In Doubt, Favour Nature? Analysing Status and Justifications for In Dubio Pro Natura as a Rule for Environmental Treaty Interpretation.

[Lavinia Stoppani is a recent graduate of the LL.B. programme in International and European Law at the University of Groningen, with research interests in environmental law and treaty interpretation]

Introduction

In a dissenting opinion to the 1977 Gabčíkovo-Nagymaros case, Judge Herczegh argued that where uncertainty exists between short term economic loss and potentially irreversible environmental harm, the latter ought to carry greater weight; citing the maxim ‘in dubio pro natura’ (p. 184), i.e. in doubt, favour nature.

While lacking the authority of a majority judgment, Judge Herczegh’s pronouncement represents the earliest explicit reference to in dubio pro natura at the international level. In recent years the notion has become increasingly used within domestic jurisdictions. Particularly in Latin-America, domestic courts have begun to prioritise the interests of nature to ensure its effective protection (see here, and here).

However, at the international level, in dubio pro natura remains ambiguous. What is the content of the notion? When can it be invoked? Is it recognised in customary international law (CIL)? Can it nevertheless be accommodated under the Vienna Convention on the Law of Treaties (VCLT)? This blog analyses the status and function of in dubio pro natura as a rule of interpretation directing the interpreter to resolve legal doubt in favour of environmental protection. Ultimately, it considers that while it may be too early for it to qualify as CIL, the notion may nevertheless be invoked through the established framework of the VCLT.

The Existence of (Legal) Doubt

In dubio pro natura, as the name suggests, works to resolve situations of ‘doubt’. In the context of judicial decisions ‘doubt’ refers to situations of uncertainty. This aspect often leads to the conflation of in dubio pro natura with the precautionary principle, which requires protective action to be taken against a potential risk of ‘serious or irreversible environmental damage’ even in the absence of scientific certainty confirming this risk (1992 Rio Declaration, Principle 15). However, whereas the precautionary principle refers to scientific uncertainty, in dubio pro natura relates to legal uncertainty.

Legal uncertainty includes, firstly, cases of linguistic ambiguity, where the meaning of a term is uncertain either because it carries more than one meaning, or because evolving societal and legal circumstances have called its meaning into question (here, evolutive interpretation may come into play. E.g.: Dispute Regarding Navigational and Related Rights, para 63 et seq). Secondly, where there is ambiguity as to the scope and/or threshold of a provision, where the applicability of the provision is uncertain. Moreover, doubt may arise due to the existence of separate, perhaps contradicting, rules dealing with equal or similar subject matters, leading to uncertainty as to which rule applies. Finally, uncertainty may arise where a treaty remains silent on an issue (either deliberately or inadvertently), at which point it may be necessary to consider whether its existing provisions may be extended in scope so as to fill this gap.

Admittedly, the boundary between legal and scientific uncertainty is not always so clear in practice, as legal uncertainty regarding the scope of an environmental obligation may be intertwined with scientific uncertainty regarding the nature/cause of harm. The Indonesian PT Kallista Alam case (Direktori Putusan No. 651 K/PDT/2015v) illustrates how uncertainty over causation can engage both the precautionary principle and in dubio pro natura (as discussed in Dwi Putro and Bedner, pp. 620-621). Nevertheless, the distinction holds at the conceptual level. Therefore, the use of in dubio pro natura hinges on the existence of legal doubt.

Lastly, in dubio pro natura functions as a supplementary means of interpretation, meaning that it can be invoked only where residual doubt remains. As such, it holds a function similar to Art. 32 VCLT, which is resorted to only to ‘confirm’ meaning established through Art. 31, or to ‘determine’ meaning when it remains ambiguous. Similarly, its function can be understood by reference to in dubio mitius, which works to resolve doubt in favour of the more restrictive reading, and which can only be invoked ‘when all other methods of interpretation have failed’ (Merkouris, p. 278).

An Emerging Rule of Interpretation in CIL?

In reconstructing the origin of in dubio pro natura, Baldin and De Vido rely on the constitutional and judicial practice of several Latin-American countries (e.g. Ecuador, Costa Rica, Mexico, and Colombia) to argue the notion may be considered an emerging rule of regional CIL (pp. 25-27). However, to constitute general CIL, in dubio pro natura would have to be supported by sufficiently widespread, representative and consistent State practice, as well as opinio juris (ILC 2018 Draft Conclusions, Conclusion 8). However, presently, practice from States outside Latin-America remains scarce, with the only significant examples emanating from Indonesia (mentioned above) and Pakistan. Such absence greatly weakens the notion’s claim to general CIL status.

