Of course it's natural Jim Cross. The story reported as such. When the water in which the coral lives heats up it bleaches or cooks the coral.
What likely isn't natural, rather it's manmade, is that higher levels of carbon dioxide in the atmosphere are heating it by way of a greenhouse effect and the surface of the sea, leading to the coral die off.
For the moment, human activity can impact the level of carbon dioxide in the atmosphere and so global warming and in turn sea temperatures and coral bleaching.
I wish more Australians cared for the health of the Great Barrier Reef - one of Australia's most significant natural assets. Mores the pity they don't.
