Once dazzling with colour and life, coral reefs around the globe are turning a spectral white as climate change warms the world’s seas. For Aotearoa New Zealand, this is not an immediate threat but for our South Pacific neighbours, the impacts are profound.
Simon Davy, professor of marine biology at Te Herenga Waka – Victoria University of Wellington, says Pacific coral reefs are declining at a rate of about 2% a year, and it may be only 40-50 years before they are completely gone.
With at least a quarter of all marine life found on coral reefs, their importance cannot be overstated. As Davy says, “The biodiversity is huge.”
It follows that a healthy reef is one teeming with top predators. “The most healthy reef I’ve ever seen, Palmyra Atoll [northwest of Kiribati], is one of the remotest places on Earth and I’ve never seen so many sharks in my life,” Davy says. However, even this lost corner of the planet occasionally gets bleached.
Bleached out
“It depends where in the world, but when you start getting much above 30°C, corals tend to bleach and potentially die,” he says. “Just one or two degrees above their normal summer temperatures, you’re going to tip them over their physiological threshold.”
When a coral bleaches, most of the algae that live in symbiosis within the coral die, leaving it without colour and its means to obtain food. If temperatures cool, a bleached coral has roughly a month for the algae to recover before its fate is likely sealed.
Another side effect of bleaching is that it reduces the coral’s skeleton, making it more susceptible to erosion.
The impacts of reef degradation can be devastating. “You have whole countries that are heavily dependent on fisheries and tourism,” says Davy. “Think how many billions [Australia’s] Great Barrier Reef is worth – and that’s a wealthy country. Now imagine the consequences to the island nations in the South Pacific.”
For Steve Simpson, professor of marine biology at the UK’s University of Bristol, a research trip to the Great Barrier Reef unveiled the sheer scale of these bleaching events. Describing the emotional impact on him and his team, he says, “We were going through this cycle of grief, where the initial response is either denial or numbness.”
For Emma Camp, coral biologist and leader of the Future Reefs team at the University of Technology Sydney, the depth of the problem hit during a 2016 visit to the Seychelles, where she saw areas of coral the size of football pitches, once alive and full of colour, that had turned completely white and died. “It was really hard to see,” she recalls.
The Great Barrier Reef saw the first-ever documented back-to-back bleaching events in 2016 and 2017, and further bleaching events in 2020 and 2022, says Camp.
“We know that about 14% of the reefs [worldwide] have been lost over recent years.”
Our planet will lose many more reefs in the next 10-20 years, warns Davy. “I don’t think all hope is lost, but people need to act very quickly, or it could be. The measures that we’re having to take are becoming more invasive, more proactive, as we become more desperate.”
Hot water
Bleaching is not the only issue to endanger the reefs. “Ocean deoxygenation is an increasing threat,” says Camp. “We’ve seen increasing evidence that reefs have less oxygen available.”
Ocean acidification, although less of a threat to tropical waters, also carries a risk.
“It’s not just the warming of our oceans, the chemistry is changing as well,” says David Aguirre, an associate professor in zoology and ecology at Massey University.
Tropical waters tend to be supersaturated with alkaline calcium carbonate, enabling corals and shells to build their skeletons. When the pH is reduced, so is the calcium carbonate and the means for corals to build their skeletons, which can leave them highly vulnerable.
Davy says there will be a transition in the structure of the reef. “Sponges appear to be less thermally sensitive than corals. When corals die, sponges may come in and occupy a lot of that space.” Though, he notes, “If they reach their thermal threshold, they will die, too.”
Bristol University’s Steve Simpson says some form of the reef will survive. “You find these extremophiles – animals that just love taking on the hardest-ever conditions – like Arabian Gulf corals that live in water ranging from 35°C in summer down to 15°C in winter, and yet they thrive.”
These species, however, are few and far between and there will be many more losers than winners in that situation, he adds.
On top of bleaching and changing ocean chemistry, there’s still the problem of overfishing and illegal fishing. Camp says in some locations dynamite fishing is still common practice. Davy explains there is “huge socio-economic pressure in poor regions to overfish the reef. Day-to-day survival takes precedence over long-term management of the reef.”
Reefs close to major human populations also tend to suffer as polluted water often brings high levels of nutrients and pathogens. These can prompt an explosion of seaweed growth that smothers corals and introduces disease.
Waves of anxiety
For many people – particularly the younger generation – the sense of helplessness and “eco-anxiety” can be overwhelming. Fostering a positive mindset is crucial, says Davy, who is seeing more anxiety than ever in his undergraduate students. “If you tell everyone everything’s going to die and there’s nothing you can do about it, then why would you bother?
“I focus on the challenge of trying to fix the problem,” he adds. “I don’t think there is room to be pessimistic. Even though the outlook is bleak, if we don’t do anything, [it will be] really bleak.”
Diametrically opposing eco-anxiety are climate change denial and its allies. Despite the relative high profile of coral bleaching, says Davy, there is “this narrative that climate change isn’t real, climate change isn’t as bad as people say it is”.
