The Ribbon Reef, on the northern side of the Great Barrier Reef, before coral bleaching occurred.(Dr Jez Roff)

Maya Srinivasan still remembers being blown away by the incredible beauty of the coral reef the very first time she put on her goggles to go for a dive.

It was an instant love affair and cemented her resolve to become a marine biologist.

But, she says, it’s been a sad summer on the Great Barrier Reef.

The Ribbon Reef, on the northern side of the Great Barrier Reef, after coral bleaching occurred.(Dr Jez Roff)

“I’ve been close to tears quite a few times in the last few weeks.”

Though it looked pretty, she knew the mix of fluorescing colours and white she saw during her surveys was bad news.

They were the signpost of a much wider story sweeping the globe.

Over the last year, widespread bleaching of coral reefs has occurred in oceans all across the planet, with America’s National Oceanic and Atmospheric Administration (NOAA) now officially declaring it a “global mass bleaching event” for just the 4th time in history.

“This is something everyone should be worried about, and everyone should be angry about, frankly,” NOAA’s Coral Reef Watch coordinator Derek Manzello said.

The news comes after a year of extreme heat in the world’s oceans, with sea surface temperatures at record levels every day.

For an event to be deemed a global event, a certain percentage of reefs in each of the Atlantic, Pacific and Indian Oceans need to have reached levels of heat stress consistent with significant bleaching throughout the previous year.

A journey across the oceans

Each of these dots represents the location of a reef around the globe. 

Depending on where they live, the corals on these reefs can handle a vast range of temperatures.

Some are used to the warm tropical waters of the Indian Ocean.

Others thrive in the cooler, sub-tropical waters off Western Australia. 

But all of them are susceptible to bleaching when the sea surface temperature rises above the typical summertime peak.

To measure the health of reefs and the stress they are under, scientists use a reef bleaching alert scale.

level 1 (l1) – Risk of reef-wide coral bleaching.

level 2 (l2) – Risk of reef-wide coral bleaching with mortality of heat-sensitive corals.

level 3 (l3) – Risk of multi-species mortality.

level 4 (l4) – Risk of severe multi-species mortality. (> 50 per cent of corals)

level 5 (l5) – Risk of near complete mortality. (> 80 per cent of corals)

Alert levels 3 to 5 were only recently added, in response to the unprecedented levels of coral bleaching heat stress witnessed last year.

Mass bleaching started in Fiji and Vanuatu as early as February during La Niña conditions.

In June, mass bleaching began to take off across the eastern Pacific and southern Caribbean and in waters around Central America.

And by the end of summer, extreme temperatures were being reported in the reefs all across the north-west Atlantic.

For the first time ever, coral was bleaching on both the Atlantic and the Pacific side of Panama at the exact same time.

“That’s never happened before,” Dr Manzello said.

“So there’s a lot things about this event that are different from previous events, and we’re just starting to understand what those differences are.”

Florida’s coral in particular had never been exposed to this magnitude of heat stress, hitting values more than three times its previous record.

Shock at Cheeca Rocks reef

Underwater, the devastation couldn’t be more clear.   

In the shallows of the Florida Keys National Marine Sanctuary, the Cheeca Rocks reef is usually brimming with life.

Colour on the reef at Cheeca Rocks before bleaching.(Supplied: AOML)

It’s a site of particular interest to researchers in the area due to its resilience.

When faced with stress in the past, it had maintained comparably high living coral cover relative to its surrounding offshore reefs.

But when Ian Enochs, who is the head of the Atlantic Oceanographic and Meteorological Laboratory, dived underneath the water to look at the site in July, the usually colourful reef corals were entirely bleached.

Cheeca Rocks post “severe” bleaching.(Supplied: AOML)

They had expelled their colourful algae inhabitants — which have a symbiotic relationship with the coral — after weeks of extreme heat stress.

“We’ve unfortunately seen bleaching at that site before, but I had never seen complete, absolutely everything, everything bleached stark white, and severely paled,” he said.

Coral bleaching doesn’t always lead to mortality.

If temperatures cool quickly enough, coral can regain its colour in a matter of weeks to months, although even mild coral bleaching events can result in lower growth rates and disrupt coral spawning, which can have significant effects on coral recovery.

Where coral is dead, however, researchers say it takes several years of good conditions for the reef to recover.

And at the Cheeca Rocks site in the Florida Keys, when Dr Enochs first surveyed the reef, there was already mortality.

He said in some cases the record-setting heat meant the coral hadn’t just bleached, but completely disintegrated.

