'Wake-up call to humanity': Research shows the Great Barrier Reef is the hottest it's been in 400 years

The Great Barrier Reef has suffered five mass bleaching events in the past nine summers .

Scientists say recent sea surface heat is unprecedented compared to the past 400 years .

They say global warming will continue to damage the reef.

Back-to-back bleaching is likely to be catastrophic for the reef, they say.

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