Invoking International Environmental Norms Through Treaty Interpretation

This article analyses the way in which international tribunals considering treaty-based disputes incorporate extraneous environmental principles through the use of interpretative mechanisms. Increasingly prominent in the international sphere, this approach allows States bringing claims under historical treaties to adopt and enforce contemporary understandings of environmental obligations. This article pursues an extensive survey of cases exhibiting this process, focusing on the interpretative techniques used; the extent to which the tribunals allowed for environmental arguments; and the basis, and use, of environmental norms. These results facilitate a comparative analysis, which concludes that tribunals’ choices regarding each of these features ultimately depends on the underlying treaty’s relationship with intertemporal law. This article thus provides a guide as to how States may effectively enforce environmental obligations, even absent explicit environmental enforcement mechanisms.


Introduction
International environmental law is largely encapsulated in soft law instruments, while the treaties that codify it often lack effective dispute resolution Brown LAPE 20 (2021) 235-266 and compensation mechanisms.1 Accordingly, for States wishing to litigate environmental issues on the international stage, the disjunct between the contemporary environmental obligations binding on States, and the treaties that provide a pathway to interstate litigation, poses a significant issue. However, methods of treaty interpretation offer a solution. Tribunals have increasingly adopted an 'evolutionary' approach to interpretation -whereby a treaty's substance is deemed capable of evolving alongside legal and factual developments2 -as a means of preventing the fragmentation of international law.3 This, in turn, facilitates the use of modern environmental norms to inform the interpretation of -and potentially re-substantiate -treaties that otherwise lack sufficient environmental obligations. The Iron Rhine arbitration provides one example of the impact such a method may have on State obligations: there, the Tribunal's use of an evolutionary approach allowed environmental norms to influence the allocation of costs pursuant to a nineteenth-century commercial agreement.4 Thus, a study modelling how States might take advantage of current judicial trends in the deployment of interpretative techniques is of great pragmatic significance, resolving issues arising from the absence of enforcement mechanisms in modern environmental treaties. This article provides a novel analysis of the interpretative processes used by international courts and tribunals to integrate international environmental norms into treaty obligations. It presents findings drawn from a detailed study of cases in which environmental norms were invoked, focusing on the interpretative devices used, and the concomitant consideration of environmental law. More particularly, this article questions the extent to which contemporary environmental norms -ranging from approaches articulated in soft law instruments, to those deemed customary by international courts -may be imported Invoking International Environmental Norms LAPE 20 (2021) 235-266 into a treaty, whether or not that treaty itself targets environmental issues. It investigates patterns across three key variables: (1) the tribunal's readiness to adopt environmental claims; (2) the interpretative techniques the tribunal adopts to incorporate environmental norms; and (3) the tribunal's engagement with, and source-based substantiation of, environmental norms. In this regard, this article finds that, while these variables are influenced by a number of factors, including the type of claim litigated and the norm itself, the determinative factor is the treaty's temporality, or, in other words, the time of the treaty's conclusion relative to the emergence of international environmental law, and the treaty's relationship with this legal regime.
This article proceeds in three parts. First, it establishes the connection between intertemporality and interpretative techniques, commenting on the paucity of scholarship considering this relationship's impact on the judicial incorporation of environmental law. Second, it outlines this study's scope, and sets out a methodology for resolving the gap identified in the literature. Third, drawing on a survey of selected cases in which environmental norms have been invoked through interpretation, it discusses trends in jurisprudence, vis-à-vis the three variables noted above. This article does not intend to consider every relevant case in depth; rather, by identifying central cases, it aims to demonstrate the malleability of interpretative techniques, and the differing treatments of environmental law that result from intertemporal constraints. Such an investigation is particularly significant for environmental law, given its relevance to, and concomitant application in, disputes across all sectors of State activity.

Outlining the Problem
An evolutive approach to interpretation, and the techniques adopted to achieve it, allow tribunals to adapt otherwise fixed obligations to factual and legal developments that alter the situations in which the obligations were originally intended to apply. This raises the question of intertemporality more broadly; namely, the extent to which a tribunal, hearing a claim long after the relevant treaty's conclusion -and thus distanced from its context -can give adequate effect to the parties' intent as expressed in the treaty's terms. Historically, the principle of contemporaneity was deemed to apply, whereby treaties were interpreted in light of the law at the time of their conclusion.5 However, this Brown LAPE 20 (2021) 235-266 approach was not adopted by the Vienna Convention on the Law of Treaties ('VCLT')6 due to the issue's complexity,7 nor by the ICJ in its advisory opinion on Namibia.8 Rather, the more satisfactory solution to the intertemporal problem incorporates the competing elements of stability -that obligations are definite and ascertainable -and change -that obligations should survive throughout a treaty's life, following developments in the corpus juris applicable to the State, or factual circumstances.9 Tribunals have largely implicitly approached intertemporality according to this balance, using party intention as the determinative factor in prioritising one element over another:10 they aim to interpret a treaty according to the law by which it was intended to be interpreted. This practice accounts for stark differences between tribunals' findings on the relevance of contemporary legal norms to older treaties. In his separate opinion in GabčíkovoNagymaros, for example, Judge Weeramantry commented that the relevant environmental standards for determining compliance were those extant at the time of application: "environmental rights are human rights … [and treaties] cannot be applied in such a manner as to constitute a denial of human rights as understood at the time of their application".11 Conversely, the Tribunal in the OSPAR Arbitration found that it was not "authorized to apply 'evolving international law and practice' and cannot do so", and that GabčíkovoNagymaros had not suggested that tribunals had an inherent authority to apply law in statu nascendi.12 (1957), 203, 212; Humphrey Waldock, "Third report on the law of treaties", Yearbook of the International Law Commission, Vol. II (1964), 6, 9. These authors have been critiqued for misconstruing the cases they cite in support of this Thus, this article posits the following hypothesis: a tribunal's willingness to engage with environmental norms is tied not to the treaty's age, but to its ability to ascertain an intention that the treaty be interpreted according to the law at the time of interpretation. Further, while this pertains to whether or not the treaty was intended to evolve at all, a tribunal will also refrain from the integration of certain subsequent developments where this implication would contradict party intention. For example, a treaty explicitly stating that environmental law is not relevant would preclude any interpretation based on environmental norms, regardless of its evolutive nature.