Camp agrees scientists must be careful when making publicly dire predictions on the state of coral reefs. “If these fail to manifest themselves in the way people expect, it simply fuels the fire for those who say it’s not really happening.”
Part of the issue is that many aren’t aware that a coral is an animal, she says. Once they realise this, there is greater understanding of what a reef is and the impact of it bleaching.
For coastal communities that rely on reefs for their livelihoods, action is becoming imperative.
Under a microscope
Genetic engineering is seen as a crucial tool in reef preservation – although, as Davy notes, this is a morally challenging dilemma. Throwing man-made organisms onto the reef for a largely unknown outcome is risky but “desperate times call for desperate measures”.
One of Davy’s goals is to research the interaction between corals and their resident algae in a bid to make the reef more thermally tolerant – a potential solution to climate change-induced bleaching.
Manipulating the complex relationship between corals and algae may prove vital, as certain corals have algae that can “shuffle” (to allow more thermally tolerant algae to dominate) under heat stress.
One solution may involve the seeding of bleached corals with more thermally tolerant algae. Negotiating this relationship is difficult, however. “Some corals are very specific for one partner, so they are completely tied to the fate of that symbiosis,” says Davy.
Part of Camp’s work involves finding more naturally tolerant corals, like those found in warm, shallow mangrove lagoons, to outplant and grow at a local scale.
A more desperate lifeline is cryopreservation – preserving at very low temperatures cells and tissues of corals and their larvae. “You keep them cryopreserved until someone sorts out the bigger problems,” Davy says.
Geoengineering is also being considered. “Can we introduce particles to reduce irradiance during bleaching events?” he asks.
Aguirre has been involved in similar research focused on shading corals from high temperatures and irradiance, as cloud cover in the eastern Pacific has been shown to reduce bleaching. “[Interventions like] using shade cloth or the floating balls used in water reservoirs to avoid evaporative loss could all help to shade corals during periods of warm water and high irradiance,” Aguirre says.
Scaling up
Simpson believes a big part of the solution lies in preserving certain habitats, species and ecosystems through the worst of what may come over the next few decades and then recolonising and restoring on a very large scale.
Aguirre agrees that preserving local variations of species on a smaller scale “gives hope that species won’t go globally extinct”. Some of his research has involved comparing how species bleach in the Maldives, for instance, with bleaching on the Great Barrier Reef. Examining how marine heatwaves are handled by different coral populations may offer some solutions.
Considering the sheer range in coral species around the world, Aguirre says, “if there is local variation in how they respond to warming, then there might be little reservoirs, little populations around the world, that might be able to survive that global warming or climate change”.
Simpson says harnessing the ability of the many small reef fish that breed at rapid rates to create a “living broodstock” could help to kickstart a recovery.
Finding the sites least affected by overfishing, pollution and heat waves is imperative. “You don’t want to draw fish into an area that’s effectively drawing them to their death,” he says.
His research in marine soundscapes (see “Call of the reef”, previous page) has proved invaluable in testing tools for restoration at Great Barrier Reef’s Lizard Island. “We found by playing back recordings of healthy coral reefs over the whole breeding season, this accelerated the settlement of fish into acoustically enriched sites, rebuilding the fish communities.”
Simpson’s team is also monitoring one of the world’s largest coral reef restoration projects, on the Indonesian island of Sulawesi.
“They’re building reefs and local communities are employed to build and guard these reefs from illegal fishing. You can see them from space, and they are thriving. We’ve been listening to those reefs over the years – we can hear the sound of recovery.”
Modern technology is also getting to the point where robots can drift over reefs and release coral larvae, says Simpson. Drones and satellites play a part and social media has a key role by giving the fishing communities in far-flung islands a voice. Simpson says social media allows good practice to be amplified, without the need for a United Nations mission to try to deliver solutions for the future “in a slightly old-fashioned, colonial way”.
No silver bullet will solve the problem of the dying reefs other than reducing carbon emissions, says Camp. Adds Davy: “You need scientists and governments to sort out climate change and CO2 levels, and you need local management of reefs for improving reef resilience. You need people like me looking at molecular tools and selective breeding.”
Indigenous care, as practised at Rangitāhua by Ngāti Kuri, also plays a crucial role in restoration efforts by promoting a holistic approach beyond the silos of any one biome, study system, institution or knowledge system. This approach allows for solutions to the many challenges coastal ecosystems face.
Countries around the world are beginning to step up. “The Maldives – which is at serious threat from sea level rise – takes it very seriously,” says Davy.
Apo Island in the Philippines has also taken up the mantle. “The locals manage the island, the fisheries and the reserve – they police it and look after it. People understand the concept of reef resilience and reef management.”
Much of this hope, says Camp, rests on young shoulders. “Young people around the world are seriously calling for change on climate action. We know exactly what the issue is, and we know exactly how to deal with it, although it’s not a simple fix. We have reef left – that’s worth fighting for.”
Aotearoa’s island ark
The Kermadec islands could be a crucial haven for coral reefs.
