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The Great Barrier Reef has experienced a third mass coral bleaching event in five years, according to the scientist carrying out aerial surveys over hundreds of individual reefs. With three days of a nine-day survey to go, Prof Terry Hughes told Guardian Australia: “We know this is a mass bleaching event and it’s a severe one.” It follows the worst outbreaks of mass bleaching on record killing about half the shallow water corals on the world’s biggest reef system in 2016 and 2017.