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Sea-Level Rise and Maritime Boundaries: From Uncertainty to Clarity?

Published online by Cambridge University Press:  14 November 2025

Nilüfer Oral
Nilüfer Oral*
Affiliation:
Director, Centre for International Law, National University of Singapore , Singapore Member of the International Law Commission
*
Email: nilufer@nus.edu.sg
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Abstract

According to the Intergovernmental Panel on Climate Change’s Sixth Assessment Report, sea-level rise will continue for thousands of years. Many small island States and low-lying coastal States are already experiencing sea-level rise together with landward regression of coastal areas. This raises several legal questions, such as whether States are obligated to revise existing baselines, outer limits of maritime zones and charts. This article examines the legal lacunae in the United Nations Convention on the Law of the Sea (UNCLOS) concerning the legal consequences of sea-level rise on baselines and maritime boundaries, including islands and archipelagos. It provides an overview of the work of the International Law Commission on sea-level rise in relation to international law. It traces the evolution of States’ views within the Sixth Committee between 2018 and 2024 and the UN, as well as the recent Advisory Opinions on climate change of the International Tribunal for the Law of the Sea and the International Court of Justice. It concludes by noting how the legal lacunae in UNCLOS have been clarified, leaving to be determined the next steps in implementing the convergence of States’ interpretations of UNCLOS and customary international law in favour of preservation of baselines and maritime boundaries despite the physical effects of sea-level rise on coastlines.

Keywords

sea-level riseInternational Law Commissionbaselinesmaritime zonesclimate changerebus sic stantibusAdvisory OpinionsSixth Committee

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Forum
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International & Comparative Law Quarterly , Volume 74 , Supplement S1: Law of the Sea: Present Challenges and Future Directions , July 2025 , pp. 163 - 178
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
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© The Author(s), 2025. Published by Cambridge University Press on behalf of British Institute of International and Comparative Law

1. Introduction

Climate change was recognised as a common concern of humankind following the historic adoption in 1988 by the United Nations (UN) General Assembly of Resolution 43/53.Footnote 1 Shortly thereafter, the now well-known Intergovernmental Panel on Climate Change (IPCC) issued its First Assessment Report, in which it highlighted the warming impacts of global warming on the ocean, including sea-level rise.Footnote 2 These important developments all took place after the adoption of the United Nations Convention the Law of the Sea (UNCLOS)Footnote 3 in 1982, following nine years of intense negotiations. Consequently, climate change and sea-level rise were not considered during any part of the UNCLOS negotiations. This gap of just a few years is having significant consequences decades later as the impacts of climate change intensify, including the increase in both actual and projected sea-level rise.

Climate change is undeniably one of humanity’s greatest existential threats today. The oceans are warming at unprecedented rates, contributing to rising sea levels through the loss of ice sheets and glaciers.Footnote 4 The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate reported that the global mean sea level (GMSL) is rising, with acceleration in recent decades due to the increasing rates of ice loss from the Greenland and Antarctic ice sheets.Footnote 5 It was also projected with virtual certainty that GMSL will continue to rise until at least 2100 because all assessed contributors to GMSL are certain to continue contributing throughout this century.Footnote 6 Moreover, sea levels will continue to rise for centuries due to continuing deep-ocean heat uptake and the mass loss of the Greenland and Antarctic ice sheets, and will remain elevated for thousands of years.Footnote 7

According to the IPCC Sixth Assessment Report, ‘[s]ea level rise, as well as other irreversible changes, will continue for thousands of years, at rates depending on future emissions’.Footnote 8 It also reported that GMSL has risen faster since 1900 than over any preceding century in at least the last 3000 years.Footnote 9 The global ocean has warmed faster over the past century than since the end of the last deglacial transition (around 11,000 years ago).Footnote 10 According to the National Ocean and Atmosphere Agency, ‘[o]ver the last 30 years, human-caused sea level rise is 10 times that of natural sea level rise’.Footnote 11

In addition, sea-level rise brings serious socioeconomic consequences for the international community, such as threats to food security, damage to marine and coastal ecosystems, displacement of populations and loss of habitable land, and will continue to do so well into the distant future. Sea-level rise also raises legal questions, including its potential impact on the status of existing maritime boundaries and their associated maritime entitlements which are measured from baselines. Normal baselines, as codified in Article 5 UNCLOS, are determined by the low-water mark. Inevitably, rising sea levels will, in theory at least, alter this, as the ‘baseline’ is a legal construct. However, it is drawn based on physical criteria, such as the coastal configuration and basepoints. Often, these basepoints are constructed upon unstable or tiny features that are susceptible to disappearance, in whole or in part, because of sea-level rise.

The possibility that existing lawfully-established maritime boundaries could be subject to change due to climate change-induced sea-level rise increases the legal uncertainty stemming from whether States can challenge established maritime boundaries. This would place at risk existing entitlements of coastal States within these maritime zones. Such legal uncertainty could spark controversies and disputes over settled maritime spaces among States. Some of the key questions that UNCLOS does not address include whether baselines are ambulatory and must be updated because of changes to the coastline from sea-level rise, or whether States can ‘fix’ or ‘preserve’ existing baselines and maritime boundaries notwithstanding physical changes to their coastline from sea-level rise. In addition, it is not clear whether States have an obligation under Article 16 UNCLOS to update nautical charts, given that the provision applies to straight baselines and not to normal low-water baselines. Furthermore, sea-level rise also raises questions as to the continued status of islands under Article 121 and archipelagos under Article 47, and their associated rights and entitlements.Footnote 12

These very real and critical legal issues have spurred the interest of many scholars,Footnote 13 the International Law Association (ILA) and organs of the UN, notably the International Law Commission (ILC), to examine these new questions of international law and consider possible solutions.

Section 2 of this article will examine the legal lacunae in UNCLOS on the questions raised by sea-level rise in relation to baselines and maritime boundaries, including islands and archipelagos. Section 3 will provide an overview of the work of the ILA and detail the work of the ILC on sea-level rise in relation to international law. Section 4 will trace the evolution of States’ views within the Sixth Committee between 2018 and 2024. Section 5 will look at how sea-level rise is being addressed by regional organisations and in other main bodies of the UN, as well as in the recent Advisory Opinions of the International Tribunal for the Law of the Sea (ITLOS) and the International Court of Justice (ICJ). Section 6 concludes that these gaps in UNCLOS have, paradoxically, helped clarify key issues. It then considers how States might implement their shared interpretations of UNCLOS and customary international law to preserve baselines and maritime boundaries despite sea-level rise.

