“(…) we have descended upon the Great Hall of Justice not unlike the high priestess Pythia once did at the Temple of Apollo — uttering a single sentence, the brevity of which belies its astonishing ramifications.”

– Judge Tomka in his Declaration to the ICJ’s Advisory Opinion of 23 July 2025

Fluvial and maritime borders have been long-standing markers of statehood and of state sovereignty. As with many concepts of contemporary constitutional (and international) law, borders are also challenged by unforeseen circumstances brought about by climate change. One such unforeseen consequence of climate change is the rise of sea levels, and in the current piece we are exploring a recent decision of the ICJ on how it handles the issue under the United Nations Convention on the Law of the Sea. 

The recent Advisory Opinion issued by the International Court of Justice (ICJ) on the July 23, 2025 on the topic of “Obligations of States in respect of Climate Change” has been considered a milestone in international climate law and climate governance (e.g. here). The Court addressed, at the request of the UN General Assembly, a plethora of topics in connection with climate change. The statements made regarding the obligations of States under the United Nations Convention on the Law of the Sea (UNCLOS) –especially concerning sea level rise– stand out due to their brevity and conciseness (paras. 336-368). However, the issue of sea-level rise is a pressing one, as emphasized inter alia by Vice-President Sebutinde (para. 8, here), Judge Carlesworth (para. 17, here) and Judge Aurescu (para. 1, here) in their separate opinions. It is therefore critical to both analyze what the Court said and –possibly even more importantly– what it did not say.

I. The court’s long-awaited opinion

While the legal issue of climate change-induced sea-level rise constitutes a rather recent point of discussion within the international community, it remains highly debated since it was first reported by the International Law Association (ILA) in 2012 (Sofia Conference Report 2012 followed by the Sydney Conference Report 2018). Particularly the reports by the International Law Commission (ILC), which were issued subsequently from 2018 until 2025, deeply analyzed various questions surrounding the issue of sea-level rise as well as State practice. As the ILC recently issued its 2025 final consolidated report, the ICJ’s Advisory Opinion comes at a time when the theoretical outline of this issue has already taken shape, but practical approaches or solutions have not been authoritatively confirmed yet. With its incomparable role, the Court had the chance to create legal certainty for those who are particularly affected by sea-level rise. In the vast field of legal issues concerning this matter, the Court has decided to pronounce on two specific topics: first, the obligations of States to update their maritime charts depicting their baselines (paras. 358-362), and second, the effects of land- and population-loss in connection with sea-level rise on the requirements of statehood (paras. 363-365).

II. Preservation of maritime boundaries (“baseline freezing”)

1. Charted vs. ambulatory baselines

The first issue has its background in the influence of sea-level rise on maritime boundaries, especially on the so-called baseline which can be measured in various ways depending on the coast’s structure (e.g. Art. 5, 7 UNCLOS). The normal baseline is defined as

“(…) the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State.” (Art. 5 UNCLOS).

Starting from this baseline, UNCLOS confers certain rights and obligations to States in different maritime zones, such as the territorial sea and exclusive economic zone. The question is whether States affected by sea-level rise lose those maritime entitlements. Simply speaking, this depends on whether the baseline automatically moves landwards with the coastal recession (“ambulatory approach”) or whether the baseline is preserved at the location last communicated by the State (“charted/fixed approach”). Two questions arise in this regard from the formulation of Art. 5 UNCLOS: first, if the “large-scale charts” are of constitutive or descriptive nature–in other words–if the normal baseline is the one reflected in the charts or the actual physical low-water line. Only if States are allowed to constitutively mark their baselines on charts by communicating the position of the low-water line at a certain point in time with the Secretary General of the United Nations, the possibility of baseline freezing is conceivable. If the normal baseline is the one reflected in the charts, second, the question arises whether Art. 5 UNCLOS imposes a legal obligation on States to update these charts in case of deviation from the actual coastline. While these two questions are interlinked with each other, the ILC already noted in its 2023 Report that the distinction remains crucial (para. 150). The first question “possibly involve(s) the creation of a new rule of law”, while the second question concerns “simply an administrative matter”.  Similarly, in its 2025 Report, the ILC expressly notes that “the absence of such an obligation [to update charts] did not necessarily imply consensus as to whether baselines were fixed” (para. 55, see also differentiation in the submission of Antigua and Barbuda to the ILC, para. 10). An obligation to update charts is only relevant for the issue of sea-level rise if the baseline in the charts itself has legal significance.

