Reef at risk. An intense cyclone shreds Australia's Great Barrier Reef at the surprising rate of every 2 to 3 centuries, on average.

Coral-Killing Cyclones

Intense cyclones pummel Australia's northeastern coast 10 times more often than previously believed, according to a new analysis of piles of destroyed coral. As a result, reefs, tropical forests, and even towns in Queensland may face devastation from storms every 200 to 300 years. The finding, published in the 3 October issue of Nature, echoes earlier studies of the history of hurricanes in the southeastern U.S.

Because written records usually go back just a century or two, researchers must use detective work to discover how often storms strike. Fortunately, the strongest hurricanes--called cyclones in the southern hemisphere--leave telltale remains. The signs include layers of sand driven far inland and massive ridges of debris near the beach, such as shattered chunks of coral reefs. The latter are especially visible along the shore of the Great Barrier Reef, which graces 2000 kilometers of the Queensland coast.

To decipher Queensland's cyclone history, coastal geologists Jonathan Nott of James Cook University in Cairns and Matthew Hayne of the Australian Geological Survey in Canberra measured parallel ridges of broken coral at seven of the reef's islands and bays. Most deposits were a few meters high, and radiocarbon dating showed that they ranged in age from a few dozen years old to more than 5 millennia. Nott and Hayne then devised a model to estimate the intensity of each cyclone. The severest storms, they reasoned, broke off the most chunks of coral and created the biggest surges of water, thus building the highest ridges. They found that "category 5" cyclones, the most destructive storms, ravaged some part of the reef every 2 to 3 centuries, on average. Previous estimates were once every several thousand years. "This is something no one predicted or expected," Nott says. Urban planners, emergency response coordinators, and insurers are "very concerned" by the finding, he notes, because their plans and rates are based on a far lower perceived risk of catastrophic storm damage.

Category 4 and 5 hurricanes appear to bombard the Atlantic and Gulf of Mexico coasts in the U.S. with the same frequency, says biogeographer Kam-biu Liu of Louisiana State University in Baton Rouge. "I welcome the addition of a new indicator for reconstructing storm histories," says Liu, who bases his own research on sand buried in coastal lagoons. But Nott's beach-ridge approach, he says, might be harder to apply elsewhere in the tropics, because it may depend on Australia's unique coastal geology.

Related sites

Tropical Environment Studies and Geography at James Cook University
Kam-biu Liu's research on past U.S. hurricanes
Studies on hurricane risks and the insurance industry