Aufa Khansazahra Bestari, Sunan Kalijaga Islamic State University, Yogyakarta

Sea level rise is primarily driven by two climate-related processes: the termal expansion of seawater and the accelerated melting of land-based ice sheets. These processes, intensified by global temperature increase, have caused unprecedented and irreversible changes to coastal geography. Sea level rise due to climate change pose an existential threat to low-lying coastal areas and particularly Small Island Developing States (SIDS) such as Tuvalu and Kiribati. Beyond the environmental destruction it causes coastal erosion, loss of biodiversity, salinization of soil, and freshwater scarcity, it also threatens their very statehood and sovereignty. As the ocean gradually encroaches upon these territories, the current maritime legal framework under the United Nation Convention on the Law of the Sea (UNCLOS) faces unprecedented challenges. 

In recent years the International Law Commission (ILC) and the International Association (ILA) have initiated discussions to adapt to international law the rising sea level, including the proposal to “freeze” maritime baselines in order to preserve states’ maritime rights. Nevertheless, the environmental implications of sea level rise extend beyond territorial loss. They encompass ecological degradation, displacement of populations, and the erosion of marine ecosystems all of which underscore the urgent need to rethink maritime law in a way that integrates environmental protection, climate justice, and the continued survival of sinking states.

UNCLOS establishes that maritime zones and state entitlements are determined by coastal baselines. However, the Convention was drafted at a time when permanent geographical changes resulting from climate phenomena were unimaginable. This has created a significant legal gap, leaving sinking states uncertain about the future of their maritime boundaries, Exclusive Economic Zones (EEZ), and even their international legal personality. The emerging debate surrounding climate-induced sea level rise demonstrates that a strict, geography-dependent reading of UNCLOS is no longer adequate. Instead, a more adaptive and environmentally informed interpretation is required to ensure that vulnerable states retain their rights and their identities in face of the changing ocean.

However, before assessing how maritime law may adapt to these environmental realities, it is essential to address a more fundamental issue: the very survival of these states as subjects of international law. The loss of territory caused by sea level rise does not merely alter maritime boundaries it challenges the core criteria of statehood and sovereignty themselves. Understanding this foundational problem is necessary to frame the broader legal and environmental implications faced by sinking states. 

According to Article 1 of the 1933 Montevideo Convention, a sovereign state must fulfill four requirements: a defined territory, a permanent population, an effective government, and the capacity to engage in international relations. Among these elements, Sinking Island States are most at risk of losing the foundational requirement of statehood: the possession of sovereign territory. 

The sea level rise also directly affects the regulation of a state’s territorial sovereignty and the determination of its coastal baseline. This becomes problematic because sea level rise threatens to shift shorelines, which serve as the reference point for determining the territorial sea and other maritime zones. Currently, the establishment of a coastal baseline is based on measuring the outermost boundary of a state’s land territory.  When changes or reductions in land occur as a result of submergence or coastal erosion, the breadth of the territorial sea may decrease. This poses a significant threat to states experiencing territorial loss, as they risk losing jurisdiction or sovereignty over the maritime areas that previously fell under their authority but have effectively disappeared due to sea level rise. 

According to UNCLOS, the outer limits of every maritime zone are measured from the coastal baseline. Article 5 of UNCLOS stipulates that the normal baseline is determined by the low-water line. When coastal areas become permanently submerged or eroded as a result of sea level rise amounting to a loss of territory the low-water line shifts landward. Such landward movement of the baseline automatically and proportionally reduces the maritime zones measured from it, including the 12 nautical-mile territorial sea, the contiguous zone, and the 200 nautical-mile Exclusive Economic Zone (EEZ). This reduction ultimately resulted in the loss of significant sovereign rights over the exploration and exploitation of natural resources, such as fisheries and hydrocarbons, within the maritime area that has diminished. Thus, the physical loss of territory directly leads to the legal loss of maritime entitlements.

Sea level rise also has significant impacts on marine ecosystems, including habitat degradation, declining biodiversity, and shifts in ecosystem functioning. One of the phenomena associated with this process is “coastal squeeze”, a condition in which intertidal zones become trapped between rising sea levels and fixed landward boundaries such as seawalls or natural topography. As a result, intertidal areas experience substantial contraction. For example, in estuaries such as Tauranga Harbour in Aotearoa New Zealand, it is predicted that approximately 75% of intertidal areas could be lost under a maximum sea level rise scenario of 1.4 meters. Sea level rise also alters the distribution of vegetated coastal habitats such as mangrove forests, salt marshes, seagrass meadows, and soft-sediment ecosystems. To survive, these habitats must either migrate landward or elevate their surface through sediment accumulation; however, this natural adaptation is often obstructed by coastal squeeze. 

