Researchers have confirmed the grim toll of an unusually hot summer on Australia's Great Barrier Reef: Mass bleaching has killed 35% of corals on the northern and central sections of the 2300-kilometer-long system. On 24 of the 84 reefs surveyed, 50% of the corals have perished, including specimens that were 50 to 100 years old. "They can't recover in anything less than that period, certainly not in 10 years," says Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies in Townsville.
Aerial surveys earlier this year found extensive and severe bleaching on the northern two-thirds of the reef. A combination of global warming and the ongoing El Niño, a periodic phenomenon that brings unusually warm water to the equatorial Pacific, warmed coastal waters. In reaction to hot water, corals lose the colorful symbiotic algae called zooxanthellae they host and turn white. The white coral skeletons are visible from the air. But brownish algae soon smother dead corals, after which the reef's condition can only be determined by close-up inspection. Hughes and his colleagues, who previously conducted aerial surveys, announced the results of their in-water confirmations in a media release today.