An analysis of systemic integration under Article 31(3)(c) of the Vienna Convention on the Law of Treaties in investor-State dispute settlement, examining treaty fragmentation, environmental and climate-related disputes, and the extent to which broader international law norms influence investment treaty interpretation.
Introduction
The relationship between international investment law and environmental protection has emerged as one of the defining tensions in contemporary international adjudication. Investor-State Dispute Settlement (ISDS), originally designed to protect foreign investment against arbitrary or discriminatory State conduct, increasingly finds itself addressing disputes arising out of environmental regulation, climate transition measures, and energy policy reforms. Regulatory interventions directed towards decarbonisation, renewable energy transition, emissions reduction, and environmental conservation have become frequent subjects of investment treaty arbitration. These developments have generated renewed attention towards the manner in which investment tribunals interpret treaty obligations in the context of broader developments in international law.1
The interpretive doctrine of systemic integration, reflected in Article 31(3)(c) of the Vienna Convention on the Law of Treaties, 1969 (VCLT), has assumed particular significance in this regard. Article 31(3)(c) provides that, together with the context, there shall be taken into account “any relevant rules of international law applicable in the relations between the parties”. The provision has frequently been understood as a mechanism designed to mitigate fragmentation within international law by ensuring that treaties are interpreted against the backdrop of the wider international legal system rather than as isolated or self-contained regimes.
In investment arbitration, systemic integration has increasingly been invoked in disputes involving environmental and climate-related regulation. States defending environment have sought to rely upon broader obligations arising under international environmental law, climate agreements, and principles relating to sustainable development. Tribunals, in turn, have been required to consider the extent to which such norms may inform the interpretation of investment treaty standards, particularly fair and equitable treatment (FET), expropriation, and legitimate expectations.
At the same time, the role of systemic integration in ISDS remains contested. Investment treaties are grounded in State consent and framed through specific treaty obligations directed towards investment protection. Environmental and climate-related norms, by contrast, often operate through broad framework obligations, progressive implementation mechanisms, and policy-oriented commitments. The interaction between these regimes raised questions concerning the extent to which environmental considerations may meaningfully shape investment treaty interpretation.
This article examined how systemic integration has enabled tribunals to engage with environmental and climate-related norms within investment arbitration. It argues that systemic integration has not fundamentally transformed the structure of investment protection, nor has it displaced traditional investment law standards as the central framework of analysis. However, systemic integration has increasingly provided tribunals with a doctrinal avenue through which environmental considerations and climate-related regulatory objectives may enter treaty interpretation. This engagement has been particularly visible in disputes arising under the Energy Charter Treaty (ECT), 1994, including the renewable energy disputes involving Spain, as well as in disputes concerning environmental regulation more broadly.
The article proceeds in six parts. Part II examines fragmentation within ISDS and the interpretative foundations of systemic integration under Article 31(3)(c) of the VCLT. Part III analyses the emergence of environmental and climate-related disputes within investment arbitration. Part IV considers the operation of systemic integration in arbitral jurisprudence, including the Argentine financial crisis cases, Vattenfall AB v. Federal Republic of Germany2, Rockhopper Italia S.p.A v. Italian Republic3, and the renewable energy disputes against Spain under the ECT. Part V evaluates the limits of systemic integration within the consent-based structure of ISDS. Part VI concludes that systemic integration does not eliminate fragmentation, it provides an important doctrinal framework through which environmental norms increasingly shape the interpretation of investment treaties.
Fragmentation and systemic integration in international investment law
1. Fragmentation and the structure of ISDS:
The fragmentation debate within international law is commonly associated with the multiplication of specialised legal regimes and adjudicatory institutions.4 International investment law exemplifies this development with particular intensity. The investment regime consists of thousands of bilateral and multilateral treaties drafted across different periods, employing overlapping yet non-identical language, and interpreted by Tribunals constituted for individual disputes.5
Unlike a permanent international court operating within a hierarchical judicial structure ISDS relies upon decentralised adjudication. Tribunals are not formally bound by precedent, notwithstanding the frequent citation of earlier awards as persuasive authority. Consequently, similar interpretive questions concerning FET expropriation, umbrella clauses, most-favoured nation treatment, police powers, and necessity have generated divergent lines of jurisprudence.6
Fragmentation in ISDS is not merely a technical problem of inconsistent awards; it reflects a deeper tension concerning the relationship between investment law and the wider structure of public international law.7 Investment disputes frequently arise from measures adopted in pursuit of public policy objectives including environmental protection, public health, financial stability, and energy transition. Where tribunals interpret investment treaties as self-contained instruments focused exclusively upon investor protection, there exists a risk that broader regulatory obligations undertaken by States receive insufficient consideration.
