The Great Barrier Reef's Latest Bleaching Event Is The Most Widespread Yet

Aerial view of the bleached reef. ARC Centre of Excellence for Coral Reef Studies

After recent news that Australia's Great Barrier Reef had experienced its third coral bleaching event in five years, but that according to experts it's "not all bad news", scientists from the James Cook University are now reporting it's actually more severe than previously thought, possibly even the most widespread event yet.

This is the fifth bleaching event the Reef has experienced in the last 20 years, and it's clear they're becoming more frequent. The first happened in 1998 and the second in 2002. The third, in 2016, was incredibly damaging to the northern portion of the barrier reef. The bleaching in 2017 expanded the damage to the central region of the reef, which is the one that suffered the most. This latest one has left no major area unaffected.  

“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,” Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said in a statement. “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.”

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The long-term effects of this new bleaching event won’t be known for a while. Bleaching is not necessarily the death of corals but prolonged and multiple events can easily be. In 2016, half of all the shallow-water corals in the northern region of the Great Barrier Reef died.

“Bleaching isn’t necessarily fatal, and it affects some species more than others. A pale or lightly bleached coral typically regains its colour within a few weeks or months and survives,” said Professor Morgan Pratchett, also from ARC's CoE for Coral Reef Studies, who led the underwater surveys to assess the bleaching. “We will go back underwater later this year to assess the losses of corals from this most recent event.”

Photos of bleached corals on the reef. Victor Huertas

Coral bleaching events of this magnitude are caused by thermal stress on the animals. The reason behind the unexpected thermal stress is the sudden increase in sea temperatures due to unusually hot summers. As temperatures have increased significantly during the ongoing climate crisis and El Niño events have become more extreme, the increase in number, frequency, and severity of bleaching events is not surprising.

“Of the five events we have seen so far, only 1998 and 2016 occurred during El Niño conditions. As summers grow hotter and hotter, we no longer need an El Niño event to trigger mass bleaching at the scale of the Great Barrier Reef,” Prof Hughes said. “We have already seen the first example of back-to-back bleaching — in the consecutive summers of 2016 and 2017.” 

Only a small number of reefs have escaped the three bleaching events and they are located far from the shore in the remote extremities both north and south of the Great Barrier Reef.

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