Scientists Discover Resilient Coral Reefs in the Marshall Islands

MAJURO, Marshall Islands—Perched on the bow of an aluminum landing craft, Anne Cohen gazed a few yards ahead of the vessel toward a yellow robot gliding across the emerald Majuro lagoon.

The unmanned surface vehicle, called Yellowfin, was quickly becoming one of the coral researcher’s most dependable guides in these Central Pacific waters.

“She’s the best dive buddy,” said Cohen, a tenured scientist at the Woods Hole Oceanographic Institution in Cape Cod. Programmed to navigate to a precise set of coordinates, the robot cut through small swells like a tiny sailboat without a mast, directing Cohen toward a destination she had traveled thousands of miles to revisit.

When the robot finally paused, hovering in place, Cohen recognized it as her cue. Somewhere below should be a patch of reef she’d been observing over the last few years, and she was eager to see how it was faring. Each visit carried a growing weight of uncertainty.

Since 2023, record-breaking marine heat waves have swept through the tropics, fueling the most severe global coral bleaching event ever recorded. More than 80 percent of the world’s reefs have been impacted in at least 83 countries and territories. Corals have been so stressed by the extreme temperatures, they’ve expelled the tiny algae living inside their tissues that provide them with food and their brilliant hues, leaving them pale, ghostly and struggling to survive. Many have not recovered.

Cohen hoped the reef beneath her might be different.

She yanked on her black and yellow snorkel fins, spit into her mask so it wouldn’t fog underwater and slid off the boat, her slight frame barely making a splash. Within seconds of peering into the blue, she let out a squeal muffled by her snorkel, astonished at the scene unfolding beneath her.

Towering pinnacles of chestnut-colored tabletop corals rose from the sandy seafloor like trees, their broad plate-like canopies sheltering fish hiding in their shadows. Dense thickets of staghorn corals stretched in every direction, their golden antler-like branches twisting across a sprawling reef extending as far as the eye could see, bursting with shades of mustard yellow, pink and lavender pastels.

“It’s like a wonderland,” Cohen said, popping her head above the surface, beaming. “I feel like Alice.”

In today’s oceans, the scene felt almost surreal, said Cohen, 62, who has spent the last 30 years studying coral reefs and the impacts of climate change on marine environments.

But it was a confirmation of something she had long believed: that even as hotter temperatures devastate coral reefs, some still possess an extraordinary ability to endure. She was determined to find out how.

Unlocking the secrets behind their resilience, she said, could one day help scientists and conservationists restore, or even cultivate, reefs better equipped to survive a warming planet.

Searching for super reefs

Over the last decade, a significant part of Cohen’s research has focused on tracking down these reefs that are somehow defying the odds.

In 2018, she started a project dedicated to this search called Super Reefs, named after a number of reefs she’d encountered around the world that seemed to be thriving even while others nearby bleached or died.

“We saw these corals that were behaving as if there was no heat wave at all,” she recalled. “I kind of felt like there was Superman or Superwoman coming in there and flexing their muscles, being super, super strong.”

Three years later she launched a joint global initiative with The Nature Conservancy and Stanford University aimed at not only finding heat-tolerant communities, but also protecting them.

Even the hardiest of reefs are not invincible, she said.

Coastal development projects such as ports or harbors that require dredging can bury corals beneath sediment. Agricultural runoff, sewage and plastic pollution introduce harmful pathogens and excess nutrients that spark coral disease or toxic algal blooms that suffocate the tiny animals. Bottom trawling—a fishing method that drags weighted nets across the seafloor—can crush entire reefs, while dynamite fishing can shatter centuries-old coral colonies in seconds.

“That would be like taking a sledgehammer to crush a hermit crab,” Cohen said.

Already, the world has lost more than half of its coral reefs to the combined pressures of climate change and other human activity. Some scientists warn that without significant intervention, more than 90 percent of tropical reefs could disappear in the next 25 years.

The goal of the new Super Reefs initiative was to specifically identify coral strongholds in places where governments had already demonstrated an invested interest in creating marine-protected areas—designated zones in the ocean where human activities are limited or prohibited to safeguard critical ecosystems.

