For years, we have been warned that low-lying coral island states will be drowned by rising sea levels. Now the first analysis of the data broadly suggests the opposite – most have remained stable, while some have even grown, despite rising sea levels, over the last 60 years.

Nanumea Atoll, Tuvalu (NASA image)
Nanumea Atoll, Tuvalu (NASA image)

Paul Kench at the University of Auckland in New Zealand and Arthur Webb at the South Pacific Applied Geoscience Commission in Fiji used historical aerial photos and high-resolution satellite images to study changes in the land surface of 27 Pacific islands over the last 60 years. Local sea levels have risen by 120 millimetres during that time but just four of the islands have diminished in size.

Tuvalu, which stands just 4.5 metres out of the Pacific, has been widely predicted to be one of the first islands to drown in the rising seas. Yet Arthur Webb and Paul Kench found that seven islands in one of its nine atolls have spread by more than 3 per cent on average since the 1950s. One island, Funamanu, gained 0.44 hectares, or nearly 30 per cent of its previous area. In the neighbouring Republic of Kiribati, the three major urbanised islands – Betio, Bairiki and Nanikai – have increased by 30 per cent.

The reason is that low-lying Pacific islands are made of coral debris. This is eroded from the reefs that typically circle the islands and pushed up onto the islands by winds, waves and currents. Because the corals are alive, they provide a continuous supply of material. Structures linking islands can boost growth by trapping sediment that would otherwise get lost to the ocean. For example, when hurricane Bebe hit Tuvalu in 1972 it deposited 140 hectares of debris, increasing the area of the main island by 10 per cent.

While the anecdotal reports that coral islands are being submerged are incorrect, it is true that people are being forced to relocate as the islands change shape.

The situation for sand islands is different. Many of these are sand bars in river deltas where, in the past, silt washed down the river has continually rebuit the island as it was eroded by the sea. Not only are these islands being slowly inundated by rising seas but diminished water flow in the rivers is reducing the amount of silt being deposited. In many cases, the main reason that delta islands are “sinking” is the construction of dams which block the normal water and silt flow.

Also see “Rising Seas and Tuvalu