Australia's Great Barrier Reef is being devastated by another mass bleaching event, officials have confirmed.
It is the fourth time in six years that such severe and widespread damage - caused by warm sea temperatures - has been detected.
Only two mass bleaching events had ever been recorded until 2016.
Scientists say urgent action on climate change is needed if the world's largest reef system is to survive.
There are particular concerns that this bleaching event has occurred in the same year as a La Niña weather phenomenon. Typically in Australia, a La Niña brings cooler temperatures.
Scientists are now fearful of the damage that could be caused by the next El Niño.
The declaration was made by the Great Barrier Reef Marine Park Authority which has been conducting aerial surveys.
Recently it warned that water temperatures in parts of the reef had been up to 4C above the March average.
Stretching over 2,300km (1,400 miles) off Australia's north-east coast, the Great Barrier Reef is one of the most biodiverse ecosystems in the world.
Bleaching occurs when under-stress corals expel the algae living within them that gives them colour and life. They can recover but only if conditions allow it.
