More than 50% of world's coral reefs at risk

More than half of the world's coral reefs - the tropical rainforests of the ocean - could be lost by the end of the century, according to the latest analysis of the marine environment's most vulnerable ecosystem.

A "triple whammy" of global warming, pollution and overfishing has put unprecedented pressure on tropical coral reefs. Until now, these reefs have survived undisturbed for thousands of years.

A study by a team of American scientists has discovered firm evidence to link the dramatic decline of coral reefs growing in warm equatorial waters with the rise in sea-surface temperatures seen in recent years.

Richard Aronson, a marine biologist at the Dauphin Island Sea Lab in Alabama and co-author of the study, said that about 10 per cent of tropical coral reefs around the world have already been affected or destroyed over the past 30 years, and the prospects for the remaining corals are not good.

"More than half will be ruined or degraded certainly by 2100, to be as conservative as possible and assuming we don't do anything about it," Dr Aronson told the American Association for the Advancement of Science in Seattle. "It definitely shows that global warming is having an impact on corals.

"The changes are unprecedented in thousands of years. On a 4,000-year timescale, things are substantially different now from the way they were before. They are substantially different in a bad way.

"Coral reef ecosystems are going to be significantly impacted by climate change. They're already being degraded by both climate change and by direct impacts such as overfishing and habitat loss, and the combination of these stresses can be devastating."

Coral reefs are built from the limestone "skeletons" of tiny sea-anemone-like animals called polyps, which live in close association with a single-celled alga that give the reefs their bright colours.

When corals suffer from environmental stress they eject the algae, with the result that they develop a ghostly, bleached appearance, which can ultimately lead to the death of the entire reef.

Since the 1980s, scientists have monitored a dramatic rise in the incidence of coral bleaching and have linked it directly to a sudden rise in sea temperatures resulting from global warming and the stronger El Niño warm currents of the South Pacific, Dr Aronson said.

About 16 per cent of the world's coral reefs were bleached as a result of a single El Niño event in 1997-98. In addition, as more carbon dioxide is released into the atmosphere, more of it dissolves in the sea. This causes a rise in acidity, which also puts corals under stress, the study found.

Dr Aronson said that he had personal experience of the dramatic decline of individual coral reefs. "If you go round the Caribbean, coral reefs look like hell," he said. "They look absolutely horrendous."

Overfishing and pollutants in rivers being washed into the ocean from densely populated areas are adding to the stress on corals, he added.

Discovery Bay in Jamaica, for instance, used to have corals covering about 50 per cent of its surface area when Dr Aronson was a research student there about 30 years ago. Now, he said, the corals cover less than 2 per cent of the bay.