Oceans and UNCLOS – Full of Life?

The International Law Commission (ILC) has adopted its report of the 76th session on 30 of May 2025 and since 9 June, an advance unofficial version of the 2025 ILC Report has been made available, including the final report of the Study Group on sea-level rise in relation to international law. With this, the ILC’s consideration on the topic of ‘sea-level rise’ finds its highly anticipated conclusion and the accompanied long-standing debate regarding the (ambulatory or fixed) nature of normal baselines (Art. 5 of the United Nations Convention on the Law of the Sea (UNCLOS)) might come to an end  (e.g. here, here and here). Since the International Law Association’s (ILA) 2012 Sofia Conference Report, the influence of sea-level rise on a States normal baseline and its accompanied academic discussion has undergone substantial transformation. While in 2012 the ILA found that coastal states “may protect and preserve territory through physical reinforcement, but not through the legal fiction of a charted line that is unrepresentative of the actual low-water line”, members of the ILC expressed in its 2023 Report a shift in perspective, stating that UNCLOS does “not prohibit the option of fixed baselines” followed by an endorsement of the ILA in 2024. Particularly the emerging State practice of baseline fixing (see here para. 32, and here para. 83) might be responsible for the shifting assessment of Art. 5 UNCLOS.

The ILC’s Study Group discussed possible ways forward, including the interpretation of existing treaties (para. 58). One specific element in the broad consideration of this topic has been touched on briefly by the ILC in the advance 2025 Report, but not analyzed to its full extent: The legal requirements of treaty interpretation in light of subsequent State practice – explicitly in the case of UNCLOS – and how the Living-Instrument Doctrine might provide a way to overcome them.

The Requirement of Uniformity

While it is uncontroversial that subsequent State practice shall be taken into account when interpreting a treaty, Art. 31(3)(b) VCLT, the legal threshold and value of subsequent State practice and especially the relation to Art. 32 VCLT were for a long time – and partly still are – an object of heated discussions. The ILC dealt with these controversies in its ‘Draft conclusions on subsequent agreements and subsequent practice’ in 2018, with commentary.

One main result of the ILC’s discussion (see Conclusions 3 and 4) is the differentiation between two kinds of subsequent State practice: On one hand, subsequent practice that constitutes an agreement between all parties of a treaty may serve as authentic means of interpretation under Art. 31(3)(b) VCLT (see Conclusion 10, para. 2f.). On the other hand, any subsequent State practice that does not constitute an agreement between the parties may only serve as supplementary means of interpretation under Art. 32 VCLT (jurisprudence on this matter by arbitral tribunals, e.g. here, pp. 103f., and WTO dispute settlement panels, e.g. here, p. 129). The legal value of both provisions – authentic and supplementary – correlates with the required threshold of participation, as the ILC stated that “greater interpretative value” (Conclusion 4 para. 33) should be ascribed to authentic means of interpretation.

Therefore, while not explicitly required in the wording of Art. 31(3)(b) VCLT, it seems to be the standpoint of the Commission that uniform State practice is required to serve as authentic means of interpretation. In this regard, Conclusion 10 (2), which clarifies that “the number of parties that must actively engage in subsequent practice in order to establish an agreement under article 31, paragraph 3 (b), may vary”, must not be understood as a deviation from this standpoint as it concerns the relevant conduct and not the required uniformity: It merely allows for the necessary uniformity to be achieved by silence of the parties, instead of active engagement (accepted by the ICJ, see here p. 23).

Concerning the matter of treaty interpretation, members of the ILC expressed their view in the sea-level rise report of 2023 (p. 93) that it must be considered “how subsequent practice satisfied the relevant legal benchmarks, as developed by the Commission”, this legal benchmark being the aforementioned requirement of uniformity. Further, the advance 2025 Report (para. 60) highlights the “diverse and contradictory” practice. The question that arises from this is whether certain treaty interpretations in light of subsequent State practice – e.g. baseline fixing under Art. 5 UNCLOS – would not be conceived as legal if one State objects to this approach. This reflects a greater issue: Especially multilateral treaties like UNCLOS are unlikely to satisfy the requirement of uniformity which effectively freezes the ability to adapt to emerging challenges such as sea-level rise.

Living-Instrument Doctrine

Here, a special legal concept comes into play that the ILC briefly raised in its commentary to Conclusion 8 (paras. 7 and 14): The Living-Instrument Doctrine. The doctrine was first articulated in the jurisprudence of the European Court of Human Rights (ECtHR) in 1978 and regularly referenced in regard to the European Convention on Human Rights (ECHR). The Court held that “the Convention is a living instrument which (…) must be interpreted in the light of present-day conditions.” In Conclusion 8, the ILC clarified that subsequent State practice under Art. 31(3)(b) VCLT can help in identifying if a term is “capable of evolving over time”. Therefore, subsequent State practice might also help to identify if a treaty is a living instrument.

However, the ILC did not say anything explicitly about the interplay of the Living-Instrument Doctrine and the requirement of uniformity of subsequent State practice. It is thus worth considering if it works in both ways: If subsequent practice can help identify whether a treaty constitutes a living instrument, can the unarguable character of a treaty as a living instrument also help overcome the requirement of uniformity under Art. 31(3)(b) VCLT as authentic means of interpretation? To put it shortly: Does a lower standard of participation for subsequent State practice apply to treaties that are considered to be living instruments?