Belize, Hawaii, and the Marshall Islands fit the bill. All had plans to create or strengthen already established marine-protected areas when the project launched.

This was important, Cohen said. She didn’t want to collect data just for the sake of it. She wanted to make sure the research her team conducted could inform practical decisions related to where and how to protect super reefs. The next challenge was narrowing their search.

Not every reef that shows signs of resilience is a super reef.

By definition, Cohen said, super reefs have to have scientifically proven capabilities of surviving hotter temperatures over time, either because they have genetically adapted to extreme heat or because local ocean conditions like cooler currents have shielded them. They also have to be able to potentially reseed other reefs.

“If we can protect these more climate-resilient reefs and make sure that they are protected from other human impacts like pollution or dredging or other things, then we’re securing those more heat-resistant strains for the future until we can really get global warming under control,” said Lizzie McLeod, global ocean director at The Nature Conservancy, who supported Cohen in the initial stages of the Super Reefs project.

Over the last few years, the Super Reefs team has identified resilient reefs in each of its target locations. But Cohen is convinced they have only scratched the surface.

“There are so many potential super reefs out there that we don’t even know exist,” she said. “We have to go find them.”

In the Marshall Islands, Cohen hopes some of these reefs might eventually become part of something larger. For years, she’d been dreaming of creating a vast network of protected super reefs that would span multiple Pacific island nations and be linked by ocean currents so that their offspring could help replenish reefs throughout the region.

“We want to create the first ‘super reef blue corridor’ across millions of square kilometers of ocean, connecting the Marshall Islands, Kiribati, and Tuvalu,” she said.

In April, Cohen visited the Marshall Islands to formally pitch her idea and test new technology she believed could dramatically accelerate the search for super reefs in the Pacific.

A nation built on coral

It was her seventh trip to the Pacific nation made up of 29 low-lying atolls and five islands. Majuro—the capital of the Marshall Islands—was one of those atolls, consisting of more than 60 tiny islands that encircle a lagoon spanning more than 100 square miles.

Each visit, she was struck by how intimately the Marshallese peoples’ lives are connected to coral. It formed the very foundation beneath their feet.

“Everything that you see, all the sand, all the land, is all made of coral,” Cohen said. “We wouldn’t be here without it.”

Long before people settled on these atolls, ancient corals built them over millions of years, slowly growing around the rims of volcanoes that eventually sank beneath the sea, leaving rings of reef encircling shallow turquoise lagoons. Over time, broken coral skeletons, rubble and sand accumulated into the thin ribbons of land where Marshallese communities have lived for thousands of years, in many places only a few feet above sea level.

Now, many of these are facing existential threats due to climate change.

A 2021 World Bank analysis shows 40 percent of existing buildings in Majuro are endangered by rising sea levels driven by global warming. Several of them are local schools.

“We’re the first to go with the sea level rise,” said Anthony M. Muller, the Marshall Islands’ minister of natural resources and commerce, speaking from his office overlooking the Majuro lagoon, where giant commercial fishing vessels flying flags from China, Panama, Liberia and Hong Kong sat anchored offshore.

For the Marshallese people, the prospect of losing coral reefs to climate change is also deeply unsettling, said Dua Rudolph, deputy director of the Marshall Islands Conservation Society (MICS), a Majuro-based non-government organization that has been collaborating with the Super Reefs team since 2020.

The majority of Marshallese people, he said, rely on fishing for subsistence or their livelihoods. And those fish depend on the reefs. It’s their home, source of food and spawning grounds. “When the reef leaves, the fish leave also,” Rudolph said. “People are going to start going hungry.”

Already, he said, many reefs throughout Majuro have experienced periods of extensive bleaching. He’d seen it firsthand while conducting in-water surveys to monitor reef health in recent years, especially in 2024, during the last El Niño event—a climate pattern that typically occurs every two to seven years and is often associated with intense marine heat waves.

The majority of Majuro’s formerly “pristine” reefs turned white, they were so stressed, he said. “To see it at that scale was pretty sad.”