2. The problem: legal lacunae in UNCLOS and under customary international law

2.1. Baselines

As noted in Section 1, the legal framework for the establishment of maritime boundaries, maritime zone islands and archipelagos as codified in UNCLOSFootnote 14 was adopted before climate change was recognised as a common concern of humankind by the UN General Assembly, and before the landmark 1989 Male Declaration on Global Warming and Sea Level Rise.Footnote 15 Interestingly, around 1990, a handful of scholars raised the possible ambulatory nature of baselines and the potential legal consequences of sea-level rise.Footnote 16

There are several provisions in UNCLOS concerning baselines. The normal baseline is the low-water baseline, as codified in Article 5, which sets out the general rule that ‘[e]xcept where otherwise provided in this Convention, the normal baseline for measuring the breadth of the territorial sea is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State’.Footnote 17 Exceptionally, straight baselines may be used in accordance with Articles 7, 9 and 10. The issue of whether the normal baseline under Article 5 UNCLOS and under international law is ambulatory because of sea-level rise has since become the subject of scholarly debate. The use of the low-water line for baselines dates back to the Geneva Convention on the Territorial Sea and Contiguous Zone,Footnote 18 an approach that was favoured during the 1930 Hague Codification Conference.Footnote 19 This was based on the assumption of relatively stable sea levels. Consequently, UNCLOS is silent on whether there is any requirement that the low-water line be periodically reviewed and updated by the coastal State, in general, and specifically in the case of changes to the low-water line. There is, moreover, no express requirement for States to update baselines or the large-scale charts that are officially recognised by the coastal State under Article 5. The only express requirements for showing baselines on nautical charts, according to Article 16, is for straight baselines (Articles 7, 9 and 10); however, there is no express reference to when an obligation to modify would be triggered.

The looming question with sea-level rise is whether the baseline must be ambulatory and, more importantly, whether under UNCLOS or customary international law, there is an obligation for the coastal State to review and revise the baseline should the coastal configuration change as a result of sea-level rise. If the answer is yes, this would have broad repercussions on the rights and obligations of the coastal State and those of third States. An obligation under international law for the coastal State to modify its baselines would mean that the outer boundaries of its maritime zones would have to change and shift landward. The result would be that part of the internal waters would become territorial sea, part of the territorial sea would become contiguous zone and/or exclusive economic zone (EEZ) and part of the EEZ would become high seas, with implications for the specific rights of the coastal State, third States and their nationals. While at first glance this does not appear to be problematic, in reality, it creates a host of consequences, in particular economic and political.Footnote 20

The consequences of a landward shift of the baseline was addressed in detail by the Study Group on Sea-Level Rise (Study Group) of the ILC, which, in summary, made the following observations:

  1. a) In general, if the baselines and the outer limits of the various maritime spaces move landward, this means that the legal status and legal regime of the maritime zones change: for example, part of the internal waters becomes territorial sea, part of the territorial sea becomes contiguous zone and/or EEZ, and part of the EEZ becomes high seas, with implications for the specific rights of the coastal State and third States, and their nationals (innocent passage, freedom of navigation, fishing rights etc).

  2. b) Sea-level rise also poses a risk to an archipelagic State’s baselines. As a result of the inundation of small islands or drying reefs, the existing archipelagic baseline could be impacted, resulting in the loss of archipelagic State status of baselines.

  3. c) The greatest loss in terms of rights of the coastal State and its nationals comes from the loss of maritime entitlements related to the EEZ should it become part of the high seas. In particular, developing States that derive important revenue from the natural resources, in particular living resources, in their EEZ could lose at least parts of this. In some cases, even a relatively small loss could have important developmental consequences.

  4. d) There is a possible effect on agreements, such as licenses for economic activities in the EEZ, such as offshore windfarms or fisheries access agreements.

  5. e) Overall, third States stand to benefit at the expense of the coastal State, and such changes in maritime entitlements do bring the risk of creating uncertainty, instability and the possibility of disputes.

  6. f) Consequently, the best option is the preservation of maritime entitlements so as to meet the need to preserve legal certainty, stability and predictability, as well as maintain the existing balance between the rights of the coastal State and the rights of third States.Footnote 21

2.2. Islands and archipelagos

In addition to the question of whether the normal baseline under Article 5 is ambulatory or not, and whether there exists an obligation to revise and update the baseline in the face of rising sea levels, there are other legal questions, such as the status of an island as opposed to a rock (and low-tide elevations) under Article 121 UNCLOS and also the potential loss of archipelagic baseline and waters under Article 47 UNCLOS, that are necessary to address in this context.

Article 121 is a rather ambiguous provision based on a contrario reasoning as to the difference between an island that is entitled to all the maritime zones, and a rock that is not entitled to an EEZ or a continental shelf of its own. First, the island must be naturally formed. Second, if it cannot sustain human habitation or economic life it is a rock that is not entitled to a continental shelf or EEZ. Much ink has been spilled on this issueFootnote 22 and, with the exception of the South China Sea Arbitration case brought by the Philippines against China,Footnote 23 the ICJ and other courts and tribunals have not provided a set of clear criteria to distinguish between a rock and an island.Footnote 24 Sea-level rise presents the legal conundrum of whether an island that could and did sustain human habitation or an economic life of its own—and had done so for a long period of time—would devolve into a rock if it were to become uninhabitable because of sea-level rise, and possibly then only be entitled to a territorial sea and a contiguous zone?

