Corals in the northern sections of the Great Barrier Reef are decomposing at an alarming rate due to mass bleaching. Powerful images released by the Ocean Agency show once-pristine and colourful corals in Lizard Island rotting away, covered by brown slime and seaweed.

"It was a deeply disturbing sight," says Richard Vevers, chief executive of the not-for-profit Ocean Agency. "The white hard corals had turned brown – they were dead and covered in algae. They looked like they'd been dead for years. The soft corals were even more shocking. They were in a state of decomposition and were literally dripping off the rocks.

"It was followed by a deeply disturbing smell as soon as we got out of the water. We smelt of the rotting flesh of animals. It was an experience we will never forget."

The devastating photos bring to focus the need to urgently tackle climate change, Vevers adds.

Great Barrier Reef
Starfish at the centre of a field of devastated corals at Lizard Island – the ground zero of the mass bleaching event at the Great Barrier ReefXL Catlin Seaview Survey
Great Barrier Reef
Dead coral pictured after the mass bleaching eventXL Catlin Seaview Survey
Great Barrier Reef
The same coral photographed five weeks before during the height of bleaching – scientists have been left shocked by the rapid rate of deteriorationXL Catlin Seaview Survey
Great Barrier Reef
Soft coral decomposing and falling off a reef. Coral bleaching is being exacerbated by man-made climate change, as oceans absorb 93% of global warming heatXL Catlin Seaview Survey
Great Barrier Reef
Like corals, giant clams live in partnership with algae called zooxanthellae. When the water stays warm for too long the algae is expelled, starving the clams of nutritionXL Catlin Seaview Survey
Great Barrier Reef
A ray gliding over a field of dead corals – one-fourth of the world's marine species rely on reefs for their survivalXL Catlin Seaview Survey
Great Barrier Reef
Dead coral covered by algae at Lizard Island. Scientists say urgent action is needed against climate change to protect the Great Barrier ReefXL Catlin Seaview Survey
Great Barrier Reef
Before and after pictures of bleached – and later dead – coral, taken between March and May 2016XL Catlin Seaview Survey
Great Barrier Reef
The Great Barrier Reef is the world's single largest 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 stress continues, they eventually dieXL Catlin Seaview Survey