How coral reefs might survive climate change

Warming waters are killing some of the world’s most spectacular coral, but scientists are scrambling to protect vulnerable areas and develop hardier species.

Opal Reef, part of Australia’s Great Barrier Reef, was damaged when ocean temperatures spiked in 2016 and 2017. “The once colorful coral was a gray ruin and all but dead—a skeletal statue created by climate change,” says David Doubilet. To document how climate change affects reefs, he and Jennifer Hayes returned to some of the most stunning corals they’d previously photographed.

The divers shrieked into their regulators, arms and legs flailing in delight. It was August 2020. Thirteen feet down on a reef in the Florida Keys, marine biologist Hanna Koch and her colleagues from the Mote Marine Laboratory & Aquarium had been hovering, waiting. Just before midnight, in a silent explosion from coral all along the reef, tiny pinkish orange bundles of sperm and eggs began to rise, speckling the sea with a pointillist eruption of life.

The team’s happy dance set off electric blue sparkles from the bioluminescent organisms in the sea around them. “It looked like we’d created our own fireworks,” Koch says. “It was beautiful.”

This sudden flurry is how many reef-building corals typically reproduce—usually once a year on a summer night a few days after a full moon. Cued by the lunar cycle, water temperature, and day length, coral species across Florida’s reefs simultaneously release trillions of sperm and millions of eggs; it’s a frenzy that boosts genetic diversity and ensures some small percentage of eggs will be fertilized, settle onto the reef as larvae, and seed the next generation.

But this was no ordinary spawning event. These mountainous star corals—Orbicella faveolata, listed as threatened under the Endangered Species Act—were cultivated and “planted” in 2015 by Mote scientists as part of a reef restoration effort. The corals survived a bleaching event that year, a Category 4 hurricane in 2017, and a disease outbreak two years later, demonstrating heartening resilience. They reached reproductive maturity years faster than their wild counterparts, and they became the first restored boulder-forming corals to spawn at sea.

It was a welcome milestone for scientists hustling to save corals from the devastating effects of climate change and other human-driven threats. Of the more than 800 known species of reef-building corals, the International Union for Conservation of Nature classifies more than a quarter as vulnerable, endangered, or critically endangered, and warns that as temperatures rise, so too does the extinction risk for corals.

Nearly 40 years ago Peter Harrison, a marine ecologist at Australia’s Southern Cross University, witnessed the first recorded large-scale coral bleaching event. Diving off Magnetic Island in the Great Barrier Reef, he was stunned by the scene before him. “The reef was a patchwork of healthy corals and badly bleached white corals, like the beginnings of a ghost city,” he says. Just months before, the same site had bustled with tropical life in Crayola colors.

“Many of the hundreds of corals that I’d carefully tagged and monitored ultimately died,” he says. “It was shocking and made me aware of just how fragile these corals really are.”

Coral exists symbiotically with photosynthetic algae, which live in its tissues and provide essential nourishment (and coloration). But high temperatures and other stresses can turn algae toxic. When this occurs, the algae may die or be ejected by the coral, a process known as bleaching because the coral’s clear tissue and white calcium carbonate skeleton are exposed. If the coral can’t reestablish its bond with algae, it will starve or succumb to disease.

The devastation Harrison saw in 1982 was repeated on many other Pacific Ocean reefs that year and the next. In 1997 and 1998 the phenomenon went global, killing some 16 percent of the world’s corals. With rising temperatures, pollution, disease, increased ocean acidity, invasive species, and other hazards, Harrison’s ghost cities are sprawling.

Scientists surmise that about four decades ago severe bleaching occurred roughly every 25 years, giving corals time to recover. But bleaching events are coming faster now—about every six years—and in some places soon they could begin to happen annually.

“The absolute key is dealing with global warming,” says marine biologist Terry Hughes of Australia’s James Cook University. “No matter how much we clean up the water, the reefs will succumb.” In 2016, a record-hot year in a string of them, 91 percent of the reefs that comprise the Great Barrier Reef bleached.

A reef is full of music. And motion. Clicks and taps, squeaks and gurgles accompany the shimmying of soft corals, the quivering of shrimp, the nibbling of fish, the skittering of crabs. Moray eels poke from hollows like open-mouthed puppets, reef sharks battle for a bite, curious cuttlefish loiter, then bolt. With thickets of elkhorn corals, and boulder corals like massive cakes iced in pinks and greens, their tiers decorated with lacy sea fans, tube worms, and feather dusters, reefs are fantastical—a Seussian stage set for a recurring daily drama. From every nook, cartoonish critters wave fins or claws or tentacles, each defending its little place in the world.