2. The opinion of the Court and others

The Court first refers to Arts. 16, 75 II and 84 I, II UNCLOS, which require States to publicize charts regarding the outer limits of maritime zones and continental shelves (para. 358). It then finds that

“(…) once the breadth of maritime zones measured from the baselines has been duly established and the State has given due publicity to the charts or lists of geographical co-ordinates in accordance with UNCLOS, (…), there is no provision in the Convention requiring States parties to update them.” (para. 359).

The Court states that this observation does not only apply to the cases mentioned in Arts. 16, 75 II and 84 I, II UNCLOS, but also to normal baselines and archipelagic baselines. In its argumentation, the Court relies both on the wording of the provision and State practice which it deems “(…) relevant for the purposes of the interpretation of UNCLOS.” (para. 360), possibly relating to subsequent State practice in the sense of Art. 31 III (b) or 32 VCLT.

While the Court did consider the evolutive manner of interpretation of UNCLOS (paras. 336 ff.) and the principle of cooperation of States concerning sea-level rise as vital and obligatory (para. 364), most of the issues discussed in the ILA and ILC remain without any consideration, such as the wording in other authoritative languages under Art. 320, 33 UNCLOS, the context of the norm, the principle of legal stability, the principle of land dominates the sea, the immutability and intangibility of boundaries, the applicability of rebus sic stantibus and its drafting history. The ICJ did mention two aspects which are both relevant to the topic of sea-level rise in other sections of the Advisory Opinion: The principle of equity (paras. 152-154) and the relationship between obligations arising from treaties and from customary international law (paras. 309-315). However, it did not expressly address the legal value of the principle of equity for the question of “baseline freezing” and only analyzed the relationship between climate change treaties and customary international law specifically.

3. The end of a lengthy discussion?

With this assessment, the Court fails to give an adequate response to the lengthy discussion of the past. Two aspects are particularly worthy of criticism:

First, while implying an answer to the “baseline freezing” discussion, it does not endorse it explicitly. Judge Aurescu, the former Co-Chair of the ILC Study Group, described the Court’s opinion in his separate opinion as “excessively and unnecessarily cautious and minimalist” (para. 1) (similar also by Vice-President Sebutinde, para. 8). The deeper issue behind the inconclusive approach of the Court is that it only refers to the second question posed regarding Art. 5 UNCLOS, namely the obligation to update charts. However, it did not address the question of whether baselines are constitutively marked in charts or ambulatory in nature. One could even understand the Court as only denying an administrative burden upon coastal States to update their charts displaying them. Bearing this in mind, Judge Aurescu also inaccurately defines “the solution of fixed baselines” as “(t)he interpretation of UNCLOS that States are not required to update their baselines (the charts or lists of geographical co-ordinates descripting the baselines), once they have been duly established in conformity with the Convention (…)” (para. 5). He makes no mention of the question of whether baselines are of ambulatory nature or constitutively marked on charts, which dominated the sea-level rise discussion for years.

That being said, it seems reasonable to understand the Court as presupposing the constitutive nature of charts when denying an obligation to update charts. Thereby, it implicitly rejects the ambulatory nature of baselines. Otherwise, the quite detailed explanation of updating charts under UNCLOS was pointless, or at least out of context. Additionally, the Court emphasizes the view of many participants that “(…) existing baselines (…) should be preserved, notwithstanding the physical effects of sea level rise, including coastal recession” (para. 355). This formulation relates to the option of fixed baselines and not simply the administrative matter of an obligation to update charts. Therefore, the Court’s answer to the lengthy discussion on “baseline freezing” could be considered as follows: Interpreting UNCLOS in an evolutive manner, baseline-freezing is allowed. It nonetheless remains problematic that one must “think around the corner” to understand this underlying interpretation, and the Court simply misses the chance to give an accurate and authoritative assessment of the complex discussion, which has been dragging on for years.

