Dead (brown) and dying staghorn coral on the central Great Barrier Reef in May.

Dead (brown) and dying staghorn coral on the central Great Barrier Reef in May.

Johanna Leonhardt

Massive bleaching killed 35% of the coral on the northern end of the Great Barrier Reef

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.

Teams found that the southern reaches of the reef were largely spared, with an average of 5% mortality. They also reported that reefs off Australia's western coast had suffered from extensive bleaching, with at least 15% of the corals dead already. Corals that bleach but survive will gradually recover their color over several months, but the bleaching negatively affects growth and reproduction. This year's bleaching is over as waters have cooled with the approach of the Southern Hemisphere winter. Hughes says the team will resurvey the reefs in October or November to verify their findings and gauge recovery. "But we don't expect [mortality] to increase from what we recorded," he says.

Hughes says this is the worst of three major bleaching events that have occurred in the last 18 years. The increasing frequency of bleaching expected to accompany global warming will give the reefs less and less chance to recover. "We’re rapidly running out of time to reduce greenhouse gas emissions,” he says.