Overall, severe bleaching of between 60 and 100 per cent of coral was recorded on 316 reefs, almost all of them in the northern half of the barrier reef. Reefs in central and southern regions of the 2300 kilometre Great Barrier Reef have experienced more moderate to mild affects. A diver checks out the bleaching on the Great Barrier Reef. Credit:XL Catlin Seaview Survey The mass bleaching event has been driven by significantly higher than average sea temperatures as a result of the current El Nino event, coupled with a long-term warming of the oceans due to climate change. While the barrier reef has experienced mass coral bleaching events in the past – notably in 1998 and 2002 – Professor Terry Hughes, convenor of the bleaching taskforce, said the current event was by far the biggest.

"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," Professor Hughes said. "Towards the southern end, most of the reefs have minor to moderate bleaching and should soon recover." Professor Hughes said the aerial work had been backed up by in-water surveys, which are still under way, of about 150 to 200 reefs. The presence of bleaching does not necessarily mean that coral will die, as they can recover when waters return to cooler temperatures. Just how much of the bleached coral across the barrier reef will ultimately die off will take months to be known. Once dead it can take a decade or more for many species of coral to return.

Coral bleaching around Heron Island on the Great Barrier Reef. Photo: Eddie Jim. Professor Andrew Baird, from the Centre of Excellence for Coral Reef Studies, said north of Port Douglas, "we're already measuring close to 50 per cent mortality of bleached corals". "At some reefs, the final toll is likely to exceed 90 per cent." But in the southern regions corals have largely escaped damaging levels of bleaching due to cooler sea temperatures, Professor Hughes said, and it is expected most will survive and regain their normal colour in coming months. The damage on the barrier reef is part of a global mass bleaching event that has hit corals hard in many places including Hawai, Fiji and New Caledonia. It is only the third global event in recorded history, with the other two occurring in 1998 and 2010.

It is not just the Great Barrier Reef being hit in Australia. Coral bleaching is now also rolling out across western reefs. Dr Verena Schoepf​, from the University of Western Australia, said the coastal area she was studying north of Broome was seeing up to 80 per cent of corals turning "snow white". Coral bleaching has also been triggered as far south as Sydney Harbour, the first time in recorded history that has occurred. Scientists and the conservation movement say the bleaching is stark evidence of the impact climate change is having on the Great Barrier Reef, which attracts approximately $5 billion in tourism each year. Environment groups in particular have sought to link the bleaching event to the recent mining approvals by the Queensland government for the proposed Carmichael coal mine, which would be Australia's largest, saying the eventual burning of the mined coal would cause further damage to the barrier reef through its contribution to global warming.

In the taskforce's statement on its final survey results, released on Wednesday, the head of the Queensland Tourism Industry Council, Daniel Gschwind​, was also quoted as saying: "thankfully, many parts of the reef are still in excellent shape, but we can't just ignore coral bleaching and hope for a swift recovery." "Short-term development policies have to be weighed up against long-term environmental damage, including impacts on the reef from climate change," he added.