Several structural characteristics of ISDS intensify this problem. First, treaty design remains heterogeneous. Some agreements contain detailed provisions concerning public welfare regulation and sustainable development, while older treaties frequently rely upon broad standards with limited textual guidance concerning regulatory autonomy.8 Second, investment arbitration remains fundamentally consent-based. Tribunals derive jurisdiction from the treaty invoked by the claimant and may therefore hesitate to rely extensively upon external norms perceived to fall outside the scope of consent. Third, the absence of a standing appellate mechanism in most treaty frameworks limit opportunities for harmonisation.
These features have contributed to criticism that investment law risks operating as a semi-autonomous regime detached from broader international legal developments.9 It is within this context that systemic integration assumes significance as a mechanism directed towards interpretive coherence.
2. Article 31(3)(c) and the doctrine of systemic integration:
Article 31 of the VCLT establishes the general rule of treaty interpretation. Article 31(3)(c) specifically provides that interpreters shall take into account “all relevant rules of international law applicable in the relations between the parties”.10 The provision reflects the proposition that treaties operate within a broader legal system and should therefore be interpreted in light of other relevant international obligations.
The doctrine of systemic integration gained particular prominence through the work of the International Law Commission (ILC) on fragmentation in international law. The ILC identified Article 31(3)(c) as an interpretive mechanism capable of mitigating fragmentation by ensuring that specialised legal regimes remain connected to the wider framework of international law.11
Scholars have differed concerning the precise scope of systemic integration. A narrower approach confines Article 31(3)(c) to rules binding upon all parties to the treaty being interpreted.12 Broader interpretations understand the provision as reflecting the systemic character of international law more generally, thereby permitting wider engagement with related legal norms.13 Regardless of the precise approach adopted, the doctrine has become increasingly important in disputes involving competing or overlapping legal obligations.
For investment arbitration, systemic integration offers a principled method for situating investment treaties within the broader framework of public international law. If investment treaties form part of international law rather than a self-contained regime, then treaty standards should be interpreted in light of other relevant obligations governing the conduct of States. Systemic integration thus provides a doctrinal basis through which tribunals may engage with environmental treaties, climate obligations, customary international law, and principles associated with sustainable development.14
At the same time, Article 31(3)(c) does not operate mechanically. Questions concerning relevance, applicability, consent, and normative conflict continue to shape the manner in which tribunals invoke systemic integration. Consequently, while the principle is widely accepted at the level of doctrine, its practical operation within ISDS remains uneven.
Environmental regulation and climate governance in investment arbitration
1. The rise of climate-related investment disputes:
Environmental regulation and climate governance have become increasingly prominent within investment arbitration. States have adopted measures directed towards renewable energy transition, emissions reduction, environmental conservation, restrictions upon fossil fuel extraction, and the phase out of carbon intensive industries. Such measures frequently affect foreign investment and may alter the regulatory assumptions upon which investments were originally made.
The resulting disputes raise complex questions concerning the relationship between investment protection and environmental regulation. Tribunals are increasingly required to evaluate whether environmental measures are proportionate, reasonable, and compatible with investor expectations. In doing so, they confront the broader question of whether climate-related obligations and environmental norms may inform treaty interpretation.
The significance of climate-related disputes is particularly visible under the ECT, which has generated substantial litigation concerning renewable energy reforms and energy transition measures.15 These disputes demonstrate how investment arbitration increasingly operates within a regulatory environment shaped by environmental and climate considerations.
2. Sustainable development and environmental norms:
The concept of sustainable development has played an important role in mediating the relationship between investment protection and environmental regulation.16 Sustainable development reflects the proposition that economic development and environmental protection should not be treated as mutually exclusive objectives. Contemporary treaty practice increasingly incorporates references to sustainable development, environmental protection, and the right to regulate.
