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bleached coral

A view of major bleaching on the coral reefs of the Society Islands on May 9, 2019 in Moorea, French Polynesia, where major bleaching is occurring. (Photo: Alexis Rosenfeld/Getty Images).

Example of 'Unknown Unknowns,' Study Detailing 'Almost Instant Mortality of Corals' Suggests Crisis Worse Than Previously Understood

"The water temperatures are so warm that the coral animal doesn't bleach... the animal dies and its underlying skeleton is all that remains."

Jessica Corbett

As the human-caused climate crisis drives up ocean temperatures at a rate that has scientists worried, a study published Thursday in the journal Current Biology reveals that warming waters are an even bigger threat to coral reefs than experts previously realized.

"Climate scientists talk about 'unknown unknowns'—impacts that we haven't anticipated from existing knowledge and experience. This discovery fits into this category."
—Scott Heron, study co-author

Past research has raised alarm about how ocean pollution and rising temperatures cause coral bleaching—which is when coral expels algae, its main food source, and turns white. Although more susceptible to disease and death, bleached coral can recover if temperatures fall, so some scientists have been hopeful that urgent climate action could revive impacted reefs.

However, the new study—conducted by researchers at the University of New South Wales Sydney, the University of Newcastle, the University of Technology Sydney, James Cook University, and the National Oceanographic and Atmospheric Administration (NOAA)—heightens concerns about the future of coral reefs in a warming world.

According to the study:

Severe marine heatwaves have recently become a common feature of global ocean conditions due to a rapidly changing climate. These increasingly severe thermal conditions are causing an unprecedented increase in the frequency and severity of mortality events in marine ecosystems, including on coral reefs... [M]arine heatwave events on coral reefs are biologically distinct to how coral bleaching has been understood to date.

"Until now, we have described coral bleaching as an event where the symbiotic relationship between coral and its microbes breaks down and corals lose their main source of nutrition, and the coral can die if the symbiosis is not restored," co-author Tracy Ainsworth, an associate professor at Australia's University of New South Wales Sydney, explained in a statement.

"But what we are now seeing is that severe marine heatwave events can have a far more severe impact than coral bleaching," Ainsworth continued. "The water temperatures are so warm that the coral animal doesn't bleach—in terms of a loss of its symbiosis—the animal dies and its underlying skeleton is all that remains."

"We find that the skeleton is immediately overgrown by rapid growth of algae and bacteria," said co-author Bill Leggat, an associate professor at the U.K.'s University of Newcastle. By CT scanning the coral skeleton, Leggat said, the team found that "this process is devastating not just for the animal tissue, but also for the skeleton that is left behind, which is rapidly eroded and weakened."

Laura Richardson at the U.K.-based Bangor University's School of Ocean Sciences—who was not involved in the study—told BBC News that the team's significant discovery was "the rapidity with which the reef skeleton breaks down when you have these severe heatwaves."

"Given that the degradation of coral reefs will result in the collapse of ecosystem services that sustain over half a billion people, we urgently need actions both globally and locally that protect and conserve these truly wonderful places."
—Tracy Ainsworth, study co-author

They are the first researchers, as Richardson noted, to document that such events are causing "almost instant mortality of corals."

"Climate scientists talk about 'unknown unknowns'—impacts that we haven't anticipated from existing knowledge and experience," said study co-author Scott Heron of Australia's James Cook University. "This discovery fits into this category."

"As we begin now to understand this impact," Heron added, "the question is how many more of these 'unknown unknowns' might there still be that could bring faster and greater damage to coral reefs from climate change."

Though the study generated alarm, the researchers expressed hope that it will spur public outcry for policymakers to pursue bolder efforts to combat the climate crisis—and, specifically, protect coral reefs, particularly considering the anticipated consequences of inaction.

PBS News Hour reported that "without the option to recover, the world may start seeing corals die off faster than expected. And the death of corals would come with a steep cost for humans: flood protection that's worth tens of millions in the U.S. alone, plus an estimated value of almost $30 billion each year globally in tourism, fishing, and other benefits."

"Across the globe coral reefs are still a source of inspiration and awe of the natural world, as well as being critically important to the communities that rely upon them," said Ainsworth. "Given that the degradation of coral reefs will result in the collapse of ecosystem services that sustain over half a billion people, we urgently need actions both globally and locally that protect and conserve these truly wonderful places."


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