This hypothesis remains untested in the literature. Scholars who provide comprehensive appraisals of interpretation, or discuss particular processes, have necessarily focused on the interpretative tools themselves, at the expense of in-depth case studies.13 Similarly, discussions of intertemporal law, and its manifestation in international decisions, provide helpful contextual and analytical guidance, but do not focus on environmental law.14 A regime-driven analysis of interpretative processes has been applied to two areas: human rights, and the law of the sea. Discussions of evolutive interpretation in the context of human rights have been fairly comprehensive,15 and at points overlap with environmental law, on account of the environmental human rights provided for in human rights instruments.16 Treaty interpretation has also been considered vis-à-vis the law of the sea, most notably in the contributions to Del Vecchio and Virzo's 2019 co-edited volume:17 again, this 13 See, e.g., Julian Arato, "Subsequent Practice and where they engage with interpretation.26 Thus, a case-driven study of evolutive interpretation in environmental law -to French, "arguably the topic-area where other sources of law have most often been utilised within the treaty interpretation process " 27 -remains a desideratum of the field; the present article intends to remedy that gap.

Methodology and Scope
This article is based on a comprehensive investigation of the jurisprudence of the International Court of Justice ('ICJ'), the International Tribunal for the Law of the Sea ('ITLOS'), ad hoc arbitral tribunals, arbitral tribunals constituted under UNCLOS, and the World Trade Organisation ('WTO')'s Dispute Settlement Body. Relevant terms were used to search the records of judgments by each tribunal, and these results were cross-referenced with scholarship outlining cases pertaining to environmental law.28 Of these cases, those in which tribunals used interpretation to incorporate environmental obligations not found in the treaty under discussion were identified.29 These key cases were then analysed with a view to articulating precisely what interpretative methods were adopted, and how the environmental norm was articulated and used, with patterns between cases identified. This article aims to represent jurisprudential approaches across a range of arbitral and judicial fora; however, it proceeds on the assumption that courts and tribunals are largely consistent in their approach: differences based on 26 See, e.g., Alan Boyle, "The Gabčíkovo-Nagymaros Case: New Law in Old Bottles", forum are not investigated.30 As the cases assessed make clear, each tribunal considers itself bound by the VCLT, on the basis that its provisions on interpretation (Articles 31 to 33) are deemed a mirror of customary international law.31 Although the VCLT provides little guidance on evolutive approaches,32 tribunals adopting such approaches seek to align their reasons with the Convention's established principles. Furthermore, tribunals tend to consider the international legal order as a homogenous system: the goal of consistencyincluding in interpretative approach33 -is implicit in their decisions.34 Similarly, although tribunals have become more willing to engage with environmental law over time, norms have only been invoked routinely over the past 20 years.35 The selected cases reflect that recency.
This survey is restricted in two further ways. First, it focuses on interstate cases concerning treaty obligations. In each case, the matter involves only States, is heard in an international forum, and concerns the interpretation and application of an international instrument, with the environmental norm arising from that interpretation. This limitation avoids complications resulting from differences in domestic environmental obligations and domestic interpretative approaches to international obligations between States. Rather, the study intends to provide a more generalist analysis of interstate relations, and the enforcement of obligations in fora open to most States.
This criterion, significantly, excludes cases where express terms in the applicable treaty, or the interpretative approach applied to it, invoke those environmental obligations imposed by domestic or supranational systems. This primarily arises in expropriation claims in the context of investor-State 30 See treaty regimes: in these, governments may seek to prevent the operation of a venture liable to cause environmental damage on the basis of domestic environmental legislation.36 Under the General Agreement on Tariffs and Trade ('GATT'),37 parties may invoke Article XX as a defence to alleged breaches: the provision permits otherwise non-compliant domestic measures made in respect of one of a list of exhaustive matters, including to "protect human, animal or plant life or health". Cases brought pursuant to these treaties have engaged with issues of intertemporality: a tribunal may interpret a provision requiring deference to domestic legislation as incorporating subsequent environmental legislation, even though that type of domestic legislation was not envisaged by the parties upon accession.38 However, as this invocation of environmental law relies on the operation of pre-existing domestic provisions, the application of which is merely facilitated by the tribunal's interpretation of an international treaty, such instances fall beyond the scope of this article. Similarly, while the European Court of Justice has actively engaged with environmental law, with environmental protection being an "essential objective" of European Community law,39 the supranational nature of this regime places it beyond this article's scope.40 Second, the article excludes from its scope all cases brought pursuant to human rights treaties, including those between States in international fora. These treaties have frequently been used by individuals seeking prevention of, or compensation for, environmentally harmful activities. The European Court of Human Rights ('ECtHR'), for example, has considerable jurisprudence extending the right to a private life under Article 8 of the European Convention on Human Rights41 to encompass a prohibition on hazardous activities near residential areas,42 and domestic European courts have interpreted the article 36 See However, tribunals in this arena tend not to apply environmental norms, but rather merely recognise environmental harm as injurious to human rights. Furthermore, these bodies have developed a distinct doctrine of intertemporality not reflected in international law more generally;46 as such, their adoption of environmental norms indicates a willingness to integrate modern social values into human rights treaties, rather than environmental law per se. While these cases were thus excluded in the present survey, scholarship on the interaction between environmental and human rights law remains a fruitful source for comparative study.47

Discussion: Intertemporality and the Interpretative Process
As above, the focus of this article is the interpretative approach taken by international courts and tribunals when incorporating contemporary environmental norms in treaties lacking these same norms, and the results thereof. This investigation considers trends across three key variables shared between the selected cases: 1. whether the tribunal was willing to accept arguments on the relevance of environmental norms to the dispute; 2. if so, the interpretative techniques relied upon to incorporate the norm within the dispute's ambit; and 3. how the tribunal then engaged with the particular norm and concomitant obligation, including its effect, breadth and basis. As the discussion below explicates, each variable is primarily dependent on the underlying treaty's interaction with intertemporality. In other words, how each variable manifests in a case is predicated on whether the treaty evinces an intention that it be interpreted according to modern developments, and whether it indicates any objection to the incorporation of environmental law itself. Given the centrality of intertemporality in this discussion, the treaties interpreted in the selected cases have been categorised according to the drafters' knowledge of, and intention with respect to, environmental law and legal change more broadly. This categorisation allows for the easier identification of jurisprudential trends.
