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. This seemingly modest change is now actively transforming coastlines—and with them, the legal and practical boundaries that nations have long relied upon. Marine boundaries, including 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 not hypothetical. Several small island states already face the prospect of their baselines being completely submerged, while coastal nations from Bangladesh to the United States are renegotiating navigation lanes and environmental protections. This article examines how rising seas are driving changes in territorial claims, maritime navigation, and environmental legal frameworks, and explores what international cooperation and adaptive law may look like in a watery, changing 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 (12 nautical miles), contiguous zone (24 nm), EEZ (200 nm), 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. For countries with gently sloping coasts, a small vertical rise can push the baseline inland by hundreds of meters or even kilometers.

Baseline Ambiguity and Disputed Zones

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

Small island developing states (SIDS) such as Tuvalu, Kiribati, and the Maldives face an existential threat: if their land territory is permanently inundated, they lose the baselines that generate their EEZs—areas often many times larger than their landmass. These zones are critical for fisheries, mineral rights, and economic survival. Several SIDS have advocated for “baseline freezing”, meaning that once a baseline is established, it cannot be changed due to sea‑level rise. This proposal has been debated within UNCLOS bodies but has not yet been formally adopted.

Case Studies: Redrawing Borders in the Pacific and Atlantic

In the Pacific, Fiji has already passed legislation to fix its baselines at 2016 measurements regardless of future sea‑level rise. Similarly, the Pacific Islands Forum has endorsed a regional declaration on preserving maritime zones against the effects of climate change. On the Atlantic side, the United States has updated its baseline maps (e.g., for the Gulf of Mexico) as barrier islands erode and shift. The U.S. National Oceanic and Atmospheric Administration (NOAA) has released new nautical charts that reflect baseline adjustments, but these updates are piecemeal and reactive.

Disputes are also likely to emerge around uninhabited claimed features—such as islands in the East China Sea—where rising waters may degrade their legal status from “fully entitled island” to “rock” (which cannot generate an EEZ under Article 121 of UNCLOS). This could dramatically alter the extent of national claims in that strategically important waterway.

Effects on Maritime Navigation and Infrastructure

Rising seas 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 position of hazards such as submerged shoals demand constant updates to marine charts and safety systems.

Shifting Shipping Lanes and Port Access

Major ports around the world—including Rotterdam, Shanghai, and New York—are already investing in raised quays and flood barriers. But beyond the port perimeter, navigational channels are affected. Siltation patterns change as sea levels rise, often requiring more frequent dredging. For example, the Panama Canal and Suez Canal, while not directly experiencing baseline shifts, face increased salinization and changes in water flow that affect ship handling. According to the International Maritime Organization (IMO), many charts used worldwide were last fully updated decades ago, before the current acceleration of sea‑level rise. The IMO’s Hydrographic Commission now recommends that coastal states prioritize re‑surveying approach channels and harbors every five years in vulnerable areas.

Arctic navigation offers a particularly stark example. Melting sea ice (driven partly by warmer oceans associated with sea‑level rise) is opening new transit routes such as the Northern Sea Route. However, these same waters experience rapid coastline change due to erosion—some Arctic coastlines are retreating at rates exceeding 20 meters per year. Charts for these areas become obsolete within a single shipping season, increasing the risk of groundings and oil spills.

Risk to Aids to Navigation

Lighthouses, buoys, and GPS reference stations often sit on low‑lying land or near the waterline. As seas rise, the physical infrastructure of marine navigation is threatened. The U.S. Coast Guard has relocated several navigational aids on the Outer Banks of North Carolina due to erosion. In the Maldives, many fixed aids have been replaced with floating buoys that adjust to water level changes—but these are less reliable in storm conditions. Without proactive adaptation, accidents and navigational errors will increase, particularly in narrow straits and approaches to crowded ports.

Rising sea levels interact with other climate impacts—warming water, ocean acidification, and increased storm intensity—to stress marine ecosystems. These environmental changes in turn strain the legal frameworks meant to protect them.