This could happen in several ways. For example, the loss of the capacity to farm, the loss of freshwater due to salinisation from the sea penetrating the freshwater sources or simply the loss of land upon which people can live could render the island uninhabitable. Would this mean that that the island would have to be reclassified as a rock? The difference between the maritime space generated by a fully-fledged island and a rock is enormous. The reclassification of a fully entitled island to the status of a rock could lead to a decrease in maritime area from 431,014 km2 to only 1550 km2.Footnote 25 Many small island States have massive EEZs from which they derive vital economic revenues, such as multi-million-dollar fisheries licensing agreements. For example, Fiji has a land area of 18,270 km2 and a total EEZ of about 1.3 million km2.Footnote 26 The loss of the EEZ to the high seas means a dramatic shift in legal regimes from one where the coastal States have exclusive sovereign rights over their natural resources, to the freedom of the high seas. The island States stand to lose economically while third States gain economically if they no longer have to pay fees. But this raises serious questions of equity as well: should the loss of one State’s territory due external factors over which it was not responsible inure to the benefit of another State?

Archipelagic States that can fulfil the complex formula under Article 47 UNCLOS may draw an archipelagic baseline connecting the outermost points of the outermost islands and drying reefs of the archipelago enclosing as archipelagic waters an area that was once part of the high seas. Archipelagic waters are similar to the territorial sea as foreign flagged vessels are granted innocent passage rights, with the difference that, unlike for the territorial sea, the right of overflight in archipelagic waters is recognised in Article 53 UNCLOS. In addition, a special regime of archipelagic sea lanes is recognised, which the archipelagic State may designate with the approval of the International Maritime Organization (IMO).Footnote 27 The question is, should the outermost points of the outermost islands and drying reefs of the archipelago become submerged due to sea-level rise, could this alter the 9:1 ratio requirement together with the formula required under Article 47(2) and (3), thereby calling into question the continuing right of the archipelagic State to employ the archipelagic straight baseline, possibly resulting in loss of the archipelagic waters and its associated rights and obligations?Footnote 28 There are some 22 States that have claimed such archipelagic status and use archipelagic straight baselines, and several of these are vulnerable to sea-level rise.Footnote 29 This marks another lacuna in UNCLOS and under customary international law as it was not taken into account during the negotiations and no answer can be found in the text of Article 47.

3. The work of the ILA and ILC

While a handful of scholars wrote about sea-level rise and the associated legal questions concerning maritime baselines and boundaries over 30 years ago,Footnote 30 these issues did not gain broader attention until the ILA, and later the ILC, undertook work on sea-level rise.Footnote 31 While the ILA Baselines Committee adopted the view that the normal baseline was ambulatory, it nonetheless recognised that sea-level rise could significantly impact baselines, stating that ‘[u]nder extreme circumstances the latter category of change could result in total territorial loss and the consequent total loss of baselines and of the maritime zones measured from those baselines’,Footnote 32 further observing that the ‘existing law of the normal baseline does not offer an adequate solution to this potentially serious problem’.Footnote 33

In light of the importance of the matter, the ILA established the Committee on International Law and Sea-Level Rise (Sea-Level Rise Committee) in 2012,Footnote 34 the work of which concluded in 2024.Footnote 35 The Sea-Level Rise Committee addressed several topics related to sea-level rise, including in relation to maritime baselines and outer limits.Footnote 36 In their detailed study in 2018, the Sea-Level Rise Committee did not challenge the Baselines Committee’s conclusion that normal baselines are ambulatory, but adopted a position in favour of the preservation of baselines and outer limits of maritime boundaries in the interests of legal certainty and predictability.Footnote 37 The final resolution of the ILA, adopted in 2024, on the issue of baselines and limits of maritime zones endorsed the recommendation by the Sea-Level Rise Committee:

that baselines and limits of maritime zones that are in compliance with the Convention and deposited with the UN Secretary-General, and have not met with objection by other States, should continue in place even if the territory involved gradually changes as a result of climate change-related impacts including sea level rise in the process or submergence …Footnote 38

and also endorsed:

the recommendation by the Committee regarding the finality of maritime boundaries agreed by treaty or settled by judicial decisions, so that existing maritime boundaries should continue in force and represent the legal extent of maritime zones, even if the territory from which the agreed or adjudicated boundaries were originally calculated gradually changes in the process of submergence.Footnote 39

The work of the Sea-Level Rise Committee was groundbreaking. However, as a private organisation of scholars, its sphere of influence was limited and lacked a direct connection to States. An important development took place when, in 2018, the ILC, a subsidiary body of the UN General Assembly, placed the topic of sea-level rise in relation to international law on its long-term programme of work and then, in 2019, on its current agenda of work. The ILC established the Study Group with five Co-Chairs responsible for three main subtopics: law of the sea; statehood; and protection of persons.Footnote 40 Notably, in 2018, a detailed written request was submitted by the Federated States of Micronesia requesting the ILC to take up the topic of sea-level rise and international law.Footnote 41 The mandate of the ILC Study Group was not to engage in normative work but to do a mapping of existing international law, including its gaps.Footnote 42

The Co-Chairs first prepared issue papers on each of the three subtopics followed by additional papers. These reports were first discussed within the Study Group of the ILC in 2021 and then commented on by the Member States of the Sixth Committee of the UN General Assembly. In 2025, the Co-Chairs submitted their final consolidated report (First Issues Paper) addressing all three subtopics in an integrated manner with specific conclusions.Footnote 43 The following discussion will focus on the first subtopic, the law of the sea.

The First Issues Paper included discussion of: the possible legal effects of sea-level rise on baselines and outer limits of the maritime spaces that are measured from the baselines; maritime delimitations; islands used for the construction of baselines and in maritime delimitations; the exercise of sovereign rights and jurisdiction of the coastal State and its nationals in maritime spaces; possible legal effects of sea-level rise on the status of islands, including rocks, and on the maritime entitlements of a coastal State with fringing islands; and the legal status of artificial islands, reclamation or island fortification activities under international law as a response or adaptive measure to sea-level rise.Footnote 44 In addition, an oral presentation on the practice of African States regarding maritime delimitation was made by one of the Co-Chairs during the first meeting of the Study Group.Footnote 45