First, some drafters, while aware of environmental norms, declined to include them in treaties in which they might otherwise be relevant (Category 1). Second, other drafters failed to include norms due to the treaties' conclusion preceding the international environmental legal regime's emergence, but evinced a broader intention that the treaties' obligations evolve (Category 2). Third, others yet, though open to environmental law, drafted treaties on the cusp of the regime's emergence, and therefore engaged with principles only tentatively and implicitly (Category 3). Only the final category of drafter was able to, and explicitly chose to, include a number of (albeit, underdeveloped) environmentally-focused provisions (Category 4).
UNCLOS, for example, falls in Category 4: it is often described as an environmental treaty,48 and its Part XII comprises an extensive set of provisions pertaining to the protection of the marine environment. However, having been drafted in the late 1970s and early 1980s, following the first major multilateral treatment of environmental law in the Stockholm Declaration,49 the Convention 48 See The discussion below is structured by variable. Within every section, each category of treaty is represented, unless insufficient evidence arises for a category. The different effects of each category on the relevant variable are discussed through a detailed explanation of the applicable cases, with additional influential factors then set out.

The Tribunal's Willingness to Invoke Norms
Under what circumstances will a tribunal allow a claim based on modern environmental norms to affect a treaty-based dispute? While a number of interpretative devices potentially facilitate the inclusion of contemporary norms in the interpretative process, the 'rules' of interpretation are such that this invocation does not necessarily need to occur. Interpretation provides a set of guiding principles to be followed; however, as there is no "mechanical method of ascertaining meaning", tribunals can justify a number of different interpretations of the same text.52 Tribunals considering treaties without, or with only limited, environmental provisions (Categories 1 and 3) are reticent to accept jurisdiction over claims based on environmental law, even when the treaty on its face allows for these. In particular, the tribunals in both Indus Waters and Pulp Mills took an unjustifiably narrow approach to treaty terms, eschewing consideration of context and purpose so as to prevent extraneous environmental claims, while still accepting some environmental norms for specific uses.53 While this may also be reflected within Category 2 treaties, there was insufficient evidence to substantiate this claim.
Indus Waters (Category 1) concerned the construction of a project on Pakistan and India's shared watercourses, pursuant to the 1960 Indus Waters Treaty.54 Pakistan disputed India's entitlement to divert the watercourses and diminish the reservoir level of a run-of-river plant in a non-emergency scenario. The Tribunal in its Partial Award limited the environmental claims pleaded by adopting a textual approach to the treaty: it found that Article IV(6)requiring parties to avoid "any obstruction to the flow in [the rivers' natural channels] likely to cause material damage to the other Party" -solely concerned the physical condition of the channels.55 This was contrary to Pakistan's contention that "material damage" encompassed loss of natural habitat and ecosystem functions, the protection of which was required by international environmental norms.56 The Tribunal's conclusion was made on the basis of the provision's ordinary meaning, informed by the travaux préparatoires. However, the VCLT, on which the Tribunal expressly relied as an authoritative source for interpretative methodology,57 promotes reliance on elements beyond the ordinary meaning of the words58 -terms are to be considered "in their context and in the light of [the treaty's] object and purpose"59 -and indeed, the Tribunal followed this formulation elsewhere.60 The approach taken is also surprising: the term 'material damage' read plainly has a broad semantic range, and would appear to cover the circumstances alleged by Pakistan. The provision's context provides further support: Article IV concerns the river system generally, and includes protections for soil against erosion and dredging.61 The Tribunal's substantiation of its textual approach through the travaux préparatoires was also inappropriate: given their unreliability,62 such extraneous documents may only confirm an ambiguous, obscure, or absurd meaning subsequent to application of the general interpretative rule.63 Altogether, the Tribunal's patchwork use of interpretation, in contrast to its full application later in its Partial Award, betrays its interest in excluding possible environmental claims. Similarly, in Pulp Mills (Category 3), the ICJ found that it lacked jurisdiction to determine breaches of obligations under other international agreements and general law through an uncharacteristically narrow analysis of treaty provisions that Argentina argued were referral clauses.64 Argentina claimed several alleged procedural and substantive breaches by Uruguay of the 1975 Statute of the River Uruguay,65 arising from Uruguay's authorisation of the CMB Mill's construction, and the Botnia Mill's construction and commission, on the river constituting the boundary between the two States. When determining whether the treaty adopted external norms and instruments, the Court used the treaty's authentic Spanish text, which relevantly provided in English translation that the parties agreed "in order to establish the joint machinery … [and] 56 Ibid., [ in strict observance of the rights and obligations arising from treaties … in force for each of the parties". The Court found that the use of y ('and') before the phrase concerning external treaties limited those treaties' applicability to the implementation of the treaty itself, as they were linked to the parties' agreement. Similarly, it found that the reference to treaties in force for cualqui era ('any') party -not 'each' -indicated that the provision only acknowledged the parties' existing commitments. The Court took a similarly restrictive approach to Article 41 of the treaty, finding that the undertaking to protect and preserve the aquatic environment by adopting measures in accordance with applicable international agreements did not incorporate those agreements, but rather required States to exercise regulatory powers in conformity with them.