A survey at the Cheeca Rocks reef shows bleaching damage happened before the summer peak.(NOAA coral reef watch)
The scientists found corals had not only bleached, but in some parts completely disintegrated.(NOAA coral reef watch)
Corals can regain their colour if they get a prolonged reprieve from heat stress.(NOAA coral reef watch)

“The soft corals, the sea fans, and sea whips that don’t necessarily provide that structure, but are very, very important for coral reef ecosystems, they hadn’t just bleached, they had disintegrated,” he said.

Relentless ocean heat

The bleaching at Florida Keys, and elsewhere around the globe, is a clear reflection of the extraordinary ocean temperatures in 2023, which have been rising for decades due to the burning of fossil fuels, scientists say.

Last year those temperatures reached new extremes.

For over a year straight, the average daily surface temperature of the world’s oceans has been the hottest on record.

“The size of the anomaly, so just how hot it has been above average, for the entire global ocean has been off the charts,” NOAA’s Dr Manzello said.

Rising greenhouse gas emissions, which reached record high concentrations in 2023, combined with the onset of El Niño, deserve the blame, University of New South Wales physical oceanographer Matt England said.

An aerial image of the Great Barrier Reef Marine Park shows the reefs from above.(Supplied: Facebook (Great Barrier Reef Legacy))

“People have been speaking about cleaner air over the oceans with the shipping industry fixing up emissions, and they definitely can contribute,” he said.

“But basically, to first order, this is a climate change problem, superimposed with some aspects of the warming that can be linked to El Niño.”

Wave of destruction moves south

Part of the global bleaching story is also how widespread the heat has been, with few areas of cooler-than-normal waters at any one time.

While the Caribbean was in the middle of its most severe heatwave on record, the Persian Gulf was also being impacted.

In the southern hemisphere, bleaching generally occurs toward the end of summer.

But pockets of the western Indian Ocean off Africa were already reaching the bleaching alert threshold as early as December and January.

So too were the remote reefs of the central Pacific.

And by the end of summer, large portions of the reefs in the southern hemisphere, including around Australia, were flashing red.

Short window, massive change

From above, aerial surveys of the Great Barrier Reef revealed a “mass coral bleaching event” was unfolding across the world-heritage site.

These images of the Lizard Island Reefs, taken by CSIRO marine ecologist Geoff Roff, show the stark transformation of the site in only three months over the summer.

This photo of Ribbon Reef, on the northern side of the Great Barrier Reef, was taken before coral bleaching occurred in December 2023.(Supplied: Dr Jez Roff)

“What really surprised us was the extent of the bleaching,” he said.

“Some sites we surveyed in December were among the healthiest reefs I’d seen in 10 to 15 years, with coral cover upwards of 80 per cent.”

But when Dr Roff surveyed the same reefs again in March this year, bleaching had impacted almost the entire coral colony, and half of them had died.

This photo of Ribbon Reef, on the northern side of the Great Barrier Reef, was taken after coral bleaching occurred in March 2024.(Supplied: Dr Jez Roff)

The coral that had bleached was the very coral they’d seen rapidly recover from the last bleaching event in 2016.

In the water at other sites of the reef system, James Cook University marine biologist Maya Srinivasan observed a “devastating” mix of vibrant colours and white.

“To the untrained eye, bleached reefs actually look incredibly beautiful because the corals fluoresce and then turn white, so you get amazing shades of bright pink, blue and purple,” she said.

“But unfortunately seeing them in those bright colours, and white, you just know that they’re stressed. “

Bleached staghorn corals and corymbose Acros corals at Turtle Island.(Supplied: Maya Srinivasan )
Lemon damsels swim around a bleached Acropora coral at Turtle Island.(Supplied: Maya Srinivasan )
Acroporas,  a small polyp stony coral, in different stages of bleaching.(Supplied: Maya Srinivasan )

The in-water surveys of the reef are still ongoing and are a crucial part of understanding the severity and depth of the bleaching.

Surveys so far have revealed varying levels of coral bleaching, ranging from minor to severe, with some mortality among it, according to the Great Barrier Reef Marine Park Authority.

Dr Srinivasan said the areas of healthy reef on her surveys at the Turtle Group islands were “few and far between”, with the worst impacts in the shallows.

“I think at the stage we’re at, if all the bleached coral die, it’s going to take some time to come back, and they may not have enough time between bleaching events to fully recover,” she said.

Even still, Dr Srinivasan holds on to some hope that some of the bleached corals at the reef will survive if conditions cool down quickly enough.

