Defining Due Diligence Obligation of States in Relation to Climate Change

Climate change and greenhouse gas (GHG) generation are global concerns, imposing an erga omnes obligation (para 33) on States to implement effective policies controlling GHG production at the source. While States are not liable for failing specific goals (para 430), they can be accountable for not reasonably pursuing these objectives, as per the due diligence concept in international law.

Two recent judgments have clarified the due diligence obligations of States regarding climate change. The European Court of Human Rights (ECtHR), in Verein KlimaSeniorinnen v Switzerland, and the International Tribunal of the Law of the Sea (ITLOS) in its Advisory Opinion on Climate Change, both outlined State responsibilities. Despite focusing on different legal areas – human rights law and maritime law – they have recognised a consistent standard of conduct for States under international law. This post analyses the due diligence obligations conceived by these judgments.

Due Diligence under International Environmental Law

Under international law, due diligence is understood in two different ways. First, it is as a standard of conduct, focusing on how obligations are fulfilled. Second, it is viewed as an obligation to endeavour, as articulated by Dupuy, which aims at mitigating the risk involved in a situation. Due diligence is accepted as a core element of State responsibility, especially in situations where no defined State obligation exists.

Due diligence, as an obligation, is one of conduct or endeavour, not of result. An obligation of result is met by achieving specified goals, while an obligation of conduct requires States to adopt certain measures to achieve a particular result. Compliance with this obligation does not depend on achieving the specified result. Thus, under a due diligence obligation, States are not responsible for failing to meet the goals, but can be held accountable for not taking appropriate measures towards them if States fail to deploy appropriate measures towards the fulfilment.

Due diligence is integral to international environmental law, stemming from the prevention principle (para 187), which requires States to act diligently to prevent environmental harm. The Long-Range Transboundary Air Pollution Convention of 1979 (specifically Article 2), the Ozone Convention of 1985 (specifically Article 2), and  the ILC Draft Articles on Prevention of Transboundary Harm from Hazardous Activities (specifically Article 3) impose due diligence obligations on States to protect the environment from further deterioration. Due diligence serves both as a standard of conduct and as an obligation in itself, such as adopting domestic legislation (ibid, Article 3 commentary). As an obligation, due diligence involves a fact-finding process, undertaken by States before introducing policies or legislation. As a standard of conduct, it specifies how States should act to comply comply with international law, exemplified by Albania’s timely notification about a minefield in the Corfu Channel case (page 22).

Apart from treaty provisions, there has been some litigation on the scope of due diligence obligations under international environmental law. The 2015 Costa Rica v. Nicaragua case is perhaps the most widely debated example (see here and here). In the Separate Opinion of Judge Ad Hoc Dugard, one of the judges on the bench, argued that due diligence is merely a standard of conduct which flows from the overarching principle of preventing harm, thereby asserting that due diligence is not an obligation in itself under international environmental law (para 9). Contrary to this, in their Separate Opinions, Judges Owada (para 18 and 21) and Donoghue (para 8-10) categorised due diligence primarily as a legal obligation of conduct, which applies at every stage of a project. They contended that due diligence is more than a mere standard of conduct, and is, in fact, an obligation in itself. This obligation can be fulfilled through various means, depending on the surrounding circumstances, such as an environmental impact assessment. Both the Judges relied on the Pulp Mills judgement (para 187) to assert that due diligence obligations of States stem from the prevention principle under international environmental law.

The latter understanding aligns with the Seabed Disputes Chamber (SDC) Advisory Opinion of 2011, which held that due diligence is an obligation in itself under international environmental law, rather than merely a standard of conduct (paras 115-116). The Chamber enhanced existing jurisprudence by noting that the conception of due diligence as an obligation is not fixed but variable. It changes according to the prevailing standards of science, technology, and the degree of risk involved in activities (para 117). Thus, judicial understanding affirms that due diligence is an obligation in itself, not just a standard of conduct. While these cases clarified the nature of this obligation, its specifics varied in each judgment depending on the facts. This contrasts with two recent landmark judgments by the ECtHR and ITLOS, which clearly defined the form of due diligence obligations in climate change matters.