A dozen years ago I spent 15 “pinch me” days among them, diving the Great Barrier Reef with photographers David Doubilet and Jennifer Hayes. The article I wrote for this magazine celebrated the reef and sounded the alarm: We could lose this extraordinary place. As a journalist with a background in conservation biology, I fretted over failing ecosystems; as a diver and lover of the sea, I feared a much deeper loss. So when I saw the more recent photos of the reefs where we had dived—some now algae-strangled rubble fields—I imagined the stillness and silence, and I cried.

Despite that devastation, the Great Barrier Reef remains a colossus—some 3,000 separate reefs strung along 1,400 miles of Australia’s northeast coastline—and a rarity: Tropical, shallow-water coral reef complexes cover less than one percent of the seafloor. The death of even a single reef has devastating effects; these ecosystems support at least a quarter of all ocean life. Reefs are also vital for human populations, buffering coastlines from storms, sustaining fisheries, and luring tourists. Experts estimate reefs directly benefit more than half a billion people, contributing tens of billions of dollars a year to the global economy through tourism alone. Meanwhile, the value of coral reefs to the human psyche—by experiencing one, or just knowing they exist—is surely incalculable.

(See a graphic that shows how corals bleach and how scientists are trying to save them.)

That so many reefs are suffering in the heat, then, is immensely significant, but the effects differ. “Climate change moves as a uniform blanket across the Earth, but there is plenty of variation in the details,” says coral ecologist Charlie Veron, former chief scientist at the Australian Institute of Marine Science. “Coral bleaching is patchy, and local weather conditions are key: You may have monsoon clouds protecting the reef over here, but over there it’s blue skies and the sun is hammering the water.” That variability makes it especially difficult to design interventions in a broad way, Veron says.

For more than 20 years the National Oceanic and Atmospheric Administration has used satellite and on-site data as well as modeling to forecast when and where bleaching is likely to occur, “giving coastal managers some lead time, a chance to ramp up protective efforts,” says Mark Eakin, the agency’s Coral Reef Watch coordinator. This early warning system has led some resource managers to limit access in vulnerable reef areas, remove rare corals proactively, and experiment with installing artificial shading.

Such “emergency” strategies aren’t cheap, and they aren’t long-term solutions. They’re also useless where coral has already died. So scientists also are trying to rebuild reefs. Helpfully, although corals are animals, they can be grown much like plants: Collect cuttings, raise them in nurseries, graft the more mature organisms onto degraded reefs, and kick-start new life.

For decades ecologists have been honing this strategy for fast-growing branching corals. But until recently, few ocean farmers tried cultivating the real building blocks of a reef—such as boulder and brain corals, slow-growing giants that can live for centuries and take decades to reach reproductive maturity. Then came a breakthrough: Mote scientists discovered that “microfragments” sawed off these corals act a bit like wounded skin, growing extremely quickly—some 10 times faster than larger cuttings. Grown side by side in lab aquariums, polyps from the same colony will fuse, reducing the time needed to reach reproductive size. Raised this way, some species that typically take a decade or more to mature have begun spawning in just a few years.

Even the best tended garden isn’t immune to bad weather, and many nursery-grown branching corals eventually have succumbed to heat. So it’s critical, says Mote senior biologist Erinn Muller, to focus on corals with high heat tolerance. Muller is also studying whether there’s a link between temperature and a disease called stony coral tissue loss, which first appeared in the Florida Keys in 2014 and now has affected nearly every part of the 360-mile-long barrier reef. “Disease is a chronic problem for corals, so we screen for disease tolerance as well as heat tolerance and ramp up reproduction of those most likely to survive both in the coming decades,” she says. “That way we integrate resilience right into our restoration pipeline.”

Mote’s strategies also support reef recovery beyond Florida’s waters. Raising Coral Costa Rica—a team led by local and American coral reef ecologists—is both farming branching species and microfragmenting boulder species to revive ancient reefs in Golfo Dulce. These corals, some of them thousands of years old, are of particular interest: With the gulf fed by four rivers and flushed by tides, they are exposed to quick fluctuations in temperature, acidity, and salinity—making them well equipped to handle changing conditions. Their genes, and those of corals living in similarly fickle conditions, could hold clues for bolstering resilience elsewhere.

