Last year was a tumultuous time for Great Barrier Reef, which suffered through the worst coral bleaching event on record. Now one year on, signs are beginning to appear that 2017 could also bring bad news for the world's largest reef system. As scientists dive in to survey the damage, they've also published a study that emphasizes the importance of curbing global warming to avoid such events becoming the norm.

The Great Barrier Reef has experienced three major bleaching events in its history. Coral bleaching occurs as a result of abnormal sea conditions, such as warmer or colder temperatures. This leads to stress on the algae living inside the coral, which is then expelled from the tissue and leaves the coral white and in danger of dying.

These events took place following heatwaves in 1998 and 2002, but the bleaching event of 2016 was particularly severe, affecting around 93 percent of the reef. There are now fears that back-to-back events could be playing out, with coral bleaching reappearing and researchers from 10 Australian institutions gearing up for a closer look. They will spend the next two to three weeks carrying out underwater and aerial surveys to that effect.

"We're hoping that the next two to three weeks will cool off quickly, and this year's bleaching won't be anything like last year. The severity of the 2016 bleaching was off the chart," says taskforce convener, Professor Terry Hughes. "It was the third major bleaching to affect the Great Barrier Reef, following earlier heatwaves in 1998 and 2002. Now we're gearing up to study a potential number four."

With their research on bleaching in the Great Barrier Reef ongoing, the scientists now have a better handle on the various factors at play. One of these is whether the past bleaching events in 1998 and 2002 made the reef more resilient to bleaching today.

"Sadly, we found no evidence that past bleaching makes the corals any tougher," says Hughes.

The team also looked at the impacts of improved water quality and protecting the reefs from fishing. It says while these will probably help reefs recover in the long term, it made zero difference to the severity of the bleaching in 2016. It warns that these mass bleaching events shouldn't be considered one-off disturbances, but a recurring risk that poses a threat to coral reefs around the world.

'Global warming is the number one threat to the reef," says co-author Dr David Wachenfeld from the Great Barrier Reef Marine Park Authority. "The bleaching in 2016 strongly reinforces the urgent need to limit climate change as agreed by world leaders in the Paris Agreement, and fully implement the Reef 2050 Plan to boost the Reef's resilience."

The research was published in the journal Nature.

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