Great Barrier Reef

Coral bleaching in March at Lizard Island on the Great Barrier Reef.

XL Catlin Seaview Survey

Survey confirms worst-ever coral bleaching at Great Barrier Reef

The devastation of coral at Australia's iconic Great Barrier Reef (GBR) "dwarfs previous bleaching events by a long mark," says Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies in Townsville. Over the weekend, Hughes and colleagues completed the final of a series of aerial surveys crisscrossing the entire reef system. Bleaching is most severe, they confirmed, along the northernmost 1000 kilometers of the reef.

Bleaching occurs when overly warm water leads corals to expel symbiotic algae called zooxanthellae. Without the colorful algae, which use photosynthesis to produce nutrients for themselves and their hosts, the corals turn white, or bleach. If the waters cool soon enough, algae return; if bleaching persists, the corals die. The world is in the midst of an unusually long El Niño, the climate phenomenon that warms water in the equatorial Pacific and affects global weather. The El Niño, abetted by global warming, has been pushing reefs worldwide into the danger zone.

NASA

The prolonged ocean warming has hit the GBR hard. Only 7% of the reef system has avoided coral bleaching entirely, according to a statement released today by Australia’s National Coral Bleaching Taskforce. The aerial survey found that middle stretches of the 2300-kilometer-long GBR system were, on the whole, moderately bleached. Only the southern reaches escaped with minimal damage, thanks to persistent cloud cover and rain in early March. The middle and southern sections will likely recover and regain color in coming months, says Hughes, who heads the taskforce. The northern section is in big trouble: Eighty percent of the reefs were severely bleached and in-water surveys have confirmed 50% mortality in some reefs, a percentage that could eventually exceed 90%.

Recovering from this degree of devastation will take decades, and even then the reef will be only a shadow of its prebleaching self. At best, in a decade or two, coral cover will be dominated by fast-growing species, without any of the 200-year-old corals that have perished recently, Hughes says. The northern GBR "won't get back to what it was, certainly not in my lifetime," he says. 

There is more bad news. Up to 80% of coral reefs on the western coast of Australia are now also bleached, according to the taskforce.