted that factors to be considered 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” (ITLOS, para. 239). Both the IACtHR and ICJ added further factors, as will be shown below. Determination of due diligence is, thus, possible, but it needs to be assessed in a case-specific context; based on what is reasonable under the concrete and specific circumstances of a State. This does not exclude identifying a standard of conduct at a general level, depending on the risk’s overall character (ICJ, para. 137).
Application of due diligence
During the proceedings before the courts, particularly before the ICJ, several parties maintained that due diligence is a self-standing norm under international law. This notion was partly enhanced by the UN General Assembly Resolution 77/276, which requested the opinion from the ICJ. The resolution called upon the Court to have particular regard, inter alia, to “the duty of due diligence” in rendering its answer to the questions put before it.
The ICJ, however, made clear that due diligence does not exist independently. It is not a norm, let alone an obligation, on its own. Rather, it is the standard applying to a State’s expected conduct in fulfilling a (main) obligation – the obligation of conduct. This can be a customary international obligation (i.e., of cooperation or harm prevention) or a treaty-based obligation (i.e., to prevent, reduce, and control pollution of the marine environment under Art. 194.1 UNCLOS or climate adaptation obligations under Art. 7 of the Paris Agreement), but there must be a (main) obligation to which the due diligence standard applies and in relation to which its compliance is assessed.
The content of due diligence
Due diligence is a variable and evolving concept, yet it does not escape determination and concretization. The ICJ, ITLOS, and IACtHR provided a comprehensive set of factors designed to support the determination of due diligence.
a) Best efforts
In general, due diligence requires that States undertake their best efforts – or to do the utmost (ITLOS, paras. 233 and 241; ICJ, paras. 253, 270; IACtHR, para. 232). This is usually defined by what can reasonably be expected of a State in a similar situation (i.e., the employment of best efforts by using all the means at its disposal (ICJ, 229; citing Pulp Mills) or as an obligation to deploy “adequate means, exercise best possible efforts, and do its utmost” (ITLOS Seabed Dispute Chamber AO)).
b) Factors
A comprehensive set of elements or factors for determining due diligence has now emerged. The factors comprise, in a non-exhaustive manner: (i) taking all appropriate measures, (ii) scientific and technological information, (iii) relevant international rules and standards, (iv) different capabilities, (v) the risk of harm and the urgency, (vi) precautionary approach or principle and respective measures, (vii) risk assessment and environmental impact assessment, (viii) notification and consultation (ITLOS, para. 239 and ICJ, paras. 231-299).
In addition to these factors, the IACtHR added the following: (ix) 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, (x) permanent and adequate monitoring of the effects and impacts of the adopted measures, (xi) strict compliance with the obligations arising from procedural rights, in particular, access to information, participation, and access to justice; (xii) transparency and accountability in relation to State climate action, (xi) appropriate regulation and supervision of corporate due diligence, and (xiii) enhanced international cooperation, particularly regarding technology transfer, financing, and capacity-building (IACtHR, para. 236).
(i) All appropriate measures
For the ICJ, taking all appropriate measures “means that States must put in place a national system, including legislation, administrative procedures and an enforcement mechanism necessary to regulate the activities in question, and to exercise adequate vigilance to make such a system function efficiently, with a view to achieving the intended objective” (ICJ, paras. 235, 281; ITLOS, para. 235 referring to Pulp Mills). This entails not only the adoption of appropriate rules and measures, but also their enforcement and the exercise of administrative control applicable to both public and private operators, such as monitoring the activities undertaken by these operators.
In the view of the ICJ, taking appropriate measures to protect the climate system also includes addressing “fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses or the provision of fossil fuel subsidies” (ICJ, para. 427).
The ICJ also clarified due diligence requirements for determining the content of Nationally Determined Contributions (NDCs). The ICJ rejected the characterization of NDCs as voluntary and discretionary and confirmed that the content, implementation, and achievement of NDCs are obligations of conduct, based on a stringent due diligence standard (for a discussion, see Voigt, 2025).
