This coral reef has given scientists hope for years. Now they’re worried. 

If you plunge into the warm, blue waters of the Caribbean today, what you’ll see in most regions is actually quite bleak. Where there were once vibrant coral reefs teeming with sharks, groupers, and lobsters, there are now piles of rubble, carpets of green seaweed, and only the meager remains of a once colorful sea of coral.

Over the last 50 years, more than half of all hard corals — colonies of tiny animals called polyps that grow skeletons and build coral reefs — have disappeared in the Caribbean. The picture is even grimmer nearby in the Florida Keys, where coral has declined by 90 percent. As corals die, seaweed often takes over, which can make it hard for the reef to recover.

A range of mostly human forces has precipitated this loss: coastal construction, diseases, pollution, and, increasingly, climate change. Ocean warming is proving to be an endgame for reefs. As temperatures soar underwater, polyps lose a beneficial kind of algae that lives within their tissues and gives them energy and their vibrant color; without these algae the coral colonies are white, or “bleached,” and can starve to death. Since early 2023, record ocean temperatures have caused one of the worst bleaching crises ever recorded.

The small island of Bonaire, however, tells a different story.

East of Curaçao in the south Caribbean, Bonaire is a volcanic island just half the size of Chicago. And within its waters is a bustling marine metropolis. A large reef circles the island with towering, centuries-old corals, where countless creatures reside, from seahorses and sea turtles to hammerhead sharks and rays.

Many marine scientists have for years considered this undersea Eden to be the healthiest coral reef in the Caribbean, and among the healthiest in the world. And in this vibrancy lies a hopeful story: long-term research suggests that this reef has — unlike most — successfully recovered from past bleaching events and storms that killed off a large portion of its coral.

“Bonaire’s coral reefs appear to have uniquely resisted the changes that have swept through the Caribbean,” Robert Steneck, a marine ecologist, and a team of scientists wrote in a 2019 study.

Over one week in July, I traveled to the island to understand how it has defied the broader Caribbean trends. Perhaps, I thought, Bonaire has lessons to share that could help corals elsewhere survive for a few more decades, as countries move to slash their carbon emissions.

As I learned from interviews with nearly 20 scientists and longtime divers, and experiencing a dozen dives myself, even the most resilient coral reefs have a limit to what they can tolerate — and Bonaire is starting to exceed it. This is a worrying reality. If this coral reef can’t survive the coming challenges, can any of them?

Life on land in Bonaire is nothing like it is underwater. Shaped like a boomerang, with the bend pointing east, the Dutch island is mostly desert, almost Martian in some places. Tall cactuses line the roads, where wandering donkeys often create traffic jams. A handful of salt flats are feasting grounds for flamingos, which hunt the crustaceans that turn their plumage pink.

On a stifling July morning, underwater photojournalist Jenny Adler and I drove up the west coast. Our destination was a dive site in the north, where I’d heard the coral was especially plentiful — an example of Bonaire’s enduring richness. Scuba tanks rattled in our truck bed as we dodged iguanas skittering across the road. Neon green parrots flew overhead.

After about 45 minutes of driving, we turned onto a road by the sea made of loose coral rubble. There was no sign of human life — no other people, no other cars, no buildings — but in the distance was a pile of old tires and driftwood. This makeshift structure marked the dive spot.

One reason why Bonaire is so popular among divers is that its reef is incredibly close to shore, often in just 10 or 20 feet of water. You don’t need a boat, as is typical elsewhere. You can just swim out and sink down.

Wearing tanks and air-filled vests, we walked into the shallows, stepping carefully through a minefield of sea urchins. The water was as warm and clear as a heated pool. Once we could no longer touch the bottom, we put on our fins, emptied the air from our vests, and descended, until all we could hear was the sound of our own breathing.

Just like that, we entered another realm.

Giant coral structures rose from the seafloor in a mind-bending array of shapes, from cones and obelisks, to boulders and miniature mountain ranges. Some of the colonies looked like messy stacks of dinner plates. Others, like chunky fingers.

We cruised around and peered into crevices, most of which were occupied. Lobsters, eels, and puffers all lurked in the shadows. A school of blue fish with pointy faces, known as tangs, briefly enveloped us. And an endangered hawksbill sea turtle swam by in the distance, with an almost mesmerizing grace. We were like alien visitors descending on another world.

Reefs like this are not just a spectacle. They’re also essential to life on Earth.

