Bleaching risk on the rise for Great Barrier Reef corals

Global warming is likely to overwhelm corals'
Global warming is likely to overwhelm corals’ built-in thermal tolerance mechanisms within the next few decades, leading to more bleaching and mortality. Photo courtesy Dr. Peter Mumby.

Study identifies bleaching and mortality thresholds for imperiled coral reefs

Staff Report

The steady rise in ocean temperatures projected for the next few decades will put more and more corals at risk of bleaching, as the warm water simply overwhelms their thermal tolerance mechanisms.

Recent research along the Great Barrier Reef shows that corals have been able to survive past bleaching events because they were acclimated to warmer temperatures by being exposed to a pattern of gradually warming waters in the lead up to each episode. But global warming is likely to change that, the scientists said.

Before long, temperature increases of as little as 0.5 degrees Celsius may push many corals over the edge as the warm water causes them to expel the algae-like dinoflagellates that help keep them alive and give them their color.

Lead author Dr. Tracy Ainsworth from Coral CoE said  bleaching is like a marathon for corals.

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

The findings, published in Science, are based on a study of 27 years of  satellite-based sea surface temperature records of the Great Barrier Reef. The scientists identified 372 thermal stress events capable of causing bleaching, then established baseline and bleaching temperature thresholds to measure how future warming will play out along the reef.

They found that, when sea surface temperatures rose to right below the threshold before decreasing again, it helped acclimate the coral, and reduced bleaching and coral cell death. But corals exposed to increases above the threshold with no pre-conditioning or recovery period were more susceptible to bleaching. The study also identified a genetic profile that was associated with lower levels of localized cell death, hinting at an underlying thermal tolerance mechanism.

“When corals lose the practice run, there is no break, or ‘relaxing’ for the corals as summer stress develops,” said co-author Dr. Scott Heron, from Coral Reef Watch at NOAA. “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.”

The study projected future coral bleaching and mortality based on historical temperatures, predicting that most of the corals that have only experienced the protective scenario to date will begin to experience single and repetitive bleaching events when sea surface temperatures are approximately 0.5 degrees Celsius higher than present — which is expected to occur within four decades based on historical warming rates.

Currently, about 75 percent of the Great Barrier Reef’s corals benefit from the protective scenario, but when sea surface temperatures increase by 2 degrees Celsius — by 2100, under current global warming scenarios — the proportion of corals benefitting from the protective scenario falls by roughly two-thirds, to only 22 percent, implying that future thermal stress events will become far more lethal for corals.

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