With respect to the content of NDCs, the ICJ made clear that States do not enjoy unfettered discretion when preparing NDCs (ICJ, para. 270). Rather, their discretion is limited (ICJ, para. 245). As an obligation of conduct, parties are obliged to exercise due diligence when putting forward their NDC and must satisfy certain expectations and standards under the Paris Agreement when doing so, as set out in Art. 4.3 (ICJ, para. 249). This provision is not a voluntary expectation. It is prescriptive in the sense it requires that “successive nationally determined contributions will represent a progression” and “reflect [a party’s] highest possible ambition”, without prescribing precisely what constitutes a progression, or what reflects a party’s highest possible ambition.” (ICJ, para. 240) These standards require, first, that NDCs represent a progression, which the court interpreted as a legal obligation of due diligence that “a party’s NDCs must become more demanding over time” (ICJ, para. 241).
Second, a party’s NDCs must reflect the highest possible ambition, to which many parties had referred to in their oral statements. The ICJ clarified that each party has a due diligence obligation to do its utmost to ensure that the NDCs it puts forward represent its highest possible ambition in order to realize the objectives of the Agreement. (ICJ, para. 270, 245). It stated that this implies that NDCs must be capable of making an adequate contribution to the achievement of the temperature goal. This provision reveals the necessity for the ambition contained in a party’s NDC to relate to the object and purpose of the Agreement set out in Article 2, i.e. to hold the increase in the global average temperature to below 1.5°C (ICJ, para. 242). Literature has provided more detail on the contours of “highest possible ambition” in Art. 4.3 (Voigt, 2016, 2023, Mayer 2024).
Parties must, third, be informed by the outcomes of the Global Stocktake in the preparation of their NDCs, according to Arts. 14.3 and 4.9 of the Paris Agreement, and when communicating their NDCs must provide the information necessary for clarity, transparency and understanding, according to Art. 4.8.
Further, the ICJ concluded that the national implementation and achievement of NDCs are obligations of conduct and not voluntary. The obligation that parties “shall pursue domestic mitigation measures” in Art. 4.2(2) is substantive in nature and creates an individual obligation of conduct for each party to the Paris Agreement (ICJ, para. 251).
This does not mean that parties are obligated to achieve their NDC targets, but rather that they must make best efforts, based on stringent due diligence (ICJ, para. 254) to obtain such a result. Parties must “pursue domestic mitigation measures” that aim to achieve the objectives of their NDCs which are proactive and reasonably capable of achieving the NDCs set by them, including in relation to activities carried out by private actors (ICJ, para. 252).
Also the fulfillment of adaptation obligations needs to be assessed against the standard of due diligence. The ICJ considers it incumbent upon parties to enact appropriate measures (examples of which are provided in Article 7, paragraph 9) that are capable of “enhancing adaptive capacity, strengthening resilience and reducing vulnerability to climate change”. (ICJ, para. 258)
In sum, with respect to the Paris Agreement, the ICJ concluded that “the compliance of parties with their obligations of conduct under the Paris Agreement is assessed on the basis of whether the party in question exercised due diligence and employed best efforts by using all the means at its disposal in the performance of that obligation” (ICJ, paras. 229 and 245).
The IACtHR added various components to identifying appropriate measures, such as the need to reflect maximum use of available resources, avoid technologies whose effects have not been fully verified, protect biodiversity and ecosystems, facilitate the continuing participation of Indigenous Peoples in decision-making, stimulate and attract investment in innovation in low-emission activities, as well as to develop new tools and standards for strengthening green finance, review existing trade and investment agreements, and settlement mechanisms for litigation between investors and States (IACtHR, paras. 336-367).
The IACtHR also details in particular what due diligence entails in relation to the regulation of private actors (IACtHR, para. 347). Private actors do not fall outside the normative “force-field” of States’ due diligence. As the activities in question are mostly carried out by private entities, due diligence requires States to regulate their conduct (i.e., “regulatory due diligence”) (ITLOS, para. 236; IACtHR, para. 231; ICJ, para. 428).