Due Diligence in Climate Change Matters – The ECtHR & ITLOS Judgments

The Verein KlimaSeniorinnen v Switzerland case addressed State responsibility concerning the human rights implications of climate change. The fundamental aim of the applicants was to compel the Swiss government to take all possible measures to protect the life and health of the applicants from climate change (para 23). The ITLOS Climate Change Advisory Opinion arose from a request by the Commission of Small Island States on Climate Change and International Law, seeking the Tribunal’s opinion on States’ obligations under the UN Convention on the Law of the Sea (UNCLOS) regarding prevention, reduction, and control of marine pollution, specifically related to climate change (para 3). Thus, while both judgments addressed different branches of international law, they arrived at the same conception of States’ due diligence obligations concerning climate change.

The ITLOS Climate Change Advisory Opinion addressed, among others issues, Article 194(1) of UNCLOS, which requires States to take measures to ‘prevent, reduce, and control pollution of the marine environment’. Building upon this language, ITLOS observed that the States are obligated only to make their best efforts to prevent, reduce, and control marine pollution, rather than guaranteeing these outcomes (para 233). Relying on the SDC  Advisory Opinion of 2011, the Tribunal emphasised that this obligation is one of conduct, not of result.

A similar conception of State responsibility was adopted by the ECtHR. In detailing the parameters to determine whether States have fulfilled their responsibility under Article 8 of the European Convention of Human Rights concerning climate change, the Court considered whether the concerned States had taken actions to reduce GHG emissions, and if they had acted in ‘good time and in an appropriate and consistent manner’ in formulating the relevant  policies (para 550). The fulfilment of the obligation does not dependent on achieving GHG reduction targets. Thus, the ECtHR also views States’ due diligence responsibility in climate change as obligation of conduct, not an obligation of result.

Having agreed on the same type of obligation, both judicial bodies have devised a similar mechanism for fulfilment their due diligence obligations. Both bodies held that meeting their obligations under the respective treaty laws requires adopting and implementing laws and policies to mitigate the effects of climate change (ECtHR, para 545; ITLOS, para 235). These laws and policies must be guided by contemporarily acceptable goals pertaining to climate change, international commitments under the IPCC, the Paris Agreement, and scientifically establishing global aims for controlling global temperatures (ECtHR, paras 546-547; ITLOS, para 243).

The two judgments not only define the nature of States’ due diligence obligations concerning climate change but also clarify the standard of compliance expected from them. Guided by the urgency of addressing climate change, both courts emphasised a rigorous standard of diligence. ITLOS set a stringent standard above mere best efforts, justified by the imminent and severe threats of climate change (para 241). However, ITLOS did not elaborate further on this stringency, whereas the ECtHR detailed a high threshold. The ECtHR required States to act promptly and consistently, aiming for substantial and progressive reduction of GHG emissions to achieve net neutrality within three decades (para 548). The Court provided a checklist to assess compliance, including setting specific targets, regularly updating them, and providing evidence of efforts made (para 550).

Lastly, both bodies agreed that the level of compliance should be tailored to the specific circumstances of each State. Rather than imposing a uniformly applicable standard, they allowed States flexibility to develop and implement necessary policies and legal frameworks according to their capacities (ECtHR, para 547; ITLOS, para 241). This underscores that due diligence obligations of States focus on conduct rather than achieving specific results. Therefore, even if States do not meet GHG reduction targets, demonstrating sincere efforts aligned with their capabilities would absolve them from heightened responsibility for exacerbating climate change.


The judgments of the ECtHR and ITLOS are landmark decisions that delineate the specific due diligence obligations of States regarding climate change. Both courts have reinforced the concept of due diligence as a distinct obligation, not merely a standard of compliance. They have clarified these obligations sufficiently, rendering specific treaty provisions unnecessary to hold States accountable for insufficient efforts to reduce GHG emissions. Additionally, the judgments afford States flexibility in determining the binding nature of their emission reduction commitments. According to McDonald, this broad interpretation of due diligence obligations fosters greater acceptance among States by allowing them to set their own policy objectives. However, while these judgments clarify State responsibilities, they do not address the consequences of failing to meet these obligations. It is hoped that the pending advisory opinions of the International Court of Justice and the Inter-American Court of Human Rights will explore this aspect further.

Divyanshu Sharma is a B.A. LL.B.(Hons.) graduate from National Law University Delhi, and will soon be joining S&R Associates.

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