In making these findings without further consideration of context, object or purpose, the Court departed from its usual holistic approach to interpretation.66 Indeed, an expansive approach in light of the treaty's object and purpose, and reflected in subsequent State practice, was available. In oral proceedings, Professor Boyle, representing Argentina, noted that Uruguay's authorisation for the Botnia Mill expressly required it to comply with the Stockholm Convention on Persistent Organic Pollutants,67 indicating that Uruguay considered itself bound by external agreements in the course of its conduct.68 Most pertinently, it appears from oral argument that Uruguay agreed with Argentina's interpretation of Article 41 as a referral clause.69 Those cases may be contrasted with ITLOS' Advisory Opinion on the Area (Category 4). The Seabed Disputes Chamber had been requested to determine States' obligations with respect to the Area, and ultimately found that developing States were held to the same standard as all States.70 Although not called upon to discuss Environmental Impact Assessments ('EIAs'), the Chamber did so regardless, and found that the more specific articulation of the obligation to undertake an EIA in the Nodules and Sulphides Regulations was applicable to all States.71 It may be suggested that this approach is attributable to tribunals' 66 See

The Tribunal's Choice of Interpretative Techniques
A tribunal invoking environmental norms may do so on the basis of a myriad of interpretative techniques that allow for this process. A number allow direct incorporation, such as VCLT Article 31(3)(c), which requires an interpretation to take into account any relevant law applicable between the parties, and UNCLOS Article 293, which requires that tribunals apply compatible international legal rules in interpreting the Convention. Other techniques depend on the treaty, such as the concepts of 'generic' evolving terms, and the treaty's object and purpose. More generally, some tribunals merely note that certain treaties betray an evolutive intention, and adopt environmental norms on that basis. The following discussion investigates patterns in the usage of these techniques: it analyses circumstances where techniques may not be referred to at all, whether certain types of treaties preclude certain techniques, and finally, given these constraints, which techniques are preferred by tribunals in interpreting particular categories of treaty.

4.2.1
Where No Interpretative Techniques are Adopted Tribunals habitually reference environmental norms without recourse to interpretative techniques justifying their invocation. In Pulp Mills (Provisional Measures), in which Argentina sought the suspension of the mills project, the ICJ raised a number of environmental norms -quoting its prior jurisprudence on the prohibition of transboundary harm,77 and commenting that the present case highlighted the importance of sustainable development78 -without relying on interpretative techniques. Ultimately, the Court found that any violation of environmental norms could be remedied at the merits stage,79 and that there was insufficient evidence to suggest irreparable damage, as required for the grant of provisional measures.80 The lack of engagement with environmental legal principles likely resulted from their irrelevance to the Court's conclusion: that the Court referred to them at all merely reflected the case's formulation by counsel.81 This same approach is also evident in the merits stage of the proceedings. The Court determined that the parties' utilisation of the River Uruguay through joint machinery should allow for sustainable development; however, quoting its prior decision in GabčíkovoNagymaros, the Court found that "[i]t is for the Parties themselves to find an agreed solution that takes account of the objectives of the Treaty".82 By reducing the concept to one of cooperation, the Court merely repeated the explicit obligation to establish joint machinery as to provide an "institutional framework for close and ongoing cooperation":83 the invocation of sustainable development thus had no impact on the interpretation of the treaty, and so no interpretative technique was required. The Court's later description of Article 27 of the treaty as embodying the "balance between economic development and environmental protection"84 similarly had no effect on its decision. Although this conclusion is somewhat progressive -neither that Article nor the notification regime to which it refers specifically require consideration of the environment85 -several other provisions indicate this balance.86 Furthermore, the Court referenced the principle of prevention in determining when the notification scheme under Articles 7-12 was triggered, noting that it derived from the doctrine of due diligence, and formed part of general international law.87 However, the Court then found that the explicit obligation under those articles -to inform the river's administering body -allowed "the initiation of co-operation between the Parties which is necessary in order to fulfil the obligation of prevention".88 As such, satisfaction of the treaty obligation fulfilled the general international law obligation of prevention ipso facto: the external principle had no work to do, and so no interpretative basis was required for its consideration. Similarly, in GabčíkovoNagymaros, the Court discussed environmental norms in the context of legitimate grounds on which to terminate a treaty, and reaffirmed that the principle of prevention formed part of the corpus of general international law.89 However, as the Court found that environmental changes could not justify termination -meaning that Hungary could not justify its termination of the 1977 treaty governing its joint dams project with Slovakia90 on the basis of modern environmental norms -no further investigation of the norms' content was necessary. As such, it appears that tribunals refrain from referencing techniques primarily where they do not intend for a principle to impact their reasoning.