While they cover less than 1 percent of the ocean floor worldwide, they support roughly a quarter of all marine species. Those include animals that humans regularly eat, like groupers and lobsters.

Beyond supplying us with food, coral reefs also defend coastal communities. During hurricanes, their large, bony structures help dampen storm surge that could otherwise flood houses, streets, and buildings. In the US alone, reefs safeguard the homes of more than 18,000 people and avert $1.8 billion in flood damage each year.

Then there’s tourism — the economic engine for Bonaire and many other Caribbean islands. An estimated 70 percent of foreigners who visit Bonaire come to dive and snorkel, according to the Netherlands government. The coral is such a popular attraction that many of the license plates in Bonaire have the slogan “Diver’s paradise.”

The reef is “what the majority of our economy depends on,” said Roxanne-Liana Francisca, senior conservation manager at an environmental group called STINAPA, which manages the island’s national parks. (One of them encompasses the reef.) “There’s a built-in appreciation for nature that’s part of Bonaire’s culture.”

It’s a good thing then that, for much of the last century, the reef in Bonaire has remained relatively healthy. Research suggests that the total cover of hard coral — a marker of reef health — is more extensive than in most other Caribbean reefs (some of these studies are dated). There’s also a bounty of certain species that have largely vanished from the wider region, such as the striking staghorn coral that grows branches that look like studded antlers.

“Bonaire is what a well-functioning, healthy reef should be,” said Sophie McCoy, a marine ecologist at the University of North Carolina Chapel Hill, who researches mats of algae-like bacteria that colonize these ecosystems. “We’ve been studying Bonaire in my lab as an example of how a reef should be functioning.”

Life appears everywhere you look. On a shallow dive up the road from my hotel, I accidentally spooked a large reef octopus as I floated over a field of staghorn. It puffed up, and then shaped itself into a torpedo and swam away. While snorkeling offshore from a popular beach bar I saw a spotted eagle ray (the same species as the teacher, Mr. Ray, in the movie Finding Nemo). The shallows were sprinkled with sand dollars that looked like loaves of sourdough and — my favorite — queen conchs. They’re those giant snails with absurd googly eyes that peer out like a periscope from their shells.

It seems that Bonaire has held onto an abundance of marine life that simply doesn’t exist in most other parts of the Caribbean.

For one, it’s in a lucky location. Bonaire, along with the islands of Curaçao and Aruba, are below the hurricane belt, the path that most major Atlantic storms often take. That means many of the region’s most damaging hurricanes — such as this summer’s record-setting Hurricane Beryl — leave Bonaire and its coral relatively unscathed. Plus, until more recently, the island had a small population and little commercial development, which has limited things like pollution and construction that could harm the reef.

But there’s another potentially critical reason why coral in Bonaire is so abundant: half a century of fishing restrictions. The island banned spearfishing in 1971 and soon after established one of the world’s first marine parks. The park, or marine protected area (MPA), encircles the entire island and prohibits certain kinds of fishing, anchoring, and other activities that can damage coral. Unlike most MPAs in the Caribbean, which fail to limit commercial activities, this park has successfully restrained fishing.

What’s key here is that certain fish — namely, parrotfish — eat seaweed, which, if left unchecked, is the enemy of coral. As seaweed, or macroalgae, spreads, it can poison corals and make it hard for baby polyps to settle on the seafloor and grow into adult, reef-forming colonies.

“The biggest impediment to coral recovery is macroalgae,” said Robert Steneck, a professor emeritus at the University of Maine, who’s been studying Bonaire’s reef for more than 20 years. “And the biggest way to control macroalgae is with herbivores.”

As Steneck and some other scientists see it, fishing restrictions lead to more parrotfish. More parrotfish — and particularly large ones — lead to less seaweed (especially in the absence of other herbivores, like long-spined sea urchins, which have largely disappeared across the Caribbean). And less seaweed leads to more coral. Put together, this suggests that strong marine protections can help reefs bounce back after they get wrecked by bleaching or other threats.

The virtue of MPAs as a coral conservation strategy is surprisingly controversial. What’s clear is that even the strongest protections, whether they prevent fishing, pollution, or any other local impacts, won’t do much for reefs during an extreme heat wave or storm. Such threats don’t care about park boundaries. That’s why some scientists have railed against MPAs as a coral-saving strategy.

“They can’t keep temperatures out,” John Bruno, a marine ecologist at the University of North Carolina Chapel Hill, told me. “They can’t keep climate out.”

