SYDNEY, Australia — When Terry Hughes surveyed the Great Barrier Reef four years ago from a small plane, mapping the bleaching and death of corals from water warmed by climate change, he hoped such a rare and heartbreaking scene would not be repeated anytime soon.

But rising temperatures sent him back to the air in 2017, when the reef bleached again. Then he returned last month, leading to another devastating conclusion: The reef was being ravaged by bleaching yet again, this time across an even wider area.

“It’s the first time we’ve seen severely bleached reefs along the whole length of the reef, in particular, the coastal reefs,” said Professor Hughes, the director of the ARC Center of Excellence for Coral Reef Studies at James Cook University. “Those are bleached everywhere.”

New aerial data from Professor Hughes and other scientists released on Monday shows example after example of overheating and damage along the reef, a 1,500-mile natural wonder. The survey amounts to an updated X-ray for a dying patient, with the markers of illness being the telltale white of coral that has lost its color, visible from the air and in the water.