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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 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 at the University of Plymouth in the UK.

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.

at James Cook University in Australia 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, 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 because of the impacts of climate change, after lobbying by the Australian government. Hughes and other researchers have 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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