Environmental considerations have also become embedded within broader instruments of international law, including the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement.17 These instruments establish frameworks for climate governance and emission reduction while recognising differentiated State responsibilities and nationally determined implementation mechanisms.
Unlike traditional investment treaty obligations, however, climate-related norms frequently possess a programmatic or framework-oriented character. The Paris Agreement, for example, relies substantially upon Nationally Determined Contributions (NDCs) and progressive implementation rather than uniform obligations imposed upon States. This structural feature affects the extent to which such norms may function as operative interpretive rules within investment arbitration.
Systemic integration in climate-related investment jurisprudence
1. Early engagement with external norms: The Argentine financial crisis cases
Early arbitral engagement with norms external to investment treaties may be observed in cases arising from Argentina’s financial crisis, including CMS Gas Transmission Co. v. Argentina Republic18 and LG&E Energy Corp. v. Argentine Republic19. In those proceedings, tribunals considered Argentina’s invocation of necessity under customary international law in response to emergency economic measures adopted during the financial crisis.
Although these disputes did not concern environmental regulation, they remain significant for understanding the operation of systemic integration within ISDS. The Tribunals were required to consider the relationship between treaty obligations and broader principles of public international law. In the CMS case, the Tribunal ultimately rejected Argentina’s necessity defence, whereas the Tribunal in the LG&E case20 accepted that the conditions of necessity had temporarily existed.
The Argentine cases illustrate both the possibilities and limitations of systemic integration. External norms were acknowledged as potentially relevant to treaty interpretation and application, but their operation remained mediated through the structure and language of the applicable investment treaty. The disputes therefore demonstrated an early willingness to situate investment obligations within a broader legal framework while simultaneously preserving the centrality of treaty-based protections.
2. Vattenfall and the regulation of energy transition:
The disputes in Vattenfall case21 arose following Germany’s decision to accelerate the phase out of nuclear energy after the Fukushima disaster. The proceedings concerned claims brought under the ECT and highlighted the growing intersection between environmental policy, energy transition, and investment protection. Germany’s measures formed part of a broader public policy shift concerning energy production and environmental risk. The dispute therefore raised important questions concerning the extent to which tribunals should account for environmental objectives while interpreting investment obligations.
Although these proceedings ultimately concluded through settlement, the disputes remain significant because it illustrated how environmental and climate-related measures may generate substantial investment claims. Discussions surrounding the arbitration frequently invoked considerations relating to sustainable development, environmental protection, and regulatory autonomy. Nevertheless, the legal analysis remained fundamentally grounded within the traditional architecture of investment law particularly the scope and protection accorded under the ECT.
The significance of Vattenfall lies in the manner through which environmental concerns entered arbitral discourse. Climate and environmental considerations formed part of the interpretive environment within which the dispute was understood, even though such norms did not operate as independently dispositive legal rules.
3. Rockhopper and offshore drilling restrictions:
A similar tension emerged in Rockhopper Italia case22. The dispute arose following Italy’s prohibition of oil and gas exploration activities within a specified distance from the coastline. Italy’s measures reflected environmental concerns associated with offshore drilling and formed part of broader regulatory developments concerning environmental protection.
The Tribunal ultimately found Italy liable under the ECT. Environmental considerations formed part of the factual background but did not fundamentally alter the structure of treaty interpretation applied by the Tribunal. The reasoning remained centred upon investment protections and the legal consequences of regulatory change.
The dispute nevertheless demonstrates how environmental norms increasingly shape the regulatory context within which tribunals assess State conduct. Even where climate related considerations do not outcomes, they influence the broader interpretive framework within which disputes are analysed.
4. Spain renewable energy cases and climate governance:
The disputes arising from Spain’s reform of its renewable energy regime have generated one of the most substantial bodies of climate adjacent investment jurisprudence under the ECT. Cases including Charanne B.V. and Construction Investments S.A.R.L. v. Spain and Eiser Infrastructure Ltd. and Energía Solar Luxembourg S.à.r.l. v. Kingdom of Spain concerned reforms affecting renewable energy incentives introduced by Spain following economic pressures within the energy sector. 23
Spain’s original regulatory framework had been designed to encourage investment in renewable energy generation through a system of incentives and guaranteed returns. Subsequent reforms altered the structure of those incentives, leading investors to initiate arbitration proceedings alleging violations of the ECT, particularly the obligation to accord fair and equitable treatment.
