Warming wears down coral’s defence system

A safeguard against coral bleaching, which has already protected parts of Australia’s Great Barrier Reef, could cease to work if waters continue to warm, experts warn.

Coral bleaching is impacting the Great Barrier Reef at an alarming rate, and much of the northern half of the reef is already severely damaged. 

Researchers at James Cook University (JCU) have identified a protective mechanism against coral bleaching, in the patterns of gradual warming that occur before major stress events in the Reef. 

The finding comes with a warning that if waters warm by another 0.5 °C, this impressive defence could lose its power.

The research team, led by JCU’s Tracy Ainsworth, says the gradual increase in water temperature appears to give coral time to adjust to warming waters.

“When corals are exposed to a pre-stress period in the weeks before bleaching, as temperatures start to climb, this acts like a practice-run and prepares the coral,” Ainsworth explains.

“Corals that are exposed to this pattern are then less stressed and more tolerant when bleaching does occur.”

During the study, published in the journal Science, researchers analysed extreme events endured by the Great Barrier Reef over 27 years, to establish how different coral species reacted. 

In three-quarters of these stress events, the coral experienced a gradual increase in temperatures, which prepared some species enough to stave off bleaching. 

However, the research warns that if water temperatures increase by another half-degree, which is currently predicted in the reef’s near future, this “practice-run” will be lost. 

“In future summers, bleaching events will occur more often and, without the practice-run, become even more severe – with a greater risk for coral mortality and a fast decline in coral cover across reefs,” says Scott Heron from the US National Oceanic and Atmospheric Administration and co-author of the paper. 

Because climate change predictions vary for different parts of the Great Barrier Reef, the researchers recommend conservation efforts should be focused on the parts that will retain their protective practice-run the longest.

“Our models suggest reefs that lose their thermal protection in the future will degrade faster and stay in a degraded state for a longer period, while reefs that maintain their protection have a better chance of maintaining coral cover – if carbon emissions are reduced in the near future,” Juan Ortiz, of the University of Queensland, explains.

Malcolm McCulloch from the University of Western Australia says the study is significant.

“It is one of the first to attempt to incorporate the past temperature history of stress events and hence acclimatisation into a predictive model of likely future tolerance to bleaching,” he says.

“The study provides further evidence that since these events are superimposed upon already warming oceans, the threshold of corals to bleaching is increasingly being exceeded.”

Related reading:
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