Aotearoa New Zealand has no coral reefs of its own – Massey University associate professor Libby Liggins says our latitude means “we don’t quite have enough light for them to make a good living” – but it is home to several coral species that form the small colonies that can help to kickstart coral reefs in warmer climates.
One such home is Rangitāhua (Kermadec Islands). This subtropical island arc 1000km northeast of New Zealand may prove surprisingly valuable in the fight to save coral.
Its latitudinal position makes Rangitāhua an important crossroads for organisms that need connections to other places globally, says Liggins, as well as a crucial barometer for measuring climate change.
“You start to see quite clearly the presence of new species or the absence of species that have been there previously,” she says.
The uninhabited volcanic island chain and marine reserve lies within the rohe of the Ngāti Kuri iwi. Surrounded by swirling currents, the islands are notoriously difficult to get to. “It’s a hard region to model from an oceanographic perspective, to try to predict how species are getting there and what warm water masses it’s affected by.”
But the lack of a settled human population makes Rangitāhua a valuable asset for reef research. “We don’t have those added effects of fishing, pollution and sedimentation,” says Liggins.
Associate Professor David Aguirre, a colleague of Liggins, says the islands lie on the same latitude as parts of Great Barrier Reef, Norfolk Island and other places where there are far more established coral communities. But their relative youth, isolation and frequent volcanic activity – “there are still gases bubbling through the seabed” – may have hindered reef development at Rangitāhua.
Despite this, the range of marine life is startling. “It’s a really eclectic mix of things that you’d usually see in temperate reef systems – macro algae, small kelps, lots of urchins and subtropical species that we’re familiar with in New Zealand,” says Liggins.
“There are also brightly coloured tropical species, crown-of-thorns starfish, megafauna – Galapagos sharks, humpback whales – a really strange mixture of characters that wouldn’t usually meet on a reef.”
Aguirre says it’s the northern limit for cold-water species and the southern limit for warm-water species. “You end up with scrappy kelps and scrappy corals.”
The climate and water chemistry are such that corals don’t form the reef platforms you have in more tropical parts of the world, says Aguirre. Instead, the corals grow on the rock and their skeletons dissolve straight back into the ocean after they die.
As a result, the islands may provide an interesting blueprint for how future reefs around the world will look as corals attempt to colonise latitudes closer to the South Pole.
Cool-water reefs may help provide crucial sanctuaries in the years to come. “Rangitāhua is one of the more southern latitudes where corals grow,” says Aguirre.
Liggins says comparing Rangitāhua’s coral species with those in the tropics gives an idea of whether tropical species would survive further south or whether high-latitude systems are actually a “completely different coral assemblage.”
Aguirre says more storms are expected with climate change. “Rangitāhua is one of those parts of the world where we’re going to see the greatest amount of change.”
Call of the reef
Australia’s Great Barrier Reef – one of the seven natural wonders of the world – is often cited as a model marine park, says Steve Simpson, professor of marine biology at the University of Bristol.
“It is the most magical coral reef ecosystem – a third of it closed off from fishing, most of it away from human pollution. It had all the right hallmarks of a perfect ecosystem, but you can’t protect against something at a climatic level.”
Simpson’s specialist interest in marine soundscape was sparked by studying the larval fish that spend their early days among the plankton in the open ocean before finding their way back to the reef.
Healthy reefs are surprisingly noisy places, with fish, lobsters, crabs, clams, oysters and the corals themselves all adding to the ruckus. By studying the biological sounds on the reef, Simpson and his team discovered young fish use acoustic cues to help find their way back.
“No one had explored marine acoustics from that perspective,” he says. “It sparked a fabulous decade of natural history and discovery.”
While Simpson was preparing for a field trip to the reef in 2016, scientists forecast a mass bleaching event. Sadly, their predictions held true and, for Simpson and his team, this marked the end of an era.
“The bleaching hit the Great Barrier Reef in the most depressingly predictable way,” he recalls. “Six months earlier, the climate scientists could see that it was going to be the perfect storm, with El Niño and the state of the ocean currents and the season.”
Simpson’s team still went out to continue their research, but the reef that greeted them was sepia coloured. “It was like swimming around in a graveyard,” he says. “The reefs we knew had been turned upside down and killed.”
What survived stood out uneasily. “Any remaining fish looked out of place. They were still embarrassingly brightly coloured; they looked like they’d been painted afterwards – like on old black-and-white photos.”
Using the quiet of the open ocean, Simpson’s team played new recordings of the degraded reef and historical recordings of the reef in its prime. They discovered the fish were still naturally drawn to the old recordings of the bustling reef. The new recordings – a quarter as loud as it had been – held no more allure than the open ocean.
It was clear to Simpson that the fish had lost their cue to find their way home. “The discoveries made 10 years previously were science history and that was really devastating – destroyed in three weeks of coral bleaching.”
Between 70-80% of coral had died in the northern Great Barrier Reef, and yet, as Simpson wryly notes, tourists were still being sold a dream to pay a fortune to swim over a paradise truly lost.