The Great Barrier Reef could be hit with severe coral bleaching and death this summer as the result of another large underwater heatwave, according to a tentative long-term forecast by one of the world's most-respected science agencies.

Key points: NOAA forecast predicts entire reef has 60pc chance of being subject to coral bleaching by March next year

NOAA forecast predicts entire reef has 60pc chance of being subject to coral bleaching by March next year If widespread bleaching happens in 2019, it would be third event in four years

If widespread bleaching happens in 2019, it would be third event in four years Predictions still very uncertain, NOAA says, as major weather patterns can change probabilities over next three months

A leading coral-reef expert said if that eventuated, it could mean the beginning of the end of the Great Barrier Reef as a coral-dominated system.

According to the forecast by the US National Oceanographic and Atmospheric Administration (NOAA), by March next year the entire reef has a 60 per cent chance of being subject to "bleaching alert level one", where bleaching is likely.

And worryingly, the southern half of the reef has a 60 per cent chance of seeing the highest "bleaching alert level 2", where coral death is likely.

About 50 per cent of shallow-water coral was killed in 2016 and 2017, when the reef experienced back-to-back bleaching events for the first time in recorded history.

Underwater heatwaves stress coral, causing them to expel the colourful algae that live inside them, leaving them a brilliant white.

That algae provides coral with most of their energy, and if the temperatures don't quickly return to normal, the coral starves and dies.

Some recovery was seen through 2018, but scientists say proper recovery on badly hit reefs takes about a decade, and bleaching again this summer will set that recovery back.

The main areas of coral bleaching in 2016 and 2017. ( Supplied: ARC Centre for Excellence for Coral Reef Studies )

Another mass bleaching would be unprecedented

If widespread bleaching happens in 2019, it would be the third event in four years.

A situation like that wasn't predicted to happen regularly until the second half of this century.

Another mass bleaching could foreshadow the decline of the reef towards a system no longer dominated by coral, according to marine biologist Professor Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland.

"It would be very serious," he said.

Professor Hoegh-Guldberg said the bleaching in 2016 and 2017 had reduced the ability of the reef to recover.

By killing so much of the coral, there is less larvae being produced and fewer stable surfaces for it to grow on.

"Killing coral has really reduced the ability of the reef to produce offspring," he said.

Back-to-back bleaching in 2016 and 2017 had reduced the ability of the reef to recover. ( Justin Marshall/coralwatch.org )

"The idea that these large remaining stocks of coral also get whacked at the same time as we're losing the ability for the reef to regenerate … It's really serious."

The now very regular bleaching, driven by climate change, puts the entire reef at risk, according to Professor Hoegh-Guldberg.

He said the continued existence of the Great Barrier Reef as a coral-dominated system depended on a balance between two factors: the events that damage coral, and its rate of recovery.

Professor Hoegh-Guldberg said they've been well-balanced in the past.

"Look forward to today where we're driving underwater heatwaves that are more and more regular on almost an annual basis. And at the same time we have acidification and pollution that are slowing the ability for corals to grow back," he said.

"When you put the two together you have impacts that are dominating recovery — and you will no longer have a reef that is coral-dominated.

"And without the coral you don't have all the fish and other wonderful animals that live there."

The existence of the reef as a coral-dominated system depends on two factors: the events that damage coral and its rate of recovery. ( Supplied: Reef and Rainforest Research Centre )

But lots of things could happen, including cyclones

The NOAA predictions to March, when the bleaching risk appears highest, were still very uncertain, said Dr Mark Eakin, head of the agency's Coral Reef Watch.

"Lots of things, including major weather patterns, can change the probabilities over the next three months," Dr Eakin said.

"I wouldn't say dire yet, but it is concerning."

In 2016, the southern half of the Great Barrier Reef avoided severe bleaching, mostly because of Cyclone Winston, which devastated Fiji.

As it approached Australia it petered out, but cooled the surface water, stopping a lot of bleaching.

"If we had cyclones move through, which have their own impacts on reefs and people, but they have one benefit: they can cool the surface water," Professor Hoegh-Guldberg said.

"It's pretty serious but we can cross our fingers and hope things won't be as bad as projected."

Professor Terry Hughes, head of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, also said cyclones could change what happens.

"We won't know for sure until January or early February, and even then a well-timed cyclone could cool things down," Professor Hughes said.

Experts are ready to monitor for any bleaching

In 2016, scientists conducted detailed surveys of the entire Great Barrier Reef to examine the severity and impacts of the bleaching, led by Professor Hughes.

In response to questioning at Senate Estimates earlier this week, the Great Barrier Reef Marine Park Authority said it hadn't yet budgeted to support monitoring of bleaching if it occurred this summer.

They said decisions about that would be made after a meeting at the end of November.

Sorry, this video has expired Vision shows parts of the Great Barrier Reef after the heatwave

But scientists said regardless of support from government, they would be doing everything they could to monitor any bleaching.

"If bleaching does occur, we are on standby to respond by re-doing our aerial surveys, and by deploying teams of scientists underwater at bleached locations," Professor Hughes said.

Professor Hoegh-Guldberg added: "All of us have research sites that we've been working on.

"With or without funding, many of us will be … jumping in the water and trying to figure out how bad this is."

He said a focus of his research would be identifying reefs that survived bleaching better than others, since those could represent an "opportunity".

It's not just the Great Barrier Reef at risk

Dr Eakin from the NOAA warned the risk of bleaching was not just on the Great Barrier Reef, but extended right around the world.

Whether that materialises will partly depend on whether an El Nino emerges.

The irregular weather phenomenon results in warm water rising to the surface across the Pacific, and heats up much of the globe.

"The model also shows a potential for bleaching more broadly in the South Pacific and southern Indian Oceans," Dr Eakin said.

"Depending on what happens with the El Nino, we could see another global bleaching event in 2019. However, it's much too early to predict that with any certainty."