Jutta Brunnée
University of Toronto

I. Introduction
In the context of a book project on ‘community obligations’ in International Law, I recently had occasion to consider the interplay between procedural duties and the due diligence aspect of the harm prevention rule under customary international environmental law.^[1] At first glance, this topic might appear to cover technical but ultimately straightforward terrain. That was certainly my own starting assumption. After all, international environmental lawyers routinely distinguish between ‘substantive’ and ‘procedural’ obligations.^[2] Substantive rules set out standards that must be met through states’ actions or conduct, the obligation to prevent serious transboundary environmental harm being one example. Procedural obligations include the duties to notify, warn, inform, or consult states potentially affected by transboundary impacts, and to undertake (transboundary) environmental impact assessments (EIA).
In teaching and writing about international environmental law over the years, I had always proceeded from the premise that, legally, these procedural obligations are both an element of the harm prevention duty and independent of it. This conclusion seemed obvious, because states do not owe an absolute duty to prevent transboundary environmental harm, but a duty to take diligent measures to avoid such harm.^[3] Hence, in addition to taking appropriate regulatory and policy measures to avoid transboundary harm, states must undertake EIAs and notify, inform or consult with potentially affected states, as the case may be. Depending on the circumstances, a failure to do so can amount to a lack of due diligence and, therefore, constitute a violation of the harm prevention duty. At the same time, the procedural obligations surrounding the harm prevention rule also have an independent existence in customary law, and so can be violated independently of the substantive duty. It all seemed clear.
But then I read the International Court of Justice’s (ICJ) most recent decision on the harm prevention rule, in the inter-connected cases of Costa Rica v Nicaragua and Nicaragua v Costa Rica,^[4] and reread some of the other recent case law on the topic. It turns out that things are not quite so straightforward, or at least the ICJ’s jurisprudence isn’t. The Court’s 2010 decision in the Pulp Mills case and the 2011 advisory opinion of the Seabed Chamber of the International Tribunal on the Law of the Sea (ITLOS) on Activities in the Area,^[5] suggested growing efforts to flesh out the due diligence standard, and to clarify the contribution of procedural obligations to environmental protection and harm prevention. The ITLOS Seabed Chamber’s opinion even cast the due diligence standard as a conceptual bridge between harm prevention and precaution. But the ICJ’s Costa Rica v Nicaragua / Nicaragua v Costa Rica does not take up this progressive aspect of the ITLOS Chamber’s understanding of the due diligence standard. Indeed, the judgment suggests that even settling the specifics of the connections between procedure and substance, especially the notion of due diligence, is far more complex than one might imagine. Particularly contentious, it seems, is the question whether the (substantive) harm prevention duty is violated when a state does not exercise due diligence, for example by failing to undertake an EIA, but when no significant transboundary environmental harm is ultimately caused.
I begin with a brief introduction to the harm prevention rule. Next, I consider the procedure-substance framework that emerges from the Costa Rica v Nicaragua / Nicaragua v Costa Rica decision. I conclude by suggesting that the Court’s approach may strengthen the preventive aspects of the transboundary harm rule by clarifying the circumstances in which the related procedural obligations are triggered. But it may also introduce some confusion regarding the relationship between procedural and substantive duties, and the role of the due diligence dimension of the central substantive rule of customary international environmental law.
II. The Duty to Prevent Environmental Harm: Procedure and Substance According to the ICJ
International environmental law remains grounded in concepts intended to balance competing sovereign interests. Under the foundational ‘no harm’ rule, states’ rights to use their territories and resources are limited by the obligation to avoid significant transboundary harm, but neighbouring states must tolerate harm that remains below that threshold.^[6] States’ duties to prevent transboundary harm caused by activities undertaken in their territories have since been affirmed and fleshed out through numerous multilateral environmental agreements and other instruments, and several judicial decisions. The ICJ first confirmed in its 1996 advisory opinion on Legality of the Threat or Use of Nuclear Weapons that ‘the general obligation of States to ensure that activities within their jurisdiction and control respect the environment of other States or of areas beyond national control is now part of the corpus of international law relating to the environment.’^[7] The Court reiterated this conclusion in its 1997 decision in the Gabčíkovo-Nagymaros case and its 2010 decision in the Pulp Mills case.^[8]
In the Pulp Mills case, Argentina alleged that Uruguay had breached both substantive and procedural obligations under an agreement pertaining to the River Uruguay. In the course of its decision, the ICJ engaged at length with the relationship between procedural duties, including a new EIA requirement ‘under general international law’,^[9] and the substantive obligation to prevent significant transboundary harm under customary law.^[10]