At the international level, explicit formulation of the principle can be found only in the 2016 ‘World Declaration on the Environmental Rule of Law’ (hereinafter: IUCN Declaration) drafted by the International Union for Conservation of Nature. Principle 5 states:

‘In cases of doubt, all matters before courts, administrative agencies, and other decision-makers shall be resolved in a way most likely to favour the protection and conservation of the environment, with preference to be given to alternatives that are least harmful to the environment.’

This formulation supports the observations made above regarding in dubio pro natura as hinging on legal doubt. Moreover, the specification that the principle is to be used in ‘all matters before courts, administrative agencies…’ further confirms its function as a rule of interpretation directing the interpreter towards environmental protection, rather than a substantive rule mandating such outcomes.

However, as a soft-law document, the IUCN Declaration remains non-binding; and although adopted in an international forum comprising State representatives, it ‘does not represent a formally negotiated outcome’ and cannot be attributed to any State actor (p. 5). Therefore, it cannot be taken as evidence of the State practice and opinio juris required to establish CIL. Yet, it could be seen as reflecting emerging consensus among institutional and legal experts as to the notion’s content. Additionally, as in the Pakistan example, the IUCN Declaration may function as a reference point for domestic courts in applying in dubio pro natura (similarly, see this case from Argentina). As such, the IUCN Declaration could over time function as a ‘catalyst’ to stimulate, domestically, a general practice capable of establishing a rule of CIL.

In Dubio Pro Natura Under the VCLT

The absence of general CIL status does not render in dubio pro natura entirely inapplicable. As a supplementary means, the notion can function as an ‘aid’ to the general rule of interpretation under Art. 31(1) VCLT, its applicability depending on the relevance of the rule to the treaty in question (see generally: Oppenheim’s International Law, p 1276). Such a determination can be done particularly by reference to the treaty’s object and purpose.

Article 31 VCLT: The Object and Purpose Route

According to Art 31(1) VCLT, a treaty must be interpreted in accordance with the ordinary meaning to be given to its terms, taken in its context, and in light of its object and purpose. This general rule has been used, for instance, to accommodate the pro homine principle of human rights law.

As explained by Judge Serghides, from the European Court of Human Rights, in cases of doubt the pro homine principle requires judges to show:

‘a preference for that interpretation which is more favourable to the right and its holder (in dubio in favorem pro liberate or in dubio pro persona)’

(p. 3).

Pro homine finds justification in the principle of effectiveness, which is also found in the more general VCLT framework of interpretation as a ‘specific application’ of the treaty’s object and purpose (VCLT Commentary, pp. 578-579, para 34). It is possible to extend the same logic to in dubio pro natura, particularly in the case of treaties that have the protection of the environment as their object and purpose. However, it should be noted that effectiveness does not necessarily mandate the most expansive interpretation available; rather, it directs towards an interpretation that gives the provision meaningful effect. In dubio pro natura would therefore operate within the boundaries set by the treaty’s text, not beyond them. This is also in line with the notion’s supplementary function, which does not aim to overlook the negotiating parties’ intentions.

However, when making this analogy, one should question whether it is possible to group environmental treaties under a single protective purpose; or more generally, what the object and purpose of environmental treaties is, and whether it is comparable to that of human rights treaties. Moreover, the pro homine principle directs to the interpretation ‘more favourable to the right and its holder’. However, when it comes to the environment, what rights does nature have? Various blog contributions have already discussed the issue of nature as a subject of rights, and their interpretative relevance (e.g. here). Therefore, present observations are limited to the argument regarding the object and purpose of environmental treaties.

Here, the analogy becomes muddled. Different from human rights treaties, environmental treaties lack a unified objective, their purpose ranging, inter alia, from conservation, to sustainable development and resource management. Given this diverse landscape, the application of in dubio pro natura would have to be analysed on a case-by-case basis, with the principle presenting a stronger affinity with certain treaties than others. This is particularly due to the fact that in dubio pro natura puts environmental protection as an end in itself (as argued here), which reflects a more ecological, rather than anthropocentric, conception of environmental law. This creates a potential tension, as the application of an ecocentric interpretative tool to treaties negotiated under anthropocentric assumptions requires careful evaluation of the treaty’s own normative foundation.