These ecological changes can disrupt the availability of marine resources, damage ecosystem dynamics, and lead to the loss of biodiversity, directly threatening the habitats and reproductive patterns of marine organisms. Ultimately, such disruptions affect the fisheries sector, one of the world’s most important sources of food and employment. In this regard, sea level rise triggers a cascade of effects that have far-reaching consequences for marine ecosystems.

The environmental and jurisdictional consequences of sea level rise ultimately expose a deeper structural gap within UNCLOS, particularly regarding the legal status of maritime features and the stability of maritime entitlements. Although UNCLOS provides detailed rules on how baselines and maritime zones are drawn, it offers no guidance on how permanent coastal changes whether through physical submergence of ecological degradation should affect these legal determinations. As sea level rise erodes and transforms islands, atolls, and coastal features, some of them may no longer meet the criteria for an “island” under Article 121 of UNCLOS and risk being reclassified as rocks or even low-tide elevations. Such a downgrade would strip these features of their capacity to generate an EEZ or a continental shelf. This situation places sinking states in an exceptionally vulnerable position, as they face a dual loss: the disappearance of physical territory and the simultaneous reduction of their maritime jurisdiction.

This issue becomes increasingly urgent considering that UNCLOS provides for the permanent determination of baselines under only two conditions: (a) the provision under Article 7(2) of UNCLOS concerning the existence of deltas; and (b) the provision under Article 76(9) of UNCLOS, which requires coastal States to submit to the Secretary-General of the United Nations charts and relevant information, including geodetic data, that permanently depict the outer limits of the continental shelf. Beyond these two conditions, several scholars, including Freestone, argue that the baseline as well as the outer limits of the territorial sea, contiguous zone, and EEZ is ambulatory. The ambulatory nature baselines means that they are dynamic and shift in accordance with changes occurring along the coastline of coastal States. Consequently, this creates negative implications in the context of rising sea levels; if the baseline shifts, the outer limits of maritime zones may also shift, while UNCLOS does not explicitly regulate whether those boundaries should follow such shifts. 

However, UNCLOS, which relies heavily on geographical considerations, has also faced critical challenges from scholars. Researchers such as Kate Purcell explicitly reject the ambulatory thesis, arguing that it threatens legal stability. Purcell contends that UNCLOS does not impose a legal obligation on coastal States to continuously update their baselines in accordance with physical changes to their coastlines; instead, it grants such action as a right available to State. This view suggests that the Law of the Sea allows States to adapt to climate change without being forced to provide a legal foundation for States to assert that, in this pressing issue, legal certainty and clarity must take precedence. This principle paves the way for solutions that prioritize stability over the geographical dynamism imposed by climate change. 

Therefore, a process of international law-making is necessary to develop new rules capable of addressing these challenges. The primary solution is to legally “preserve” (sustain or freeze) the status of maritime features and the maritime zones they generate, regardless of the physical changes caused by sea level rise. This principle of maintaining coastal states’ entitlements must ultimately be codified in treaty law. A procedural solution proposed in the literature is a hybrid approach, since a formal amendment to UNCLOS is widely considered impractical. A more feasible pathway involves adopting supplementary agreements or internationally supported resolutions, which could later evolve or be codified through customary international law. Furthermore, states most threatened by sea level rise such as small island states should be recognized as ‘specially affected states”. Such recognition would enhance the legal weight of their practice in the formation of customary international law.

The environmental implications of sea level rise expose not only the physical vulnerability of sinking states but also a profound legal gap within the UNCLOS framework. The threat of losing territorial integrity, coupled with the instability posed by the ambulatory thesis, necessitates a shift from a strict geographical reading to one that prioritizes climate justice and legal stability. The proposed solution of legally “freezing” maritime baselines and developing supplementary agreements offers a pragmatic path forward. Ultimately, the continued survival and sovereign rights of Small Island Developing States depend on the international community’s willingness to adapt and amend the law of the sea, ensuring that the principle of statehood is protected against the irreversible changes brought by a warming world.

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