The Great Barrier Reef is currently experiencing the most widespread bleaching ever recorded, with 60 per cent of reefs across all three regions affected, according to a detailed survey of the system.

Key points: Warmer sea temperatures have led to coral bleaching along the length of the Great Barrier Reef

Warmer sea temperatures have led to coral bleaching along the length of the Great Barrier Reef More coral reefs were bleached in 2016, although the damage was concentrated in the north

More coral reefs were bleached in 2016, although the damage was concentrated in the north Marine biologist Terry Hughes says the reef is rapidly adapting to climate change

It is the third mass bleaching event on the reef in five years — a phenomenon primarily caused by greenhouse gas emissions, and one that had never been recorded before 1997.

"We were hoping that this year would be a relatively mild bleaching event, but unfortunately that's not the case," said Professor Terry Hughes, head of the ARC Centre of Excellence in Coral Reef Studies.

"We were really shocked. No scientist expected to see three severe bleaching events in just five years."

Professor Hughes spent nine days in an aeroplane surveying the damage to the reef.

Loading

In that time, he covered more than 10,000 kilometres to visually inspect more than 1,000 reefs, retracing surveys he did after previous bleaching events.

Coral bleaches when the water is too warm for too long.

The coral becomes stressed and expels the algae that live inside it.

That algae gives coral its colour, and also provides it with most of the energy it needs to survive.

If temperatures do not recover quickly, the coral starves and dies.

It is too early to know yet how much coral will die as a result of this bleaching, but Professor Hughes said he feared the worst, particularly in the southern part of the reef.

"The southern bleaching was very severe and we were most concerned about the south because of the naivete of the corals that are there," he said.

Terry Hughes spent nine days surveying 10,000 kilometres of the reef. ( Supplied: Ed Roberts/ARC Centre of Excellence for Coral Reef Studies )

"They hadn't bleached before, which means there are more corals and more of the corals that are particularly susceptible to heat stress," he said.

Great Barrier Reef's 'huge transformation' to adapt

In the last two bleaching events in 2016 and 2017, about half the coral on the Great Barrier Reef was estimated to have died.

Bleaching in 2016 was more severe than 2020, but was concentrated in the north of the Great Barrier Reef. ( Supplied: ARC Centre of Excellence for Coral Reef Studies )

In those events, the areas hit hardest were in the remote northern stretches of the reef, around Lizard Island and beyond.

The southern and central reef had mostly been spared.

This time those parts were not so lucky.

Coral bleaching at Magnetic Island, around the centre of the Great Barrier Reef. ( Supplied: Victor Huertas )

"For the first time, the [Great] Barrier Reef has bleached in all three major regions: in the north, in the central region and in the southern regions," Professor Hughes said.

He said this time we might see coral die in the southern stretches in a similar way to what happened in the northern region a few years ago.

Professor Hughes said one reason the north did not bleach so badly was because global warming was rapidly transforming its makeup.

"Quite frankly, [I'm] astonished by the speed of these changes," he said.

This year's bleaching event was the second worst on record. ( Supplied: ARC Centre of Excellence for Coral Reef Studies )

"What we're seeing on the Great Barrier Reef now is a huge transformation in the mix of species.

"We're seeing less and less of the heat-sensitive, susceptible species — the so-called losers — compared to the tougher corals, which are persisting.

"The Great Barrier Reef is transitioning to a new mix of species that are tougher.

"Optimistically, we think it will take more heat in the future to cause the same level of bleaching and the same level of mortality as it does today."

But he said the species of coral that did not recover well after that previous bleaching — known as branching coral — provided important habitat for many fish species.

"Branching corals make all of the nooks and crannies that the rest of the biodiversity — the fish and so on — depend on," he said.

"So having fewer and fewer corals is having a much broader effect, not just on the corals themselves, but in the broader ecosystem."

Scientists are hopeful some of the coral will recover from the bleaching. ( Supplied: Kristen Brown/ARC Centre of Excellence for Coral Reef Studies )

The big problem for any coral, he said, was repeated bleaching events in quick succession.

"As these bleachings occur more and more frequently, the damage from them accumulates," he said.

"There's less and less opportunity for a proper recovery between them — there isn't enough time," he said.

'Really, really hot' summer in the south

Dr Selina Ward leads the University of Queensland's research station on Heron Island in the southern section of the reef, offshore from Gladstone.

The reefs directly around Heron Island were spared in 2016 and 2017 and she is hopeful they will recover from the limited bleaching that did occur this year.

"The station manager told me it was great that they had 11 straight days of rainy, cloudy, windy weather," she said.

"That might just mean that it won't be as severe or last as long. It's hard to tell until you see the recovery — when the colour starts to come back."

Twenty kilometres east sits One Tree Island — the tiny coral cay lies within a scientific zone of the World Heritage Site.

One Tree Island is shaped like a boomerang and surrounded by a lagoon , which is home to colonies of coral. ( ABC News: Elena de Bruijne )

It is shaped like a boomerang, surrounded by a lagoon, and is home to a research station for the University of Sydney.

The island's only inhabitants are the station managers, Ruby Holmes and her partner.

"We have this emotional connection to this place because we live here, and we see the ocean; it's like our playground," Ms Holmes said.

"We know lots of the colonies and corals and spots really well. So when it started to change like that, I definitely felt emotional."

Coral within the surrounding lagoon started bleaching in early February when Ms Holmes said water temperatures were around 30 degrees Celsius.

Since then, she estimates around 80 per cent of coral in the lagoon has been bleached.

It is estimated 80 per cent of coral in the One Tree Island lagoon was bleached in this summer's warmer temperatures. ( Supplied: Ruby Holmes, One Tree Island Sydney University Research Station )

She too hopes cooler, overcast, weather in March will help the coral recover, rather than die, but she fears this mass bleaching event is a sign of things to come.

"With global climate change, unfortunately, this is the kind of stuff that we're seeing in areas that were kind of 'safe havens'," she said.

"Because we're further down south and we have a lot of tidal movement and water rushing in and out of the lagoon system and around them … it helped keep the coral cooler and the water moving [in 2016 and 2017].

"But it's just been really, really, hot over summer. Unfortunately, this is what happens."

Lone scientists remain to study coral

The coronavirus pandemic is also creating some problems for research on both Heron Island and One Tree Island, because scientists can no longer travel to the island to carry out their research.

Coral near One Tree Island was not affected by previous mass bleaching events. ( ABC News: Elena de Bruijne )

On One Tree Island, Ms Holmes has had to try to fill some of those gaps.

"It puts us in that unique position where we can provide updates of what's happening out here for all the researchers and for the director of the station," Ms Holmes said.

One observational experiment set up in the nearby waters, by a scientist from the University of Wollongong, is to study if certain coral colonies are more resistant to bleaching than others.

This event could provide crucial insights, and the pair will monitor the coral, take photographs and collect data from sites within the lagoon.

"It's nice to be able to keep that research going for them or at least provide them with some insight into what's happening," Ms Holmes said.

Scientists hope to return to the Great Barrier Reef later this year to determine if any coral has died as a result of this year's mass bleaching event.