4.2.2
Where Treaty Categories Preclude Certain Techniques Consideration of the treaty drafters' intention is fundamental to an assessment of intertemporal law. Where their intention to incorporate a given norm cannot exist -either because the relevant treaty was concluded prior 84 Ibid., [177]. to the emergence of the surrounding regime, or because it excludes by omission the relevant category of norms -tribunals avoid the direct adoption of that norm (i.e., in Categories 1 and 2). This precludes the use of broadly applicable techniques, primarily the principle of systemic integration (as at VCLT Article 31(3)(c)). Rather, tribunals in these decisions demonstrate a preference for establishing that the drafters intended that the treaty evolve so as to incorporate the contemporary norm, with Article 31(3)(c) being of secondary importance in their reasoning. The Tribunal in Indus Waters (Partial Award) (Category 1) determined that India's right to divert the watercourses and diminish the reservoir level of a run-of-river plant in a non-emergency scenario was not absolute: Pakistan, under an express treaty provision, was owed a minimum flow, a reading confirmed upon interpretation in light of contemporary customary environmental law.91 In so interpreting, the Tribunal articulated the treaty's express terms on interpretation -which the Tribunal in fact relied on -as a mirror of the content of VCLT Article 31(3)(c). The former, however, provides a far more limited basis for invoking external norms, ultimately reflected in the Tribunal's findings.92 Similarly, the Tribunal drew on broader notions of systemic integration, as expressed by the ICJ in GabčíkovoNagymaros, merely to justify its ability to take into account contemporary norms, noting that this was required "even when (unlike the present case) interpreting treaties concluded before the development of that body of law" (emphasis added).93 This reflects the treaty's relationship with intertemporal law, and the consequent impact on interpretative techniques: as the relevant treaty was concluded after the emergence of environmental law,94 the drafters could have integrated this regime within the treaty, but, unlike many instruments governing international waters concluded at that time, chose not to.95 The explicit inclusion of a mechanism for systemic integration may have also tended against the Tribunal relying on other interpretative techniques; however, the hesitant approach it took to its discussion of evolutive techniques generally indicates a broader opposition to affording environmental norms too large a role. Indeed, while the Tribunal could have relied on interpretative approaches associated with following party intention, such as the treaty's object and purpose, or generic terms, to incorporate new norms,96 the Tribunal avoided their consideration, likely due to the fact that the drafters explicitly did not intend the result. Tribunals determining cases dealing with Category 2 treaties, on the other hand, are more willing to rely on VCLT Article 31(3)(c) itself. However, they deem the treaty evolutive as an initial step. The Tribunal in Iron Rhine, for example, in determining how the Treaty of Separation of 183997 construed the modernisation of the railway built pursuant to it, as well as the apportionment of this project's costs, adopted a two-step interpretative process. The Tribunal only relied on Article 31(3)(c) after it had first found that the treaty was intended to evolve, on account of the treaty's object and purpose;98 this was despite the arbitral agreement explicitly allowing the direct incorporation of present norms.99 The Tribunal, in noting that the treaty's terms could evolve, abstracted the treaty from its historic circumstances prior to invoking Article 31(3)(c), and so allowed for incorporation of the current legal order (including environmental law), rather than the legal order at the time of the treaty's conclusion two centuries earlier. Such reticence likely arises from the notion that systemic integration conforms to drafter intention:100 for it to be used to integrate contemporary law, an evolutive intention must be established.
The Appellate Body in ShrimpTurtle (Category 2) took a similar two-stage approach. This decision concerned whether the US' discriminatory prohibition on shrimp imports based on turtle protection mechanisms fell within Article XX of the GATT, which permitted otherwise discriminatory measures 96 These techniques still could be used even where the treaty being interpreted deliberately excludes environmental law, as they merely require an intention that the treaty evolve generally: Sondre Torp Helmersen, "Evolutive Treaty Interpretation: Legality, Semantics and Distinctions", 6(1) for the conservation of "exhaustible natural resources". 101 The Body set out the environmental object and purpose of the 1994 update to the GATT prior to deeming Article XX a generic term. 102 Notably, however, the latter determination is sufficient to sustain evolution of itself, and, significantly, the 1994 treaty's object and purpose ought to be irrelevant to the interpretation of Article XX, which remained unchanged from 1949. By contrast (and as will be discussed in Section 4.2.3 below), the tribunals in cases under Categories 3 and 4 do not consider the treaty's evolutive nature separately: both generic terms and systemic integration directly permit environmental considerations. 103 This distinction reflects the greater significance of justifying the integration of environmental norms in those situations where party intention could not have foreseen the regime's development.

4.2.3
Where Tribunals Prefer Certain Techniques for Invoking Norms It appears that -unless the treaty is explicitly environmental in naturetribunals prefer to base evolutive treaty interpretation on generic terms, likely due to this technique's direct relationship to party intention and capacity to be ascertained from the text alone. If the parties intended a term to evolve, finding such evolution is justifiable; conversely, the use of VCLT Article 31(3)(c) may appear to overstep intention in some cases, as it allows for the importing of external legal norms, regardless of whether the drafters intended this to occur.104 Further, the ICJ has found that generic terms must be presumed to evolve, as a consequence of intention.105 As French has argued, this allows tribunals to integrate developments under the guise of intention, regardless of the parties' actual intention, so long as the terms are sufficiently open.106 In situations where this is palatable -namely, where the treaty has not excluded environmental norms -it provides an accessible means to integrate modern law without further justification.
