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Coral Atolls Holding On Despite Rising Sea Levels

June 3, 2010

Researchers revealed Thursday that some South Pacific coral atolls have held their own or even grown in size over the past 60 years, despite rising sea levels.

Some scientists worry that the tiny, low-lying islands throughout the South Pacific will eventually disappear under rising sea levels.

However, two researchers who measured 27 islands where local sea levels have risen 4.8 inches over the past 60 years, found just four had diminished in size.

The reason is that coral islands respond to changes in weather patterns and climate, with coral debris eroded from encircling reefs pushed up onto the islands’ coasts by wind and waves.

Professor Paul Kench of Auckland University’s environment school and coastal process expert Arthur Webb of the Fiji-based South Pacific Applied Geoscience Commission both used historical aerial photographs and high-resolution satellite images to study the changes on the islands.

According to the research, published in the scientific journal Global and Planetary Change, while four had gotten smaller, the other 23 had either remained the same size or grown bigger.

The islands changed their size through what the scientists describe as ocean shoreline displacement toward their lagoons, lagoon shoreline growth or extensions to the ends of elongated islands.

Kench said it had been assumed that islands would “sit there and drown” as sea levels rose.  However, the islands responded as the sea rose.

“They’re not all growing, they’re changing. They’ve always changed … but the consistency (with which) some of them have grown is a little surprising,” he told The Associated Press (AP) on Thursday.

Tuvalu has its highest point just 14 feet above sea level.  This is a coral island group that climate change campaigners have repeatedly predicted will be drowned by rising seas.  The researchers found seven of its nine islands had grown by over 3 percent on average over the past 60 years.

Cyclone Bebe dumped 346 acres of sediment on the eastern reef of Tuvalu in 1972, increasing the area of Funafuti, the main island, by 10 percent.  Funamanu, another island, gained 30 percent of its previous area.

Kiribati saw a similar trend as well.  Betio expanded by 89 acres, or 30 percent, Bairiki grew 16.3 percent, and Nanikai by 12.5 percent.

Kiribati President Anote Tong warned on World Environment Day in 2008 that parts of his island nation were already being submerged, forcing some of Kiribati’s 94,000 people living in shoreline village communities to be relocated from century-old sites.

He said at the time that worst case scenarios showed Kiribati would disappear into the sea within a century.

However, Kench said the study shows the islands are coping with sea-level change, with higher waves and water depth supplying sand and gravel from coral reefs.

“In other words, they (the islands) are slowly moving … migrating across their reef platforms,” he said. “As the sea-level conditions and wave conditions are changing, the islands are adjusting to that.”

However, he warned an accelerated rate of sea level rise could be “the critical environmental threat to the small island nations,” with “a very rapid rate of island destruction” possible from a water depth beyond a certain threshold.

John Hunter, an Australian sea level oceanographer, told AP the findings “are good news and not a surprise.”

“Coral islands can keep up with some sea-level rise, but (there’s also) ocean warming … and ocean acidification … that are certainly problematic for the corals. Sea-level rise can actually make the islands grow “” as it apparently is doing,” said Hunter, who did not participate in the study.

Hunter said while coral might adjust to ocean warming, ocean acidification “will probably be the death knell of the coral reefs,” leaving coastal management by humans as the only way of retaining and rebuilding atolls.  Hunter is a researcher at the University of Tasmania’s Antarctic Climate and Ecosystems Cooperative Research Center.

New Scientist magazine said on the new findings that “Erosion of island shorelines must be reconsidered in the context of physical adjustments of the entire island shoreline, as erosion may be balanced by progradation on other sectors of shorelines.”

Image Caption: A beach at Funafuti atoll, Tuvalu, on a sunny day. Credit: Stefan Lins (Wikipedia)

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