‘Loizidou v. Turkey’: An Intersection

One answer may be found in the commentary to Conclusion 10. There, the ILC cited the preliminary objections to the case ‘Loizidou v. Turkey’ of 1995 by the ECtHR when discussing the meaning of an “agreement of the parties regarding the interpretation” in Art. 31(3)(b) VCLT (para. 6). In the decision, the Court reaffirmed the ECHR’s nature as a living instrument and continued to seek an interpretation of the relevant provisions under Art. 31(1) VCLT and Art. 31(3)(b) VCLT. While it found deviations from the practice in question by the Respondents’ government as well as by a third State, it still restated the existence of “uniform and consistent State practice” as these deviations “do not disturb the evidence of a practice denoting practically universal agreement amongst Contracting Parties”.

Specifically, the word “practically” shows the difference to the legal benchmark established by the ILC which requires uniform subsequent State practice. Even though the reasoning seems to be a contradiction in itself, the Court’s specific reference to authentic instead of supplementary means of interpretation leads to the assumption that the ECtHR might deduce from the Living-Instrument Doctrine not only modifications of Art. 31 VCLT concerning its temporal dimension, but also regarding the requirement of uniformity. The outcome would be that – similar to the creation of customary international law (c.f. here, para. 74) – virtually uniform practice would be sufficient under Art. 31(3)(b) VCLT, as long as the treaty in question falls under the Living-Instrument Doctrine.

The ILC acknowledged in its commentary to Conclusion 10 para. 6 that “interpreters, at least under the European Convention, possess some margin when assessing whether an agreement of the parties regarding a certain interpretation is established.” Thus, the Commission equally suggests that courts may exercise certain discretion regarding the uniformity-requirement of subsequent State practice in treaties similar to the ECHR.

Extension to UNCLOS

The question then arises whether a lower standard than uniformity of subsequent State practice is applicable to UNCLOS as well. In May 2024, the International Tribunal for the Law of the Sea (ITLOS) referenced the Living-Instrument Doctrine in an Advisory Opinion and confirmed its application to UNCLOS. For States that argue for the legality of baseline fixing, this could serve as a strong argument. While referring to a contemporary treaty interpretation (para. 58), the ILC’s Study Group fails to mention the doctrine directly – possibly because numerous issues might be raised, which ultimately do not hinder the doctrine’s application, however:

First, the principle of consent requires States to not unilaterally go their own path but instead ensure that a certain interpretation is supported by all parties as “the parties (…) remain the masters of their treaty” (Villiger 2009, Art. 31 para. 16). Practically ignoring the objection of one State to a certain interpretation would contradict one of the most fundamental principles in public international law.

Second, the Living-Instrument Doctrine was developed in respect of temporal aspects in treaty interpretation (“contemporary” or “evolutionary”) and not the number of States participating in a certain interpretative practice. However, that might just be academical pettiness as the Living-Instrument Doctrine is not restrictively defined by its requirements but more so characterized through its pragmatic aim to effectively protect certain rights in areas of law which are influenced by quickly evolving circumstances. Further, its underlying idea is that certain treaties – considering their nature and their number of parties – need a modified way of interpretation.

Third, another crucial question is, whether the ECHR and UNCLOS are actually comparable in this aspect. While both are deemed to be living instruments, the ECtHR has based the special character of the ECHR on its “collective enforcement of human rights and fundamental freedoms.” UNCLOS’ character as a living instrument is brought up for example in connection with the protection of the marine environment and its relation to climate change and sea-level rise. However, while the substance of the treaties differs, it would be difficult to conceive which aspect of these differences would prohibit a deviation from the uniformity-requirement of subsequent State practice.

Conversely, the uniformity requirement produces a severe dilemma: As UNCLOS currently (July 2025) has 170 Parties, it is difficult to accurately assess the relevant State practice and nearly impossible to achieve uniform practice. In practice, this means that with its “veto-power”, one State could hinder the evolvement of the interpretation of relevant provisions such as Art. 5 UNCLOS, even though the majority of States (see here para. 32, and here para. 83) advocate that UNCLOS does not forbid baseline freezing or have even implemented such legislation. In this regard, the principle of consent should not overstay its welcome as it is questionable whether the lack of consent of one State should be able to dictate the future of a treaty. In order to not entirely disregard the opposing State’s view, one could – again similar to customary international law – grant it a right to object to the new interpretation or the right to terminate its treaty membership.

Conclusion

The ILC’s 2025 Report on ‘sea-level rise’ has concluded its consideration of the topic regarding the law of the sea. However, questions and practical solutions to issues such as the fixing of baselines remain open, as States seek to implement them through an interpretation of UNCLOS, rather than through a rule of customary international law (e.g. here, here and here). The recent reaffirmation of UNCLOS as a living instrument and the severe effect of sea-level rise calls for a repositioning of the ILC concerning its legal benchmark of uniformity and the adoption of the approach taken by the ECHR.