Parts of the Great Barrier Reef (GBR) will never fully recover from repeated bleaching of its corals, caused by spikes in the water temperature, scientists say.

Key points: Repeated bleaching means many corals do not have time to recover

Repeated bleaching means many corals do not have time to recover Even pristine reefs are not immune

Even pristine reefs are not immune Researchers call for tough action on global warming to save Great Barrier Reef

Professor Terry Hughes of James Cook University and colleagues analysed the impact of three major bleachings in the past two decades on Australia's reef.

The research confirmed last year's event, which also affected reefs in places like Japan and parts of the Caribbean, was the worst on record.

With even the most pristine areas affected by heat, the researchers warned tough action on global warming was needed to ensure the reef's future.

"Globally these events have become the new normal, and the time between them is getting shorter and shorter," Professor Hughes said.

Last year the northern third of the reef was worst hit, with some reefs in the area losing 80 per cent of their corals, he said.

Among the casualties were 50 and 100-year-old corals, which take a long time to be replaced.

The findings, published today in the journal Nature, come as evidence shows a new bleaching event is occurring.

"As these events become more frequent, and more severe as temperatures keep going up, there's less and less time between them for anything like a full recovery," Professor Hughes said, adding that "weedier", faster-growing corals were replacing older corals.

"The mix of species in the north has changed forever."

Aerial surveys played a key role in the research. ( Supplied: James Kerry / ARC Centre of Excellence for Coral Reef Studies )

Tough action on climate change required

Professor Hughes and colleagues argue that while protecting the reef from run-off and over-fishing is important, immediate action to curb climate change is the only thing that will limit damage done to the reef.

Their study used aerial and underwater surveys to document the extent and severity of last year's bleaching, and National Oceanic and Atmospheric Administration (NOAA) satellite data on sea surface temperatures to look at the cause.

The researchers found bleaching occurred regardless of how protected the reef was from run-off and over-fishing.

"The only thing that made the difference was the intensity of the heat," Professor Hughes told The Science Show on RN.

"Near shore, turbid reefs that are quite muddy had the same loss of corals, the same level of bleaching as offshore reefs in crystal clear water."

While the northern third of the reef bore the brunt of the heat in 2016, the southern parts of the reef were rescued by the cooling effect of two weeks of rain from Cyclone Winston, which devastated Fiji in February last year, Professor Hughes said.

"That's an incredibly precarious situation to be in where the health of the GBR at a huge scale depends on a chance weather event."

Many reef fishes depend on the habitat provided by healthy corals. ( Supplied: Tane Sinclair-Taylor )

A fourth mass bleaching

Professor Hughes is currently doing aerial surveys of the reef following evidence of a 2017 bleaching event.

"We know enough now from underwater surveys to categorise it as a major event," he said.

But, said Professor Hughes, while observations suggest the middle of the reef is being affected, it is too early to know how widespread the problem will be.

"We're hoping the north will escape," he said.

Significantly, this is the first "back-to-back" bleaching of the reef to occur, and it does not coincide with El Nino, a weather pattern that has been associated with bleaching events.

"Climate change is already dangerous for the Great Barrier Reef," Professor Hughes said, adding that even a 2 degrees Celsius global temperature rise (the Paris Agreement) "won't be a comfortable place for coral reefs".

"We've got a window of opportunity, but it's getting narrower and narrower to quickly move away from fossil fuels and curb emissions," he said.

Sorry, this video has expired Aerial surveys help reveal how extensive coral bleaching is on the Great Barrier Reef. ( ARC Centre of Excellence for Coral Reef Studies )

Dr Andrew King, a climate scientist from the University of Melbourne who studies climate extremes and variability, said Professor Hughes' research established just how strong the link was between coral bleaching and warmer water.

"These findings, in conjunction with other studies, point to the fact that warmer waters are damaging the reef and climate change means that large parts of the reef won't recover," he said.

Dr King said recent research looking at the frequency of bleaching events emphasised the need for strong action.

He said even if global temperature rise was limited to 1.5C, corals were in for a tough time, with bleaching set to occur every second year on average.

I don't think people are aware just how bad it is," he said.

Hear the interview with Professor Terry Hughes this Saturday on The Science Show.