Threats to Coastal and Marine Ecosystems

Mangrove forests, salt marshes, and coral reefs provide natural protection against erosion and serve as critical habitats. Many can keep pace with slow sea‑level rise by building upward or migrating inland, but the current rapid pace of 3–4 mm/year globally—and faster in some regions—can outstrip their ability to adapt. When these ecosystems are submerged or squeezed against coastal development, biodiversity declines. For example, the Sundarbans mangrove forest in Bangladesh and India has lost over 10% of its area in the past 40 years due to a combination of sea‑level rise and upstream water diversion.

These losses reduce fish nursery grounds, weaken carbon sequestration, and decrease the natural buffer against storm surges—making coastal communities more vulnerable. From a legal perspective, environmental protection zones (such as marine protected areas) may need to be redrawn as ecosystems shift. A marine protected area established with fixed coordinates risks protecting only open water if its boundaries were originally set around low‑water lines that have moved.

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. Sea‑level rise does not directly alter the geomorphology of the continental shelf, but it can complicate the baseline measurements used to determine the starting point for shelf claims. Furthermore, 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 limit of the shelf may shift, potentially causing overlaps with neighboring claims.

The legal debate over “baseline ambulatory” vs. “baseline fixed” is not purely academic. A 2022 study in Nature Climate Change projected that by 2100, up to 6.8 million square kilometers of ocean area currently within EEZs could become high seas if baselines shift (under a high‑emissions scenario). This would mean a massive transfer of resources—fisheries, oil, gas, manganese nodules—from national jurisdiction to international waters, with profound implications for governance, revenue, and exploitation.

Adapting International Law to a Changing Climate

International bodies, including the International Law Association and UNCLOS state parties, are actively discussing how to adapt the law of the sea to rising waters. Proposals include:

  • Baseline freezing – legally fixing baselines at a specific date regardless of physical changes.
  • Zonal boundary freezing – freezing the outer limits of maritime zones (e.g., EEZ lines) even if baselines recede.
  • State continuity for submerged states – recognizing that a country does not cease to be a state even if its land territory is fully inundated, allowing it to retain its maritime zones.

Some progress has been made: in 2023, the Pacific Islands Forum secured a declaration that maritime zones should remain stable despite sea‑level rise, and this principle has been endorsed by the UN General Assembly. However, binding treaty amendments have not yet been adopted. Without such changes, legal uncertainty will persist, increasing the risk of international disputes and undermining investment in marine industries.

Environmental Management in Transitional Zones

Beyond boundary disputes, the practical management of marine environments is complicated by shifting baselines. Marine spatial planning—the process of allocating space for fishing, shipping, conservation, and energy—relies on fixed zones. If the low‑water line moves, the zoning designations (e.g., “within 3 nautical miles of the coast”) become ambiguous. Countries like Australia and the Netherlands have begun using “dynamic baselines” in their spatial plans, where the boundary is set relative to a moving reference (such as the high‑water mark or a contour of water depth). This approach is technologically feasible with modern satellite data but requires legal acceptance and integration into national legislation.

Conclusion: A Call for Proactive Governance

Rising seas are not a future problem; they are actively reshaping marine boundaries today. The impacts on territorial claims, maritime navigation, and environmental legal frameworks are profound and interconnected. Without proactive adaptation—through baseline freezing, updated charts, and reformed international law—the world risks a cascade of conflicts, economic losses, and ecological damage. Nations must work together through the UNCLOS framework and regional bodies to create stable, predictable maritime boundaries that can accommodate physical change while preserving the rights and responsibilities of coastal states and the global community.

The ocean has always been a dynamic space, but human governance has largely been static. As waters rise, so too must our laws and cooperation—or the maps we have trusted for centuries will become relics of a lower‑sea past.

For further reading on the legal dimensions, consult the United Nations Division for Ocean Affairs and the Law of the Sea. For the latest scientific projections, see the IPCC Sixth Assessment Report and NOAA’s Sea Level Rise Viewer.