The Spain of renewable energy disputes are significant because they arose directly within the context of energy transition policy. The cases concerned investments in renewable energy infrastructure and involved measures adopted against the backdrop of broader climate and sustainability objectives.
Tribunals addressing these disputes frequently engaged with the broader regulatory environment within which the reforms were adopted. In the Charanne case24, for example, the Tribunal recognised that States retain authority to adapt regulation in response to changing circumstances.25 Similarly, tribunals in subsequent proceedings considered the extent to which investors operating within heavily regulated sectors could legitimately expect complete regulatory stability.
At the same time, the analytical framework applied by tribunals remained rooted in traditional investment law doctrine. Questions concerning legitimate expectations, proportionality, and regulatory reasonableness occupied a central role within arbitral reasoning. Climate-related objectives and renewable energy policy considerations were generally treated as part of the regulatory background rather than as independently operative legal norms under Article 31(3)(c) of the VCLT.
This feature of the Spain jurisprudence is particularly significant for understanding the practical operation of systemic integration within ISDS. Environmental and climate related considerations entered tribunal reasoning indirectly through the assessment of regulatory context and the scope of permissible State action. Tribunals acknowledged the legitimacy of policy evolution in the renewable energy sector, but such acknowledgement was generally accommodated within existing investment law concepts rather than through direct reliance upon environmental treaties or climate obligations.
Nevertheless, the Spain renewable energy cases demonstrate that systemic integration has facilitated greater engagement with environmental considerations than might previously have been expected within investment arbitration. The disputes reflect an increased recognition that investment treaties operate within a broader legal and regulatory environment shaped by climate governance and environmental policy.
The limits of systemic integration
While systemic integration has enabled tribunals to engage more openly with environmental and climate-related norms, its operation remains subject to important limitations.
Firstly, investment arbitration remains fundamentally consent-based. Tribunals derive jurisdiction from the treaty invoked by the claimant and remain bound by the obligations contained within that instrument. Environmental norms may therefore inform interpretation, but they do not automatically override or displace treaty protections.
Secondly, many climate related obligations possess a flexible and programmatic character. Instruments such as the Paris Agreement establish broad frameworks for progressive implementation rather than precise and directly enforceable obligations. This feature affects the extent to which such norms may operate as determinative interpretive rules.26
Thirdly, tribunals continue to rely primarily upon traditional investment law concepts as the principal framework through which environmental considerations are accommodated. FET, police powers, proportionality, and legitimate expectations remain the central doctrinal mechanisms governing analysis. Environmental norms therefore frequently enter arbitral reasoning indirectly rather than as independently operative legal standards.27
Fourthly, systemic integration itself may become a source of interpretive contestation. Article 31(3)(c) does not specify how tribunals should identify relevant rules, determine their normative weight, or reconcile competing obligations.28 Consequently, the operation of systemic integration often depends upon judicial methodology and interpretative discretion.
These limitations suggest that systemic integration functions less as a mechanism through which environmental law transforms investment law, and more as an interpretive framework through which environmental considerations may be incorporated into arbitral reasoning. The doctrine facilitates engagement with climate governance while preserving the fundamental structure of investment protection.
Conclusion
The increasing intersection between environmental regulation and investment arbitration has renewed attention towards the doctrine of systemic integration under Article 31(3)(c) of the VCLT. Climate-related investment disputes, particularly those arising under the ECT, demonstrate that tribunals are increasingly required to interpret investment obligations within a broader regulatory environment shaped by environmental protection and energy transition policy.
This article has argued that systemic integration has enabled tribunals to engage with environmental and climate-related norms within investment arbitration without fundamentally displacing the traditional architecture of investment protection. Tribunals increasingly acknowledge the broader legal context within which climate-related disputes arise, even where environmental norms do not independently determine arbitral outcomes.
The jurisprudence examined in this article suggests that environmental considerations now form part of the interpretive environment within which investment treaties are understood. This development is particularly visible in disputes concerning renewable energy transition and environmental regulation under the ECT. At the same time, the practical operation of systemic integration remains mediated through established investment law concepts including FET, legitimate expectations, proportionality, and police powers.
