Ask four Australians why the Great Barrier Reef is bleaching, and you’ll hear four different answers.

A travel agent in Brisbane says it’s because of the cyclone that hit last year. Another one just down the street blames “some sort of starfish.” A tour guide in Cairns says bleaching is a natural part of the coral life cycle; another, in nearby Port Douglas, calls it propaganda. He proclaims the reef isn’t bleaching at all.

Research Scientist Dr. Neal Cantin (right) carries a tray of collected samples of the coral Pocillopora acuta from Rib Reef, one of the sections of the Great Barrier Reef in Queensland. 
Photographer: David Maurice Smith/Oculi/Redux

About the size of Japan,...