1450km of Great Barrier Reef has bleached badly

Scientists just back from a 8045km aircraft survey of Australia's Great Barrier Reef pronounced a dire verdict: Warm waters have severely bleached large swaths of its corals for the second year in a row in a deadly one-two punch.
In 2016, two thirds of corals in the northern sector of the reef died after severe bleaching from unusually warm waters.

Now this year, researchers with the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, who reported the previous findings, say that the reef's central sector has been hit by another year of damaging warmth.

"We've had a back-to-back bleaching for the first time," said Terry Hughes, who directs the centre.

"So we redid our aerial surveys again, which was a bit tough. I was hoping to never have to do it again."

After that first survey, Hughes tweeted: "I showed the results of aerial surveys of #bleaching on the #GreatBarrierReef to my students, And then we wept."

Coral bleaching occurs when unusually warm waters provide a stress to corals that in turn trigger a mass exodus of photosynthetic algae, called zooxanthellae, from their cells. The corals lose colour and turn white, an outward indicator that their metabolism has been upended. The stronger the bleaching and the longer it goes on, the more likely corals are to die.
 

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