But according to Steneck, Bonaire is living proof that MPAs can indeed make reefs more resilient, even in the ailing Caribbean. You just need loads of herbivores — which the island has.

In Bonaire, the total biomass of parrotfish (a figure that incorporates both abundance and size) is among the highest in the region. That number has declined in recent years, for reasons that aren’t entirely clear, though the island still has far more parrotfish than most other Caribbean reefs.

I saw dozens of them on every dive, some as large as human toddlers. They cruise around, stopping often to scrape algae off of rocks and coral with a set of big, beak-like teeth. (Parrotfish also eat the coral itself — and their excrement turns to sand — but the impact to coral is minimal, Steneck said.)

This profusion of parrotfish has helped Bonaire’s reef swiftly recover from damaging events in the past, Steneck said. In 2008, Hurricane Omar lashed Bonaire, damaging homes and many of its shallow reefs. Then in 2010, a marine heat wave drove a massive bleaching event in the southern Caribbean. Together, these events killed off 22 percent of the live hard coral, according to a 2019 paper led by Steneck.

But remarkably, the coral recovered. By 2017, coral cover had returned to its pre-destruction state, he found — a truly significant feat. “This is the first example of a resilient Caribbean coral reef ecosystem that fully recovered from severe climate-related mortality,” Steneck and his coauthors wrote in the 2019 study.

What corals in Bonaire appear to show is critical: Long-term protection can help coral reefs survive. Again, the marine park is not the only reason why the reef is still standing. It’s also not representative of other MPAs, which are often poorly managed and allow industrial fishing practices. Still, it shows there are ways to help coral withstand warming, short of putting an end to climate change.

“You would be hard pressed to find reefs elsewhere that are in as good shape as the healthy reefs of Bonaire,” said Nancy Knowlton, a renowned marine scientist and author, formerly with the Smithsonian National Museum of Natural History, “which, as the data show, are a result of strong management.”

There’s a pretty enormous caveat to this otherwise hopeful message. While well-protected reefs with an abundance of coral may be able to recover from a few severe blows, it’s hard for them to survive a neverending deluge of threats. Coral grows slowly, on the scale of a few centimeters each year. Recovery takes time. No ecosystem — no matter how protected, no matter how healthy — can withstand the combined force of bleaching, pollution, and disease, year after year. Even the largest, most pristine stretches of the Great Barrier Reef aren’t safe.

Neither is Bonaire. While its reefs may be famously beautiful and packed with coral, this undersea ecosystem has still declined significantly in recent decades due to rampant coastal development, bleaching, and disease. And I saw these sorts of threats playing out in real time. On every dive we noticed a handful of corals that looked pale, either because they were starting to bleach or because they hadn’t yet recovered from bleaching last year. That’s the problem with climate change: As the baseline ocean temperature rises, bleaching events are becoming more common, giving corals less time to recover.

Right now the surface water in Bonaire is close to 85 degrees. That’s far hotter than it typically is this time of year, and it’s enough to cause another widespread bout of bleaching. This could happen any day now.

What’s even more alarming is the spread of disease. Over the last decade, a sickness called stony coral tissue loss disease (SCTLD) has been spreading in the Caribbean and killing off a huge portion of the region’s hard, reef-building corals. Last spring, it appeared for the first time in Bonaire, where its impact has been quick and brutal. Preliminary research by STINAPA suggests that SCTLD — which essentially liquifies the coral colony from the inside out — has already wiped out nearly all of some species around the island, including maze, flower, and boulder brain corals. “It’s just devastating,” said Caren Eckrich, an ecologist at STINAPA.

On a breezy morning, we joined biologists from STINAPA on a pair of dives to evaluate the damage from this disease. As we cruised along the reef, the biologists, including Francisca, pointed out signs of infection. The sickness appeared as white blotches radiating outward, or as a wave washing over the colony; SCTLD eats away at the colorful tissue, leaving only the calcium carbonate skeleton behind. Some corals, which were large enough to be decades or centuries old — colonies can live for thousands of years — were completely dead and covered in green and red algae.

This reef may be one of the healthiest in the Caribbean — but it isn’t healthy. It’s sick, and it is literally dying before our eyes.

People who haven’t been diving in Bonaire for decades struggle to grasp the scale of this destruction, Francisca told me, as we took off our vests and wetsuits after the first dive. They suffer from what she calls “shifting baseline syndrome.”