In GabčíkovoNagymaros, the ICJ found that several provisions relating to the creation of a joint contractual plan to protect the environment were Brown LAPE 20 (2021) 235-266 evolutive in nature:107 as such, while modern environmental norms did not justify Hungary's termination of the 1977 treaty, any newly developed norms could be incorporated, by agreement, into the parties' relations.108 Similarly, in Pulp Mills, the obligation "to protect and preserve" the aquatic environment under Article 41 of the treaty was considered generic.109 In neither of these cases did the Court defend this analysis deeming the terms generic. Further, the Appellate Body in ShrimpTurtle labelled the concept of "exhaustible natural resources" generic: while the Body supported the finding by reference to the underlying treaty's object and purpose, it too did not justify the 'generic' label.
Indeed, these approaches may be contrasted with the cases explored above. In Iron Rhine (Category 2), the Tribunal's use of Article 31(3)(c), instead of identifying a generic term, was out of necessity. The Tribunal initially discussed the evolutive effect of generic terms, but discounted this approach's applicability in the circumstances, as the dispute required consideration of external technical developments.110 On the other hand, the Tribunal's reliance in Indus Waters (Category 1) on systemic integration was likely due to the difficulty in legitimately finding an intention for terms to evolve to incorporate environmental law where this regime was clearly excluded. Rather, it was the treaty's express mandate there that provided a textual basis for the Tribunal to objectively ascertain an intention to draw in external norms.
Identifying generic terms is less pertinent where the underlying treaty is itself 'environmental' . In the cases brought under UNCLOS (Category 4), each tribunal's approach responds to that treaty's intentionally explicit environmental purpose, which implicitly justifies the invocation of environmental norms. Both the South China Sea Arbitration and the Advisory Opinion on the Area demonstrate the effects of this expansive basis: having discussed the significance of environmental protection to the UNCLOS regime, neither tribunal fully engaged with other interpretative techniques; rather, systemic integrationboth as a general principle,111 and through the explicit mechanisms of VCLT Article 31(3)(c),112 UNCLOS Article 293,113 and UNCLOS Article 237114 -could be relied upon without further consideration of intention. 115 Evolutive interpretation thus ultimately depends on ascertaining party intention,116 with tribunals chiefly considering objective intention -determined through the treaty text -rather than the drafters' state-of-mind,117 likely because the latter is less accessible and verifiable. The technique tribunals adopt tends to pursue drafter intention, to the full extent the treaty allows. It is likely for this reason that VCLT Article 31(3)(c) -a provision in a separate treaty, with little connection to party intention -is the least used: it appears in a supporting role, where all other techniques are unavailable, or where the underlying treaty inherently accepts the inclusion of environmental norms.

Engagement with and Basis of Environmental Norms
The above discussion on tribunals' habit of referencing norms without further engagement highlights a third key variable between cases: the character of the norm itself. This in turn raises three distinct issues: whether the norm invoked in fact affects the interpretation of a treaty provision, or indeed the outcome of the dispute; what content this norm is given; and what sources are used to justify its existence. Each is discussed in turn below. The Effect of Environmental Norms on the Case's Outcome How a tribunal engages with the content of legal norms absent from, but relevant to, the treaty being interpreted, correlates to the underlying treaty's relationship with intertemporal law. Necessarily, the more a treaty allows for the integration of environmental law, the more the tribunal is willing to engage in this process. As such, where the underlying treaty excludes environmental law (Indus Waters), the effect of this body of law on the dispute is negligible; where the treaty either lacks the capacity to incorporate environmental law (Iron Rhine; ShrimpTurtle) or does so to a minimal extent (Pulp Mills; GabčíkovoNagymaros), there is some engagement and application; and where the treaty's purpose concerns environmental protection, expansive integration and the application of a range of environmental norms is common (South China Sea Arbitration; Advisory Opinion on the Area).
By way of example, in Indus Waters (Category 1), the Tribunal's primary use of environmental law in the Partial Award was to substantiate its finding that a minimum flow for Pakistan was required: the Tribunal made this decision on the basis that the treaty terms indicated an intention to balance Pakistan's unrestricted use of the rivers against the express curtailments in the treaty;118 however, it noted that this conclusion "also stems from the Treaty's interpretation in light of customary international [environmental] law".119 Indeed, the Tribunal ultimately clarified that the parties had agreed that a minimum diversion was required, and differed only as to the level of the minimum, which it reserved for the Final Award.120 However, in the Final Award, the Tribunal severely constrained its references to environmental law: while the need to mitigate significant harm to the environment was to be taken into account, any findings beyond that were deemed prohibited by the treaty; it was neither appropriate nor necessary "to adopt a precautionary approach and assume the role of policymaker".121 Ultimately, the Tribunal settled on 9 cumecs as the minimum flow to Pakistan, a value lower than its determination of the minimum when exclusively considering environmental conditions -12 cumecs -and far below Pakistan's claim of 20-40, but nonetheless higher than India's posited flow of 3. 94 In Iron Rhine (Category 2), each party contended that the relevant treaty, controlling the construction of a road (extended to encompass the treaty's railway), imposed monetary liability according to a binary categorisation of the alterations: where the project constituted a new road or canal, Belgium was liable, while the Netherlands bore the full burden for maintenance.123 The Tribunal found that environmental protection measures were necessary for the modernisation, and so were fully integrated into the project and its costs, on the basis of the principles of sustainable development and prevention (thus burdening Belgium). However, the Tribunal mitigated this finding: where measures exceeded what was necessary to balance the environmental impacts of the railway's operation, and conferred some benefit to the Netherlands, the Netherlands was required to contribute a sum proportionate to that benefit.
