Scientists with the Great Barrier Reef Marine Park Authority have already found "significant bleaching" at three reefs in the far north, the authority's chief scientist David Wachenfeld told The Sydney Morning Herald. Wachenfeld said the reefs in the north were relatively protected from most human activity, but not from the rising of ocean temperatures caused by the climate crisis.

"Climate change is the great leveller," he said. "There is nowhere that is not affected by it."

Coral bleaching occurs when corals expel the algae that give them their color and nutrients in response to heat stress. Back-to-back bleaching events in 2016 and 2017 killed as many as half of the corals in the reef.

Currently, temperatures across the reef are around two to three degrees Celsius above normal, and summer temperatures do not usually peak until mid March, professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, told The Guardian.

"We are down to the wire," he said.

What happens will ultimately depend on the "vagaries of the weather," he said.

The National Oceanic and Atmospheric Administration has placed the reef on Alert Level 1 for the next seven days, which means a significant bleaching event is likely, ABC News reported. However, the agency's Coral Reef Watch also forecast that the bleaching would not be as intense as the bleaching in 2016 and 2017.

The atmospheric buildup of greenhouse gasses from the burning of fossil fuels, which is responsible for heating ocean temperatures, has also been blamed for Australia's more extreme wildfire season. But Prime Minister Scott Morrison has been criticized for his pro-coal policies and his seeming dismissal of climate concerns.

"Surely after this summer of bushfires, and this marine heatwave hitting the reef, we cannot keep burying our heads in the sand," Leck told The Guardian. "We need a reef-safe climate policy that's consistent with a 1.5C global warming target, or less. No more excuses will be tolerated."