The Quest for Standard of Proof Required for Invoking Climate Change Obligations before the ICJ

[Quazi Omar Foysal is a Bangladeshi-qualified international lawyer, currently pursuing a PhD at La Trobe University, Australia]

Given that the Request for Advisory Opinion on Obligations of States in respect of Climate Change was limited to questions related to the obligations of States concerning climate change and the legal consequences of their breach in international law, the International Court of Justice (ICJ or Court) confined itself from making any in concreto determinations in its Advisory Opinion of 23 July 2025 (para. 106). It is projected that the consolidation and clarification of climate change related international obligations and confirmation of the erga omnes (and also erga omes partes) character of such obligations may prompt States to consider contentious proceedings before the ICJ. Interstate proceedings will likely give rise to evidentiary issues. In this context, considering the science-heavy nature of the climate change obligations, Judge Nolte noted that science has a necessary role in the determination of the breach of climate change-related obligations, but the essence remains the legal determination (Separate Opinion, para. 20). For this reason, the issue of “standard of proof” lies at the heart of such legal determination exercise. 

Against this background, this post argues that the evidentiary standards applicable to climate change-related obligations at the ICJ should be examined through the theoretical framework, which has been developed from the Court’s case law. In this context, the ICJ’s environmental jurisprudence can serve as a guide for selecting an appropriate standard of proof, while simultaneously providing a means to validate that choice through the application of the theoretical framework.

Defining the Standard of Proof: Inside the ICJ’s Invisible Rulebook on Proof

Kazazi defined “standard of proof,” in respect of international litigation, as “a measure against which the value of each piece of evidence as well as the overall value of the evidence in a given case should be weighed and determined.” It should be distinguished from “burden of proof,” which determines who should furnish the evidence, “forms of evidence,” which deal with different categories of evidence admissible before the courts and tribunals, and “standard of review”, which refers to the degree of scrutiny or deference the Court applies when evaluating an assessment of facts or law by any domestic authority. Though it is practicable to have a clearly defined standard of proof for the ICJ, as highlighted by Judge Higgins in her Separate Opinion in the Oil Platforms case (2003), the absence of any reference to standard of proof in the ICJ’s legal framework, the wide array of cases entertainable by the Court, and the divergence of legal traditions (civil law and common law) of the judges mean that the Court tends to rely on its inherent power to adopt a standard of proof for a particular case or a particular element of a case (see Riddell and Plant, and Farnelli). As a result, the ICJ has resorted to different standards of proof in different cases, and sometimes in a single case, without articulating them (see Farnelli).

An Attempt to Theorize the Standards of Proofs of the ICJ

In the absence of any explicit reference to standards of proof in the ICJ’s legal framework, scholars such as Riddell and Plant, and Del Mar have sought to theorize the Court’s choice of particular standards by analysing its jurisprudence. To start with, Riddell and Plant categorized three types of cases for the purpose of determining the standard of proof. The first type involves cases where the Court is entrusted with assessing competing claims not amounting to international responsibility. This type of case can be illustrated by boundary disputes, and the required standard of proof is “on the balance of probabilities” or “preponderance of evidence.” This standard implies that a claim is proven if it is more likely than not to be true (or there is more than 50% of certainty). The second type of case involves cases where the Court is tasked with determining international responsibility, but the claims fall short of “charges of exceptional gravity.” This type can be illustrated by disputes related to treaty application and interpretation, and it requires a standard higher than balance of probabilities but lower than that required for “charges of exceptional gravity.” The third type includes “charges of exceptional gravity,” exemplified by the Genocide Convention cases or the use of force cases. In those cases, the appropriate standard of proof is “fully conclusive.”

Del Mar attempted to theorize the choice of a standard of proof from three points of view. First, the ICJ’s choice of a standard may be subject to the nature of its adjudicatory functions. According to Del Mar, the ICJ has two broad categories of functions, i.e., declarative and determinative. On the one hand, the Court engages in declarative functions while dealing with territorial or maritime boundary disputes. On the other hand, the Court is tasked with the determinative function while assessing allegations of breaches of obligations under international law. Generally, the ICJ applies a lower standard of proof while discharging declarative functions and a relatively higher one while engaging with determinative functions. Second, the ICJ adopts different standards of proof depending on the manner of the breach of an obligation. For instance, the ICJ requires, as exemplified by the Corfu Channel (1949) and Bosnian Genocide (2007) cases, a higher standard of proof for breaches by commission, while it requires a lower one for breaches by omission. Third, the choice of standard is also dependent on the importance of the norm allegedly breached. For instance, the ICJ requires a higher standard of proof for breaches of jus cogens norms (like genocide) and a lower one for ordinary obligations.

