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Shape-shifting islands defy sea-level rise

02 June 2010 by Wendy Zukerman - NEW SCIENTIST Magazine issue 2763. Subscribe and save For similar stories, visit the Climate Change Topic Guide

Not drowning but growing? (Image: George Steinmetz/Corbis)

Not drowning but growing? (Image: George Steinmetz/Corbis)

AGAINST all the odds, a number of shape-shifting islands in the middle of the Pacific Ocean are standing up to the effects of climate change.

For years, people have warned that the smallest nations on the planet - island states that barely rise out of the ocean - face being wiped off the map by rising sea levels. Now the first analysis of the data broadly suggests the opposite: most have remained stable over the last 60 years, while some have even grown.

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. During that time, local sea levels have risen by 120 millimetres, or 2 millimetres per year on average.

Despite this, Kench and Webb found that just four islands have diminished in size since the 1950s. The area of the remaining 23 has either stayed the same or grown (Global and Planetary Change, DOI: 10.1016/j.gloplacha.2007.11.001).

Webb says the trend is explained by the islands' composition. Unlike the sandbars of the eastern US coast, 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. "Atolls are composed of once-living material," says Webb, "so you have a continual growth." Causeways and other structures linking islands can boost growth by trapping sediment that would otherwise get lost to the ocean.

All this means the islands respond to changing weather and climate. For instance, when hurricane Bebe hit Tuvalu in 1972 it deposited 140 hectares of sedimentary debris onto the eastern reef, increasing the area of the main island by 10 per cent.

Kench says that while the 27 islands in his study are just a small portion of the thousands of low-lying Pacific islands, it shows that they are naturally resilient to rising sea levels. "It has been thought that as the sea level goes up, islands will sit there and drown," he says. "But they won't. The sea level will go up and the island will start responding."

It's been thought that as the sea level goes up, islands will sit there and drown. But they won't

John Hunter, an oceanographer at the University of Tasmania in Australia, says the study is solid, and good news for those preparing evacuations. The shifting shape of the islands presents a challenge, however. Even on islands where the total land mass is stable or grows, one area may be eroded while another is being added to. It's not possible to simply move people living in highly urbanised areas to new land, says Naomi Biribo of the University of Wollongong in New South Wales, Australia.

Webb and Kench warn that while the islands are coping for now, any acceleration in the rate of sea-level rise could overtake the sediment build up. Calculating how fast sea levels will rise over the coming decades is uncertain science, and no one knows how fast the islands can grow.

Barry Brook, a climate scientist at the University of Adelaide in Australia and a supporter of the 350 campaign - which calls for the most stringent global emissions targets in the hope of saving low-lying states from sea-level rise - points out that sea-level rise is already accelerating. But, while he was initially surprised by the findings, he agrees with Webb and Kench's analysis. "It does suggest that islands have been able to adapt to sea-level rises," he says. And Biribo, who lives on the Pacific island of Kiribati, says: "It gives me that sense that we can still live on this island."

Good news, but the warnings stand

At its highest point, Tuvalu stands just 4.5 metres out of the Pacific. It is widely predicted to be one of the first islands to drown in the rising seas caused by global warming. 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.

Similar trends were observed in the neighbouring Republic of Kiribati. The three major urbanised islands in the republic - Betio, Bairiki and Nanikai - increased by 30 per cent (36 hectares), 16.3 per cent (5.8 hectares) and 12.5 per cent (0.8 hectares), respectively.

Yet warnings about rising sea levels must still be taken seriously. Earlier this year, people living on the low-lying Carteret Islands, part of Papua New Guinea, had to relocate. Kench says anecdotal reports that the islands have been submerged are "incorrect", saying that instead erosion has changed the shape of the islands, forcing people to move.

Issue 2763 of New Scientist magazine
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