Coral reefs are about as colorful as the ocean gets—except when they bleach. Overly warm water can cause corals to spit out the colorful, photosynthetic, single-celled symbiotes that live inside them and produce most of their food. If the heat passes before the corals starve to death, their symbiotes can return, bringing color and health back to the coral.

As the globe warms, widespread bleaching events are occurring with disturbing frequency. These tend to occur during times of El Niño conditions in the Pacific, which add a temporary boost to the warming water at some reefs. The current record-strength El Niño is sadly no exception.

Researchers contributing to Australia’s National Coral Bleaching Taskforce recently completed a survey of the state of the iconic Great Barrier Reef. The results show that it is currently experiencing the worst bleaching event we’ve ever seen there. Overall, 93 percent of the Great Barrier Reef has bleached to some degree. The northern half of the reef has been hit the hardest, with about 80 percent categorized as severely bleached. The far southern portion has escaped the warmest water, and the area of severe damage there drops to around 1 percent.

In a press release for the survey, James Cook University’s Andrew Baird said, “North of Port Douglas, we’re already measuring an average of close to 50 percent mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90 percent. When bleaching is this severe it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return.”