Regarding charges of exceptional gravity, the ICJ has not yet provided a clear definition or articulated the rationale for applying a higher standard of proof. Del Mar suggests that such charges may be identified based on (i) the manner in which a norm is violated and (ii) the importance of that norm, typically involving breaches of jus cogens norms by commission. She further observes that the serious legal consequences and potential political repercussions of violations of this kind justify the adoption of a higher evidentiary threshold.

In relation to standards of proof for charges falling short of exceptional gravity, as mentioned by Riddell and Plant, and for breaches of norms not amounting to jus cogens, as mentioned by Del Mar, Farnelli illustrates that the ICJ mentioned different standards of proof in different cases but summarizes that these standards may qualify as a single standard. He suggests that the Court adopts the probabilistic approach in employing the standard of proof for those cases. Based on the literature available in this area, he suggested that this standard, which requires the conviction of the Court by high probability, is “‘clear and convincing evidence,’ rather than ‘preponderance of evidence.’” According to Grossen (see also O’Connell), this standard, borrowed from the US legal system, demands proof higher than the civil standard of preponderance of evidence, but lower than the criminal standard of beyond reasonable doubt (or the “fully conclusive” standard required for charges of exceptional gravity). Notably, the tribunal in the Trail Smelter Arbitration (1941), one of the earliest international cases involving environmental components, employed the “clear and convincing evidence” standard in resolving the dispute regarding transboundary pollution between the US and Canada.

Standard(s) of Proof Employed by the ICJ in Earlier Cases Involving Similar (Environmental) Obligations

In order to test the appropriateness of the theoretical framework discussed above in climate change-related obligations, this post suggests making an inquiry into the standards of proof in cases involving similar (mostly environmental) obligations. Although environmental and climate change obligations differ in certain respects, they, as outlined in the next section, share important similarities, particularly in respect of their nature. Accordingly, the standards of proof employed by the ICJ in environmental cases can help substantiate those applicable to climate change–related obligations as derived from the application of the theoretical framework developed in the preceding section.   

Four cases are particularly relevant. To begin with, in the Pulp Mills case (2010), Argentina alleged that Uruguay breached certain obligations of the Statute of the River Uruguay 1975, including the obligation to prevent transboundary pollution in the River Uruguay and certain procedural obligations. At the outset, it can be said in general that these obligations under scrutiny in that case share similarities with climate change-related obligations. In the judgment, the Court referred both to “clear” (paras. 225, 243, 257, 259, 264) and “convincing” (paras. 189, 228), together with “sufficient” (paras. 254, 262). Thus, it is safe to conclude that the Court employed the “clear and convincing evidence” standard in that case.

Interestingly, Judge Greenwood, while accepting that the violations of environmental obligations do not amount to charges of exceptional gravity, considered that the required standard of proof for the obligations in question was the “balance of probabilities” standard, the applicable one for civil litigation in common law (para 26). However, it appears that he equated “clear and convincing evidence” standard with “balance of probabilities” standard, rather than proposing a different standard in that case. This position can be adduced from his comparison between allegation of breach of environmental obligations under a water sharing treaty and that of charges of exceptional gravity under the Genocide Convention (para. 26).

In the Whaling case (2014), Australia alleged that Japan breached certain obligations related to the prohibition of commercial whaling under the International Convention for the Regulation of Whaling 1946. The ICJ, as Farnelli remarked, took a probabilistic approach and found the facts plausible based on the evidence presented (para. 206). The use of “convincing” in the Separate Opinions of Judges Xue (para. 14) and Trindade (para. 82) and the Dissenting Opinion of Judge Owada (para. 22) indicates that the Court employed the “clear and convincing evidence” standard.

In the Certain Activities and the Construction of a Road cases (2015), Costa Rica, on the one hand, alleged that Nicaragua breached its substantive obligations concerning transboundary harm and its procedural obligations regarding conducting environmental impact assessments and furnishing notification. On the other hand, in the Construction of a Road case, Nicaragua contended that Costa Rica violated similar substantive and procedural environmental obligations. In its Merits Judgment of 16 December 2015, the ICJ assessed the allegations on the basis of the “convincing” standard (para. 119).