A State is responsible for its own actions or omissions when failing to exercise regulatory due diligence. In the context of state responsibility, the question of attributing the conduct of private actors to a State therefore does not arise in such circumstances, as a State’s own regulatory performance is at stake. “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.” (ICJ, para. 428; see also IACtHR, para. 345)
A final aspect of due diligence with regard to appropriate measures to be highlighted here is vertical and horizontal policy coherence. The IACtHR observed that “[t]aking into account the standard of enhanced due diligence […], States have the obligation to ensure coherence between their domestic and international commitments and their obligations concerning the mitigation of climate change. […] among other measures, States should ensure that public finance and incentives aimed at activities that generate [greenhouse gas (GHG)] emissions are conditioned on strict compliance with national mitigation norms and policies” (IACtHR, para. 344).
Overall, while the respective climate measures of each state will necessarily differ depending on its emission portfolio, drivers, and other national circumstances, the courts have provided comprehensive guidance for states on due diligence for adopting appropriate measures.
(ii) Scientific and technological information
The availability of, and the need to acquire and analyse scientific and technological information, are important factors of due diligence, as scientific information may be relevant to assess the probability and seriousness of possible harm (ICJ, 283).
This is not a passive obligation. Due diligence requires States to actively pursue the scientific information. In this regard, reports by the IPCC constitute comprehensive and authoritative restatements of the best available science about climate change at the time of their publication (ICJ, para. 284). The standard of due diligence may also become more demanding in the light of new scientific or technological knowledge (ICJ, para. 284).
(iii) Relevant international rules and standards
In determining the required due diligence, current international rules and standards must be taken into consideration which may arise from binding and non-binding norms (ICJ, para. 287 referencing Gabcikovo-Nagymaros). Such standards may therefore not only be contained in treaties and in customary international law, but “may also be reflected in certain decisions of the [conferences of the parties (COPs)] to the climate change treaties and in recommended technical norms and practices, as appropriate.” (ICJ, para. 288)
Regarding COP decisions, the ICJ narrowed their application in the context of determining due diligence to those decisions that have acquired the status of customary international law, in so far as they reflect State practice and express an opinio juris and noted that such legal significance can only be determined in concreto. It did, however, not indicate when COP decisions – and which – may have acquired such status.
The ICJ´s approach to international rules and standards in the context of due diligence builds on that of ITLOS. For climate change, ITLOS observed that international rules and standards are found in various climate-related treaties and instruments, including the UNFCCC and the Paris Agreement, MARPOL, the Chicago Convention, and the Montreal Protocol, including its Kigali Amendment (ITLOS, paras. 214, 239). ITLOS relied predominantly on the 1.5oC global temperature goal and the timeline for emission pathways (i.e. net-zero emissions by 2050) set forth in the Paris Agreement in the interpretation of “all necessary measures” under Art. 194.1 of UNCLOS and due diligence (ITLOS, para. 222). In the same vein, the ICJ observes that “the climate change treaties establish standards that may enable or facilitate the identification and application of the diligence that is due in specific instances”. (ICJ, 313).
The reference to international rules and standards is also important for another reason: to create consistency and harmony in international law. By interpreting UNCLOS in the light of the Paris Agreement as the primary legal instrument for addressing climate change (ITLOS, para. 214), ITLOS ensured that the standard applied to states´ obligations to address climate change under UNCLOS was no different to the one they agreed to when adopting the Paris Agreement. Similarly, the ICJ applies the Paris Agreement standard in the determination of due diligence pertaining to other treaties as well as to customary law. It goes so far to state that “at the present stage, compliance in full and in good faith by a State with the climate change treaties, as interpreted by the Court […], suggests that this State substantially complies with the general customary duties to prevent significant environmental harm and to co-operate” (ICJ, para. 314). This, according to the ICJ, also applies to States that are not a party to the climate change treaty or treaties. Though it noted that “if a non-party State does not co-operate in such a way, it has the full burden of demonstrating that its policies and practices are in conformity with its customary obligations”. (ICJ, 314)
The ICJ reiterated ITLOS´ observation that obligations under other treaties (such as under UNCLOS) (ITLOS, 222 and 223) or customary obligations would not be fulfilled simply by States complying with their obligations under the climate change treaties. Treaty law and customary international law are different in character and do not entirely overlap. However, they inform each other. Therefore, the ICJ noted with respect to UNCLOS, that measures under Art. 192 and 194 “must be adopted in accordance with the obligations incumbent upon States under the UNFCCC and the Paris Agreement, in so far as the States concerned are parties to those instruments“ (ICJ, 343).
