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Earlier this year, scientists warned that the Great Barrier Reef could be on the brink of its most widespread bleaching event ever recorded. That fear has been realized.

Surveys conducted by scientists at Australia's James Cook University and the Great Barrier Reef Marine Park Authority show that a summer of extreme heat has caused the reef, which is a World Heritage Site, to suffer a mass bleaching of unprecedented scale. Corals from the far north to the southern tip of the 1,400-mile ecosystem are experiencing severe impacts.

It was also one of the reef's worst mass bleaching episodes in terms of intensity, second only to 2016, which killed half of all shallow-water corals on the northern Great Barrier Reef.

Unlike the summer of 2016, when an intense marine heat wave coincided with one of the strongest El Nino events on record, this past summer brought a bleaching event without any assistance from the Pacific climate oscillation.

El Nino can elevate ocean temperatures in that part of the world, making bleaching events more likely. To scientists, this is another clear sign that human-caused climate change is the primary driver behind these devastating events.

Mark Eakin, coordinator for the National Oceanic and Atmospheric Administration's Coral Reef Watch program, described the rate of recurrence of these events as "truly disturbing." Bleaching from the 2016 event was followed by a recurrence in 2017, when there was also an absence of an El Nino.

"In 2016 and 2017, the Great Barrier Reef had their first back-to-back bleaching events. Now we have the third bleaching event in five years," Eakin said.

"That is unprecedented on the Great Barrier Reef."

Bleaching is a response to heat stress that occurs when corals spend too much time in water that's too hot for them to handle. Exposure to prolonged heat causes the reef-building animals to temporarily evict their zooxanthellae, symbiotic algae in which the corals shelter in exchange for food.

Because these algae also give corals their vibrant colors, mild bleaching causes corals to grow pale. Severely bleached corals turn bone white, and if their algal partners stay away for too long, they can starve to death.

As heat built across the reef in February, the Great Barrier Reef Marine Park Authority began reporting pockets of bleaching in the far north toward the end of the month. By early March, vast swaths of the ecosystem had accumulated eight or more "degree heating weeks," a metric that scientists use to describe recent cumulative heat exposure.

At this threshold, reef scientists expect to see widespread bleaching and mortality from thermal stress, according to NOAA.

Researchers decided to conduct aerial and waterborne surveys to assess the extent of the damage. The surveys, which took place during the last two weeks of March, quickly confirmed the reef has undergone its third mass bleaching event in the past five years.

Now, more details about the extent and severity of the event are emerging. A new map produced by Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, confirms what scientists with NOAA and Australia's Bureau of Meteorology predicted: This year's bleaching was more widespread compared with 2016, which hammered the reef's northern third, and 2017, which struck the reef's midsection hardest.

This year, some 35 percent of the 1,036 reefs the scientists surveyed experienced moderate bleaching, while a quarter were severely bleached. Scientists saw severe bleaching on coastal reefs from Torres Strait in the far north to the southern border of the Great Barrier Reef Marine Park, at levels only eclipsed during 2016.

"For the first time, the south was hot as well as the middle and the north," Hughes said. "After 2016-17, when the north and middle went, I said to somebody our worst nightmare is if the next region to bleach is the south."

A Section on 04/07/2020

Print Headline: Heat hits Great Barrier Reef

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