As explained here, the United Nations Convention on the Law of the Sea – UNCLOS, 1982, brings maritime delimitations in order to legally divide the ocean into zones to be explored by States (Inland Waters, Territorial Sea, Contiguous Zone, Economic Zone Exclusive, Continental Shelf, High Seas and Area). These zones have different legal characteristics, so that each one has its own size and objective, based on the provisions of UNCLOS. For this separation to be possible, the aforementioned convention provides baselines, which can be normal or straight. According to article 5 of UNCLOS, normal baselines are:

Except 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.

Straight baselines, in turn, are conceptualized by article 7 of UNCLOS as being:

1. In localities where the coastline is deeply indented and cut into, or if there is a fringe of islands along the coast in its immediate vicinity, the method of straight baselines joining appropriate points may be employed in drawing the baseline from which the breadth of the territorial sea is measured.

2. Where because of the presence of a delta and other natural conditions the coastline is highly unstable, the appropriate points may be selected along the furthest seaward extent of the low-water line and, notwithstanding subsequent regression of the low-water line, the straight baselines shall remain effective until changed by the coastal State in accordance with this Convention.

3. The drawing of straight baselines must not depart to any appreciable extent from the general direction of the coast, and the sea areas lying within the lines must be sufficiently closely linked to the land domain to be subject to the regime of internal waters.

4. Straight baselines shall not be drawn to and from low-tide elevations, unless lighthouses or similar installations which are permanently above sea level have been built on them or except in instances where the drawing of baselines to and from such elevations has received general international recognition.

5. Where the method of straight baselines is applicable under paragraph 1, account may be taken, in determining particular baselines, of economic interests peculiar to the region concerned, the reality and the importance of which are clearly evidenced by long usage.

6. The system of straight baselines may not be applied by a State in such a manner as to cut off the territorial sea of another State from the high seas or an exclusive economic zone.

The aforementioned articles demonstrate that, if normal baselines are applied, the separation between the Inland Waters and the Territorial Sea of ​​the coastal State is made from the lowest tide line along the coast. If, on the other hand, straight baselines are adopted (in cases where the State's coast is too jagged or irregular due to the existence of islands), the separation between maritime zones is made using a straight line, joining specific points at the lowest tide on the coast. These straight lines cannot, however, deviate considerably from the general direction of the State coast. Furthermore, according to article 14 of UNCLOS, both forms of lines can be used by the same State.

Unfortunately, global warming and rising sea levels have caused difficulties in maritime delimitations, and even made it possible for States to disappear. In November 2023, scientists specializing in climate change and global warming announced that the ice at the poles is melting faster than expected and, if this situation is not overcome, the rise in sea levels will be “disastrous” for society. Legally, with the rise in ocean levels, sea water has increasingly retreated towards the continental area of ​​coastal States, making low tides above normal. This causes the jurisdictional water spaces to also recede, reducing their extension and increasing the High Seas area.

In September 2023, the Prime Minister of Japan requested the United Nations (UN) for the UNCLOS interpretation of maritime delimitation to be updated, maintaining the count by low tide lines, without considering the rise in sea level in the delimitation of the beginning of the Territorial Sea. In this way, the delimitation of maritime zones would remain as it is today, regardless of sea level rise. If the interpretation is not updated to consider this situation, the Japanese government fears the reduction of its EEZ, a space in which it holds exploration, conservation and resource management rights up to 200 nautical miles from the baseline. This could be positive from the point of view of the greater expansion of the High Seas, which would allow freedom of fishing in areas that were previously exclusive to coastal States, but this scenario will certainly also bring new legal problems to be discussed.

Furthermore, islands in the Pacific Ocean, such as Tuvalu, are at risk of disappearing due to sea level rise, or of no longer even being considered islands from the legal point of view of UNCLOS. According to the aforementioned convention, in its article 121, islands are defined as being:

1. An island is a naturally formed area of land, surrounded by water, which is above water at high tide.

2. Except as provided for in paragraph 3, the territorial sea, the contiguous zone, the exclusive economic zone and the continental shelf of an island are determined in accordance with the provisions of this Convention applicable to other land territory.

3. Rocks which cannot sustain human habitation or economic life of their own shall have no exclusive economic zone or continental shelf.

In the case of Tuvalu, rising sea levels have caused concern about the island's ability to support human life, which would disqualify the place from being an island, as per item 3 above. If this occurs, Tuvalu will no longer be considered a coastal State with the right to jurisdictional waters, and will be considered a rock. This situation creates the so-called “environmental refugees”, since the residents of these islands who are at risk of disappearing will have to leave their homes and seek residence elsewhere. Because of this, according to Rodrigo More, the rise in sea levels generated by global warming has consequences in multiple dimensions, and not just in the legal scope of the delimitations of maritime zones. According to the author (MORE, 2020, p. 39):

Sea level rise has far-reaching, multidimensional implications. As the problem grows and goes unresolved, we will see the rapid emergence of increasingly complex interrelated issues and dimensions. The geographical dimension involves, for example, the flooding of agricultural and urban coastal areas, the definitive submersion of exposed shallows, the total or partial disappearance of small islands. The humanitarian dimension, in the displacement of populations, the accentuation of poverty and the impacts on the health not only of the displaced people, but also of those in whose territories the displaced people will seek refuge. The legal dimension, in the change of base points and baselines that are used to measure the territorial sea and, consequently, all other maritime spaces with impacts on the exploration and economic use of resources, including fishing and economic use of Hydrocarbons. The economic dimension, in more severe conditions of tides and winds in ports with negative impacts on the flow of maritime trade, as well as impacts on tourism, a source of income and livelihood for the vast majority of small island developing States.

In view of the above, the seriousness of the current scenario is clear. Climate change substantially affects the oceans, and one of the consequences of global warming is sea level rise. As waters rise, baselines, usually counted by the lowest tide line under UNCLOS, become a legal problem for coastal States. The more the tide rises, the more the line “recedes” and the smaller the space of the States' jurisdictional waters becomes. Furthermore, islands in the Pacific (such as Tuvalu) are at risk of disappearing, or losing their legal island status (and losing the right to have jurisdictional waters, as a consequence), a situation that has not only legal consequences, but also political, social and economic as well.