Scientists are collecting underwater recordings from healthy reefs at Ningaloo. (Supplied: Andre Rerekura, Australian Institute of Marine Science)

Meanwhile in Western Australia, the world-heritage-listed Ningaloo Reef, and those in the Pilbara, are expected to have avoided falling to the same fate as other reefs around the globe, according to the Australian Institute of Marine Science (AIMS).

The consequences of losing coral reefs are catastrophic for the oceans, providing a home to a quarter of the world’s marine species — including fish, turtles and algae.  

They are like the bricks and mortar of ecosystems. If they break down due to death, then species like fish aren’t able to find refuge from predators and everything starts to change. 

The death of coral also represents a huge economic loss, with the Great Barrier Reef alone contributing more than $6.4 billion each year to the Australian economy and around 64,000 full-time jobs. 

Scientists expect the worst

All up, NOAA’s data shows bleaching has been confirmed across 54 countries and territories since February 2023, spanning all major ocean basins. 

This is how severe bleaching was in reefs in the Gulf of Mexico and the Caribbean Sea by the end of March this year.

A sea of white, which means most reefs are at the highest alert level.

A look at the same time the year prior shows just how extreme this bleaching event has been.

A full global account of how many corals have survived will take months, if not longer, due to the ongoing nature of climate events in the southern hemisphere.

But there will be significant mortality, coral scientists say.

Sprawling reef shelves around North Point, on Lizard Island, are visible from the air.(Supplied: Geoff Roff)

Spatially, the amount of bleaching is on par with the last global event, which was the worst on record, but the hope is it won’t be as severe.

Though coral reefs can regrow, the luxury of time without bleaching hasn’t been afforded to the last two mass bleaching events.

Scientists are worried that will be the case for this event, too.

“What we’re seeing now is really the start of severe, large-scale, persistent impacts that are just continuing to occur with increasing frequency, and that’s the really alarming thing,” Dr Manzello said.

Giving reefs time to breathe

An aerial image of Turtle Island in the Great Barrier Reef, where marine biologist Maya Srinivasa surveys reefs.(Supplied: Sina Straehl)

But, Dr Manzello added, “by no means does this mean all is lost for coral reefs, at this point in time”.

The key characteristic of large-scale events was the variability of bleaching among the reefs, which gave them a fighting chance of recovery in years like this.

“So, much of the Great Barrier Reef is undergoing bleaching right now, however not all reefs are going to be impacted the same,” he said.

“And this is really crucial, because it’s those reefs that escape the most severe impacts that act as sources of larvae for the reefs that did get impacted to recover.”

The corals at Eyrie Reef in December 2023.(Jez Roff)
The corals at Eyrie Reef in December 2023.(Jez Roff)
The corals at Eyrie Reef in December 2023.(Jez Roff)
Coral formations at Eyrie Reef in March 2024.(Jez Roff)
Coral formations at Eyrie Reef in March 2024.(Supplied: Geoff Roff)
Coral formations at Eyrie Reef in March 2024.(Jez Roff)

The top row shows corals at Eyrie Reef in December 2023. The row below shows just how much these corals have changed in just three months.

This is particularly true for reefs like the Great Barrier Reef, where Dr Manzello said there were still enough healthy corals around spawning and reproducing that it could settle on new sites.

“All is not lost, yet,” he said.

Similar glimmers of hope have been reiterated in Florida, where Dr Enochs said, despite the widespread mortality, some coral was already starting to show colour again.

“If I had to distil it all down, I would say that we have seen how bad things can be but we also have seen that it’s not too late to do something meaningful for our coral reefs,” he said.

“The magnitude of the challenge we are facing is huge, but it is not impossible.”

At the Great Barrier Reef, Dr Srinivasan is hoping Australia’s deep connection to coral will spur on necessary change to make sure that hope doesn’t fade.

The magnificent corals on the Great Barrier Reef are heritage-listed and are considered one of the Seven Natural Wonders of the World. (Supplied: Geoff Roff)

“Coral reefs are just incredible ecosystems, and you only have to snorkel or dive on one once to feel a connection,” she said.

“I’m just hoping people use that feeling of appreciation and admiration, and wanting to show their children reefs, to do all they can to tackle climate change.”


Reporter: Tyne Logan

Developer: Ashley Kyd

Producer: Fran Rimrod

Designers: Ben Spraggon and Teresa Tan

Editors: Tim Leslie, Matt Liddy and Edwina Farley

Posted , updated