The Great Barrier Reef also appears to be dying

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.

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Global heating caused by escalating atmospheric greenhouse gases is a major threat to the world’s coral reef ecosystems. The UN’s Intergovernmental Panel on Climate Change found the published evidence suggested a majority of tropical coral reefs would disappear even if heating was limited to 1.5C and would be “at very high risk” at 1.2C. The globe has warmed about 1C since the industrial revolution.

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Not all bleached corals die. Corals bleach when they sit in waters that are unusually hot for too long. They can recover if temperatures fall, but are often killed when high temperatures are sustained.

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Hughes said he was worried about corals in areas yet to be surveyed in the south. Those reefs had higher proportions of corals that were more susceptible to bleaching. “If they do bleach severely, we will see significant losses,” he said.

In 2016 and 2017, there was no severe bleaching south of Hydrographers Passage, a natural shipping channel north-east of Mackay. Hughes said by late Tuesday they had surveyed about 10 reefs south of the passage and found all were severely bleached.

  Guardian