The treaty underlying Pulp Mills (Category 3) allowed for a greater exploration of environmental norms: concluded in 1975, it provided for the protection of the marine environment, while maintaining a strong developmental focus. Environmental principles permeate the judgment. The Court held that Article 41, which required parties to protect and preserve the aquatic environment, reflected the prohibition against transboundary harm,124 and necessitated a "certain level of vigilance in [the rules'] enforcement and the exercise of administrative control".125 Further, as noted above, the Court found that the notification scheme incorporated the principle of prevention, and that EIAs were compulsory under customary international law, as a component of due diligence. On the other hand, the Court tempered this progressive and novel finding, and declined to substantiate what minimum requirements an EIA ought to possess for this obligation to be satisfied. Narrowing the broad finding to the present facts, the Court concluded that -despite Argentina's reliance on several instruments -no external law binding on the parties could provide guidance; rather, it was for each State to determine the content in its domestic legislation. This was an avoidable conclusion: as Fitzmaurice comments, the Court's appraisal of the obligation was "less onerous than obligations imposed by multilateral conventions",126 which largely require public consultation. Accordingly, customary law might impose certain obligations in EIAs; however, the Court's failure to survey State practice meant that it could merely Brown LAPE 20 (2021) 235-266 assert a de minimis standard.127 Additionally, the Court rejected the argument that the precautionary approach shifted the burden of proof to Uruguay, requiring it to establish compliance.128 Ultimately, both parties agreed on the obligation to conduct an EIA, and Uruguay had, in fact, conducted one, pursuant to its domestic legislation;129 rather, it was only the sufficiency of that EIA that was in dispute. By not articulating a minimum standard, the Court did not engage with the issues in dispute: indeed, it ultimately found no breach of Article 41.
Finally  (5), ultimately adopting the expansive definition articulated in CBD Article 2. Taken together, these provisions imposed a due diligence obligation, preventing the harvesting of species considered at risk of extinction. China, by tolerating and protecting harvesting, had breached this obligation, despite adopting regulations to the contrary.133 The impact of the underlying treaty category on the use of norms is evident by comparing Pulp Mills (Category 3): the applicable treaty in that case similarly imposed an obligation "to protect and preserve" the aquatic environment; however, as above, the Court there limited the environmental norms invoked under that provision. 127  It ought to be noted that these differences between categories may also result from aspects inherent to the treaty, not merely its relationship with environmental law. Kolb, for example, argues that the more a treaty provision is targeted at protecting the public good, the more likely it is to attract an evolutive interpretation.134 Similarly, where the treaty is attached to a specific project or relationship, tribunals articulate obligations with greater precision of substance, and their potential for alteration diminishes over time. On the other hand, 'constitutional' treaties, such as UNCLOS, confer broad, durable rights without specific content, such that modern legal developments are clearly encompassed by the treaty's purpose, and fit easily within the wording used. However, at the very least, the intertemporal explanation posited here serves to account for differences in the judicial treatment of treaties within Categories 1-3.
Similarly, differences in the effect of norms on a dispute may be explained by factors inherent to the norm invoked. Material difficulties arise when elucidating the content of, or applying, norms that are vague in nature, as a greater contribution is required of the interpreter.135 Sustainable development and the precautionary approach cause issues in this arena, as they rely upon State policy by their very nature. Sustainable development lacks a norm-creating character,136 and cannot be applied as a standard ipso facto;137 in the cases surveyed in which it was raised, it merely provided an expression of the balance otherwise inherent in the relevant treaty,138 or had little to no effect.139 Similarly, the content of the precautionary 'approach' is unclear, as are the Brown LAPE 20 (2021) 235-266 circumstances in which it may be breached. Of the cases in this study, this approach appears only in the Advisory Opinion on the Area, being rejected as inapplicable by tribunals in Indus Waters and Pulp Mills. Here, however, the Chamber merely suggested that the approach should be taken by the parties. 140 Indeed, even where the precautionary approach is found within a treaty, tribunals avoid drawing in external articulations of the norm: in the WTO proceedings in Beef Hormones,141 for example, the Appellate Body -despite finding that the relevant provision incorporated the approach -expressly avoided deciding on the approach's legal status, as it could not override the express provisions or the application of general rules of treaty interpretation.142 However, if the approach had been deemed customary, assumedly VCLT Article 31(3)(c) -a usual principle of treaty interpretation -would have rendered it relevant.

4.3.2
The Breadth and Content of the Norm Articulated Conterminous with the effect of environmental norms is the breadth of their articulation within each case. The above analysis indicates this tendency: while the Court in Pulp Mills (Category 3) articulated an obligation to undertake an EIA without further expanding on the specific conduct required, the Tribunal in the South China Sea Arbitration (Category 4) set out broad obligations to take positive steps to avoid harm.
However, there are four further factors that affect the breadth of the norm articulated.
