This article is more than 1 year old

This article is more than 1 year old

Great Barrier Reef corals that survived bleaching in 2016 were more resistant to a second marine heatwave the following year, “astonished” scientists have observed.

A study, published in the journal Nature Climate Change, outlines how a process called “ecological memory” emerged in the northernmost reefs during back-to-back heatwaves in 2016 and 2017.

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An international research team, led by Terry Hughes from the ARC Centre of Excellence for Coral Reef Studies in Townsville, highlighted the extent of damage to the reef from coral bleaching events in 1998, 2002, 2016 and 2017.

They found that only 7% of the Great Barrier Reef has escaped bleaching since 1998, and that 61% of individual reefs had been severely bleached at least once.

This year, the same research team published research in Nature that focused on the impact of the 2016 marine heatwave. It found there had been a “mass mortality” of corals in 2016, and that the northern section was the most severely affected.

In the north, some reefs lost almost all of their coral cover in 2016.

The latest study, published on Tuesday, is a companion that considered the second year of unprecedented back-to-back bleaching events. The corals that survived in 2016, when exposed to even more extreme conditions the following year, were found to be more resilient.

“We were astonished to find less bleaching in 2017, because the temperatures were even more extreme than the year before,” Hughes said. This was partly due to the mass deaths of of more susceptible species, while hardier corals survived.

“Dead corals don’t bleach for a second time,” Hughes said. “The north lost millions of heat-sensitive corals in 2016, and most of the survivors were the tougher species. As a result of bleaching, the mix of species is changing very rapidly.

“The outcome in 2017 depended on the conditions experienced by the corals one year earlier. We called that ‘ecological memory’ and show that these repeating events are now acting together in ways that we didn’t expect.”

The study said scientists needed to understand the way the cumulative impacts of climate change-driven events, as they increase in frequency, impact on vulnerable ecosystems.

“Climate change is radically altering the frequency, intensity and spatial scale of severe weather events,” the study said. “As the time interval shrinks between recurrent shocks, the responses of ecosystems to each new disturbance are increasingly likely to be contingent on the history of other recent extreme events.

“Ecological memory – defined as the ability of the past to influence the present trajectory of ecosystems – is also critically important for understanding how species assemblages are responding to rapid changes in disturbance regimes due to anthropogenic climate change.

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“During unprecedented back-to-back mass bleaching of corals along the 2,300 km length of the Great Barrier Reef in 2016, and again in 2017 ... the impacts of the second severe heatwave, and its geographic footprint, were contingent on the first.”

Hughes said urgent action to curb greenhouse emissions was needed to save the world’s coral reefs.

The 2017 bleaching event most severely affected corals in the central section of the reef. The southern section of the reef was cooler in 2016 and 2017 and was not severely bleached in either year.

One of the report’s co-authors, Andrew Hoey, said it was only a matter of time before another mass bleaching event occurred. “One of the worst possible scenarios is we’ll see these southern corals succumb to bleaching in the near future,” Hoey said.