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Part I of the series argues that the fixed baseline approach violates the good faith obligation under UNCLOS, as it contradicts the treaty’s ordinary meaning and intent by disregarding actual coastal geography. While equity and climate justice are invoked, such interpretations risk distorting UNCLOS’s foundational principles and legal coherence. Part II evaluates whether the fixed baseline approach qualifies as valid subsequent practice or customary international law, ultimately concluding it lacks sufficient state support, opinio juris, and legal coherence.
The Limits Of ‘Subsequent Practice’ In UNCLOS Interpretation
Article 31(3)(b) of the VCLT recognizes the subsequent practice of parties in the application of a treaty which establishes their agreement as to the treaty’s interpretation. Island nations contend that their subsequent practice endorsing fixed baselines should be given weight in interpreting the UNCLOS. However, this argument faces several lacunae.
The International Law Commission had observed that subsequent practice must be utilised to assist in determining “whether or not the presumed intention of the parties upon the conclusion of the treaty was to give a term used a meaning which is capable of evolving over time.” The UNCLOS entered into force in 1994, when the international collegium was not aware about the disastrous and permanent effects that climate change could induce. The term “baseline” was not given a loaded meaning. Instead, it was meant to affix the EEZ based on the real-time coastline of the country. Thus, it is unwarranted to assume that the treaty-makers intended an interpretation of the text of the treaty that disregards actual geographical changes of the country and remains oblivious to the accuracy of these baselines, solely because they are “recognized” by the country.
While states often exercise caution in adopting an ‘evolutive interpretation’ of treaty terms to avoid unforeseen obligations, the UNCLOS Preamble explicitly acknowledges the need to accommodate ‘the evolution of international law’ and ‘changing circumstances’. This aligns with the International Law Commission’s (ILC) Draft Conclusions on Subsequent Agreements and Subsequent Practice (2018), which recognize subsequent practice as a legitimate tool for interpreting treaties dynamically. The International Court of Justice (ICJ) has affirmed this principle in cases such as Legal Consequences for States of the Continued Presence of South Africa in Namibia (1971), where it emphasized that treaties must be interpreted in light of ‘subsequent developments in international law’.
Thus, while contemporaneous interpretation remains a default approach to preserve clarity, evolutive interpretation is not inherently incompatible with UNCLOS or state practice. Rather, it reflects the treaty’s adaptability to existential challenges like climate change, provided such interpretations are grounded in widespread and consistent subsequent practice. Another aspect is the number of parties that must be subject to the subsequent development. The ILA Baselines Committee and the ILC have subsequently and consistently concluded that there is no established Customary International Law (CIL) governing the nature of baselines. Both bodies emphasize that the question of whether baselines are ambulatory or fixed remains unresolved under CIL. Consequently, an interpretation diverging from the UNCLOS framework would require explicit consent from State parties to the Convention.
The VCLT does not stipulate any fixed number of parties that can consent to the subsequent practice, which causes ambiguity among legal scholars. Some scholars are proponents of the view that all or most parties to the treaty must be advocates of the subsequent practice to be held valid. If this approach is upheld, the fixed baseline argument still faces significant challenges. While the Pacific Island Forum and the Small Island Developing States (SIDS), representing approximately 39 countries, have formally endorsed fixed baselines through joint declarations (2021). Moreover, several other States have supported the approach in submissions to the ILA, ILC, and ICJ, proponents remain a minority among the 167 UNCLOS parties. Notably, no State has explicitly objected to fixed baselines, which could imply tacit acquiescence. Nonetheless, under the VCLT’s requirements for subsequent practice, widespread and representative participation—not mere silence—is essential. With advocates constituting less than a quarter of UNCLOS signatories, the threshold for establishing a binding interpretation remains unmet. Thus, while the absence of objections weakens counterarguments, the lack of affirmative consensus retains the ambulatory baseline as the prevailing norm.
Specially-Affected States
An alternate approach taken is to look at States that are specially affected by the aforementioned climatic catastrophe. Arguably, these small island nations are specially affected, as they face potentially extreme territorial loss and subsequent loss in EEZ. This concept was developed in the North Sea Continental Shelf case, where it was held that for a custom to be developed, “State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked”. This case is the primary source of this concept, since it is not mentioned in any legal text. In this judgment, it is evident that there are two qualifiers for this doctrine to be applicable.
Firstly, as articulated in the North Sea Continental Shelf case, the formation of general customary international law requires State practice to be both extensive and uniform, which must include the practice of States whose interests are specially affected. This threshold ensures that the custom reflects broad acceptance, particularly by those most impacted by the issue. Currently, the said principle has not been extensively applied by these States as it is still in its nascent stage and it requires further development, since the global acceptance of the catastrophic effects of rising sea levels has come recently.
