Current coral bleaching event is the longest known

Heat stress has made corals eject algae that feed them and give them color

coral bleaching

Healthy coral (left) is a riot of color. When stressed, corals eject the colorful algae they depend on for food (right). Such “bleaching events” threaten coral reefs’ survival. Global warming may make long bleaching events more common, scientists say.

Jim Maragos/U.S. Fish and Wildlife Service (CC BY 2.0)

Coral reefs won’t be out of hot water for quite a while. These normally colorful undersea ecosystems are under increasing stress, mostly because of warming oceans. Now, researchers report that a global coral bleaching event began in June 2014. The longest on record, it has sapped the color out of vast areas of coral — and now threatens their health. The reefs affected cover a larger area than ever before. What’s worse, the bleaching shows no signs of ending.

Corals are tiny animals that dwell mainly in tropical oceans. They live in colonies, on structures made up of the hard exoskeletons of dead corals. These are called reefs. Corals depend on algae that live inside their tissues. Those algae are a source of food and give corals their vibrant colors. But when stressed by heat, corals may eject the colorful algae living inside them and turn a ghostly white. With their major food source gone, reef corals are at risk of dying.

Several climate conditions have combined to foster the long bleaching event. That’s what scientists at the National Oceanic and Atmospheric Administration, or NOAA, now report. The first of these is a long-term warming of the planet. The second is a short-term period of warmer surface waters in the Pacific Ocean (called an El Niño). Scientists with NOAA, which studies ocean resources and works to preserve them, presented their findings on June 20. They were taking part in the 13th International Coral Reef Symposium in Honolulu, Hawaii.

Since 1979, coral bleachings across distances spanning hundreds of kilometers have occurred now and again. But none lasted for more than “a year or so,” said NOAA’s Mark Eakin. This current event has already dragged on for two years. And it has threatened more than 40 percent of reefs globally. A similar bleaching threatens more than 70 percent of reefs in U.S. coastal waters.  

NOAA scientists aren’t sure what will end this episode. It could extend into 2017, they said. And more frequent events are possible in the future. Computer programs often are used to predict — or model — what Earth’s future climate will look like. Such climate models “suggest that most coral reefs may be seeing bleaching every other year by mid-century,” Eakin notes. “How much worse that gets will depend on how we deal with global warming.”

coral bleaching infographic
Corals feed on the algae that live inside them. When stressed, corals cast out the algae. This strips the coral of its color and makes it more prone to disease. NOAA



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