To illustrate, consider the Convention on Biological Diversity (CBD). The CBD follows three aims: the conservation of biological diversity, its sustainable use, and the fair and equitable sharing of the benefits to be derived from its resources (Art. 1). Given its protective scope, in dubio pro natura easily fits with the conservation of biodiversity. However, does the notion similarly fit within the Convention’s human-centered provisions on sustainable use? The CBD’s preamble recognises ‘the intrinsic value of biological diversity’, classifying its conservation as a ‘common concern of humankind’. Recognising the intrinsic value of biological diversity reinforces the importance of its conservation. Indeed, conservation functions as the ‘fundamental goal’ of the CBD, with the sustainable use of biodiversity components ‘in a way and at a rate that does not lead to the long-term decline of biological diversity’ (Art. 2) being a ‘prerequisite’ for achieving that goal (IUCN Guide, p. 4). This points to an ecological orientation, in which conservation is prioritised while maintaining a balance between the Convention’s objectives. This is reinforced by COP decisions, e.g. COP 2 Decision II/8 which recognises the ecosystem approach as the Convention’s ‘primary framework of action’ (see also COP Decision V/6), as well as subsequent agreements to the CBD, e.g. the Kunming-Montreal Biodiversity Framework which recognises the need to ensure ‘ecosystems of high ecological integrity’ as a means to reduce ongoing threats to biodiversity (Target 1). In light of this, the use of in dubio pro natura is arguably consistent with the CBD’s object and purpose, as its aims are made to serve conservation.

In dubio pro natura becomes harder to justify in the interpretation of treaties that do not concern ecological integrity per se, such as the LOSC, whose rationale behind its environmental regime stemmed from the fact that States considered it necessary to address issues of marine conservation as an extension of the fact that various activities regulated under the Convention have negative impacts on the marine environment (Virginia Commentary, part XII para 2). Given that States’ interests as to the use of their own maritime zones is an equally important objective under the Convention, prioritising nature becomes harder to justify.

However, the LOSC is recognised as a ‘living instrument’, capable of evolving with time. Here, the Agreement on Marine Biological Diversity (BBNJ Agreement), entered into force in January 2026, reflects an important development which may inform the interpretation of the LOSC’s Part XII regime (as argued here), and thus facilitate the use of in dubio pro natura. The objective of the BBNJ Agreement is the ‘conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction’ (Art. 2). The BBNJ Agreement additionally includes reference to the ecosystem approach, and encourages the restoration and maintenance of ecosystem resilience and integrity (Art. 7(f) and (h)). Reference to ecosystem integrity reflects ecological framing, and the inclusion of these approaches among ‘guiding principles’ further points to their interpretational relevance (as explained here). Together, these observations point to a stronger affinity with in dubio pro natura. Given its relationship to the LOSC, the BBNJ Agreement could be invoked through Art. 31(3)(c) VCLT (or ‘systemic integration’), as a ‘relevant rule of international law’ for the interpretation of the LOSC’s Part XII provisions, at which point its ecological framing could be used as an argument for relying on in dubio pro natura even within the LOSC.

Article 32 VCLT: The ‘Supplementary Means’ Route

Finally, in dubio pro natura, as non-binding soft-law, may be invoked through Art. 32 VCLT. Art. 32 allows particularly for reference to be made to the preparatory work of the treaty, which has been interpreted by the ICJ as to include soft-law documents, however only insofar as they played a significant role in the drafting and adoption of a treaty’s text, or functioned as the inception of the drafting process itself (de Vries-Zou, pp. 43-44). However, given that the IUCN Declaration was issued in 2016, this pathway is limited to treaties concluded after that date, and its interpretive weight would depend on a number of factors, including the extent to which it reflects an emerging consensus among the negotiating parties (VCLT Commentary, at p. 624 para 20).

Conclusion

Ultimately, absent independent status, within the VCLT in dubio pro natura finds its stronger justification through Art. 31(1), particularly by reference to object and purpose. That is where, as a subsidiary rule of interpretation, the notion best operates to give effect to a treaty’s environmental protection/conservation objectives. Where a treaty lacks a direct environmental link, the other available route, namely Art. 32 VCLT, imposes a more demanding threshold. While invoking in dubio pro natura through Art. 32 is not per se impossible, stricter requirements as to what qualifies as ‘preparatory work’ may impede its application. Nevertheless, treaty interpretation is not static. As context, subsequent practice, and applicable rules continue to develop, and as environmental obligations increasingly permeate international law, the range of treaties whose object and purpose can accommodate in dubio pro natura may expand. Consequently, those appearing to be limitations, may indeed be subject to change, particularly if State practice and opinio juris continue to develop in its favour.

Photo attribution: by Vlad Hilitanu on Unsplash