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14/10/2020 9:14 AM IST

Half Of Corals On The Great Barrier Reef Have Died Since 1990s, Study Finds

“What we need is for the temperature to stop rising," a scientist said, noting climate change remains the most urgent danger to the delicate ecosystems.

SYDNEY — Half of the corals on Australia’s iconic Great Barrier Reef have died since the 1990s, according to a troubling new study that analyzed just how devastating years of catastrophic mass bleaching have been for one of the most biodiverse structures on Earth.

The paper, published Tuesday in the Proceedings of the Royal Society B, found dramatic losses of corals large and small, of nearly every major species, from branching colonies of pale yellow staghorn to massive protrusions that look like underwater tables.

The loss of so many corals along nearly every section of the 1,600-mile reef is shocking. Larger corals can take years or decades to grow, and effectively act as both the parents and home for a new generation of coral polyps that replace them. The researchers analyzed the number of corals along sections of 30 reefs up and down the Great Barrier 20 years apart, in 1996 and in 2016, and found populations of elder, adolescent and baby corals had all fallen by more than 50% in just 20 years.

Dr. Andreas Dietzel, a researcher at the ARC Centre of Excellence for Coral Reef Studies and the lead author of the paper, compared the loss of coral colonies across the reef system to an international conflict.

“We’ve seen this in human populations,” Dietzel told HuffPost. “After a big war, you can see a massive dent in the number of middle-aged men.”

With corals, he added, it’s worse: “They’ve declined pretty much across all sizes. We’re losing a lot of the big ones, but we’re also losing a lot of small ones.”

The Great Barrier Reef is still recovering from a series of devastating, back-to-back mass bleaching events in 2016 and 2017 when hotter-than-usual seawater washed over the delicate structures, effectively cooking them. The environmental change can cause ruby- and topaz-colored corals to turn ghostly white as the algae that feeds them flees their skeletons. If temperatures return to normal, the algae may return and the corals can recover, but if it stays too hot for too long, the corals will die.

“As a marine biologist, I know that a bleached coral is very unhealthy, but it can provide some very breathtaking colonies,” Dietzel said, saying some reefs he’s visited in recent years looked like ghost towns. “Tourists say, ‘Whoa, this looks beautiful.’ But as a scientist, you’re well aware they’re either starving or dead.”

“Some of these bleached colonies, once the external algae grow over the dead skeleton, that colony is gone,” he added. “It may take hundreds of years to replace a colony of that size.”

NOAA
Coral bleaching leaves the delicate structures sick and vulnerable.

The back-to-back events were declared a one-two punch to large swaths of the Great Barrier by scientists at the time, who likened the event to an underwater apocalypse while expressing hope that action on climate change could help the structures recover and grow.

But earlier this year, researchers said the reef was suffering from mass bleaching again, the third time in just five years. In February, average temperatures on the reef were the highest ever recorded since record-keeping began in 1900.

Dr. Terry Hughes, a professor at the ARC Centre and a co-author of the paper, has long said climate change remains the single greatest threat to the future of the Great Barrier as we know it.

“What we need is for the temperature to stop rising, and to even out at a level that’s not too hot,” he told HuffPost, expressing hope that the reef will survive, albeit as a different ecosystem. “We’re on this journey, we have a ways to go, and the endpoint will be determined by greenhouse gas emissions.”

Andreas Dietzel/ARC Centre for Excellence
Giant table corals and branching corals were dramatically impacted by three bouts of mass bleaching along the Great Barrier Reef. "They are important because they provide the three-dimensionality, the nooks and crannies that fish depend on," Dr. Terry Hughes said.

The study is one of the first long-term analyses of the Great Barrier Reef and its coral density. Traditionally, scientists have flown over hundreds of miles of the structure via helicopter and monitored the “coral cover,” or how much of the reef appeared either healthy or bleached from afar. This time, Dietzel and Hughes got in the water, which Hughes said allowed their team to see that the size of the many corals now resembles something out of “Honey, I Shrunk the Kids.”

“The loss of big corals right across the board, in terms of every species, is important because they’re the mature adults that make the next generation of juvenile corals — they’re the breeders,” he continued. “The capacity of the reef to bounce back is increasingly compromised.”

Hughes added that the idea the reef is under threat doesn’t convey just how urgently the world needs to address climate change if it hopes to save the Great Barrier Reef and other reef systems like it around the globe.

“The word ‘threat’ is funny,” he said. “If you threatened to punch me on the nose, it’s something you might do. We’ve been measuring the impacts on the Great Barrier Reef for 22 years … it’s certainly not a future threat. It’s been part of the ongoing saga for a long time.”

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