Second, the Court only deals with the consequences of sea-level rise in a few paragraphs,  ignoring most of the contentious issues. This was also criticized by Judge Aurescu in his separate opinion (e.g. paras. 4, 5, 14f.) and seems disproportionate to the scale of the issue. It is surprising that the Court does not mention the statement by the ILC, which recognizes a strong indication of the customary nature of fixed baselines (ILC Report 2025 para. 483; separate opinion of Judge Aurescu, paras. 5, 12). This could be even more problematic than the imprecise legal assessment of “baseline freezing” given the fact that sea-level rise constitutes an urgent issue for many States and especially indigenous people (similar by Judge Aurescu, para. 1, Vice-President Sebutinde, para. 6, and Judge Charlesworth, para. 17) which are in need of legal clarity.  

What remains is a bitter aftertaste. Either the Court, contrary to Judge Aurescu’s definition, only pronounced on the administrative question of an obligation to update charts, or it generally endorsed and argued in favor of the option of fixed baselines under Art. 5 UNCLOS in a few paragraphs with an imprecise formulation, which leaves room for ambiguity (similar declaration of Judge Tomka).

III. The effects of sea-level rise on statehood

Additionally, the Court discusses the effects of sea-level rise on statehood. The brevity and simplicity with which the Court softens the requirements for statehood in this section for the benefit of low-lying States, while morally appealing, is simultaneously surprising in a negative way: States still constitute the original subjects of public international law, and the topic of statehood is at the forefront of current political discussions, especially considering international recognition of a Palestinian State. Judge Tomka vividly notes in his declaration that “(…) the Court’s manner of addressing the issue is itself troubling and warrants attention for its ambiguous and potentially far-reaching implications” (para. 2). Likewise, Judge Aurescu criticizes the inaccurate wording in this regard (para. 20).

The Court first notes that “in the event of the complete loss of a State’s territory and the displacement of its population, a strong presumption in favour of continued statehood should apply” (para. 363). The consideration of a presumption has been recently contrasted by the ILC in its 2025 Report (para. 62) with the possibility of a principle of continuity. The ILC stated that a presumption would place the burden of proof on the claiming State and create a discussion regarding rebuttable and irrebuttable presumptions, while the characterization as a principle would better align with the specific situation of States. The Court’s dissent from this assessment by the ILC comes without reason and regrettably includes no further information regarding the burden of proof and a possible connection with the strength of the presumption.

Further, the Court states, that “(…) once a State is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood” (para. 363). In this context, Judge Tomka provides two possible interpretations: It could either mean that international law does not preclude the possibility of continuity or that the Court endorses the “deconstruction of the conditions of statehood as such” (para. 2). The establishment of statehood must be distinguished from the maintenance of statehood. The role of the Court is not to “anticipate the law before the legislator has laid it down” (ICJ Fisheries Jurisdiction 1974, para. 53), the legislators being the States themselves. Therefore, in the absence of an explicit agreement, the rules of maintenance of statehood must be established through a rule of customary international law. However, Judge Tomka argues that such a rule has not yet crystallized (para. 6). Based on the lack of (nearly) flooded low-lying States, there is simply not enough State practice regarding the recognition of such States despite territory- and/or population-loss. Additionally, while the ILC has found a growing tendency in respective opinio juris, it does not suffice for the qualification of an unequivocal rule of customary law.

While there remains plenty to discuss concerning this topic, Judge Tomka has accurately found that “to pronounce itself upon statehood as such was not only unwise, but also unnecessary” (para. 10), as the question of statehood was not even part of the mandate given by the General Assembly and the Court’s initiative in this aspect disregarded that the progressive development of international law is a role assigned to the General Assembly – and subsequently to the ILC – under Art. 13 (1) (a), not to a judicial organ.

IV. Conclusion

The ICJ’s determinations regarding the obligations of States in relation to sea-level rise and related issues do not provide the clarity that may have been intended by the Court. Both the considerations concerning the obligation to update charts under UNCLOS and statehood stand out due to their brevity. It is exactly that brevity that is the foundation for the ambiguity of the Court’s statements. In the interest of all States endangered by sea-level rise, and referring to the statement of Judge Tomka at the beginning, having regard to these astonishing ramifications of the Court’s words, we hope that it will revisit this topic.

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Lukas Herich is a PhD Student and Research Associate at the Ruhr-University Bochum.

Katharina Thiehoff is a student assistant at Ruhr University Bochum’s Institute for International Law of Peace and Armed Conflict (IFHV).