First is the breadth of the treaty obligation founding the interpretation. The tribunals in the South China Sea Arbitration and Pulp Mills found that the treaty obligation to "protect and preserve" constituted the foundation of a number of other environmental norms. This term thus constitutes a 'framework principle' , which can incorporate other environmental norms according to the dispute, and, in turn, allows more contentious norms to be attached. ITLOS, in its Advisory Opinion on the Area, for example, viewed the precautionary approach as an aspect of due diligence:143 although the former's content and status in general international law is controversial,144 the latter -an accepted understanding of the obligation "to protect and preserve" -offered a convenient stepping-stone. In that case, the Chamber's earlier finding that Article 139 created an obligation of due diligence meant that this attribution embedded the precautionary approach within the UNCLOS regime,145 in parallel with the Chamber's extension of the approach in the Nodules Regulations and Sulphides Regulations to the UNCLOS regime more broadly. Articulating more controversial concepts as merely elements of established norms -being far more palatable than attempting to justify them as customary in and of themselves -is an approach facilitated by broad treaty terms. However, as exemplified by the differing uses of the framework principle in the South China Sea Arbitration and Pulp Mills outlined above, the breadth of the imported norms largely depends on the underlying treaty's relationship with environmental law.
A second factor impacting the breadth of the norm articulated is whether the parties in fact agree that the particular norm is applicable. In Indus Waters (Category 1), for example, India concurred that Pakistan ought to receive a minimum flow,146 such that the invocation of a number of broad norms in the Partial Award had no impact on the parties' situation. When India disagreed on the application of the norms -vis-à-vis the determination of the level of minimum flow -the Tribunal reverted to a far narrower approach.147 Similarly, the ICJ's progressive finding in Pulp Mills (Category 3) that EIAs were required by general international law was acceptable given that Uruguay had already conducted an EIA:148 the identification of a customary obligation did not retroactively place Uruguay in breach.
Third, where there is an instrument related to the treaty being interpreted that shares the same parties, the articulations contained in the secondary instrument are often transplanted across, facilitating the substantiation of broader norms in the primary instruments. This is notably seen in ITLOS' Advisory Opinion on the Area,149 in which the Chamber extended the precautionary approach's articulation in the Nodules and Sulphides Regulations, to the broader UNCLOS regime: the complete overlap in parties assisted in legitimising the imposition of this norm.
Finally, tribunals' effective enforcement of environmental norms appears to rely on the development of environmental principles in tribunals generally. While the Court's labelling of EIAs as custom in Pulp Mills had no effect there, this characterisation -and its characterisation of due diligence more generally -resulted in the imposition of EIAs as a customary norm in later cases. 150 The development of environmental law in ITLOS' provisional measures cases is emblematic of this tendency. In Southern Bluefin Tuna,151 ITLOS did not explicitly rely on the precautionary approach, although implied its possible relevance. However, in Ghana/Côte d'Ivoire,152 it cited this earlier comment, and relied upon it to directly incorporate the approach. 153 The more expansive these prior decisions, the more expansive an obligation is identified; conversely, where only a restricted norm is identified -such as the Tribunal in Iron Rhine's articulation of the principle of prevention -the more narrow its subsequent articulation (as in Indus Waters). 154 The considerable impact of judicial decisions in the environmental law regime has been noted;155 the development of the law by international tribunals, as judicial bodies, thus has a legitimising effect.156

4.3.3
The Basis of the Norm Articulated As noted above, tribunals indicate a preference for relying on statements in prior jurisprudence as the 'source' for an environmental norm's customary nature. This practice occurs even when not required: the Tribunal in the South China Sea Arbitration, for example, relied on statements in Nuclear Weapons, Indus Waters, and Iron Rhine, in addition to the CBD and CITES, despite its (acknowledged) ability to directly apply those treaties, through application of UNCLOS Article 237.157 Tribunals also eschew external sources in determining an external norm's customary nature where the underlying treaty obligation connotes an environmental norm itself. The tribunals in the South China Sea Arbitration and Pulp Mills both imported broad norms into the obligation to "protect and preserve", without exploring the customary bases on which their evolutive expansions relied. The ICJ in GabčíkovoNagymaros similarly held that the duty to protect water quality incorporated present environmental standards, including the prevention of harm, without reference to practice.158 Furthermore, tribunals rely on international instruments primarily as a source for norms' basic definitions, rather than their content per se: several cases cite the Rio Declaration for its expression of the precautionary approach,159 but fail to engage further with its content. Instruments are also used to substantiate evolutive definitions, as exemplified in the South China Sea Arbitration and ShrimpTurtle, both of which involved updating a term -'ecosystem' and 'exhaustible natural resources' , respectively -in accordance with current practice. The tribunals in each matter relied largely on environmental treaties -CITES and the CBD -with ShrimpTurtle relying on UNCLOS as an additional source.160

Final Thoughts
Overall, modern courts and tribunals, to varying degrees, are willing to adopt environmental norms in their findings, using a range of interpretative approaches to justify their invocation, each to various effect. These invocations, however, ultimately depend on the underlying instrument, and its relationship to both environmental law and the evolution of its terms. Whether a tribunal will allow an environmental claim, which interpretative devices it will use, and the likely extent of its engagement with norms are broadly predictable according to this model. With regard to the problem originally articulated -that the lack of recent environmental norms in older treaties, and the lack of enforcement mechanisms in contemporary ones, hamper States' ability to litigate issues of environmental law -this determination is of great significance. This article, in demonstrating when and how environmental norms are effectively used in litigation concerning historic treaties, provides direct guidance to States. In particular, where States interested in pursuing an environmental claim can choose between treaties as a basis for jurisdiction, the above discussion indicates that tribunals are more willing to incorporate and use expansive, contemporary norms in Category 3 and 4 treaties, than in those in Category 1.
In any case, the results of this article highlight the central role tribunals play in developing and enforcing international environmental law: in the absence of new multilateral treaty regimes, these institutions provide the ideal fora for progressive legal change, and it is imperative States use them for this purpose.