The First Issues Paper presented a number of preliminary observations, notably that: first, at the time of the negotiation of UNCLOS, sea-level rise was not an issue that needed to be addressed; second, that the ambulatory baseline approach does not respond to the concerns expressed by Member States in relation to the effects of sea-level rise, especially the need to preserve legal stability, security, certainty and predictability; and, third, that the preservation of baselines and outer limits of the maritime zones would respond to these concerns. In addition, the First Issues Paper observed that UNCLOS does not prohibit the preservation of baselines and maritime zones. The obligation under Article 16 in relation to charts and lists of coordinates only refers to straight baselines and not to normal baselines. Even in the case of straight baselines, UNCLOS does not indicate an obligation to draw and notify new baselines when coastal conditions change. Moreover, nothing under UNCLOS prevents Member States from refraining from updating charts and notifications to the Secretary-General in order to preserve their entitlements. In addition, the ambulatory approach could bring into question effected maritime delimitations, which in turn would create legal uncertainty. Importantly, the First Issues Paper observed the necessity of preserving existing maritime zones notwithstanding the coastal changes produced by sea-level rise so as to maintain legal stability, certainty and predictability. On the question of whether a fundamental change of circumstances under Article 62(2) of the Vienna Convention on the Law of Treaties (VCLT) applies to sea-level rise, the First Issues Paper took the position that, as maritime boundaries enjoy the same regime of stability as any other boundaries, they would not be subject to it.Footnote 46

The First Issues Paper was followed by the Additional Paper, in which the Co-Chairs addressed specific topics that the Study Group had proposed, such as the relationships between legal stability, certainty and predictability and the preservation of baselines and maritime zones.Footnote 47 In addition, it examined the immutability and intangibility of boundaries, including the principle of uti possidetis juris. Footnote 48 It engaged in a more detailed examination of the fundamental change of circumstances principle under Article 62(2) VCLT.Footnote 49 In addition, the Additional Paper studied: the effects of the potential situation whereby overlapping areas of the EEZs of opposite coastal States, delimited by bilateral agreement, no longer overlap; the issue of objective regimes; the effects of a situation whereby an agreed land boundary terminus ends up being located out at sea; and the judgment of the ICJ in Maritime Delimitation in the Caribbean Sea and the Pacific Ocean (Costa Rica v Nicaragua). Footnote 50 The Additional Paper examined a number of principles, including: that the land dominates the sea;Footnote 51 historic waters, title and rights;Footnote 52 equity;Footnote 53 and permanent sovereignty over natural resources.Footnote 54 It also conducted a detailed examination of the possible gains and losses of third States in the case of a landward shift of baselines and maritime zonesFootnote 55 as well as nautical charts and their relationship to baselines, maritime boundaries and safety of navigation.Footnote 56

A detailed summary of the preliminary observations made on these issues would be too long for this article; however, some can be highlighted. It should be noted that both the First Issues Paper and the Additional Paper benefitted from the written statements of Member States of the Sixth Committee and submissions made to the ILC in response to a request to States for information on their practices. By the time of the Additional Paper, a clear trend was emerging in favour of the preservation of baselines and maritime boundaries as being directly linked to the principles of legal stability, predictability and certainty. This was highlighted in the preliminary observations of the Additional Paper based on the numerous statements from the Sixth Committee. Likewise, many States made clear their view that the principle of a fundamental change of circumstances under Article 62(2) VCLT does not apply to maritime boundaries being also subject to the principle of the immutability of boundaries. In addition, there was a very clear trend from States that, under UNCLOS and in practice, there is no obligation for States to update nautical charts in relation to baselines and sea-level rise. The IMO and the International Hydrographical Organization provided the ILC with valuable information on the use and nature of nautical charts, which is detailed in the Additional Paper. The relationship between permanent sovereignty over natural resources and the preservation of maritime zones and their associated entitlements was also highlighted by States as a principle of customary international law.

The final meeting of the Study Group was held during the seventy-sixth session of the ILC during which the Co-Chairs presented their Final Consolidated Report which combined all three subtopics, including cross-cutting issues that were common to the three subtopics, such as the principles of stability, certainty and predictability, the duty of cooperation, equity and other issues.Footnote 57 Notable was the finding of the convergence of views from States across regions on the issues concerning sea-level rise and maritime boundaries.Footnote 58 The Final Consolidated Report also included a set of draft conclusions for the work of the Study Group.Footnote 59 Following discussions, the Study Group adopted the Final Consolidated Report with a set of conclusions and suggested ways forward,Footnote 60 which were adopted unanimously by the ILC.Footnote 61

4. The Sixth Committee and sea-level rise

The Sixth Committee, also known as the Legal Committee, is the legal body of the UN General Assembly that considers legal questions with representation of all UN Member States. Part of its work programme involves consideration of the work of the ILC. As an expert technical subsidiary body of the UN General Assembly that is responsible for the promotion of the progressive development of international law and its codification, the views of the Member States of the Sixth Committee are integral to the ILC’s work. As stated in Section 3, the ILA does not have this direct connection with States.

The possibility of the ILC taking up the topic of sea-level rise was discussed as early as 2017 during the seventy-second session of the meeting of the Sixth Committee when 15 States requested the inclusion of this topic in the programme of work.Footnote 62 Following the decision of the ILC to place the topic on its long-term programme of work in 2018, some 50 States commented, of which 26 were in favour of the ILC taking up the topic.Footnote 63 However, a small number of States expressed negative views,Footnote 64 while other States were ambiguous or silent. By the following year, the support for the topic and the work of the ILC grew to 57 delegations referring to the topic in their interventions, of which 49 delegations (including statements on behalf of regional groups or organisations) expressed support for the decision taken by the ILC in 2019 to include the topic in its current programme of work, with a few expressing certain concerns.Footnote 65

In the period between 2020 to 2024, in addition to a groundswell of support for the work of the ILC on sea-level rise, a very clear trend emerged across all regions, in particular Europe, Asia, Latin America and Oceania, recognising the lacunae in UNCLOS in addressing the questions concerning the legal consequences for baselines, maritime boundaries and associated entitlements and the need to find practical solutions. A common thread in the statements and submissions of States was the overarching importance of maintaining legal stability and predictability.

