Menu

Our Blue Planet

Catastrophe on the reef

The worst ever mass bleaching event and successive powerful cyclones have devastated many of the world’s tropical coral reefs.

It has been a catastrophic few years for the world’s coral reefs: the worst ever mass bleaching event has devastated many of the world’s tropical corals, while back-to-back cyclones along the Great Barrier Reef have pummelled the already weakened mighty structure.

The not-for-profit organisation The Ocean Agency has been recording the damage and the eerie aftermath caused by tropical storms and bleaching.

“We visited the Great Barrier Reef after Cyclone Ita in 2014 and the devastation was mind-blowing,” says Richard Vevers, founder and CEO of The Ocean Agency, which uses innovative technology to chart and communicate the plight of corals. Recently, the not-for-profit has been taking thousands of 360 degree, GPS tagged “before and after” photographs of damaged reefs, to create a baseline record for scientists and to publish eye-catching underwater “street views” via Google.

“There has been a lot of media attention about coral bleaching over the last couple of years but little mention of the devastating impact of the growing number of high intensity storms,” says Vevers.

Before: The Great Barrier Reef in 2012 before Cyclone Ita, photographed for the XL Catlin Seaview Survey (Credit: The Ocean Agency / XL Catlin Seaview Survey)
Before: The Great Barrier Reef in 2012 before Cyclone Ita, photographed for the XL Catlin Seaview Survey (Credit: The Ocean Agency / XL Catlin Seaview Survey)

Although cyclones are common, since 2009 the Great Barrier Reef has been hit by the most intense cyclones since records began in the 1970s, says Dr Anne Hoggett, director of Lizard Island Research Station, Australia.

Cyclone Ita was followed 11 months later by Cyclone Nathan. Nathan caused “absolutely horrifying” damage “like we’d never seen before” to the northern stretch of the reef off the island, she recalls. Corals that were normally robust were “scoured off… like somebody had just come along with a bulldozer.” And pieces of reef “as big as a room” broke off as they were smashed by powerful waves and “rolled down the slope and smashed everything in their path… it was annihilation.”

Unfortunately for the Great Barrier Reef, tropical cyclones Ita, Nathan, Marcia and Debbie hit just as it was enduring the third and worst global coral bleaching event, which started in late 2014 and came to a “likely” end in 2017, according to the US’s NOAA (National Oceanic and Atmospheric Administration). “We’ve had four years in a row of utter disaster,” says Hoggett. The cyclones in 2014 and 2015 were precursors for the bleaching that followed at Lizard Island in 2016. Even corals in a lagoon in the island’s centre that had survived the tropical storms were destroyed by bleaching.

“We’ve had a really, really bad time.”

After: The same stretch of The Great Barrier Reef just after Cyclone Ita pummeled it in 2014 (Credit: The Ocean Agency / XL Catlin Seaview Survey)
After: The same stretch of The Great Barrier Reef just after Cyclone Ita pummeled it in 2014 (Credit: The Ocean Agency / XL Catlin Seaview Survey)

Healthy coral reefs are extraordinary natural metropolises that cover just 0.1 percent of the sea floor while supporting about a quarter of marine life, making them the most biodiverse habitats in the ocean. They are made up of thousands of tiny coral polyps, which are animals closely related to jellyfish and anemones.

Reefs are of vital importance to millions of people, providing food, jobs and acting as breakwaters protecting coral islands from storms.

Some academics have tried to estimate the corals’ economic value; one such study suggested reefs provide a range of services which total around $352,000 (£263,000) per hectare per year. But despite their importance, over the past 30 years half of the world’s corals have died, according to The Ocean Agency.

During the recent mass bleaching event (the other two were in 1998 and 2010) more than 70 per cent of tropical coral reefs around the world experienced the prolonged high temperatures that can cause bleaching, and all have experienced above-normal temperatures. The Great Barrier Reef was the most high-profile victim to suffer during the last bleaching bout, with two thirds of the 2300km (1,400 mile) long mighty structure sustaining damage. US corals too were especially badly hit, with severe bleaching seen over extended periods in Florida, Hawaii, the Mariana Islands and Guam. Experts say increased ocean temperatures caused by climate change is the main driver of coral bleaching.

Bleaching happens when coral becomes stressed and the algae living in its tissue leaves. Without the algae coral is left bleached and vulnerable to disease. Corals may survive bleaching, but if the algae don’t return they will die.

Coral reefs have weathered storms for millions of years and they’re usually good at withstanding them. However, changing patterns in tropical storms (linked to climate change) have compounded the already huge strain from mass bleaching, pollution and overfishing, creating a grim outlook for reefs. 

In June, Unesco warned that under a “business as usual” scenario, by 2100 all 29 World Heritage reefs would cease to exist as functioning coral reef ecosystems unless CO2 emissions drastically reduce.