What can be drawn from this is that the Paris Agreement permeates other international obligations of states, both under treaty and customary law, and informs their interpretation and the exercise of due diligence. Thus, the standards and norms included in the Paris Agreement become the benchmark against which, to a large extent, the performance of states in respect to climate change is assessed under other international treaties and under customary law (See Voigt, 2023). Such an approach ensures that states are consistently held to the 1.5oC temperature threshold and to acting with the necessary due diligence in this regard, also in fulfilling their other obligations under international law.
iv) Differentiation
The need to act with due diligence applies to all states. However, consistent with the principle of common but differentiated responsibilities and respective capabilities (CBDR), the standard varies (ICJ, para. 247). The ICJ noted that CBDR reflects the need to equitably distribute the burdens of the obligations in respect of climate change.
The ICJ observed that CBDR does not categorically place different burdens on a state based on whether is a developed or developing country. Rather, in its view, the principle calls for taking into account the circumstances of the state in question, such as historical and current contributions to cumulative GHG emissions, and their different current capabilities and national circumstances, including their economic and social level of development and other national circumstances of the party in question (ICJ, para. 148). The court also observed that the inclusion of the phrase, “in the light of different national circumstances” in the Paris Agreement, adds nuance to the principle by recognizing that the status of a State as developed or developing is not static (ICJ, 226) (see, on the nuanced and dynamic nature of CBDR, Voigt and Ferreira, 2016)
Importantly, the ICJ notes that in between the most developed and least developed states “are States that have progressed considerably in their development since the conclusion of the UNFCCC in 1992 […] and some of 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” (ICJ, para. 150).
Moreover, in referring to the ITLOS climate change advisory opinion, the ICJ noted that CBDR “requires a State with greater capabilities and sufficient resources to do more than a State not so well placed,” but that, based on CBDR, “implementing the obligation of due diligence requires even the latter State to take all the means at its disposal to protect the climate system in accordance with its capabilities and available resources” (ICJ, para. 291). It stated:
“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” (ICJ, para 150).
v) Precautionary approach or principle and respective measures
Due diligence is also informed by the risk at stake and the urgency to act. In general, “[t]he standard of due diligence has to be more severe for the riskier activities” (Seabed AO). The notion of risk in this regard should be appreciated in terms of both the probability or foreseeability of the occurrence of harm and its severity or magnitude. With respect to climate change, the risks are exceptionally high as it poses a “quintessentially universal risk to all States” which is “of a general and urgent character” (ICJ, para. 138) and “an existential problem of planetary proportions that imperils all forms of life and the very health of our planet” (ICJ, para. 456). States are therefore required to take all appropriate measures to prevent significant harm where reliable scientific evidence of a risk of significant harm exists (ICJ, para. 293).
Where scientific uncertainty exists, States should not refrain from or delay taking precautionary actions of prevention in the face of risk. According to Principle 15 of the Rio Declaration, precaution requires that where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation (see also Article 3, paragraph 3, of the UNFCCC).
The obligation of due diligence is closely linked with precaution. As ITLOS already confirmed (ITLOS, paras 213 and 242), it is “an integral part of the general obligation of due diligence” (Seabed AO). The ICJ agreed with this conclusion that “where there are plausible indications of potential risks,” a State “would not meet its obligation of due diligence if it disregarded those risks” and, in that sense, the “precautionary approach is also an integral part of the general obligation of due diligence” under the duty to prevent significant harm to the environment” (ICJ, para. 294).
(vi) Risk assessment and environmental impact assessment
Due diligence also requires States to take certain procedural measures, which are distinct from substantive measures, to prevent risks (ICJ, para. 295), However, as ITLOS noted, “procedural obligations, such as the requirement to conduct an environmental impact assessment [EIA], may, indeed, be of equal or even greater importance than the substantive standards existing in international law”. (ITLOS, para. 345 with reference to Chagos Award)
In this context, the ICJ pointed to the need to undertake an EIA, which it considered a rule of customary international law (with reference to Certain Activities and Pulp Mills). This aligns with the confirmation by the IACtHR that, “in compliance with the standard of enhanced due diligence, States must conduct a meticulous assessment of activities that could result in significant harm to the climate system before granting approval” (IACtHR, para. 363).
