How Rising Sea Levels Are Redrawing the World’s Maritime Map

Global mean sea level has risen by approximately 8–9 inches (21–24 cm) since 1880, with the rate accelerating over the past two decades. While this change may seem modest, its cumulative effects are actively transforming coastlines worldwide—and with them, the legal and practical boundaries that nations have long relied upon. Marine boundaries such as territorial seas, exclusive economic zones (EEZs), and continental shelf claims are defined by reference to baselines drawn from the low‑water line along the coast. As that line moves inland or is permanently submerged, entire national maritime zones can shrink, shift, or become ambiguous. Understanding these transformations is essential for international diplomacy, maritime safety, environmental management, and sustainable development.

The consequences are no longer hypothetical. Several small island states already face the prospect of their baselines being completely submerged, threatening their sovereign rights over vast maritime spaces. Coastal nations—from Bangladesh’s vulnerable delta to the United States’ extensive coastlines—are renegotiating navigation lanes and environmental protections in response to changing shorelines. This article examines how rising seas are driving changes in territorial claims, maritime navigation, and environmental legal frameworks, and explores the emerging landscape of international cooperation and adaptive law in a rapidly changing marine world.

Impact on Territorial Claims and Maritime Zones

Under the United Nations Convention on the Law of the Sea (UNCLOS), a coastal state’s maritime zones—territorial sea (up to 12 nautical miles), contiguous zone (up to 24 nautical miles), EEZ (up to 200 nautical miles), and continental shelf rights—are measured from baselines that normally follow the low‑water line along the coast. When sea level rises, that low‑water line retreats landward, effectively redrawing the starting point for maritime claims. For countries with gently sloping coasts, a small vertical rise can translate into a significant horizontal shift—sometimes pushing the baseline inland by hundreds of meters or even kilometers, with profound implications for national jurisdiction.

Baseline Ambiguity and Disputed Zones

Because baselines are dynamic, rising seas create immediate legal ambiguity. If a baseline moves landward, the outer limits of the EEZ and territorial sea also shift accordingly. This can lead to overlapping claims with neighboring states, especially in regions where maritime boundaries are already contested, such as the South China Sea, the Eastern Mediterranean, or the Persian Gulf. For instance, if an island that previously generated an EEZ becomes submerged due to inundation, the surrounding maritime space may revert to the high seas or become subject to new claims by other states, potentially escalating geopolitical tensions.

Small island developing states (SIDS) such as Tuvalu, Kiribati, and the Maldives face an existential threat from rising seas: if their land territory is permanently inundated, they lose the baselines that generate their EEZs—zones often many times larger than their landmass. These maritime zones are critical for sustaining fisheries, mineral rights, and national economic survival. Several SIDS have advocated for “baseline freezing,” a legal principle that would fix baselines at the date they were initially established, preventing changes due to sea‑level rise. While this proposal has been debated within UNCLOS bodies and some regional organizations, it has not yet been formally adopted, leaving these vulnerable nations in legal limbo.

Case Studies: Redrawing Borders in the Pacific and Atlantic

In the Pacific Ocean, Fiji has proactively passed legislation to fix its baselines at 2016 measurements regardless of future sea‑level rise, setting a precedent for other nations in the region. The Pacific Islands Forum has similarly endorsed a regional declaration aimed at preserving maritime zones against the effects of climate change, emphasizing state continuity even if physical territory is lost. These measures reflect a collective effort to safeguard national rights and resources amid rising oceans.

On the Atlantic coast, the United States has begun updating its baseline maps, particularly for areas like the Gulf of Mexico, where barrier islands are eroding and shifting rapidly. The U.S. National Oceanic and Atmospheric Administration (NOAA) has released updated nautical charts reflecting these baseline adjustments. However, these updates remain piecemeal and reactive rather than part of a comprehensive strategy, highlighting the challenge of keeping pace with dynamic shorelines.

Disputes are also emerging around uninhabited or sparsely inhabited features, such as islands in the East China Sea. Rising waters may degrade their legal status from “fully entitled island” to “rock,” which under Article 121 of UNCLOS cannot generate an EEZ. This subtle but critical distinction could dramatically alter the extent of national claims in strategically vital waterways, affecting access to fisheries, seabed minerals, and shipping routes.