So when Cohen first reached out to him via email about the possibility of working together to find and protect coral refugia defying such trends, he thought it sounded almost too good to be true—practically a “fairy tale,” he said.

It felt like this was an opportunity, he said, to “fight back” the “one big enemy that we’ve all been facing.”

Science for action

To begin searching for Majuro’s super reefs, Cohen worked with Woods Hole oceanographers Weifeng Zhang and Yan Jia to build a computer model that could simulate a decade’s worth of temperatures, currents and wave energy throughout the atoll’s lagoon. The goal was to pinpoint Majuro’s hottest waters—places where any surviving corals would likely possess an unusual ability to withstand extreme heat.

To test the model, Cohen asked Rudolph’s team to deploy underwater temperature loggers and current sensors on reefs throughout the Majuro lagoon. They identified several sites of interest, but one in particular stood out just offshore a community on the southwestern edge of Majuro, named Laura. There, the water temperature appeared to run nearly two degrees hotter than much of the rest of the capital, Cohen said.

Then Rudolph’s team from MICS collected samples from a variety of coral species at each site to test for heat tolerance with scientists from the Resist, Recover and Rebuild group at Stanford University, which studies how corals adapt to climate change.

Together, they built their first coral-testing lab on a local dock using picnic coolers, aquarium heaters, chillers and temperature controllers. Inside the coolers, they exposed the collected coral fragments to carefully controlled bursts of heat “designed to mimic the extreme temperatures reefs experience during hot days at low tide,” said marine biologist Stephen Palumbi, who oversees the coral recovery program at Stanford University’s Hopkins Marine Station. It soon became apparent, he said, which corals bleached quickly under heat stress and which ones could endure.

By the end of the experiments, Palumbi said they were able to rank which corals appeared most capable of surviving extreme heat and pinpoint where they came from. True to what Cohen’s model had indicated, some of the toughest were from Laura, inside the Majuro lagoon.

Rudolph and his team at MICS, along with staff from The Nature Conservancy, have since shared these findings with the community of Laura, with the hopes of building support for creating a locally managed marine area around the super reef identified off their shores. The effort is being guided by a process called Reimaanlok—Marshallese for “looking to the future”—a community-led conservation framework that brings together local leaders, landowners and residents to determine whether and how an area should be protected based on both traditional knowledge and modern science.

“The Reimaanlok process is designed to ensure conservation areas are community-led, culturally appropriate and sustainable over the long term,” said Alicia Edwards, the protected areas’ network coordinator for the Marshall Islands Marine Resources Authority, the government agency responsible for managing the country’s marine resources and fisheries.

At first, Rudolph said, some leaders were hesitant. The idea of limiting fishing in any area did not sit well in a community of about 900 people—almost all of whom are fishers.

The super reef site is a common fishing ground. At night, community members wade along the shallow reef flat using spears and machetes to snag fish and octopus, oftentimes crushing fragile corals beneath their feet without knowing it, said Jina David, a local councilman in Laura who has been advocating for the proposed protected area. Fishing boats regularly drop their heavy anchors onto the reef too, he said.

But after explaining the science and reasoning behind the idea of creating a protected area around the reef, Rudolph said, some attitudes have begun to shift, especially after learning that it would not only benefit Laura residents long-term, but likely other communities too.

“We don’t just do things that would benefit only one or two people,” Rudolph said of Marshallese culture. “More often, you see communities agreeing on things that will benefit more people.”

Research has shown that reducing fishing pressure and other human disturbances inside marine protected areas can help fish populations rebound and even spill into surrounding areas. Coral reefs that retain their diversity and ecological balance are also generally better equipped to recover from the impacts of climate change such as coral bleaching, said Edwards.

Cohen’s team had also found evidence that Laura’s super reef could serve as a source of recovery for the broader atoll. Using ocean current data to model where coral larvae released from the reef would likely travel, they found that the offspring from Laura’s most resilient reef could spread throughout Majuro, potentially helping populate distant reefs with the next generation of heat-tolerant corals.

Still, David said, it will probably be at least two years before the community comes to consensus to protect Laura’s super reef.

“We’re going to have to find a way to convinc