The number of coral reefs worldwide that will be spared the harm of warming is predicted to plummet within just a few decades.
That's the finding of research published today in the journal PLOS Climate, which comes as reef scientists "anxiously" eye off the potential for bleaching events on the Great Barrier Reef and in Western Australia over the coming weeks.
Currently about 84 per cent of the world's coral reefs are buffered from ocean warming by influences like cool upwellings, deep water, and cooler ocean currents.
But today's research shows that figure will drop to just 0.2 per cent of reefs when global warming breaches 1.5 degrees Celsius above pre-industrial levels.
We're currently forecast to hit 1.5C in the 2030s, and it's believed that there's now almost no chance of staying under that limit without extremely aggressive cuts to emissions and unproven carbon capture technologies that can draw CO2 from the atmosphere.
The Great Barrier Reef suffered bleaching events in 2016, 2017, and 2020, and the co-author of today's paper, Scott Heron from James Cook University, said scientists were anxiously watching weather patterns in Queensland's north, and off the northern WA coast.
"Here we are at the end of January — summertime in Australia — and we're seeing heat stress developing to a point of potential widespread coral stress on the Great Barrier Reef," Dr Heron said.
Some overcast monsoonal weather has given the reef a brief reprieve from the heat, and scientists are hoping for more of the same to stave off yet another catastrophic hit just two years after the last, Dr Heron said.
But there are already signs of corals fluorescing — a sign of distress in some species — and some whiteness has been observed, according to marine biologist Jodie Rummer from JCU.
She says temperatures are already high and if there are cumulative days of heating over the next weeks or months, bleaching is likely to occur.
"We've got temperatures that are warmer than expected [and] there's not as much mixing [of water] as we'd hoped, despite it being a La Nina," Dr Rummer said.
Marine biologist Mike van Keulen from Murdoch University in Perth says there are already "signs of coral stress" in north-west Western Australia as well.
Warm waters in the north can cause coral stress down much of the WA coastline, Dr van Keulen says.
"Particularly for WA, that becomes a problem because the warm water to the north of us seeds the Leeuwin current that runs down our coast.
"The last big event we had in 2011 bleached corals all the way down to Rottnest Island [off Perth]."
At 2C, no reef will be spared
In order to understand the future of coral reefs globally, Dr Heron and colleagues from the University of Leeds and Texas Tech University modelled the impact of warming on reefs down to a resolution of 1 square kilometre.
At 1.5C of warming, they found only a few reefs in Polynesia and the Coral Triangle in the western Pacific would be spared recurring "thermal stress events" at least once every 10 years, according to Dr Heron.
"There were very few places around the world [that would be spared] and they were focused in the area of French Polynesia and the Coral Triangle," he said.
The scientists found that more than 90 per cent of reefs will suffer these thermal stress events at least every five years.
Thermal stress events are periods of elevated sea-surface temperature capable of causing "significant coral bleaching".
No reefs in Australia will be immune to regular thermal stress events once warming reaches 1.5C, they concluded.
At 2C of warming, no reefs worldwide will be spared the impact of climate change.
Today's findings, and the prospect of yet another bleaching event, underline how important climate action is for the reef, Dr Rummer said.
"All of these secondary and tertiary issues the reef is facing, it's not going to matter if we fix them without addressing the real problem, which is climate change," she said.
A number of scientists, including Dr Rummer, were critical of last week's pledge by the federal government for $1 billion over nine years for the reef, without also committing to ambitious climate action.
Dr van Keulen says programs that will be funded under the government's pledge, which are aimed at improving the resilience of corals, are "too little, too late".
"We've got climate change that causes consecutive years of bleaching; there's no way we can return to normal from here.
As we approach 1.5C, Dr van Keulen says the delicate, colourful corals we associate with tropical holidays will be gone.
As they go, we will see "huge flow-on effects" in fish abundance and diversity, which will also impact many commercial industries like seafood and tourism.
Dr Heron says the more we study the impacts of climate change on reefs, the more we realise we're heading toward a "cliff face".
