The story is better in the central and southern regions, which respectively lost just 6 percent and 1 percent of their corals in 2016. "The corals have now regained their vibrant color and these reefs are in good condition," ARC Centre Professor Andrew Baird said. (We've reached out to the ARC Centre for additional information about this year's die-off and will update this story as we hear back.)

Bleaching can occur when water temperatures rise and stay too warm, causing corals to expel the algae zooxanthellae, which gives them their color and provides them with food. The corals turn white and they're immensely weakened, though not dead quite yet. If they survive predators and disease, they can be revitalized, though ARC Centre researchers expect it will take 10 to 15 years to regain the northern region's corals. Another bleaching event could of course slow down the recovery process.

This is the third major die-off to strike the Great Barrier Reef, following less destructive events in 1998 and 2002. Coral bleaching is yet another sign that the planet is warming in unprecedented ways, largely because of human activity.