The Great Barrier Reef is experiencing a major coral bleaching event right now

diver looking at bleached corals

Corals along large swaths of the Great Barrier Reef have lost their symbiotic algae, removing their vibrant colors and primary food supply, new observations show.

Greg Torda/ARC Centre of Excellence for Coral Reef Studies/James Cook Univ.

A severe coral bleaching event spurred by high ocean temperatures has struck the Great Barrier Reef for an unprecedented second time in 12 months, reveal aerial surveys released April 10 by scientists at James Cook University in Townsville, Australia. While last year the northern third of the reef was hardest hit, this time around the reef’s midsection experienced the worst bleaching. The two bleaching events together span around 1,500 kilometers of the 2,300-kilometer-long reef.

“It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016,” James Kerry, one of the researchers behind the finding, said in a statement.

Bleached corals aren’t dead. Trauma, disease or warm water can cause an exodus of the symbiotic algae that provide corals with food and vibrant color schemes. If better conditions, such as cooler waters, return, the algae may return to their homes. If they don’t come back, though, the corals starve.

Warming caused by El Niño exacerbated last year’s bleaching event. With El Niño now long gone, the researchers blame this year’s bleaching largely on global warming. If humans don’t curb emissions of planet-warming greenhouse gases, scientists warn that the entire reef could be in jeopardy.

map of 2016 and 2017 bleaching events
For the second time in a year, a severe coral bleaching event has hit the Great Barrier Reef. While last year’s event mostly affected the northern third of the reef, this year’s bleaching hit the middle third. ARC Centre of Excellence for Coral Reef Studies/James Cook Univ.

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