Effects on Maritime Navigation and Infrastructure

Rising sea levels do not only move baselines; they also alter the physical environment for ships, ports, and navigational aids. Changes in water depth, coastline shape, and the location of hazards such as submerged shoals demand constant updates to marine charts and safety systems, which are crucial for the safety and efficiency of global maritime traffic.

Shifting Shipping Lanes and Port Access

Major ports around the world—including Rotterdam, Shanghai, Singapore, and New York—are investing heavily in raised quays, flood barriers, and resilient infrastructure to adapt to rising seas and increased storm surges. However, beyond port perimeters, navigational channels are also affected. Sea-level rise alters siltation patterns and underwater topography, often requiring more frequent and extensive dredging to maintain navigable depths.

For example, the Panama Canal and Suez Canal, while not directly experiencing baseline shifts, face increased salinization and altered water flows that complicate ship handling and canal operations. According to the International Maritime Organization (IMO), many nautical charts used worldwide were last fully updated decades ago, before the current acceleration of sea‑level rise. In response, the IMO’s Hydrographic Commission now recommends that coastal states prioritize re‑surveying approach channels, harbor basins, and critical shipping lanes at least every five years in vulnerable areas to ensure safe navigation.

The Arctic region presents a particularly stark example. Melting sea ice—driven partly by warmer ocean temperatures associated with climate change—is opening new transit routes such as the Northern Sea Route and the Northwest Passage, shortening shipping distances between Asia, Europe, and North America. However, these same waters experience rapid coastline change due to permafrost thaw and erosion, with some Arctic coastlines retreating at rates exceeding 20 meters per year. Navigational charts for these areas become obsolete within a single shipping season, increasing the risk of groundings, collisions, and environmental disasters such as oil spills.

Risk to Aids to Navigation

Lighthouses, buoys, GPS reference stations, and other aids to navigation often sit on low‑lying land or near the waterline. As sea levels rise and shorelines erode, the physical infrastructure of marine navigation faces increasing threats. In the United States, the Coast Guard has relocated several navigational aids on the Outer Banks of North Carolina due to rapid shoreline retreat. Similarly, in the Maldives and other low-lying island nations, many fixed aids have been replaced with floating buoys that adjust to changing water levels, although these are less reliable during storms or rough seas.

Without proactive adaptation and sustained investment, accidents and navigational errors are expected to increase, particularly in narrow straits, harbor approaches, and congested shipping lanes. This not only threatens maritime safety but also economic activities dependent on reliable shipping routes.

Rising sea levels interact with other climate impacts—such as warming waters, ocean acidification, and increased storm intensity—to stress coastal and marine ecosystems. These environmental changes, in turn, strain the legal frameworks established to protect these ecosystems, complicating conservation and sustainable use efforts.

Threats to Coastal and Marine Ecosystems

Mangrove forests, salt marshes, coral reefs, and seagrass beds provide natural protection against erosion and serve as critical habitats for fish, birds, and other wildlife. Many of these ecosystems can keep pace with slow sea‑level rise by building upward through sediment accumulation or migrating inland. However, the current rapid pace of sea-level rise—averaging 3–4 mm per year globally and higher in some regions—can outstrip their ability to adapt, particularly where human development blocks inland migration.

For example, the Sundarbans mangrove forest, straddling Bangladesh and India, has lost over 10% of its area in the past 40 years due to a combination of sea‑level rise, upstream freshwater diversion, and storm damage. These losses reduce fish nursery habitats, weaken carbon sequestration capacity, and diminish the natural buffer that protects coastal communities from storm surges and flooding.

From a legal perspective, environmental protection zones—such as marine protected areas (MPAs)—may need to be redrawn as ecosystems shift. A marine protected area established with fixed coordinates risks protecting only open water or degraded habitat if its boundaries were originally set around low‑water lines that have moved inland or been submerged. This challenges the effectiveness of conservation efforts and may require dynamic or adaptive legal frameworks.

