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Great Barrier Reef suffers first mass bleaching under cooling La Niña

Corals have turned white across all four of the reef’s main areas, despite the cooling influence of the La Niña climate phenomenon, in the natural wonder’s sixth mass bleaching event of modern times

Environment 25 March 2022

This picture taken on March 7, 2022 shows the current condition of the coral on the Great Barrier Reef, off the coast of the Australian state of Queensland. - The Great Barrier Reef has again been hit with "widespread" bleaching, authorities said on March 18, 2022, as higher-than-average ocean temperatures off Australia's northeast threaten the already struggling World Heritage site. (Photo by Glenn NICHOLLS / AFP) / The erroneous mention[s] appearing in the metadata of this photo by Glenn NICHOLLS has been modified in AFP systems in the following manner: <figure class="article-image-inline" data-method="caption-shortcode" ></figure> instead of [Cairns]. Please immediately remove the erroneous mention[s] from all your online services and delete it (them) from your servers. If you have been authorized by AFP to distribute it (them) to third parties, please ensure that the same actions are carried out by them. Failure to promptly comply with these instructions will entail liability on your part for any continued or post notification usage. Therefore we thank you very much for all your attention and prompt action. We are sorry for the inconvenience this notification may cause and remain at your disposal for any further information you may require. (Photo by GLENN NICHOLLS/AFP via Getty Images)

Bleached coral on the Great Barrier Reef in March

GLENN NICHOLLS/AFP via Getty Images

Unusually warm ocean temperatures have turned corals white on Australia’s Great Barrier Reef in the first-ever mass bleaching under the cooling conditions created by the La Niña weather pattern.

An official analysis of aerial surveys published today finds mass bleaching across all four of the reef’s management areas, with the north and central parts of the World Heritage Site worst hit. The impact has been less severe in the south of the reef.

“What we’re seeing at the Great Barrier Reef is very worrying,” says Miriam Reverter at the University of Plymouth in the UK.

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Warmer oceans under climate change have led to an increase in mass bleaching events at the world’s largest reef: this is the sixth since modern records began in 1988, and the fourth in just seven years. Ocean temperatures at the reef during March have been between 0.5°C and 2°C above average in most places, and up to 4°C higher in some spots. Normally, the water would be expected to start getting cooler in March.

The bleaching is particularly notable for happening when the region is in a cooling phase brought about by La Niña. The worst mass bleaching event happened in 2016, the planet’s hottest year on record, when an El Niño warming phase was in effect.

Terry Hughes at James Cook University in Australia tweeted that the latest mass bleaching was “a grim milestone during what should have been a cooler (La Niña) summer”.

Reverter says the milestone means there is increasingly little respite for coral. “Coral reef scientists were thinking there would be some years when coral reefs could recover,” she says. “We thought it [La Niña] could be a safe period. Turns out it’s not.”

Whether the cumulative impact of more frequent mass bleaching events makes coral more vulnerable to new bleaching is still being researched, says Reverter. But she says there is evidence that the coral reef species dying off in the greatest numbers during mass bleaching are those with a physically complex, more three-dimensional structure. Their loss hurts the reef’s ability to provide a habitat for fish and mitigate coastal flooding.

The Great Barrier Reef Marine Park Authority, which conducted the surveys using helicopters and small planes over the past week, said on its website that the bleached coral could still recover if the waters cool, as happened in 2020 when there was relatively little coral die-off despite the most widespread bleaching ever.

UNESCO, which awarded the reef World Heritage status, last year stopped short of placing the natural wonder on a list of sites in danger because of the impacts of climate change, after lobbying by the Australian government. Hughes and other researchers have said the decision was denying the scientific evidence.

While the Great Barrier Reef is being affected now, Reverter says it will be important to monitor other coral in the Pacific and Indian oceans in the coming months, to see whether heat stress triggers more widespread bleaching.

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