“I’ve been looking at this dive site for six years, and there used to be so much more coral,” said Francisca, who grew up in the nearby island of Curacao. “But if you’re coming in new or if you’re coming in from other places, you’re like, ‘What are you talking about? This site is amazing!’ It has definitely declined.”

I’m susceptible to this syndrome myself.

Earlier in the week I went diving on the east side of the island, where few people get in the water. The coastline here is lined with sharp rocks and battered by large waves. You typically need a guide to scuba dive, and mine was a Dutch man named Bas Tol, who’s been diving in Bonaire for three decades. With tanks strapped to our back, we jumped off a small cliff into the choppy water below.

Through my eyes the reef was mesmerizing. There were massive sea sponges and coral structures twice my size. We also encountered what might be the cutest creature in the entire ocean: a baby trunkfish. When they’re young, these animals are perfectly spherical and not much larger than a chickpea. This one looked like a black and white polka-dotted orb floating in the ocean.

But to Tol, this scene was a bit depressing. “What you saw today is half of what was there last year,” he said as we dried off by his truck. The reef is “under constant bombardment,” from bleaching, and now SCTLD, he said. “This is what I’ve been seeing for years. It’s just not getting better. It’s getting worse.”

Comments like this raise a much graver concern: If Bonaire’s reefs, which are among the healthiest and best protected in the world, are starting to fail, it’s hard to imagine that those elsewhere, and especially in the Caribbean, are anything but doomed.

What I saw in Bonaire is, on one hand, a stinging indictment of marine protection. Why invest in restricting fishing and other harmful activities if bleaching and disease will just sweep through the reef anyway?

From another perspective, it’s those decades of marine protection that have given Bonaire something to lose. The island is facing ocean warming and SCTLD with a greater amount of coral to begin with. And that inherently makes the reef more resilient. Because some corals are naturally more resistant to diseases and the effects of extreme heat, a larger baseline population should have more survivors. Then there are all of those parrotfish. The years to come will be the ultimate test of Steneck’s theory: that a squadron of herbivores will help the reef bounce back.

“Even though we have disease and climate change, we’re doing way better than most reefs around the world because of the conservation for more than 60 years,” said Ebby Jules, a local dive instructor who’s been diving in Bonaire since the late 1980s.

In the long term, there’s no way to ensure a future for corals, at least as they appear today, without quickly slashing carbon emissions. That much is clear. Yet what’s happening in Bonaire reveals that there are, in fact, ways to strengthens the ability of coral reefs to withstand the threat of warming. Here’s how Steneck’s 2019 paper put it: “It is easy to conclude that the long-term prognosis for coral reefs is poor, but it is inaccurate to say there is nothing humans can do to halt or slow the decline of these beleaguered ecosystems.”

And this point is critical for Caribbean islands, because they have very little influence over the global climate. Comprised of small nations and territories that lack huge industries, the region’s share of carbon emissions worldwide is less than half a percent.

“There is absolutely nothing the Caribbean can do about lowering carbon emissions,” Francisca told me. “But we can do something about those local stressors. Hopefully, by addressing those local stressors, we continue to build resilience into the system.”

Beyond patrolling the marine park for illegal fishing and anchoring, STINAPA is now using antibiotics to treat corals that are infected with SCTLD. Though slow-going, this approach appears to be blunting the disease’s impact. Other organizations, like Reef Renewal Bonaire, meanwhile, are growing baby corals in nurseries to eventually plant on the reef, like saplings in a degraded forest.

“What we were trying to do is keep as many individuals as we can alive,” Francisca said. “The longer they’re alive, the longer they’re reproducing, and the longer they’re reproducing, the more babies are out there.”

As Francisca sees it, the goal is to keep the ecosystem functioning until other countries — and their politicians and corporations — figure out how to wrestle down greenhouse gas emissions. This approach has a time limit; marine protections will only stave off coral collapse for so long. Heat is killing these animals today. But even buying reefs a few more decades helps, said Francisca, especially in a place like Bonaire.

“Reefs are not just ecological systems, they are cultural and historical systems,” Francisca said. Even if protecting corals only buys 20 years, she said, that helps. “That is 20 years of fishing for the people who depend on fisheries, that is 20 years of tourism for the people who depend on tourism.”

“We can’t afford to just give up,” she added. “If we just give up, we’re giving up on our history, giving up on our heritage, but also giving up on our livelihoods.”