It will be months before scientists understand how much damage the Great Barrier Reef has sustained from its latest mass bleaching event.
A long-awaited report released on Tuesday has revealed 91 per cent of reefs surveyed along the length of the natural wonder suffered some level of coral bleaching from hotter-than-average water temperatures.
Of 719 reefs inspected from the air between the Torres Strait and the southern end of the reef 654 were affected.
The central region most popular with tourists, from Cape Tribulation south to the Whitsundays, suffered the most severe damage.
In that area, bleaching ranged from major, with 31-60 per cent of coral cover bleached, to extreme, with more than 90 per cent bleached.
The event was the result of the reef's hottest December on record, followed by further heat stress in January and then a late summer heatwave in March.
It was the fourth mass bleaching event since 2016 and the sixth since 1998. But it's the first to occur under the La Nina weather pattern, which usually keeps conditions cooler.
Dr Britta Schaffelke is a reef researcher with the Australian Institute of Marine Science, which co-authored the report along with the Great Barrier Reef Marine Park Authority, and the CSIRO.
She says the true toll won't be known until September onwards when long-term monitoring teams head out to assess coral mortality.
Bleaching can kill corals but doesn't always. Corals can recover but heat stress leaves them more susceptible to diseases that can kill them. Reproduction is also affected.
"Corals that don't die immediately can get sick and die over the course of the winter," Dr Schaffelke says.
"We need to wait a few months, then the full consequences will be visible."
She says the reef's prospects are not good in the face of mounting pressures including climate change, and monitoring its decline is frustrating.
"We are only providing the data. It's not, in itself, an action, obviously. Hopefully it will help people make good decisions."
The Intergovernmental Panel on Climate Change, the leading scientific authority on the global threat, has said up to 90 per cent of tropical coral reefs will vanish even if the world achieves Paris pact ambitions to keep warming to 1.5C.
United Nations Secretary-General Antonio Guterres recently said the world is on a pathway towards to 3C and beyond.
The climate action group Climate Analytics recently analysed the 2030 emissions reduction targets set by the Coalition, Labor, the Greens and the teal independents.
It found the Coalition's pledge is consistent with 3C of global warming bordering on 4C, Labor's with 2C, and the Greens and teal independents with 1.5C.
The Great Barrier Reef Marine Park Authority says early indications suggest the most recent event will be more like the one in 2020, when coral mortality was low, and less like 2016 when severely bleached areas suffered extreme mortality.
Chief scientist David Wachenfeld says the authority can't comment on individual emissions plans but current trajectories are taking the world's reefs to very bad places.
"If we implement everything we know how to do, and we do it as quickly and assertively as we can, there's still a chance to protect at least some of the world's coral reefs."
Australian Associated Press
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