After the storm: The Ocean Agency and others monitor and photograph reefs before and after bleaching and extreme weather (Credit: The Ocean Agency / XL Catlin Seaview Survey)
After the storm: The Ocean Agency and others monitor and photograph reefs before and after bleaching and extreme weather (Credit: The Ocean Agency / XL Catlin Seaview Survey)

Dr Emma Kennedy, a coral reef ecologist at the University of Queensland, Australia, saw an alarming scale of destruction by Tropical Cyclones Marcia and Nathan, along the Great Barrier Reef, in 2015. She had just finished setting up experiment sites along the reef when the cyclones hit. “I was amazed at the damage the cyclones had caused. In the Keppels, one of our study reefs, and all my experiments, had been transformed to rubble by Tropical Cyclone Marcia,” she says.

“There was nothing left alive for a 50m stretch… and even more dramatically, most of the corals and presumably my experiments had been tossed 10m up onto island cliffs behind the reef.”

Kennedy witnessed the carnage off Lizard Island, where Cyclone Nathan had stripped a once-spectacular reef slope site completely bare.

“It looked like a concrete car park. All the corals were gone, the structure was flattened and there were no fish to be seen.”

“We were faced with a sheer wall of vacant brown rock. The only way we knew it was definitely our reef was a large metal post that I’d hammered into the reef to mark our spot.”

When a cyclone hits a reef it is not the tropical storm itself, but the height and duration of the waves it generates that cause damage.

Waves may also stir up sediments, dirtying water and releasing nutrients such as nitrogen and phosphorus that can cause algal blooms which smother corals.

Tropical storms cause patchy, rather than wall-to-wall, devastation, Kennedy explains. “As you travel along the length of a cyclone-damaged reef you can pass swathes of utter destruction where the all your see is rubble, followed by comparatively healthy looking complex reef structure.”

Since 2005, the Great Barrier Reef has been hit by many more strong cyclones than was typical in 1969 when reliable records began. “We’ve since had category 4 and 5 cyclones in 2005 (Ingrid), 2006 (Larry), 2009 (Hamish), 2011 (Yasi), 2014 (Ita), 2015 (Nathan, Marcia) and 2017 (Debbie),” says Dr Marji Puotinen, a spatial and ecological data scientist at the Australian Institute of Marine Science. The Caribbean too is going through an “upswing of activity at the moment”, she acknowledges.

So is there something unprecedented going on with cyclones?

“Not necessarily,” says Dr Puotinen. “We just don’t have enough reliable data from the past to know.” Predicting how tropical storms fit into the global picture of climate change is a hot debate in the scientific community.

“In general, scientists are predicting that extremes of weather of all kinds will be more likely in future,” says Puotinen.

“The overall consensus, on a global basis by cyclone experts, is that there will be fewer cyclones in future but that the cyclones that do form will be more likely to reach high intensity.”

Warmer seas don’t guarantee more cyclones on their own, since the tropical storms need a specific set of conditions to happen.

American Samoa in the South Pacific Ocean, photographed in December 2014, before tropical coral reefs were hit by bleaching (Credit: The Ocean Agency / XL Catlin Seaview Survey)
American Samoa in the South Pacific Ocean, photographed in December 2014, before tropical coral reefs were hit by bleaching (Credit: The Ocean Agency / XL Catlin Seaview Survey)

“Cyclones are powered by warm ocean waters and cannot form unless water temperatures are at least 26.5C [80F],” Puotinen explains.

“However, they also require high humidity, atmospheric instability, a pre-existing disturbance, low vertical wind shear (winds that blow straight down) and enough ‘spin’ from the Coriolis Force [the effect that causes a deflation in global wind patterns] to develop a low-pressure centre.”

Kennedy says there also may be occasional “silver linings” to storms, such as where “storms and extreme warming occur together” causing a phenomenon known as “beneficial cooling” where some cyclones developing near reefs “can help cool the area [by] providing a temporary refuge from underwater heatwaves and protect corals against bleaching”.

And some storm damage may actually be healthy for shallow reefs - an ecological theory called “intermediate disturbance hypothesis”, Kennedy explains: “By levelling the playing field it can prevent one type of coral from dominating and boost diversity on a reef, in the same way… infrequent bush fires can be healthy for forests.”

However many experts fear corals may not have time to recover from intense pressure from powerful cyclones, bleaching, pollution and overfishing before the next trauma.

The reef in American Samoa in April 2015, after it had been bleached (Credit: The Ocean Agency / XL Catlin Seaview Survey)
The reef in American Samoa in April 2015, after it had been bleached (Credit: The Ocean Agency / XL Catlin Seaview Survey)

Anne Hoggett believes the biggest problem coral reefs face right now are “repeated, massive insults on way too close a timescale”. She points out reefs can bounce back from huge disturbances on a decadal scale, but not on an annual one.