The ICJ emphasized that procedural obligations have a separate existence – they can be violated, for example, even when significant transboundary harm does not ultimately occur.^[11] At the same time, as I already noted, the substantive harm prevention obligation is not absolute. Rather, the Court confirms, it demands that states meet the requirements of due diligence in their efforts to prevent significant transboundary impacts.^[12] In satisfying these requirements, states must ensure ‘not only the adoption of appropriate rules and measures, but also a certain level of vigilance in their enforcement and the exercise of administrative control applicable to public and private operators.’^[13] Furthermore, due diligence requires appropriate procedural steps to avert transboundary impacts, such as an EIA and notifying, informing or consulting potentially affected states.^[14]
In view of the outcome of the Costa Rica v Nicaragua / Nicaragua v Costa Rica, to which I turn below, it is worth noting that the ICJ did not hold in Pulp Mills that a failure to meet a procedural requirement necessarily implies a violation of the harm prevention duty, as Argentina had argued.^[15] The Court’s decision is ambiguous to the extent that its comments on this point focused on the interpretation of the treaty between the parties whereas, elsewhere in the judgment, it clearly connected procedural duties and due diligence.^[16]
This ambiguity carried over to the Court’s handling of the inter-related disputes regarding sovereignty over territory and various activities near a boundary river between Costa Rica and Nicaragua. In its decision on Costa Rica’s complaint concerning Nicaraguan dredging, the ICJ observed that ‘to fulfil its obligation to exercise due diligence in preventing significant transboundary harm, a State must … ascertain if there is a risk of significant transboundary harm, which would trigger the requirement to carry out an environmental impact assessment.’^[17] The Court also held that, if the assessment confirmed the risk, due diligence may require the state planning the activity ‘to notify and consult in good faith with the potentially affected State.’^[18] Then, however, the Court opined that, given the absence of the requisite risk, ‘Nicaragua was not under an obligation to carry out an environmental impact assessment … [and so also] was not required to notify, or consult with, Costa Rica’ regarding its plans.^[19] This conclusion might be explained by the Court’s view that, on the facts of the Costa Rica v Nicaragua case, none of the procedural or due diligence requirements had been triggered. But the Court’s assessment of Nicaragua’s complaint concerning Costa Rica’s road building suggests there is more to its approach.
In the Nicaragua v Costa Rica case, the ICJ found that the road building plans did trigger the obligation to undertake an EIA, an obligation that Costa Rica had failed to discharge.^[20] The Court then concluded that there was no need to examine whether Costa Rica had a duty to notify or consult, since it had ‘not complied with its obligation under general international law to perform an environmental impact assessment prior to the construction of the road.’^[21] It is unclear whether the Court considered states’ procedural duties to be strictly sequential, with notification and consultation being contingent upon a prior finding of risk through an EIA, or whether it considered that the failure to conduct an EIA had already established a failure to act diligently. The former may be the more plausible reading of the decision, given that the ICJ drew on the International Law Commission’s Draft Articles on Prevention of Transboundary Harm, which set out a sequenced approach to procedural duties.^[22] In any event, as in the Pulp Mills case, the ICJ concluded that, absent evidence of significant transboundary harm, there was no violation of the ‘no harm’ rule.^[23]
Strongly worded separate opinions on the procedure-substance question hint at just how much the Court wrestled with the relationship between the two. ‘In the planning phase [of a project],’ wrote Judge Donoghue in her opinion, ‘a failure to exercise due diligence to prevent significant transboundary harm can engage the responsibility of the State of origin even in the absence of material damage to potentially affected States.’^[24] In emphasising the importance of due diligence throughout all phases of a given project, Judge Donoghue added forcefully that she did ‘not find it useful to draw distinctions between “procedural” and “substantive” obligations, as the Court has done.’^[25] It is important to note, however, that Judge Donoghue questioned whether the obligation to conduct an EIA had in fact emerged as an independent procedural obligation at customary law. Accordingly, she chose to read the Court’s description of it as an obligation under ‘general international law’ in Pulp Mills as referring to its grounding in due diligence requirements.^[26]
Judge Dugard’s separate opinion reached the opposite conclusion: the term ‘general international law’ had to be understood as denoting ‘a rule of customary international law requiring an environmental impact assessment to be carried out where there is a risk of transboundary harm.’^[27]  He found that this duty was independent from the obligation to take diligent steps to prevent transboundary harm, as well as subject to its own due diligence standard, which served to determine its scope.^[28] Judge Dugard shared Judge Donoghue’s dissatisfaction with the Court’s failure to distinguish between breaches of the harm prevention obligation and of the obligation not to cause transboundary harm. However, according to the judge, treating the due diligence obligation as the source of the EIA obligation risked allowing a state to argue in hindsight that, in the absence of proven harm at the time of the proceedings, ‘no duty of due diligence arose at the time the project was planned.’^[29] In Dugard’s view, the ICJ adopted ‘[t]his backward looking approach’ in its judgment. Assessing the EIA obligation separately, he added, would help avoid the shortfalls of that approach, notably by clarifying that the threshold for triggering the obligation ‘is not the high standard for determining whether significant transboundary harm has been caused but the lower standard of risk assessment.’^[30]
