Around 93% of reefs on the Great Barrier Reef have been hit by coral bleaching, according to the survey revealing the full extent of the devastation caused by abnormally warm ocean temperatures sweeping the globe.

There have only been three mass bleaching events recorded on the reef, and all of them have happened since 1998. Scientists say this episode is the worst they’ve ever seen.

If coral remains bleached for an extended period, it is likely to die. Hughes said that the northern region is already seeing mortality as high as 50%, and he expected that at some reefs it will exceed 90%. “By that metric, this event is five times stronger.” And in those two years, only 18% of reefs were severely bleached. “This time it’s 55%.