"The really valuable thing about documenting coral bleaching is that it is this straight, very direct visual indicator of how hot the oceans are getting. If the temperature of the water passes a certain threshold, the corals turn white. It's that simple. There's nothing natural about the cycle that's going on right now. In 2016, we lost 29 percent of the Great Barrier Reef. So 29 percent of the Great Barrier Reef died in a single year, because the water was hot."