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 Niño events on record, this past summer brought a bleaching event without any assistance from the Pacific climate oscillation.

El Niño events 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 Niño.

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“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 wrote in an email.

“That is unprecedented on the Great Barrier Reef.”

Heat stress can be deadly to corals

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 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.

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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 scientists use to describe recent cumulative heat exposure.

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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.

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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.

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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.”

That’s because the south, having escaped the previous two events, is relatively unaccustomed to bleaching and contains large numbers of heat-sensitive Acropora corals, including branching and table-shaped species that give the reef its three-dimensional structure and provide habitat for fish.

In the northern and central Great Barrier Reef, these corals were largely annihilated by bleaching in 2016-17, transforming vast swaths of the reef into a “highly altered, degraded system,” according to a 2018 paper in the journal Nature.

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Now the south seems poised to slide into a similar ecological disrepair. Hughes cautioned that bleaching doesn’t necessarily lead to mortality and said he would be conducting repeat surveys in about eight months to see which corals survived and which ones didn’t.

“I have to admit, I’m devastated to learn that the southern reefs are taking such a hit right now, as they were a rare bright spot during the 2016 mass bleaching event,” said Kim Cobb, a coral reef and climate scientist at the Georgia Institute of Technology who was not involved in the new survey.

Hughes is expecting less mortality in the north this year, because many of the heat-sensitive corals have already been killed. But reefs that bleached this year in addition to three or four years ago are likely to be set back in terms of recovery, he said.

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“Underwater and even from the plane, we could see very many small corals that have recruited to the reef since the previous bleaching … so that recovery, which was in its early phases, has been interrupted by this new bleaching event,” Hughes said.

Getting closer to yearly bleaching events

Mass bleaching events have often been associated with El Niño, a recurring climate pattern characterized by above-average sea surface temperatures in the eastern equatorial Pacific, which leads to shifts in ocean heat distribution, atmospheric circulation, and weather patterns around the world.

Across the Great Barrier Reef, changes in local weather patterns related to El Niño, including higher than average air and ocean temperatures, clear skies and lots of sunshine, can help fuel bleaching.

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But while the first recorded mass bleaching event on the Great Barrier Reef in 1998 and the most intense mass bleaching event on record coincided with El Niño, mass bleachings in 2002, 2017 and now 2020, did not.

This, along with the fact that the gap between severe bleaching events is shrinking, suggests that as summers grow warmer due to climate change, the reef will suffer heat stress more regularly regardless of whether the tropical Pacific is in a favorable state. It is telling, Hughes said, that February 2020 brought the highest monthly sea surface temperatures ever recorded across the Great Barrier Reef, with no El Niño to assist.

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“It’s now clear that we can have major bleaching events caused by global climate change alone with no tropical forcing,” Eakin said, adding that we may be seeing “early signs” of a world where the reef bleaches on a near-yearly basis.