The Great Barrier Reef is in the throes of its worst-ever coral bleaching event, scientists say. Only 7% of the 2,300km-long reef system has been left untouched by bleaching, with researchers expressing alarm over the health of one of the world's greatest natural wonders. Climate change and an unusually strong El Nino have been blamed for the latest mass bleaching event, which has resulted in vast swathes of coral reefs turning white. Bleached corals can die if they are under prolonged stress.

Great Barrier Reef
Coral bleaching at Lizard Island on the Great Barrier ReefXL Catlin Seaview Survey
Great Barrier Reef
Bleached corals in New Caledonia. Bleaching occurs when corals expel algae called zooxanthellaeXL Catlin Seaview Survey
Great Barrier Reef
El Nino and climate change have been blamed for the latest bleaching eventXL Catlin Seaview Survey
Great Barrier Reef
Bleaching drains the colour from corals before turning them completely whiteXL Catlin Seaview Survey
Great Barrier Reef
Coral bleaching has been detected as far south as Sydney HarbourXL Catlin Seaview Survey
Great Barrier Reef
Aerial shot of coral bleaching in New CaledoniaXL Catlin Seaview Survey
Great Barrier Reef
The Great Barrier Reef is the world's single biggest structure made by living organisms and can be seen from spaceXL Catlin Seaview Survey
Great Barrier Reef
Corals are known to recover from bleaching but if the algae loss is prolonged they eventually dieXL Catlin Seaview Survey
Great Barrier Reef
The Great Barrier Reef supports many vulnerable and endangered speciesXL Catlin Seaview Survey
Great Barrier Reef
Reef tourism brings A$5bn (£2.7bn) of revenue annually to AustraliaXL Catlin Seaview Survey