AUSTRALIA – Almost 93 per cent of reefs on the Great Barrier Reef have been hit by coral bleaching, according to a comprehensive survey revealing the full extent of the devastation caused by abnormally warm ocean temperatures sweeping the globe.

There have only been three mass bleaching events recorded on the reef , and all of them have happened since 1998. Scientists say this episode is the worst they’ve ever seen.

The world is in the midst of a global bleaching event, which is a result of a pulse of warm water flowing around the Pacific Ocean caused by El Niño, and the background global warming caused by man-made greenhouse gas emissions.

Corals bleach when they experience temperatures above their normal summer maximum for a month or two.

The situation is made worse if there are few clouds, and a high level of UV radiation blasts the coral.

The Great Barrier Reef aerial survey began last month, and has now covered 911 individual reefs along the 2300km structure from helicopters and planes.

Coral bleaching on the Great Barrier Reef shows up as white and yellow patches visible from aerial surveys.

Professor Terry Hughes, from James Cook University and head of the National Coral Bleaching taskforce, said it appeared that only 68 of those reefs had escaped bleaching entirely.

Guardian Australia reported last week, more than half the surveyed reefs have been severely bleached, with as many as 81 per cent of reefs north of Port Douglas experiencing severe bleaching.

If coral remains bleached for an extended period, it is likely to die.

Hughes said that the northern region is already seeing mortality as high as 50 per cent, and he expected that at some reefs it will exceed 90 per cent.

In the central section between Port Douglas and Mackay, a third of the reefs are severely bleached and 57 per cent have seen moderate or minor bleaching.

South of Mackay, one per cent of reefs are severely bleached, but even there, only a quarter of the reefs have escaped bleaching entirely.

Hughes told Guardian Australia that in the previous two mass bleaching events – 1998 and 2002 – 40 per cent of the reefs escaped bleaching.

“By that metric, this event is five times stronger.” And in those two years, only 18 per cent of reefs were severely bleached. “This time it’s 55 per cent.”

“We’ve never seen anything like this scale of bleaching before. In the northern Great Barrier Reef, it’s like 10 cyclones have come ashore all at once,” said Hughes.

“Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover.”

Hughes says the southern part of the reef was partially saved by weather conditions that brought clouds and cooler weather.

“This time, the southern third of the Great Barrier Reef was fortunately cooled down late in summer by a period of cloudy weather caused by ex-cyclone Winston, after it passed over Fiji and came to us as a rain depression.

“The 2016 footprint could have been much worse.”

The aerial work is being confirmed by teams of divers who are examining the coral up-close.

Hughes said there have been 100 people underwater for the past month, and so far they’ve collected data from about 150 reefs.

- Guardian Australia