By 2024, the views of many members of the Sixth Committee had emerged in strong support for an interpretation of UNCLOS and international law that included the following: lawfully-established baselines are not required to be updated because of changes to the coast due to sea-level rise; the existence of a direct relationship between legal stability, certainty and predictability and the preservation of lawfully-established baselines and maritime zones; that there is no requirement to update nautical charts under UNCLOS; that a requirement for States to change their baselines or maritime zones due to sea-level rise would produce inequitable outcomes and create situations of uncertainty; that there is no provision in UNCLOS that prevents States from preserving existing and lawfully-established baselines and maritime zones; the principle of the immutability of boundaries applies to lawfully established maritime boundaries; that maritime boundaries are not subject to the fundamental change of circumstances principle under Article 62(2) VCLT;Footnote 66 and that the preservation of baselines and maritime zones is also part of the principle of permanent sovereignty over natural resources, which is a principle of customary international law that also applies to marine resources.

5. Beyond the Sixth Committee

Sea-level rise started off primarily a concern for small island States, in particular those from the Pacific Region who, throughout the years, have been vocal in leading the initiative for the international community to address these pressing legal issues of great consequence to their livelihoods. In 1989, the Alliance of Small Island States (AOSIS) adopted a historic declaration warning the world about the potential consequences of global warming on rising sea levels.

Decades later, when the international community and scholars gave serious attention to this pressing issue for humanity, the Pacific Islands Forum (PIF) adopted the Declaration on Preserving Maritime Zones in Face of Climate-Related Sea Level Rise (2021 PIF Declaration).Footnote 67 In the 2021 PIF Declaration, the 18 Member States presented their view that UNCLOS ‘imposes no affirmative obligation to keep baselines and outer limits of maritime zones under review nor to update charts or lists of geographical coordinates once deposited with the Secretary-General of the United Nations’.Footnote 68 Moreover, the PIF Member States declared that they would preserve their maritime boundaries once they have established and notified them to the UN Secretary-General in accordance with UNCLOS. The States further declared that they would not review and update the baselines and outer limits of maritime zones as a consequence of climate change-related sea-level rise.Footnote 69 The 2021 PIF Declaration was reaffirmed subsequently with the adoption of the 2023 Declaration on the Continuity of Statehood and the Protection of Persons in the Face of Climate Change-Related Sea-Level Rise (2023 PIF Declaration).Footnote 70

The landmark 2021 PIF Declaration received widespread support from States. It was first endorsed by the 39 Member States of AOSIS on 22 September 2021,Footnote 71 followed by the Climate Vulnerable Forum (58 members: 27 members from Africa and the Middle East, 20 members from Asia and the Pacific and 11 members from Latin America and the Caribbean);Footnote 72 and the Organization of African, Caribbean and Pacific States (79 members from Africa, the Caribbean and the Pacific).Footnote 73 In 2024, the 56 Commonwealth Heads of Government adopted the Apia Commonwealth Ocean Declaration for One Resilient Common Future, supporting an interpretation of UNCLOS that allows for the preservation of maritime zones.Footnote 74 On 23 September 2024, the Heads of State and Government of AOSIS adopted a Declaration on Sea-Level Rise and Statehood.Footnote 75

The topic of sea-level rise has also spilled into the main organs of the UN.Footnote 76 On 14 February 2023, a debate in the UN Security Council was held on the implications of sea-level rise for international peace and security, which was followed with an informal plenary meeting of the UN General Assembly on the existential threats of sea-level rise amid the climate crisis on 3 November 2023.Footnote 77 On 25 September 2024, the UN General Assembly’s High-level Meeting to Address Existential Threats Posed by Sea-Level Rise was held in New York,Footnote 78 with the overall theme of addressing the threats posed by sea-level rise.Footnote 79

Lastly, two important Advisory Opinions have been delivered on the topic of climate change and the responsibility of States. The first one was issued by ITLOS following a request made by the Commission on Small Island States (COSIS) on 12 December 2022,Footnote 80 and the second was issued by the ICJ following the unanimous adoption by the UN General Assembly of Resolution 77/276.Footnote 81 Both Advisory Opinions garnered broad interest from States, the latter attracting over 90 States to submit written statements and appear for the oral hearings held from 2 to 12 December 2024. ITLOS delivered its unanimous Advisory Opinion on 21 May 2024; however, sea-level rise and issues related to maritime boundaries were not addressed as these issues were not part of the request made by COSIS.Footnote 82

By contrast, in the proceedings before the ICJ, while not expressly part of the questions included in Resolution 77/276, sea-level rise was raised by many States in their written and oral statements, including matters relating to the impacts of sea-level rise on their coastlines, baselines and maritime entitlements. These States also made clear their national positions in support of the preservation of baselines and maritime zones,Footnote 83 including endorsements of the PIF Declarations.Footnote 84 The written statement submitted by COSIS, examining different sources, in particular the work of the ILC, asserted that ‘at least 104 States—representing a strong majority of island and coastal States—acknowledge that maritime baselines remain fixed at their current coordinates notwithstanding physical coastline changes brought about by sea level rise’.Footnote 85

The ICJ delivered its much-anticipated Advisory Opinion on 23 July 2025. The voluminous opinion also addressed sea-level rise and expressly acknowledged the work of the ILC.Footnote 86 In relation to the law of the sea questions, the ICJ expressed its unanimous opinion:

that the provisions of UNCLOS do not require States parties, in the context of physical changes resulting from climate-change related sea level rise, to update their charts or lists of geographical co-ordinates that show the baselines and outer limit lines of their maritime zones once they have been duly established in conformity with the Convention. For this reason, States parties to UNCLOS are under no obligation to update such charts or lists of geographical co-ordinates …Footnote 87

thus endorsing the conclusions of the ILC.Footnote 88

6. Conclusion

A gap of a few years between the adoption of UNCLOS and the emergence of international awareness of climate change led to significant gaps in the legal framework of maritime boundaries, islands and archipelagic baselines some decades later. While a handful of scholars raised concerns as early as 1990, the issue was not taken up formally until 2012 when the ILA Sea-Level Rise Committee was established, followed in 2018 by the ILC. During this time, small island States, in particular the Pacific Island States, were active in raising concerns about sea-level rise.