Systemic integration therefore operates best as a corrective rather than a cure. It does not eliminate fragmentation within international law, nor does it fundamentally transform the consent-based structure of ISDS. Nevertheless, it provides an important doctrinal framework through which investment treaties may be interpreted in light of broader developments within international environmental law and climate governance. As climate-related disputes continue to increase, systemic integration is likely to remain central to the evolving relationship between investment protection and environmental regulation.
*FCIArb, FPD, BA LLM (Edinburgh), Advocate, Bombay High Court (India) and Barrister (UK). Author can be reached at: mikhailbehl@gmail.com.
1. Stephan W. Schill, “System-Building in Investment Treaty Arbitration and Lawmaking” (2011) 12(5) German Law Journal 1083.
2. ICSID Case No. ARB/12/12.
3. ICSID Case No. ARB/17/14, Award, 23-8-2022.
4. International Law Commission, Fragmentation of International Law: Difficulties Arising from the Diversification and Expansion of International Law, U.N. Doc. A/CN.4/L.682 (2006).
5. Hersch Lauterpacht, The Function of Law in the International Community (Clarendon Press, 1933) 51, 436.
6. Anthea Roberts, Is International Law International? (Oxford University Press, 2017) 214.
7. Stephan W. Schill (Ed.), International Investment Law and Comparative Public Law (Oxford University Press, 2010).
8. Kate Miles, The Origins of International Investment Law: Empire, Environment, and the Safeguarding of Capital (Cambridge University Press, 2013) 347.
9. Stephan W. Schill, The Multilateralization of International Investment Law (Cambridge University Press, 2009) 25.
10. Vienna Convention on the Law of Treaties, 1969, Art. 31(3)(c).
11. International Law Commission, Fragmentation of International Law: Difficulties Arising from the Diversification and Expansion of International Law. Report of the Study Group of the International Law Commission/finalized by Martti Koskenniemi (Geneva: United Nations, 2006).
12. Richard Gardiner, Treaty Interpretation (2nd Edn., Oxford University Press, 2015) 320.
13. Panos Merkouris, Article 31(3)(c) VCLT and the Principle of Systemic Integration Vol. 17 (Queen Mary Studies in International Law, Brill, Leiden, 2015) 34.
14. Campbell McLachlan, The Principles of Systemic Integration in International Law (Oxford University Press, 2024).
15. Valentina Vadi, Public Health in International Investment Law and Arbitration (1st Edn., Routledge, London, 2013) 112.
16. Jorge E. Viñuales, Foreign Investment and the Environment in International Law (Cambridge University Press, 2012) 112.
17. Paris Agreement, 2015.
18. ICSID Case No. ARB/01/8, Award, 12-5-2005.
19. ICSID Case No. ARB/02/1, Decision on Liability, 3-10-2006.
20. LG&E Energy Corp. v. Argentine Republic, ICSID Case No. ARB/02/1, Decision on Liability, 3-10-2006.
21. Vattenfall AB v. Federal Republic of Germany, ICSID Case No. ARB/12/12.
22. Rockhopper Italia S.p.A. v. Italian Republic, ICSID Case No. ARB/17/14, Award, 23-8-2022.
23. See generally SCC Case No. 062/2012, Final Award, 21-1-2016; ICSID Case No. ARB/13/36, Award, 4-5-2017.
24. Charanne B.V. and Construction Investments S.A.R.L. v. Spain, SCC Case No. 062/2012, Final Award, 21-1-2016.
25. See generally Charanne B.V. and Construction Investments S.A.R.L. v. Spain, SCC Case No. 062/2012, Final Award, 21-1-2016; Eiser Infrastructure Limited and Energía Solar Luxembourg S.à.r.l. v. Kingdom of Spain, ICSID Case No ARB/13/36, Award, 4-5-2017.
26. Campbell McLachlan, The Principles of Systemic Integration in International Law (Oxford University Press, 2024).
27. Paris Agreement, Art. 4.
28. Campbell McLachlan, “The Principle of Systemic Integration and Article 31(3)(c) of the Vienna Convention” (2005) 54(2) International and Comparative Law Quarterly 279.