Under UNCLOS, a coastal state can claim an extended continental shelf beyond 200 nautical miles if it can demonstrate that the seabed is a natural prolongation of its land territory. Although sea‑level rise does not directly alter the geomorphology of the continental shelf, it can complicate the baseline measurements used to determine the starting point for shelf claims. If a state’s baselines are fixed by law (as Fiji has done), its extended continental shelf claims remain stable; if baselines are not fixed, the outer limits of the shelf may shift, potentially causing overlaps and disputes with neighboring claims.

The legal debate over “baseline ambulatory” (moving with physical changes) versus “baseline fixed” (frozen at a point in time) is not purely academic. A 2022 study published in Nature Climate Change projected that by 2100, under a high‑emissions scenario, up to 6.8 million square kilometers of ocean area currently within EEZs could revert to high seas if baselines shift inland. This would mean a massive and unprecedented transfer of resources—fisheries, oil, gas, manganese nodules—from national jurisdiction to the international commons, with profound implications for governance, revenue streams, and resource exploitation.

Adapting International Law to a Changing Climate

International bodies, including the International Law Association, UNCLOS state parties, and regional organizations, are actively discussing how to adapt the law of the sea to rising waters. Several proposals have emerged to address the challenges posed by shifting baselines and submerged territories:

  • Baseline freezing: Legally fixing baselines at a specific date regardless of future physical changes to prevent loss of maritime claims due to sea-level rise.
  • Zonal boundary freezing: Freezing the outer limits of maritime zones such as EEZ lines, so that even if baselines recede, the maritime claims remain stable.
  • State continuity for submerged states: Recognizing that a country does not cease to be a state due to permanent inundation of its land territory, allowing it to retain its maritime zones and sovereign rights.

Some progress has been made. In 2023, the Pacific Islands Forum secured a declaration affirming that maritime zones should remain stable despite sea‑level rise, a principle subsequently endorsed by the UN General Assembly. However, binding treaty amendments to UNCLOS have yet to be adopted. Without such changes, legal uncertainty will persist, increasing the risk of international disputes, undermining investment in marine industries, and leaving vulnerable states exposed.

Environmental Management in Transitional Zones

Beyond boundary disputes, the practical management of marine environments is complicated by shifting baselines and evolving shorelines. Marine spatial planning—the process of allocating space for competing uses such as fishing, shipping, conservation, and renewable energy—relies on fixed zones. If the low‑water line moves inland or is submerged, zoning designations like “within 3 nautical miles of the coast” become ambiguous and challenging to enforce.

Countries like Australia and the Netherlands have begun experimenting with “dynamic baselines” in their spatial plans, where the boundary is set relative to a moving reference point such as the high‑water mark or a particular contour of water depth. This approach leverages modern satellite and remote sensing data for real-time updates but requires robust legal acceptance and integration into national legislation to be effective and enforceable.

Dynamic marine spatial planning offers a promising pathway to reconcile the physical realities of rising seas with governance needs, but it also raises complex questions about legal certainty, enforcement, and rights of users. International cooperation and harmonization will be essential to avoid conflicting claims and cross-border management challenges.

Conclusion: A Call for Proactive Governance

Rising seas are not a distant future problem; they are actively reshaping marine boundaries and coastal environments today. The impacts on territorial claims, maritime navigation, and environmental legal frameworks are profound, interconnected, and accelerating. Without proactive adaptation—through baseline freezing, more frequent and comprehensive chart updates, and reformed international law—the world risks a cascade of conflicts, economic losses, and ecological damage.

Nations must collaborate through the UNCLOS framework, regional bodies, and global institutions to create stable, predictable maritime boundaries that can accommodate physical change while preserving the rights and responsibilities of coastal states and the international community. Equally important is the integration of cutting-edge science, technology, and flexible legal instruments to manage shifting ecosystems and maritime spaces effectively.

The ocean has always been a dynamic space, but human governance has largely been static. As waters rise, so too must our laws, cooperation, and adaptive capacity. Only through foresight, innovation, and solidarity can we secure a just and sustainable future in a world where the sea—and the boundaries it defines—are in constant flux.