The need for practical solutions that would preserve legal certainty, stability and predictability through the preservation of baselines and maritime boundaries was endorsed by the ILA Sea-Level Rise Committee. However, it was as a result of the ILC taking on the topic of sea-level rise that a process was started with the direct involvement of Member States in the Sixth Committee. In a relatively short space of time, between 2018 and 2024, a clear convergence of views from States on issues relating to the law of the sea emerged. First, States agreed on the importance of legal stability, certainty and predictability and the relationship to maritime boundaries. Second, there was a very clear convergence of views that there is no obligation under UNCLOS for States to revise and update baselines or maritime boundaries duly established because of sea-level rise. Third, there was a clear convergence of views by States that there is no requirement under UNCLOS (Article 16) for States to update nautical charts because of changes to baselines or maritime boundaries due to sea-level rise. Fourth, there was broad agreement that sea-level rise, as a fundamental change of circumstances, would apply to maritime boundaries. Moreover, there was also a broad concurrence of views that the principle of the immutability of boundaries applies to maritime boundaries. Many States also agreed that the principle of permanent sovereignty over natural resources is a rule of customary international law that applies to the marine resources of the coastal State.

Sea-level rise has also garnered much attention in other bodies and organs of the UN, including the Security Council, the General Assembly and the ICJ, and also outside the UN in ITLOS. The Advisory Opinion of the ICJ has contributed significantly to the clarification of the status of maritime baselines and boundaries and associated entitlements in relation to climate change-induced sea-level rise by endorsing the conclusions of the ILC.

In conclusion, sea-level rise may present a situation where States, through the opportunities afforded by the multilateral framework of the UN and its subsidiary bodies, are able to cooperate and develop a common interpretation of a legal instrument, in this case UNCLOS, and fill gaps in the interests of legal stability, certainty and predictability. This can be considered within the framework of international law-making. Options available to States include the adoption of an interpretative statement under UNCLOS, or possibly subsequent agreement or practice under Article 31(3) VCLT or the statements of States at the Sixth Committee may be considered as evidence of the crystallisation of customary international law. The ball is in the court of States as to the next steps.

Acknowledgements

The author thanks Vincenzo Elia for his assistance.

References

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13 e.g. DD Caron, ‘When Law Makes Climate Change Worse: Rethinking the Law of Baselines in light of a Rising Sea Level’ (1990) 17 EcologyLQ 621; AHA Soons, ‘The Effects of a Rising Sea Level on Maritime Limits and Boundaries’ (1990) 37 NILR 207; J Lisztwan, ‘Stability of Maritime Boundary Agreements’ (2012) 37 YaleJIL 153; C Schofield and D Freestone, ‘Options to Protect Coastlines and Secure Maritime Jurisdictional Claims in the Face of Global Sea Level Rise’ in MB Gerrard and GE Wannier (eds), Threatened Island Nations Legal Implications of Rising Seas and a Changing Climate (CUP 2013) 141; C Lathrop, ‘Baselines’ in DR Rothwell et al (eds), Oxford Handbook on the Law of the Sea (OUP 2016) 69; D Freestone, A Torres Camprumbí and D Vidas, ‘Sea Level Rise and Impacts on Maritime Zones and Limits: The Work of the ILA Committee on International Law and Sea Level Rise’ (2017) 5 KoreanJIntlCompL 5; C Grundy-Warr (ed), International Boundaries and Boundary Conflict Resolution, Proceedings of the 1989 IBRU Conference (Durham University 1990) 279; TG Puthucherril, ‘Rising Seas, Receding Coastlines, and Vanishing Maritime Estates and Territories: Possible Solutions and Reassessing the Role of International Law’ (2014) 16 ICLR 38; K Purcell, Geographical Change and the Law of the Sea (OUP 2019); M Lando, ‘Baseline Preservation as a Response to Sea-Level Rise’ (2025) 56 ODIL 100.

14 UNCLOS (n 3) arts 5, 7, 9, 10.

15 Small States Conference on Sea Level Rise, ‘Male Declaration on Global Warming and Sea Level Rise’ (18 November 1989) Doc No MDV/SLR/15. UNGA, ‘Letter dated 20 November 1989 from the Permanent Representative of Maldives to the United Nations addressed to the Secretary-General’ (20 November 1989) UN Doc A/C.2/44/7.

16 Caron (n 13) 621; Soons (n 13) 207; E Bird and V Prescott, ‘Rising Global Sea Levels and National Maritime Claims’ (1989) 1 Marine Policy Reports 177; D Freestone and J Pethick, ‘International Legal Implications of Coastal Adjustments under Sea Level Rise: Active or Passive Policy Responses?’ in J Titus and N Psuty (eds), Changing Climate and the Coast: Volume 1: Adaptive Options to Sea Level Rise Report to the Intergovernmental Panel on Climate Change (IPCC 1990) 237.

17 UNCLOS (n 3) art 5.

18 Convention on the Territorial Sea and the Contiguous Zone (adopted 29 April 1958, entered into force 10 September 1964) 516 UNTS 205.

19 Lathrop (n 13) 74.

20 N Oral and B Aurescu, ‘Sea Level Rise and Maritime Boundaries: The Case for Stability, Legal Certainty, and Peaceful Relations’ in CN Brower et al (eds), By Peaceful Means: International Adjudication and Arbitration: Essays in Honour of David D Caron (OUP 2024) 437, 447.

21 ILC, ‘Sea-Level Rise in relation to International Law: First Issues Paper by Bogdan Aurescu and Nilüfer Oral, Co-Chairs of the Study Group on Sea-Level Rise in relation to International Law’ (28 February 2020) UN Doc A/CN.4/740 (First Issues Paper) para 190.

22 e.g. JM Van Dyke and RA Brooks, ‘Uninhabited Islands: Their Impact on the Ownership of the Oceans’ Resources’ (1983) 12 ODIL 265, 271; J Charney, ‘Rocks that Cannot Sustain Human Habitation’ (1999) 93 AJIL 863, 870–71; G(J) Xue, ‘How Much Can a Rock Get?’ (2011) 6 China Oceans Law Review 1; YH Song, ‘Okinotoroshima: A “Rock” or an “Island”? Recent Maritime Boundary Controversy between Japan and Taiwan/China’ in SY Hong and JM Van Dyke (eds), Maritime Boundary Disputes, Settlement Processes, and the Law of the Sea (Martinus Nijhoff 2009) 145.

23 South China Sea Arbitration (Philippines v China) PCA Case No 2013-19, Award (12 July 2016); MST Gao, ‘The Interpretation of Article 121(3) of UNCLOS by the Tribunal for the South China Sea Arbitration: A Critique’ (2019) 50 ODIL 49.