Furthermore, the uniformity in application is questionable. These States are often willing to accept new territories into their States, which are frequently created through volcanic eruptions or other natural causes, which increases the EEZ of these countries. However, these States conveniently decide to affix their EEZ boundaries to restrict any reduction in EEZ. Hence, there is ambiguity and inequality in the application of the fixed baseline approach by these States. Secondly, the word “including” in the judgement means that the State practice of the specially affected States is a valid ground for accepting a custom as CIL, but it is not sufficient. The requirement of a custom to be “widespread and representative” as per North Sea Continental Shelf case necessitates the support of other States. In the present case, despite the ILC Special Rapporteur’s recognition of sea-level rise as a ‘disaster’ and the IPCC’s warnings of its accelerating pace, developed nations motivated by their economic reliance on high-seas navigation and resource exploitation continue to oppose the fixed baseline approach.
Seeking Solace In Customary Law: Can Island Nations Establish Fixed Baselines As CIL?
An ideal way forward for the island nations would be to push for the fixed baseline approach to be considered as Customary International Law. Under Article 38 of the ICJ Statute, CIL is a valid and binding rule in international law. To qualify as Customary International Law (CIL), a practice must meet two criteria: widespread and consistent state practice, coupled with opinio juris. Unfortunately, this practice is currently not capable of being termed as CIL because it comprises neither. The practice has officially been recognized by the Pacific Forum, which consists of only 18 countries. However, as noted in the analysis of subsequent practice under Article 31(3)(b) of the VCLT, such limited endorsement falls short of establishing a binding interpretation of UNCLOS. The International Law Commission observed that ‘more than 70 States are or are likely to be directly affected by sea-level rise,’ and over 167 countries have ratified the UNCLOS. Crucially, the advocating Pacific nations represent hardly one-fourth of the affected countries, and their practice lacks the widespread and uniform acceptance required to form customary international law (CIL).
Coming to the second requirement, the first-issues paper by the ILC in 2018 had observed that although the countries that advocated for a fixed baseline approach preferred permanent maritime boundaries for securing their economic interests, it cannot be equated to opinio juris, which requires “a sense of legal right or obligation” on the part of the concerned State. The ILC explicitly noted that such advocacy ‘stems not from a belief in legal obligation but from pragmatic considerations to preserve maritime entitlements in the face of environmental threats’. Herein, the States were not acting out of a legal obligation, but were, motivated by socioeconomic imperatives rather than juridical duty.
In the absence of a valid CIL to justify the practice of these States, internal laws of such States cannot be viewed as justifications for the non-application of the UNCLOS, under Article 27 of VCLT. For instance, Tuvalu has developed a project that uploads a digital image of their nation into the metaverse, in order to affix their position in the ocean and consequently affix their maritime boundaries. However, such laws are inconsistent with the ambulatory approach of the UNCLOS, which has garnered the status of CIL. Therefore, it is evident that there is no general CIL in lieu of the approach advocated by the island nations.
However, a pyrrhic victory for the relevant States would be to render the practice the status of regional customary international law. A regional custom is specifically binding only on States that recognize such practice. The fixed baseline approach is currently only officially recognized by countries in the Pacific region. Due to their geographic homogeneity and their potential to be recognized as specially affected States, this approach can be deemed to be a regional custom. The ILC has concluded that no regional custom exists in favour of fixed baselines, emphasizing the absence of opinio juris, the belief that such practice reflects a legal obligation. Without evidence that these States act out of a sense of legal duty, even regional recognition remains untenable. Thus, geographic homogeneity and ‘specially affected’ status alone cannot substitute for the requisite opinio juris to establish binding regional custom. However, being recognized as a regional custom will not be suitable for the States’ interests, since maritime boundaries create rights and obligations that extend to and emanate from the entire international community. Regional customs do not create obligations on States outside the relevant region.
Furthermore, in the Asylum case, it was held that it must be established by the party that seeks the enforcement of a custom that such a custom is binding on the other party, and that it creates a duty on the other party’s side. While general custom acts as law for all States, regional customs do not apply to the States outside the relevant region. This is crucial in the present scenario, since the high seas are majorly plundered and utilised by the developed and large nations, who do not support the fixed baseline approach. This regional custom, once developed, will not create any corresponding duties on the developed nations, and hence, will have very little actual utility.
Conclusion
The impact of climate change is increasingly visible in matters of Statehood, when dwelling into international law. Its permanent impacts, such as retreat of coastlines, has massive economic impacts on the reduction of territory as well as a proportional reduction in EEZ, under the ambulatory baseline approach of UNCLOS. Despite advocacy for fixed baseline approach by the Pacific island-nations, it is still not viable for this approach to be accepted under general CIL. It is far from being accepted by all countries, and this interpretation of the UNCLOS not only violates the VCLT, but also lacks the components for being granted the status of an alternative interpretation of the said treaty. This regional custom’s non-binding nature on the rest of the world, and its lack of viability under States’ good faith obligations render it invalid. Therefore, these countries must advocate for widespread acceptance of this interpretation, especially the influential and developed countries, whose State practices often coalesce into the custom for international law. Securing the assent of these countries as either a custom or a binding treaty will be a bulwark for these States against the catastrophic impacts of climate change.
Kshitij Nair is a final year law student at Jindal Global Law School.
Picture Credit: Ministry of Justice, Communication and Foreign Affairs Tuvalu Government/ REUTERS
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