To sum up, the ICJ’s approach regarding standards of proof in the aforementioned cases highlights that it has consistently applied the “clear and convincing” standard in environment-related cases.

Climate Change Related Obligations and the Standards of Proof of the ICJ

Before moving to identify the relevant standard(s) of proof, it is practicable to shed some light on the nature of the obligations involved in the context of climate change. While this post does not aim at providing a complete analysis of the (most directly relevant) obligations related to climate change discussed in the Advisory Opinion, both under treaty law and customary international law, an attempt to explore their generalized nature will, as highlighted in the previous section, assist us in pointing out the standard(s) of proof of the ICJ.

In the Advisory Opinion, the Court analysed the obligations of States under the Charter of the United Nations, the United Nations Framework Convention on Climate Change (UNFCCC), Kyoto Protocol, Paris Agreement, the United Nations Convention on the Law of the Sea, several environmental treaties, human rights law, and customary international law. The Court held that climate change-related obligations may be categorized as both obligations of result, which require strict compliance, and obligations of conduct, which require the best efforts of the States concerned (para. 175). For example, it concluded that certain obligations enumerated in Article 4, paragraph 1 of the UNFCCC are obligations of result (e.g., the obligation to formulate and publish national programmes) while certain obligations are obligations of conduct (e.g., the obligation to co-operate in the development and diffusion of technologies and practices related to control, reduction, or prevention of anthropogenic emissions of greenhouse gases) (para. 203). However, it will not be an overstatement to conclude that the majority of climate change-related obligations are obligations of conduct, and States are required to satisfy the (stringent) due diligence standard of conduct to fulfil such obligations (see paras. 280-292).

As noted earlier, climate change–related obligations under customary international law are erga omnes, while those arising from treaty law are erga omnes partes (Advisory Opinion, para. 440). However, they do not possess the status of jus cogens norms. This can be particularly adduced from the Declaration of Judge Tladi, who has earlier served as the Special Rapporteur of the International Law Commission on jus cogens. While he has highlighted the Court’s tendency to conflate the effects of obligations erga omnes (and erga omnes partes) with those of jus cogens norms, he notably refrained from characterizing climate change–related obligations as jus cogens.

Returning to the quest for the standard of proof, it is now pertinent to test the climate change obligations through the theoretical prisms. Following Riddell and Plant’s categorization, the allegations of breaches of climate change obligations involve treaty application and interpretation, and their positive findings would lead to State responsibility. However, since these obligations do not possess jus cogens status and are typically breached through omission rather than commission, such allegations cannot properly be characterized as “charges of exceptional gravity.” Thus, the appropriate standard of proof for those allegations would be “clear and convincing evidence.” Following Del Mar’s categorization, (1) the Court will be engaged with determinative functions while dealing with climate change obligations, (2) the obligations related to climate change, though erga omnes (and erga omnes partes) in nature, do not have the character of jus cogens, and (3) the Court will mostly deal with omissions rather than commissions in respect of climate change obligations. This deduction leads to the assumption that the ICJ would resort to the “clear and convincing evidence” standard. It is evident that the deductions based on both theoretical frameworks lead to the same result. This finding is also consistent with the previous case laws discussed in the earlier section.

Concluding Remarks

While there are still some remaining uncertainties regarding the standard of proof in the law and practice of the ICJ, the suggestion that the “clear and convincing” standard is the most appropriate applicable standard of proof in climate change-related cases would be both realistic and agreeable for the disputant parties, especially for Applicants. In the words of Judge Greenwood, “the nature of environmental disputes is such that the application of the higher standard of proof would have the effect of making it all but impossible for a State to discharge the burden of proof.” However, there are certain evidentiary issues that both potential litigating parties and the Court may need to explore in further detail. These include the treatment of scientific evidence, the interlinkage between causation and reparation, and the standards of review, among others. In particular, the ICJ’s treatment of the evidentiary standard for assessing the causation of damage at reparation phase (see Advisory Opinion, paras. 433–438) merits a separate inquiry. 

Though the ICJ has already dealt with science-heavy environmental disputes involving similar (or near-similar) obligations, the future climate change-related cases will be more like a déjà-vu than a jamais-vu case for the Court. Nevertheless, the Court should remain mindful that its Great Hall of Justice must not become an unfitting battleground for international lawfare, but rather a forum for advancing climate justice, as reflected in its Advisory Opinion of 23 July 2025.

Photo attribution: Photo by Marc Schulte on Unsplash