24 e.g. Continental Shelf (Libyan Arab Jamahiriya/Malta) (Judgment) [1985] ICJ Rep 13, para 64; Maritime Delimitation and Territorial Questions between Qatar and Bahrain (Qatar v Bahrain) (Judgment) [2001] ICJ Rep 40, 99, para 195; Territorial and Maritime Dispute (Nicaragua v Colombia) (Judgment) [2012] ICJ Rep 624, 674, para 139; Eritrea/Yemen Arbitration (Second Stage: Maritime Delimitation) (2006) XXII RIAA 335, 368, paras 147–148, 404; Sovereignty over Pulau Ligitan and Pulau Sipadan (Indonesia/Malaysia) (Judgment) [2002] ICJ Rep 625; Maritime Delimitation in the Black Sea (Romania v Ukraine) (Judgment) [2009] ICJ Rep 61.

25 See C Schofield, ‘The Trouble with Islands: The Definition and Role of Islands and Rocks in Maritime Boundary Delimitation’ in SY Hong and JM Van Dyke (eds), Maritime Boundary Disputes, Settlement Processes, and the Law of the Sea (Martinus Nijhoff 2009) 19, 21.

26 UN Office of the High Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island Developing States, ‘Small Island Developing States in Numbers’ (2020) 4–5 <https://www.un.org/ohrlls/sites/www.un.org.ohrlls/files/sids_in_numbers_oceans_2020.pdf>.

27 UNCLOS (n 3) art 53.

28 Freestone and Schofield (n 12) 381–83.

29 ibid.

30 See sources in n 13, especially Caron and Soons.

31 ILC, ‘Report of the International Law Commission on the Work of Its Seventy-First Session (29 April–7 June and 8 July–9 August 2019)’ (2019) UN Doc A/74/10.

32 Committee on Baselines under the International Law of the Sea, ‘Final Report’ in International Law Association (ILA), ‘Report of the Seventy-Fifth Conference’ (ILA Conference, Sofia, 2012) 31 <https://www.ila-hq.org/en/documents/conference-report-sofia-2012-13>.

33 ibid.

34 The ILA Committee on International Law and Sea Level Rise was established following Resolution 1/2012 of the 75th ILA Conference held in Sofia in 2012. ILA Committee on International Law and Sea Level Rise, Resolution No 1/2012: Baselines under the International Law of the Sea (2012) <https://www.ila-hq.org/en/documents/conference-resolution-english-sofia-2012-5>.

35 ILA, ‘International Law and Sea Level Rise Committee: Final Report’ (ILA Conference, Athens, 22 May 2024) <https://www.ila-hq.org/en/documents/athens-2024-final-report-committee-on-international-law-and-sea-level-rise-26-06-2024-1>.

36 The other topics included the impacts on statehood and the rights of affected populations.

37 ILA, Resolution No 5/2018 (ILA Conference, Sydney, 2018).

<https://www.ila-hq.org/en_GB/documents/conference-resolution-sydney-2018-english-2>; ILA, ‘Final Report of the Seventy-Eighth Conference’ (ILA Conference, Sydney, 2018) <https://www.ila-hq.org/en/documents/conference-report-sydney-2018-5>.

38 ILA, Resolution No 1/2024 (ILA Conference, Athens, 25–28 June 2024) 2 <https://www.ila-hq.org/en/documents/ila-resolution-1-2024-e-committee-on-international-law-and-sea-level-rise-1>; ILA, ‘Final Report’ (ILA Conference, Athens, 2024) <https://www.ila-hq.org/en/documents/athens-2024-final-report-committee-on-international-law-and-sea-level-rise-26-06-2024-1>.

39 ibid.

40 The Co-Chairs were Mr Bogdan Aurescu, Mr Yacouba Cissé, Ms Patrícia Galvão Teles, Ms Nilüfer Oral and Mr Juan José Ruda Santolaria.

41 ILC, Document ILC(LXX)/LT/INFORMAL/1 (31 January 2018) (on file with the UN Codification Division), cited in First Issues Paper (n 21) n 37.

42 ILA, ‘Report of the International Law Commission on the Work of Its Seventieth Session’ (2018) UN Doc A/73/10, annex B (syllabus on sea-level rise).

43 First Issues Paper (n 21).

44 ibid.

45 ILC, ‘Report of the International Law Commission on the Work of Its Seventy-Second Session (26 April–4 June and 5 July–6 August 2021)’ (18 August 2021) UN Doc A/76/10, ch IX, paras 259–261 (presented by Mr Yacouba Cissé).

46 First Issues Paper (n 21) para 141.

47 ILC, ‘Additional Paper to the First Issues Paper (2020) by Bogdan Aurescu and Nilüfer Oral (2023), Co-Chairs of the Study Group on Sea-Level Rise in relation to International Law’ (13 February 2023) UN Doc A/CN.4/761, paras 20–76.

48 ibid paras 99–111.

49 ibid paras 112–125; Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155 UNTS 331 (VCLT).

50 ibid paras 126–147.

51 ibid paras 148–155.

52 ibid paras 156–168.

53 ibid paras 170–183.

54 ibid paras 184–192.

55 ibid paras 195–214.

56 ibid paras 215–245.

57 ILC, ‘Final Consolidated Report of the Co-Chairs of the Study Group on Sea-Level Rise in relation to International Law, Patrícia Galvão Teles, Nilüfer Oral and Juan José Ruda Santolaria’ (3 February 2025) UN Doc A/CN.4/783.

58 ibid para 491.

59 ibid annex.

60 UNGA, ‘Report of the International Law Commission on the Work of Its Seventy-Sixth Session’, UNYBILC, vol II, part two (2025) UN Doc A/80/10, annex I.

61 ibid ch IV, paras 30–77.

62 First Issues Paper (n 21). The States were Indonesia (UN Doc A/C.6/72/SR.24, para 126), Marshall Islands, on behalf of the Pacific small island developing States (i.e. Fiji, Kiribati, Marshall Islands, Micronesia (Federated States of), Nauru, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, Tuvalu and Vanuatu) (UN Doc A/C.6/72/SR.22, paras 51–53), Micronesia (Federated States of) (UN Doc A/C.6/72/SR.20, paras 63–66), Peru (UN Doc A/C.6/72/SR.22, para 116), Romania (on file with the Codification Division) and Tonga (UN Doc A/C.6/72/SR.20, para 32).

