Coral bleaching on the Great Barrier Reef is 'a wake up call'

Coral reefs around the world are experiencing global bleaching for the fourth time - a result of warming ocean waters - driven in part, by climate change. The event is expected to be the most extensive on record. For Australia's Great Barrier Reef, it's the fifth mass bleaching event in eight years.

A fish swims near coral showing signs of bleaching. Credit: AP

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