SYDNEY, Australia — A wide stretch of the Great Barrier Reef has been hit by a sixth mass bleaching event, the marine park’s authority said on Friday, an alarming milestone for the coral wonder that points to the continued threat of climate change and greenhouse gas emissions.
Government scientists who used helicopters and small planes to survey 750 separate reefs across hundreds of miles last week found severe bleaching among 60 percent of the corals.
Bleaching events have now occurred in four of the past seven years, with 2022 offering a disturbing first — a mass bleaching in a year of La Niña, when more rain and cooler temperatures were supposed to provide a moment of respite for sensitive corals to recover.
“We’re seeing that coral reefs can’t cope with the current rate of warming and the frequency of climate change,” said Dr. Neal Cantin, a coral biologist who led one of the teams observing the state of the reef. “We need to slow down that warming rate as fast as possible.”
Coral bleaching is often called a climate change warning system, a canary in the coal mine of a struggling earth. It indicates that corals are under intense stress from the waters around them, which have been growing steadily warmer. Last year, scientists recorded the hottest year on record for the world’s oceans — for the sixth year in a row.
First, the stress shows up on coral reefs in bright, almost neon colors as coral, which is an animal, expels the algae that lives inside it and provides the coral with food. The corals go on to turn white as bone but can still recover if temperatures cool for a long enough period.
Along the 1,500 miles of the Great Barrier Reef — a stunning ecosystem that can be seen from space — there are still large, healthy sections of coral, with sharks, turtles, rays and fish the color of crayons.
But all along the natural wonder, there are also signs of damage. The blocks of underwater graveyards, with gray fields of brittle, dead coral covered in wisps of ugly algae, have been growing with each mass bleaching since the first one occurred in 1998.
In Australia, that decline has become increasingly politicized. The government of Prime Minister Scott Morrison, which has done little to cut the country’s fossil fuel reliance or exports, has repeatedly pushed the United Nations to defy its own scientific advice and keep the reef from being placed on a list of endangered world heritage sites.
Instead of aggressively pursuing emissions cuts, Australia has thrown hundreds of millions of dollars at long-shot projects that aim to help the reef by cleaning up agricultural runoff, killing invasive species or finding and cultivating the most heat-resistant species of coral.
Climate protests across the country have also been intensifying, some led by children, others by activists who have tried to block trains and traffic.
U.N. scientists are now in Australia checking the status of the reef. Dr. Cantin said he met with them on Friday afternoon and explained what the surveys had found.
The image of the reef (and Australia’s stewardship of it) stands to be severely tarnished if the United Nations suggests it is slowly moving toward extinction. But the damage to the world’s reefs go far beyond threats to tourism or a country’s reputation.
While coral reefs cover a tiny fraction of the ocean floor, they collectively support an estimated $2.7 trillion per year in goods and services worldwide, according to a recent report from the International Coral Reef Initiative. Their fish supply food to hundreds of millions of people worldwide — and in Australia and elsewhere, they provide protection from the severe storms that are also becoming more common with climate change.
Dr. Cantin said he was especially disappointed by the spatial footprint of this year’s bleaching damage. Reefs closer to the shore experienced the most extreme bleaching, but he said the bleaching seemed to cover an area wider than back-to-back outbreaks in 2016 and 2017.
He said it was the product of a summer that started early.
“In December we were already warmer than the historical February summer maximums,” he said. There was a cooling period in February, he added, but then the last two weeks of this month saw little rain and continued heat.
“With the frequency of big stressful summers, we’ve been on bleaching watch almost every year,” he said. “We’re in concerning times.”