63 First Issues Paper (n 21) para 9.

64 ibid para 16.

65 ibid paras 19–20.

66 VCLT (n 49).

67 Pacific Islands Forum (PIF), ‘Declaration on Preserving Maritime Zones in the Face of Climate Change-Related Sea-Level Rise’ (6 August 2021) <https://www.forumsec.org/2021/08/11/declaration-on-preserving-maritime-zones-in-the-face-of-climate-change-related-sea-level-rise/>. See also Alliance of Small Island States (AOSIS), ‘Alliance of Small Island States Leaders’ Declaration’ (2021) para 41 <https://www.aosis.org/launch-of-the-alliance-of-small-island-states-leaders-declaration/>.

68 PIF (n 67) 2.

69 ibid.

70 PIF, ‘Declaration on the Continuity of Statehood and the Protection of Persons in the Face of Climate Change-Related Sea-Level Rise’ (2023) <https://forumsec.org/publications/2023-declaration-continuity-statehood-and-protection-persons-face-climate-change>.

71 AOSIS (n 67) para 41.

72 Climate Vulnerable Forum, ‘Dhaka-Glasgow Declaration’ (2 November 2021) para 8 <https://cvfv20.org/wp-content/uploads/2024/08/Dhaka-Glasgow-Declaration-of-the-CVF_Final-1.pdf>.

73 See OACPS, ‘Declaration of the Seventh Meeting of OACPS Ministers in Charge of Fisheries and Aquaculture’ (8 April 2022) 8 <https://www.oacps.org/wp-content/uploads/2022/05/Declaration_-7thMMFA_EN.pdf>.

74 The Heads of Government of the Commonwealth met in Apia, Samoa, in October 2024. As a result of this meeting, the following documents were issued: the Leader’s Statement, the Samoa Communiqué of the Commonwealth Heads of Government Meeting and the Apia Commonwealth Ocean Declaration ‘One Resilient Common Future’: see The Commonwealth, CHOGM 2024: Samoa Communiqué, Leaders’ Statement and Declarations on ‘One Resilient Common Future’ <https://thecommonwealth.org/news/chogm2024/Samoa-communique-leaders-statement-and-declarations>.

75 AOSIS, ‘Leaders’ Declaration on Sea Level Rise and Statehood’ (25 September 2024) <https://www.aosis.org/aosis-leaders-declaration-on-sea-level-rise-and-statehood/>.

76 ILC, ‘Additional Paper to the Second Issues Paper (2022) by Patrícia Galvão Teles and Juan José Ruda Santolaria, Co-Chairs of the Study Group on Sea-Level Rise in relation to International Law’ (19 February 2024) UN Doc A/CN.4/774, paras 177–178.

77 ibid.

78 UNGA, ‘High-Level Plenary Meeting on Addressing the Existential Threats posed by Sea Level Rise: Draft Decisions submitted by the President of the General Assembly’ (8 January 2024) UN Doc A/78/L.35; UNGA, Decision 78/544 (16 January 2024) UN Doc GA/12581; UNGA Res 78/319 (1 August 2024) UN Doc A/RES/78/319. See also UNGA, ‘High-Level Meeting on Sea Level Rise’ (New York, 25 September 2024) <https://www.un.org/pga/78/high-level-meeting-on-sea-level-rise/>.

79 UNGA Res 78/319 (n 78) para 1.

80 Commission of Small Island States (COSIS), ‘Request for an Advisory Opinion on Climate Change and International Law’ (12 December 2022) <https://itlos.org/fileadmin/itlos/documents/cases/31/Request_for_Advisory_Opinion_COSIS_12.12.22.pdf>.

81 For further information on the advisory proceedings before the International Court of Justice (ICJ) with regard to the obligations of States in respect of climate change, see ICJ, Obligations of States in respect of Climate Change <https://www.icj-cij.org/case/187>.

82 Request for an Advisory Opinion Submitted by the Commission of Small Island States on Climate Change and International Law (Advisory Opinion) (ITLOS, Case No 31, 21 May 2024).

83 Costa Rica (Written Statement, paras 125–127); Burkina Faso (Written Statement, para 345); Colombia (Written Comments, para 1.21); El Salvador (Written Comments, para 9); Cook Islands (Written Comments, para 111); Mauritius (Written Comments, para 147); Timor-Leste (Written Comments, para 82); Cote d’Ivoire (Verbatim Record 2024/39, 40); Papua New Guinea (Verbatim Record 2024/43, 26); Romania (Verbatim Record 2024/48, 41); Tonga (Verbatim Record 2024/51, 41); AOSIS (Verbatim Record 2024/52, 48); Bahamas (Written Statement, para 221; Written Comments, 4) United States (Written Statement, para 1.13); COSIS (Written Statement, para 71); PIF (Written Statement, paras 14, 16; Verbatim Record 2024/53, 41).

84 New Zealand (Written Statement, para 13; Verbatim Record 2024/46, 31); Dominican Republic (Written Statement, para 4.40); Australia (Written Statement, para 1.17); El Salvador (Verbatim Record 2024/39, 71); Fiji (Verbatim Record 2024/40, 67); Jamaica (Verbatim Record 2024/43, 22); Latvia (Verbatim Record 2024/44, 14); PIF Fisheries Agency (Written Statement, paras 38–40); PIF (Oral Statement, 8–9).

85 COSIS (Written Statement, para 72); Tuvalu (Verbatim Record 2024/51, 60); Bahamas (Written Comments, para 6(g) referring to ‘broad support for – and no opposition to – the fixing of baselines and maritime entitlements, as well the continuation of statehood, irrespective of physical changes to the affected States’ coastlines resulting from sea level rise’); Kiribati (Written Comments, para 41).

86 Obligations of States in Respect of Climate Change (Advisory Opinion) (General List No 187, 23 July 2025) para 361.

87 ibid para 362.

88 See also Obligations of States in Respect of Climate Change (n 86) (Separate Opinion of Judge Aurescu).