The Ocean Agency/XL Catlin Seaview Survey.

Extreme water temperatures that led to a massive coral bleaching event in 2016 effectively "cooked" large parts of Australia's Great Barrier Reef and damaged it forever.

That's the verdict of Australian researchers who have tracked the health of the reef over time to measure the extent of coral bleaching and death on the world's largest coral reef systems.

The researchers from Australia's James Cook University released their findings in the scientific journal Nature on Wednesday, revealing the mass bleaching event in 2016 which gained worldwide attention transformed one third of reefs in the GBR system.

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Coral bleaching occurs when corals expel the tiny algae (called zooxanthellae) that live inside them and provide their food. This algae -- which also gives coral its beautiful colour -- is put under stress by rising ocean temperatures. In the case of the 2016 event, the water temperatures rose enough for huge stretches of the reef to expel this algae, turning the coral stony white, starving it of food and ultimately killing it.

The James Cook University team conducted extensive aerial surveys of the reef, as well even more detailed underwater surveys, shortly after the mass bleaching event in March and April 2016, and again eight months later.

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The team found that the northern third of the reef was hit hardest, with many corals dying immediately from "heat stress," while corals with more complex branching structures were also more susceptible to bleaching. The bleaching not only harmed the corals themselves, but also impacted the broader reef ecosystem by robbing fish and other animals of their habitat.

The worst hit coral species in the worst-affected parts of the reef had mortality rates of 90 percent or more, according to JCU lead researcher Professor Terry Hughes. Hughes also said a quarter of corals died in two to three weeks during the 2016 bleaching event.

"They didn't die slowly of starvation, they died directly of heat stress," Hughes told the ABC. "They cooked because the temperatures were so extreme."

Researchers across Australia and the world are attempting to save the Great Barrier Reef and other coral reefs like it by developing heat-resistant coral, creating 3D simulations to track erosion and showing the world (in VR no less) what we stand to lose.

"If we fail to curb climate change, and global temperatures rise far above 2 degrees Celsius [above preindustrial levels], we will lose the benefits they provide to hundreds of millions of people," Hughes said.

"The Great Barrier is shifting radically, a trend that will continue for the next century or more."

Rebooting the Reef: CNET dives deep into how tech can help save Australia's Great Barrier Reef.

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