New Scientist - USA (2022-04-02)

2 April 2022 | New Scientist | 7

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 on 25 March

found 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.

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 as the southern

hemisphere autumn arrives.

The bleaching is particularly

notable for happening when

the Pacific 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

The Great Barrier Reef is being affected by warm seas despite

the cooling effect of La Niña, reports Adam Vaughan

No respite for coral

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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

to mitigate coastal flooding.

The Great Barrier Reef Marine

Park Authority, which conducted

the surveys using helicopters and

small planes, 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 at the moment,

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. ❚

Bleached coral on

the Great Barrier

Reef in March

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News

“ We thought La Niña could

be a safe period when

coral reefs could recover.

Turns out it’s not”