Judge Dugard’s conclusion that an independent EIA obligation arises at customary law finds support in the ITLOS Seabed Chamber’s advisory opinion on Activities in the Area.^[31] In this assertive opinion, and building on the ICJ’s observations concerning due procedural requirements and due diligence in the Pulp Mills case, the ITLOS Chamber also emphasised the contextual nature of the due diligence standard, which it considered may change over time and in light of the risks involved in a given activity.^[32] As such, due diligence provides a bridge between the duty to prevent environmental harm and the proposition that, even in the absence of ‘full scientific certainty’, states must take precautionary measures to ‘prevent environmental degradation.’^[33] According to the Chamber, ‘the precautionary approach is … an integral part of the general obligation of due diligence.’^[34] For the ITLOS Chamber, therefore, the due diligence obligation requires states ‘to take all appropriate measures to prevent damage … [and] applies in situations where scientific evidence concerning the scope and potential negative impact of the activity in question is insufficient but where there are plausible indications of potential risks.’^[35] The Chamber concluded that a state ‘would not meet its obligation of due diligence if it disregarded those risks.’^[36]
While this perspective seems sensible in many respects, it remains to be seen whether its sliding scale approach to prevention and precaution will be embraced by international practice.^[37] The most significant dimension of this approach to prevention and precaution is the increased importance that it accords to procedural obligations, including in particular EIA obligations, and the lowering of thresholds that it entails for the triggering of these obligations. However, whereas Judge Dugard’s separate opinion in Costa Rica v Nicaragua / Nicaragua v Costa Rica built on these dimensions of the ITLOS Chamber’s Advisory Opinion, the ICJ as a whole did not engage with the Opinion in its own discussion of due diligence and of the risk threshold necessary to trigger the EIA obligation. One might speculate that, if the Court was unable to agree on the specifics of the legal relationship between the procedural and substantive aspects of the harm prevention rule, endorsing due diligence as a bridge between prevention and precaution, as the ITLOS Seabed Chamber’s Advisory Opinion had done, was more than one bridge too far for the Court to cross.
III. Conclusion
The recent instances of transboundary environmental dispute settlement suggest that procedural obligations have considerable potential to strengthen the preventive aspects of the transboundary harm rule, as well as flesh out – or support, depending on one’s reading of the ICJ case law – its due diligence standard. Procedural obligations can also serve useful purposes when states, or judges, are reluctant to entertain substantive arguments, or find it difficult to establish that environmental harm has been, or is being, caused by another state. Violations of procedural obligations are more easily established and, by holding states to their procedural duties, they can sometimes be prompted to correct harmful conduct, or at least to take more effective preventive measures going forward.
Although the ICJ did not follow the progressive course charted by the ITLOS Chamber, and notwithstanding the ambiguities in its case law, the procedure-substance framework revolving around the harm prevention rule could be a potentially powerful tool in protecting the environment, potentially even in a global context. Rather than having to prove, for example, that a given state’s greenhouse gas emissions have caused or will cause specific harmful impacts, such as sea level rise, concerned states could focus on the preventive dimension of the no harm rule, holding the emitting state to its procedural duties.^[38] The emphasis would shift from a relatively amorphous ‘negative’ duty to avoid harm to a ‘positive’ duty to take concrete steps to protect the environment,^[39] including the substantive steps a state may have to take to meet its due diligence obligation once risks are revealed.^[40] Furthermore, an initial finding of risk through an EIA would not only specify the triggers of the duties to notify and consult, but also reinforce the proposition that international law requires states ‘not merely to notify what is known but to know what needs to be notified.’^[41]
At the level of substantive law, however, it appears as if the ICJ distinguishes between the duty to take diligent steps to prevent significant transboundary harm, which it then deals with under the rubric of separate procedural obligations, and the duty to take diligent steps not to cause harm, which it considers cannot be violated simply by a failure to act diligently. While the latter conclusion is clearly correct, international environmental lawyers have commonly considered that violations of procedural obligations do breach the harm prevention duty, even if no transboundary harm results from that breach.^[42] That view appears to have been shared by some of the judges who wrote separate opinions in Costa Rica v Nicaragua / Nicaragua v Costa Rica. The upshot is that the substance-procedure framework of international environmental law, as well as the legal status of the EIA obligation, may be less settled than it might appear at first glance.
What then to make of the ICJ’s recent decisions on the harm prevention rule? One might say that the Court has taken two steps forward and one step back. The Court moved forward in concretizing the procedural aspects of the harm prevention framework, in particular the circumstances in which various procedural obligations are triggered. It appeared to move backward, however, in obscuring the relationship between procedure and substance, notably the due diligence duty of the harm prevention rule. We have to wait and see whether this turn to procedure proves to be mainly an effort to sidestep substance, or a pragmatic approach to enhancing the preventive power of customary international environmental law, as I certainly hope. For now, it seems that the price for greater clarity in preventive practice is confusion at a higher (read conceptual) level.