On the other side of the globe, Harrison knows that no matter how genetically outstanding a coral’s parents, any single larva has only about a one-in-a-million chance of surviving. He wants to improve those odds dramatically. “Larvae have limited control over where they go,” he says. The vast majority drift away, and if they do eventually bump into suitable substrate, he notes, there’s “a wall of mouths waiting to eat them.”

So Harrison’s teams scoop up slicks of eggs and sperm released by corals that have survived bleaching and proven their heat tolerance. Amassing the gametes in mesh enclosures near the ocean surface promotes fertilization and larval formation; those offspring can then be drizzled over damaged reefs. Harrison is testing two distribution methods: remote-controlled “LarvalBots” that squirt larvae onto reefs and ceramic plugs with larvae attached that can be stuck into gaps in the reef.

Targeted larval settlement has proven effective on research plots in the Philippines and on the Great Barrier Reef, but Harrison knows he needs to scale way up, spreading billions of larvae over miles of seafloor to make a difference.

The spawning events at which Harrison pilfers gametes are one of nature’s ways of maintaining strong genetic diversity—as eggs and sperm from different parents mingle. But as reef health declines, fewer corals are spawning successfully. After the 2016 and 2017 bleaching events on the Great Barrier Reef, Hughes and colleagues found that larval colonization had dropped 89 percent.

In a lab at the Australian Institute of Marine Science, geneticist Madeleine van Oppen is giving a push to the natural adaptations that could reduce such losses. By teasing out genes related to heat management in algae and bacteria that live in corals, she says, “we are starting to have a good understanding of these intimate associations and to make use of them.” By exposing this lab-grown algae to stepwise temperature bumps over years, she lets natural selection and random mutation do the work of boosting the algae’s heat tolerance—but on a fast track. Corals that accept these experimentally evolved algal partners have proven less prone to bleaching. Van Oppen also plans to “lab evolve” the bacteria that live in coral microbiomes.

“If we could inoculate coral with lab-grown algae and bacteria that can help neutralize heat stress,” she says, “we see the potential to increase thermal bleaching tolerance in the wild.”

The institute’s scientists are also creating hybrids, crossing corals adapted to warmer waters with cool-water ones of the same species to see if heat tolerance is passed to the offspring. The initial results are promising. “And we are creating hybrids between species, which can have enhanced climate resilience compared to their purebred counterparts,” van Oppen says.

Encouragingly, in some instances corals already are doing the job themselves: Scientists working on reefs around the world’s largest atoll—Kiritimati in the central Pacific—discovered corals that were recovering from bleaching during a heat wave. They did it by taking in naturally heat-tolerant algae.

These comeback corals, however, weren’t in crisis from other human-caused stresses when they bleached; those that had been stressed previously did not recover as well, illustrating the devastating impact of a one-two punch. Still, it’s a hopeful sign that scientists hadn’t seen in the wild.

Reefs worldwide are battered, having absorbed, on average, more than one degree Celsius of warming. “And they’re still there,” Hughes says. “The mix of corals has changed. It’s very different from five years ago, but that’s a source of resilience.”

He doubts, though, that the animals can survive two- to three-degree warming, and he worries we’re putting too much faith in reviving reefs. “Restoration is something of a distraction. The urgent need is to deal with the root reasons,” he says. “What’s happening to reefs is a crisis of governance—of water quality, of fisheries, and especially of greenhouse gases—and there’s work to be done in all three realms.”

Meanwhile, as global temperatures trend up, some scientists have taken to prepping—stashing hard corals in “living biobanks” to conserve as much diversity as possible. A holding facility in Sarasota, Florida, is already accepting U.S. specimens, while the nonprofit Great Barrier Reef Legacy and its partners have established the Living Coral Biobank in Australia, where a seaside “ark” will house the more than 800 hard-coral species from around the world.

“This is something we can do right now: Collect every species and tag them and keep them alive indefinitely, for genetic studies and, if possible, to repopulate the oceans with species extinct in the wild sometime in the future,” says Veron, one of the biobank’s founders. “It’s up to us to use every tool we have to keep reefs alive. My belief is, we can’t not do this.”

Jennifer S. Holland is a science writer and longtime contributor to the magazine. David Doubilet and Jennifer Hayes plan to work with Indonesian scientists to explore Coral Triangle conservation efforts.

This story appears in the May 2021 issue of National Geographic magazine.

The National Geographic Society is committed to illuminating and protecting the wonder of our world. Learn more about the Society’s support of ocean Explorers.

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