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  • Bleached coral is photographed on Australia's Great Barrier Reef near Port Douglas, February 20, 2017 in this handout image from Greenpeace.

    Bleached coral is photographed on Australia's Great Barrier Reef near Port Douglas, February 20, 2017 in this handout image from Greenpeace. | Photo: Reuters/Greenpeace

Published 25 December 2019

The mass bleaching of corals occurs when sea temperatures rise from 1-2 degrees Celsius above normal for an extended period of weeks, accompanied by strong sunlight.

The phenomenon of bleaching and the impact of sea warming affects even corals found in deep water, according to a study published Tuesday in Australia.

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“Our analysis of 1,453 heating events found that most of the time there was no escape from the heat down to 38 metres depth,” said Mark Eakin, co-author and coordinator of the U.S. National Oceanic and Atmospheric Administration’s Coral Reef Watch, in a statement from James Cook University (JCU).

The investigation measured the subsurface temperatures at 457 coral reefs among 49 islands in the central and western Pacific Ocean between 2001-2017, and compared the data with that of satellites used to read sea temperatures.

“Scientists primarily use satellite-derived sea surface temperatures to understand heat stress and predict coral bleaching,” said Scott Heron, JCU associate professor.

“But because the measurement is only at the very surface of the ocean, there (was) some uncertainty about how well it reflects what is actually happening on deeper reefs,” he added.

The onsite monitoring determined that the satellite readings underestimated heat stress across the Pacific by around 40 percent.

Sea temperatures decrease as depth increases, but corals accustomed to colder conditions away from the surface are also affected when these cooler conditions are above the usual range.

“Heat stress isn’t so much about the temperature itself, but how different today’s temperature is from what the corals are used to experiencing,” Eakin said.

The scientists stressed that urgent action is needed from national and international governments against climate change.

“What is critical now is that, as a global community, we need to act upon the clear science of climate change to reduce emissions of greenhouse gases,” Heron said.

“We urgently need international leadership, without evasion, to contain future impacts as only ‘bad,’ and not ‘worse,’” he added.

In 2016, mass bleaching occurred on the Great Barrier Reef, which surveys found had killed at least 29 percent of the reef’s shallow water corals, and in some parts an estimated 70 percent of shallow water corals died.

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