Scientists are now certain the Great Barrier Reef is in the grip of another mass bleaching event driven by climate change.
Flights over the length of the 2300km World Heritage-listed site have confirmed the worst - that there is bleaching in all parts of the marine park. Some of it is severe and coral mortality is evident from the air.
But it will be months until the full extent of mortality is known because it is possible for bleached corals to recover, although their immune and reproductive systems inevitably suffer.
A United Nations monitoring mission, which is in Australia to evaluate the federal government's efforts to protect the reef from climate change and other events, will be briefed on the findings on Friday afternoon.
Dr Neal Cantin from the Australian Institute of Marine Science led the aerial surveillance work, in conjunction with the Great Barrier Reef Marine Park Authority. He'll be among those to brief the mission.
"Yes, it's crystal clear now," he told AAP when asked if the reef is in mass bleaching territory.
"There are clear signs of bleaching through all four regions of the marine park and this is the fourth mass bleaching event to occur in the last seven summers. And it's the sixth to hit the Great Barrier Reef since 1998."
The stretch of the reef from the Whitsundays north to about Cooktown is the worst hit, with severe bleaching and coral deaths observed.
Further north and into the Torres Strait, bleaching is variable from minor to severe.
The southern part of the reef, where heat stress is lower, is less affected.
The bleaching event is the result of the reef's hottest December on record, followed by further heat stress in January and then a late summer heatwave in recent weeks.
While conditions have cooled in the past week or so, the Bureau of Meteorology is still forecasting above average temperatures.
Dr Cantin said the scale of bleaching in a La Nina year, when ocean temperatures are typically lower, is particularly concerning.
"The frequency of concerning heat stress is happening faster than we ever thought it would.
"The fact it's impacting such a large area of the marine park, during a La Nina year, is a clear sign of climate change-driven ocean warming. We expect these trends to continue and only accelerate in the future."
It's reasonable to expect that in the seven summers ahead, the reef could see another four bleaching events, Dr Cantin said.
Given it takes about 10 years for decent recovery of fast-growing corals, and longer for slow-growing species, the reef will not have enough time to recover between hits.
The UN mission is in Australia to speak with scientists, conservation groups, and government authorities. It will then report back to the World Heritage Committee, which may decide to list the reef as in danger.
The marine park authority's chief scientist David Wachenfeld said the mission's two experts have been shown photos of the latest bleaching and will be taken to bleached sites in the north next week.
"Keeping them fully informed of what's happening with the current situation on the reef is a critical part of what we are doing," he said.
"Every time we see a climate change-driven impact to the reef it is extremely concerning. We need to act as strongly as we possibly can to reduce global greenhouse gas emissions and protect the resilience of the reef ..."
Dr Wachenfeld urged people to travel to the reef and connect with it so they would feel energised to be part of efforts to protect it.
"My greatest concern is that people read about events like this and, as concerning as they are, lose hope. That worries me because people who lose hope don't act. And what we need now is action."