Since customary international law does not specify the scope and content of an EIA, the specific character of the respective risk and the specific circumstances of each case need to be taken into account (ICJ, paras. 296, 298). Such specific assessment could also “identify previously unknown information about possibilities for reducing the quantity of GHG emissions by relevant proposed individual activities” (ICJ, para. 298).
The obligation to conduct an EIA concerns planned activities and must be carried out on the basis of the best available science. It must be conducted prior to the implementation of a project and applies to both those planned by private entities and by States (ITLOS, 358). Moreover, it should include not only the specific effects of the planned activities concerned but also the socio-economic impacts and cumulative impacts of these and other activities (ITLOS, para. 365). Planned activities may not be environmentally significant if considered in isolation, whereas they may produce significant effects if evaluated in interaction with other activities (ITLOS, para. 365).
Ultimately, “it is for each State to determine in its domestic legislation or in the authorization process for the project, the specific content of the environmental impact assessment required in each case, having regard to the nature and magnitude of the proposed development and its likely adverse impact on the environment as well as to the need to exercise due diligence in conducting such an assessment’”. (ICJ, para. 298)
(vii) Notification and consultation
Due diligence also contains an obligation of States to notify and consult in good faith with other States where planned activities within the jurisdiction or control of a State create a risk of significant harm, and notification and consultation is necessary to determine the appropriate measures to prevent that risk. (ICJ, para. 299 with reference to Certain Activities)
This applies in particular “where an activity significantly affects collective efforts to address harm to the climate system” (ICJ, para. 299). This can be understood as notification and consultation to be especially warranted when planning fossil fuel extraction or combustion projects, which may affect other states´ fossil fuel phase-out policies or their transition to renewable energies.
c) Qualifier: Stringent/Enhanced Due Diligence
As explained above, the catalogue of factors to assess whether the obligation to act with due diligence has been met is detailed and specific, and allows for the determination of lawful – or unlawful – state behavior in relation to their obligations to address climate change. This constitutes a significant hardening and concretization of international climate law and circumscribes the conduct to be exercised in the compliance with international legal obligations.
The courts also qualified the level of due diligence. ITLOS observed that, given the high risks of serious and irreversible harm to the marine environment from GHG emissions, the standard of due diligence under UNCLOS should be set high and considered it to be stringent (ITLOS, para. 243). With respect to transboundary pollution affecting the environment of other States, the standard is even more stringent (ITLOS, paras. 248, 256, 258).
Similarly, the ICJ confirmed that the standard of due diligence is stringent because of the seriousness of the threat posed by climate change (ICJ, paras. 138, 246, 399) and on account of best available science indicating that the “[r]isks and projected adverse impacts and related losses and damages from climate change escalate with every increment of global warming” (ICJ, para. 254).
Given the “extreme gravity of climate impacts” and “the urgency of effective measures to avoid irreparable harm to the individual,” the IACtHR found that 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 (IACtHR, paras. 233-236).
Accordingly, states must do their utmost in addressing climate change (ICJ, para. 246). Due diligence takes on a particular quality in the context of climate change, requiring a heightened degree of vigilance and prevention. This includes the exercise of regulatory due diligence in taking all necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under a state´s jurisdiction and control. There is now a legal imperative – alongside a moral one – to address this “existential problem of planetary proportions that imperils all forms of life and the very health of our planet” which not only is daunting, but also self-inflicted (ICJ, para. 456).
Conclusion
The Advisory Opinions issued by the ICJ, ITLOS, and IACtHR make clear that nothing short of the utmost effort by each individual State would satisfy the duty of States. Due diligence – once believed to be soft and weak – has now emerged as a potent and powerful standard, against which to assess compliance of international obligations. There is no hiding behind discretion and sovereign entitlements anymore. States must act with stringent due diligence and do the utmost in addressing climate change or they will incur the consequences of international responsibility: restoration, in the form of restitution, satisfaction, and – importantly – compensation for climate harms.