Repeated bleaching events are reducing the Great Barrier Reef's ability to recover and scientists say the events can no longer be viewed as isolated incidents.
According to a paper published by James Cook University, only two per cent of the reef has escaped a bleaching event since 1998 and about 80 per cent has been severely bleached at least once since 2016.
Lead author Terry Hughes said it was a complex situation because different parts of the reef had been impacted in vastly different ways.
He said surveys of different parts of the reef found the amount of heat needed to trigger bleaching depended on the history of each location.
"Reefs that are naive to exposure to extreme heat bleach more severely compared to more experienced reefs that have bleached before," Dr Hughes said.
He said corals that survived bleaching events were usually tougher.
"The Great Barrier Reef has gone through one hell of a natural selection event, during each of these five [bleachings]," Dr Hughes said.
Bad news and good news
Another University of Queensland paper published today examined years of bleaching events.
University of Queensland professor Peter Mumby said until recently it had been difficult to predict what parts of the reef were more or less likely to bleach.
But he said it had been theorised that some spots heat up more than might be expected.
"What we're learning is that there are in fact some areas that are predictably less likely to get hot," Dr Mumby said.
He said scientists now believed that cooler spots were receiving colder water from outside the reef, which "flushed" the areas.
"The warmer spots are likely to be in places there isn't as much mixing, so you get a kind of puddling, and the water gets very, very warm," Dr Mumby said.
He said those spots seemed to face more regular bleaching and were largely clustered in northern parts of the reef.
But many of the cool "refugia" on the southern end of the reef were connected in some way to about half of the other reefs in the Great Barrier Reef, Dr Mumby said.
Those could be a source of recovery for bleached areas of the reef, because bleached reefs have lower coral spawn survival, but spawn can travel in from reefs further away.
Dr Mumby said satellite mapping showed that the refuges were rarer in the northern stretches of the reef.
"It does mean that those reefs in the far north especially are probably more vulnerable to recurrent bleaching," he said.
Dr Hughes disagrees somewhat over how specific the mapping can be and where sources of new coral spawning will come from.
"What we're going to see, I think, is recovery will be local from nearby reefs that have had a chance for a half-decent recovery since the last time they bleached," he said.
But both researchers say knowing more about which areas are susceptible to bleaching events is vital to helping improve resilience.
Dr Mumby said it was early days, but protecting a selection of reefs based on this information could help the process of adaptation, or evolution, occur as quickly as possible.
The ultimate need
Dr Mumby said while managing corals was crucial "to help them through the warming that's coming", he said it came second to the need to cut CO2 emissions.
Dr Hughes also said while the findings around some reefs toughening to bleaching events was "a sliver lining" the impact was still negative.
"We're seeing a general downward trend in coral cover, we're seeing the recovery phases, the window of opportunity between one bleaching event and the next is shrinking," he said.
He said the Great Barrier Reef's long-term survival was dependant on how successful the world was in reducing carbon emissions.
"Business as usual, that might take us to three or four degrees of global average warming," Dr Hughes said.
"There unfortunately won't be a recognisable Great Barrier Reef if temperatures are allowed to go that high."