Kennedy adds: “The global mass bleaching event that ran all the way through until 2017 was the worst in recorded history, and was nothing short of catastrophic, with devastating impacts on reefs worldwide, including the Great Barrier Reef.”

Emeritus professor Charles Sheppard at the University of Warwick, whose work has focused on the impact of climate change the world’s coral reefs, says: “Think of it this way: if you caught at the same time flu, measles, chicken pox, mumps, all the rest of them, malaria… you’re not going to survive.”

“The intervals between the mass mortality events that corals have are getting closer together,” he says. “What happens if you start to kill a population before they’re old enough to breed? You’re on a downward slope.”

Coral reefs cover just 0.1 percent of the ocean floor while supporting about 25 percent of all marine life (Credit: The Ocean Agency / XL Catlin Seaview Survey)
Coral reefs cover just 0.1 percent of the ocean floor while supporting about 25 percent of all marine life (Credit: The Ocean Agency / XL Catlin Seaview Survey)

After the 1998 mass bleaching, the corals Sheppard studied in the British Indian Ocean Territory recovered well. Those that survived needed a few years to regain strength and breed. It took three to four years before Sheppard saw signs of new growth. The corals grew until warming happened again in 2015 and destroyed them.

“I would have to say though that many reefs are not either alive or dead – it’s not a black or white story,” he explains.

As you go deeper down into cooler water you may get a larger proportion of surviving corals. So, says Sheppard, you might get total wipe-out down to 10m depth, and then half the corals might have survived between 10 and 20m and deeper down still you might get a greater amount of survivors.

“There’s no ‘switch’ day when you’ll wake up and find the reefs are not there anymore,” he says. “Imagine the nature of forest trees: you start to chop them down. And you chop down a certain amount every month or every year - they’re growing - but sooner or later someone will come along and say, ‘well that really isn’t a forest anymore’.”

One direct effect of tropical coral reef erosion is they stop acting as breakwaters for islands.

“Many people don’t realise what a vital ecosystem service coral reefs provide by creating this immensely strong barrier between huge damaging storm-generated waves and the communities of people living in coastal areas,” Emma Kennedy says. “A healthy, strong barrier reef can literally save lives by protecting low-lying coastal areas from damaging waves and inundation.”

Sheppard adds: “Whole countries and parts of other countries are dependent on the coral animal growing, I believe, to build a reef and to keep that island, that nation above water.”

“If the corals are dead they don’t grow the reef. And not only that but you have storm erosion and you have bio erosion which is where little animals and plants attaching to it cause a drop in the reef height, the breakwater height of a few millimetres a year.”

Some hypersensitive, small islands he has seen have already lost 15-20 per cent of their land area to erosion on their shores, he says.

“If you damage your reef, you’re damaging your breakwater. And some islands around the world have had to build concrete breakwaters.”

In 2003 Sheppard published a paper forecasting coral mortalities in the central Indian Ocean would happen more frequently than they can survive and recover from, in roughly 10 years from now.

Anne Hoggett admits she is pessimistic about the future of corals.

“If I could see the world getting together and doing something about our carbon emissions I would feel, yes, we can do things, we can get over the hump, the corals are resilient enough to last for 50 years, and we could do all those technical things and keep them into cold storage and put them out again once we’ve solved the weather problem. But we’re not solving the weather problem. And so I am very, very pessimistic.”

But there are still glimmers of hope.

Off Lizard Island, reefs are showing signs of life, albeit very sparse. “There are still corals. They’re about as big as the size of your hand, they’re very spaced apart, so don’t look like anything like they’re supposed to look like, but that’s the seeds of our hope,” says Hoggett.

Richard Vevers, Emma Kennedy and Marji Puotinen are all working on a major conservation effort called 50 Reefs, launched by The Ocean Agency, which will identify the reefs least vulnerable to climate change. “Our aim is to rapidly bolster conservation efforts in these reefs which have the best potential for reseeding the world when the climate system is stabilised,” says Vevers.

In 2015 the Paris climate agreement attempted to unite all the world’s nations in a single deal to tackle climate change for the first time in history.

However in 2017 the US withdrew from the agreement, and scientists have pointed out the deal must be stepped up if it is to have any chance of curbing climate change.

“The difficult bit is not the science; the difficult bit is trying to convince the government that they really should do something quite radical to help,” argues Charles Sheppard. He blames the media too, for failing to effectively communicate problems with coral reefs to people.

“I’d like to think if people knew how bad it was they’d say well we’ve got to do something about it.” He adds: “I’m afraid it’s not a matter of can we do anything about it, it’s a matter of will we do anything about it.”

For more incredible ocean stories, follow OurBluePlanet on Twitter. Get in touch with to share your most magical ocean moments, and for inspiration watch the launch video on the BBC Earth YouTube Channel.

OurBluePlanet is a collaboration between BBC Earth and Alucia Productions.

Never miss a moment. Sign-up now for the BBC Earth newsletter.

By Michelle Douglas