Australia’s Great Barrier Reef is experiencing one of its worst bleaching events since monitoring began nearly four decades ago, authorities say, with much of the famed reef showing signs of damage as warming ocean temperatures blight reefs worldwide.

Bleaching occurs when heat-stressed coral turn white after expelling symbiotic algae that provide food and color. It’s a result of abnormal ocean temperatures in the past year that scientists worry could represent a major change to Earth systems.

In the Great Barrier Reef marine park, 73 percent of the reefs surveyed have prevalent bleaching — which means that more than 10 percent of the coral cover is bleached, the Great Barrier Reef Marine Park Authority, which manages the area, said Wednesday. Very high and extreme bleaching was observed across nearly 40 percent of the reef system.

“Climate change is the greatest threat to the Great Barrier Reef, and coral reefs globally,” said Roger Beeden, the authority’s chief scientist. “The Great Barrier Reef is an incredible ecosystem, and while it has shown its resilience time and time again, this summer has been particularly challenging.”

The dire update on Earth’s largest reef system comes just days after scientists with the U.S.-based National Oceanic and Atmospheric Administration and the International Coral Reef Initiative said anomalous ocean temperatures are afflicting reefs worldwide.

According to NOAA scientists, the world is experiencing its fourth global bleaching event, and the second in the last decade. At least 53 countries and local regions have experienced mass bleaching across the Atlantic, Pacific and Indian oceans, they said.

Ocean temperatures began building in the Great Barrier Reef area in late December and continued to rise throughout the Southern Hemisphere summer, causing “the highest levels of thermal stress on record,” the marine park authority said.

The most intense and prolonged heat stress occurred at inshore reefs in the southern part of the marine park, with sea surface temperatures peaking at 2.5 degrees Celsius above average.

This is the Great Barrier Reef’s fifth major bleaching event in nine years. Scientists say it could be the biggest test yet of the 1,400-mile-long world wonder’s ability to recover. For the first time, extreme bleaching — where more than 90 percent of coral cover on a reef is bleached — was observed in all three regions of the marine park.

“Southernmost parts of the reef, which had been largely spared previously, have been hit particularly hard this time, with bleaching affecting many more species, extending to greater depths, and affecting some of the oldest and most resilient corals,” said Simon Bradshaw, a research director with the nonprofit Climate Council. “This is a disaster at our doorstep.”

The giant reef system — which is so large it can easily be spotted from space — has bounced back from disturbances in the past, including underwater heat waves in 2016 and 2017 that triggered coral bleaching events so severe that scientists worried the reef would never look the same again.

Scientists say the recovery from those events was driven by fast-growing Acropora corals, which are more vulnerable to thermal stress and coral bleaching.

“The Great Barrier Reef has seen increases in coral cover to high levels in recent years, indicating it is still a resilient system. But this resilience has its limits,” said David Wachenfeld, research program director at the Australian Institute of Marine Science, a government agency.

Wachenfeld said the level of heat stress, and the results of the aerial surveys, indicate that the latest bleaching event is “one of the most extensive” the reef has experienced since the agency began monitoring the reef nearly 40 years ago.

Research divers had observed coral fatalities in every region of the reef, he said.



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