SINGAPORE – Australia’s Great Barrier Reef has suffered widespread coral bleaching after sea temperatures reached record levels last month, scientists say.

How severe the event is will be known in a few weeks once aerial surveys of the 2,300km reef are completed and sea temperatures fall.

It will be the latest blow to the reef and tourism in the area after marine heatwaves in 2016 and 2017 caused widespread damage. Visitor numbers have already taken a hit from the widespread bush fires in the spring and now the coronavirus pandemic.

“There’s quite a lot of bleaching out there. It is the third widespread bleaching event in five years,” said Professor Terry Hughes, head of the ARC Centre of Excellence for Coral Reef Studies, at James Cook University in Townsville.

He told The Straits Times on Thursday (March 12) the signs so far are that the present event is not as severe as the back-to-back bleachings in 2016 and 2017, when up to half of all shallow water corals in the northern and central sections died.

The southern portion was spared then. But this year, temperatures have soared in the southern section, particularly for the inshore reef areas. “It does appear that parts of the south are bleaching that haven’t bleached before or for a very long time,” he said.

Prof Hughes, who has been studying the Great Barrier Reef for several decades, said data from the US National Oceanic and Atmospheric Administration shows the amount of heat exposure this summer for corals is the third-highest since satellite records began in 1985.

That’s bad news because the reef needs time, upwards of a decade, to recover and rebuild from severe bleaching events. Another event so soon after 2016-17 risks further degradation of the reef.

The Great Barrier Reef is built by hundreds of different coral species, many of which are sensitive to small increases in sea temperatures. Because of climate change, summer water temperatures and extreme marine heatwaves have been increasing and the corals have struggled to adapt.

The sea temperatures along the reef in February were the highest since records began in 1900, according to the national weather agency, the Bureau of Meteorology.

When heat-stressed, corals expel photosynthetic algae, called zooxanthellae, which give the corals their colour. The algae provide food for the corals but once expelled, the corals turn pale when stressed, and a ghostly white when severely heat-stressed or when they die.

Since January, temperatures along parts of the reef have been 1 deg C or more above the summer maximum, while some area have reached 3 deg C above the maximum.

That has led to a large amount of what scientists call accumulated heat stress, which can bleach corals and eventually kill them.

If water temperatures cool in the next week or so, it is possible for some corals to eventually recover, Prof Hughes said. But for corals exposed to very high temperatures, that is less likely.

“Once the corals are white, they stay white for weeks to months. Two things can happen: They can die if it’s really severe or they can slowly regain their colour.

“But in 2016 and 2017 and in February this year, we saw reefs that were exposed to temperatures that were about 3 deg C above the normal summer maximum. That level of extreme heat kills the corals directly. They don’t die slowly of starvation, they cook.”

So far, most of the reports of bleaching are along inshore reefs, while the main offshore reefs appear to be less affected. But aerial surveys to be conducted by Prof Hughes and the Great Barrier Reef Marine Park Authority are needed to confirm this.

For now, scientists are monitoring sea temperatures. Widespread cloud and rain have lowered temperatures but summer temperatures and bleaching events usually peak in March, so there is a possibility that temperatures could rise again before the end of the month.

“The most important thing is that over the next two to four weeks we anticipate the peak of the bleaching,” said Dr David Wachenfeld, the marine park’s chief scientist.

The peak areas of heat stress are up in the far northwest of the marine park in coastal and slightly offshore reefs and in coastal reefs in the central and southern areas, he said on Thursday (March 12) during a weekly update on the reef.

“Of course, what we really want to know is how much bleaching there is on offshore reefs and that’s one of the things we will learn from the aerial surveys that begin next week.”

Prof Hughes is worried that climate change means more and more heat stress for the reef and more frequent bleaching events, robbing corals of recovery time. Some of the strongest bleaching events, in 1998 and 2016, have been during El Nino years, when global temperatures spike.

“We haven’t had an El Nino now since 2016 and it’s almost guaranteed that the next El Nino we get, which could be next year, will see severe bleaching, probably more severe than the one we are dealing with now and it could well be the second back-to-back example.”

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