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Great Barrier Reef has most coral in decades. Global warming could reverse it.

Schools of fish in the waters of the Great Barrier Reef in 2017. (J. Sumerling/AP)
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Marine scientists have found that parts of Australia’s Great Barrier Reef have recorded their highest levels of coral cover since monitoring began nearly four decades ago, although they warn the reef’s recovery could be swiftly undone by global warming.

The Australian Institute of Marine Science, a government agency, began monitoring Earth’s largest reef system 36 years ago. Its latest report indicates that the northern and central parts of the reef are on the mend after an “extensive bout” of disturbances over the past decade, said Mike Emslie, a senior research scientist at the institute.

The results of the institute’s annual survey show that the reef “is still vibrant and still resilient, and it can bounce back from disturbances if it gets the chance,” Emslie said in an interview Thursday.

The Great Barrier Reef has been hit hard by rising temperatures in recent years. In 2016 and 2017, underwater heat waves triggered coral bleaching events so severe that scientists worried the reef would never look the same again.

UNESCO, the United Nations’ educational, scientific and cultural agency, threatened last year to add the Great Barrier Reef to a list of world heritage sites that are “in danger.” A June meeting to discuss the status of the reef was canceled after Russia’s invasion of Ukraine.

Australia’s Great Barrier Reef should be listed as ‘in danger,’ U.N. body says

News of the recovery in the reef’s northern and central parts was partly offset by the report’s finding that there was a loss of coral cover in the southern region. There, the reef fell prey to an outbreak of crown-of-thorns starfish, which feed exclusively on live coral, the scientists said.

About half of the reefs were surveyed before the most recent coral bleaching event in February and March. Emslie said researchers won’t know the full extent of the coral cover lost from that event until next year. The sheer size of the Great Barrier Reef system — it spans some 1,700 miles and is so large it can easily be spotted from space — means the survey is staggered over seven or eight months of the year.

Among the 87 reefs surveyed for the latest report, average hard coral cover in the north increased to 36 percent, up from 27 percent in 2021, and to 33 percent in the central Great Barrier Reef from 26 percent last year. Average coral cover in the southern region decreased from 38 percent in 2021 to 34 percent this year.

Much of the recent reef recovery was driven by the fast-growing Acropora species — whose delicate branching and table corals have adorned countless postcards for tourists.

Marine scientists worry that these corals are some of the most vulnerable to the impacts of global warming, including marine heat waves, coral bleaching and damaging waves, such as those generated during tropical cyclones.

“They’re susceptible to thermal stress and mass coral bleaching. They’re a preferred prey of crown-of-thorns starfish. And, they are easily toppled over and broken up by large storms,” said Emslie, the research scientist. He noted that Tropical Cyclone Yasi — the worst storm to hit the area in recent years — destroyed vast swaths of coral in a matter of hours in 2011.

Zoe Richards, a senior research fellow who leads the Coral Conservation and Research Group at Curtin University, said the report’s findings were “good news because the corals provide habitat for thousands of other plants and animals.”

However, she noted that the recovery was driven by a species that often grows in a “boom-and-bust pattern” and is vulnerable to thermal stress, as the report’s authors also detailed.

Some models on the effects of global warming predict there will be more severe cyclones as the planet heats up, along with more frequent and severe marine heat waves.

“Even though it’s promising this year, we’re probably only one bad summer away from a reversal of what we’ve seen, unfortunately,” Emslie said.