Great Barrier Reef suffers worst-ever bleaching due to warming, scientists say

Australia's iconic Great Barrier Reef has suffered the most widespread coral bleaching event on record, owing to rising temperatures due to climate change, scientists said on Tuesday. After five bleaching events, the number of reefs that have so far escaped severe bleaching continues to dwindle said the researchers from James Cook University in Australia.

Those reefs are located offshore, in the far north, and in remote parts of the south, according to the researchers. "We surveyed 1,036 reefs from the air during the last two weeks in March, to measure the extent and severity of coral bleaching throughout the Barrier Reef region," said Professor Terry Hughes from James Cook University (JCU).1

"For the first time, severe bleaching has struck all three regions of the Great Barrier Reef the northern, central and now large parts of the southern sectors," Hughes said. The researchers explained that coral bleaching at regional scales is caused by thermal stress due to spikes in sea temperatures during unusually hot summers.

The first recorded mass bleaching event along the Great Barrier Reef occurred in 1998, then the hottest year on record, they said. Four more mass bleaching events have occurred since more temperature records were broken in 2002, 2016, 2017, and now in 2020, according to the researchers.

This year, February had the highest monthly temperatures ever recorded on the Great Barrier Reef since the sea surface temperature records began in 1900, they said. "Bleaching isn't necessarily fatal, and it affects some species more than others," said Professor Morgan Pratchett, also from JCU, who led underwater surveys to assess the bleaching.

"A pale or lightly bleached coral typically regains its color within a few weeks or months and survives," he said. However, many corals die when bleaching is severe. In 2016, more than half of the shallow-water corals died in the northern region of the Great Barrier Reef.

"We will go back underwater later this year to assess the losses of corals from this most recent event," Pratchett said. "The north was the worst affected region in 2016, followed by the central region in 2017. In 2020, the cumulative footprint of bleaching has expanded further to include the south," he said.

The distinctive footprint of each bleaching event closely matches the location of hotter and cooler conditions in different years. "As summers grow hotter and hotter, we no longer need an El Nino event to trigger mass bleaching at the scale of the Great Barrier Reef," Hughes said.

"Of the five events we have seen so far, only 1998 and 2016 occurred during El Nino conditions," he said. The gap between recurrent bleaching events is shrinking, hindering a full recovery, the researchers said.

"We have already seen the first example of back-to-back bleaching in the consecutive summers of 2016 and 2017," Hughes said.

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