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Bleaching had damaged almost all Great Barrier Reef coral surveyed this year in the fourth such mass event in seven years, Australian government scientists said.

The world’s largest coral reef ecosystem suffered its sixth mass bleaching on record after being hit by three marine heatwaves, the Great Barrier Reef Marine Authority (GBRMA) said in its snapshot report of summer 2021-22.

Bleaching is a heat stress response caused by global warming. This is the reef’s first bleaching event during a La Nina weather pattern, which is associated with cooler Pacific Ocean temperatures, the authority said.

Simon Bradshaw, a researcher at the Climate Council, an Australia-based group that tracks climate change, said the report shows that the reef’s survival depends on steep global emission cuts within the decade.

"This is heartbreaking. This is deeply troubling," Mr Bradshaw said.

"It shows that our Barrier Reef really is in very serious trouble indeed."

Surveys were conducted after the final heat wave in March 2022. Bleached coral is stressed but still alive, so can recover and regain its colour as the ocean cools off after summer, as happened in 2020 when much coral survived a mass bleaching event.

"The early indications are that the mortality won’t be very high," said David Wachenfeld, chief scientist at GBRMA, which manages the reef ecosystem.

"We are hoping that we will see most of the coral that is bleached recover and we will end up with an event rather more like 2020 when, yes, there was mass bleaching, but there was low mortality."

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Bleaching in 2016, 2017 and 2020 damaged two-thirds of the coral in the famed reef off Australia’s east coast. The earlier two events led to "quite high levels of coral mortality", Mr Wachenfeld said.

Last July, Australia garnered enough international support to defer an attempt by Unesco, the United Nations’ cultural organisation, to downgrade the reef’s World Heritage status to "in danger" because of damage caused by climate change.

A United Nations delegation visited the reef again in March to assess the reef’s World Heritage listing.

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