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Last year the Great Barrier Reef – the largest coral structure on Earth – saw unprecedented bleaching due to extremely warm ocean temperatures. In major parts of the remote northern sector of the reef, two-thirds of the corals ultimately died.

This was the reef’s third and worst severe bleaching event – prior events occurred in 1998 and 2002. But now, scientists say, yet another event is unfolding that is also quite severe, meaning that the reef is experiencing its first back-to-back bleaching in two successive years.

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“This one won’t be as bad as 2016, but it could be more comparable to 1998 or 2002,” said Terry Hughes, the lead author of the new study and director of the ARC Centre of Excellence in Coral Reef Studies at James Cook University in Townsville, Australia. “It’s an open question whether it’s the third- or second-most-severe.”

Bleaching occurs when stresses from temperature cause coral cells to push out the colorful algae, called zooxanthellae, that they rely on for energy. It isn’t an automatic death sentence for coral – that depends on whether the bleaching is mild or severe and how long the corals go without the algae. The hotter the waters and the longer they’re around, the greater the stresses and the more likely the corals are to succumb to them.