The Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies released their estimated fatalities of the Great Barrier Reef on May 30 after conducting intensive aerial and underwater investigations. The researchers reveal that mass coral bleaching killed about 35 percent of corals in the northern and central portion of the largest coral reef system in the world.

“We found on average that 35 percent of the corals are now dead or dying on 84 reefs that we surveyed along the northern and central sections of the Great Barrier Reef, between Townsville and Papua New Guinea,” points out Terry Hughes, a professor and the director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University (JCU). “Some reefs are in much better shape, especially from Cairns southwards, where the average mortality is estimated at only five percent.”

Coral bleaching is a result of several unusual environmental situations which includes increased ocean temperatures, that drives away zooxanthellae, a group of small photosynthetic algae. Consequently, the disappearance of the algae bleaches the corals or turns them into white.

Great Barrier Reef

Bleached mature staghorn coral. Credit: ARC Centre of Excellence for Coral Reef Studies

“This year is the third time in 18 years that the Great Barrier Reef has experienced mass bleaching due to global warming, and the current event is much more extreme than we’ve measured before,” adds the professor. “These three events have all occurred while global temperatures have risen by just one degree C above the pre-industrial period. We’re rapidly running out of time to reduce greenhouse gas emissions.”

The coral bleaching changes drastically going from north to south of the Great Barrier Reef, which will most likely worsen in time. Coral bleaching least affected the corals at the southern part of the reef. Nevertheless, this damage still hampers their recovery and growth.

However, the researchers say that the corals can still go back to normal if the water temperature decreases and the algae will return to them. The corals are expected to recover in the following months.

“Fortunately, on reefs south of Cairns, our underwater surveys are also revealing that more than 95 percent of the corals have survived,” says Mia Hoogenboom, who is also from JCU. “We expect these more mildly bleached corals to regain their normal colour over the next few months.”