Marine heat waves now reaching the world’s most remote reefs
Joe Pollock, senior reef strategy lead with The Nature Conservancy, calls the waters around Palmyra Atoll “one of the most stunningly beautiful reefs that you could ever imagine.” More than 1,000 miles south of Hawaii, the U.S. national wildlife refuge surrounds an atoll with no permanent residents and a marine ecosystem that researchers say remains largely preserved — from tiny reef fish to a thriving population of sharks.
The atoll’s remote location has long shielded it from human disturbance, but humans have left traces. The atoll was likely visited by Polynesians who crossed the Pacific Ocean and later became part of the U.S. after it colonized Hawaii. During World War II, the military constructed runways, roads and small buildings, but for the most part it’s had few human inhabitants. Since 2000, the atoll has been a hotspot of scientific research, where marine biologists can study a coral ecosystem that’s functioning as it should be.
The underwater world itself can feel like rush hour in a busy downtown. Schools of brightly-colored fish move among the towering corals. A peacock grouper rests on a piece of coral where a tiny cleaner wrasse darts out to nibble its dead skin and parasites, a visit to what’s essentially a fish spa. An octopus peeks from a hiding spot. Blacktip reef sharks slowly cruise the outskirts.
The corals themselves are the size of cars, sporting their own rainbow of colors. Some have delicate branching arms like deer antlers. Others are flat like undulating ribbons of fabric or round like massive boulders. “Almost every last square inch is covered in live coral, so much to the point that corals are trying to grow over each other,” Pollock says. “It’s coral on coral, which is exactly what you want to see.”
Climate change is now reaching even this remote location. Palmyra Atoll’s corals are already showing signs of heat stress this year due to abnormally warm ocean temperatures. Corals are highly vulnerable to heat, which turns them a bone-white color, a process known as coral bleaching. Prolonged heat can kill corals. Worldwide, the current bleaching may become one of the worst on record due to the powerful El Niño climate pattern that’s raising temperatures globally.
Three years ago, a marine heat wave hit Palmyra Atoll, raising water temperatures for more than half a year. Pollock remembers seeing some of the atoll’s most vibrant corals turn completely white. “It was devastating,” Pollock says. “You could feel in your body the suffering. Honestly, I didn’t know that it would bounce back.” Heat disrupts a vital relationship for corals. They get most of their food from their roommates — tiny algae that live inside the corals’ tissues. When stressed, those algae get expelled, making coral lose their color. Bleached corals slowly starve and die if a marine heat wave lasts too long. But if temperatures drop, corals can recover when the algae move back in.
But the corals are showing remarkable signs of bouncing back. “It is just an absolute sea of baby corals,” Pollock says, pointing to what look like small cauliflowers. “The parents of these corals were the survivors of that bleaching event.” Corals that survive bleaching often have a higher tolerance to heat, scientific research has shown. That ability can be passed on to the next generation, meaning these baby corals may have an advantage during future marine heat waves, thanks to their parents.
Palmyra Atoll’s coral may still have other lessons to impart. While some corals bleached during the marine heat wave in 2023, others nearby were largely untouched. The reefs also recovered at different rates. Understanding these differences could help uncover more about what makes some corals more heat-tolerant.
“Coral reefs are not doomed,” Pollock says. “Palmyra shows us that we’re not ready to write the obituary on coral reefs.”
“We actually know so little about the biodiversity that exists on the reefs,” says Tali Vardi, executive director of the Coral Restoration Consortium, a non-profit focusing on reef restoration. “Most of it is unknown and there are still species being discovered.”
Researchers studying Palmyra’s ecosystem have identified three ingredients that appear to support that resilience. The first is grazing fish. All over Palmyra’s reefs, throngs of brightly-colored parrotfish nibble at the corals. They’re eating algae, which grow quickly in the warm marine environment. The fish continually clean the surfaces with their grazing.
After a bleaching event, those janitorial services are crucial. Algae can quickly move in, covering the bleached and dead corals. “They’ll take any chance they can get to take over that real estate,” Pollock says. “Once corals have given up that ground to the algae, it’s really hard to come back from that.” Fish help keep the real estate open for corals, preventing them from getting overwhelmed by algae and making space for the next generation. When baby corals first form, they float in the current, searching for a place to settle. When a reef is largely clear of algae, there are more open landing spots available.
The second ingredient is a bubblegum-pink crust that grows on the reef, known as crustose coralline algae. This stony seaweed is a friend to corals, unlike many other algae. Its hard surface fills in gaps on the reef. Many baby corals prefer to settle on top of it when looking for a place to grow. “What it does do is it saves space for those corals to come back,” Pollock says. “It’s basically a placeholder that keeps the algae out and it keeps that area open for when the environment is right for the baby corals to come back, to have a place to settle and to have a place to thrive.”
The third ingredient is sharks. Palmyra’s corals also benefit from species at the top of the food chain. When biologists first started diving at Palmyra, they were astounded at the number of sharks. Studies show it has one of the highest densities of predators in the world. It’s a sign that the food chain is largely intact, which helps keep all species in balance.
The grazing fish that help clear algae are eaten by larger, predatory fish. But with plenty of sharks, those predatory fish that eat the grazing fish are kept in check, meaning populations of grazing fish don’t have too many predators and stay abundant. “It’s really that interconnected ecosystem that we have here that unfortunately is fairly rare in more impacted areas around the world,” Pollock says. Due to overfishing, sharks have become sparse on many reefs around the world. Globally, sharks and rays have declined by 71% since 1970.
As coral restoration projects ramp up around the world, researchers are trying to recreate some of the missing ingredients of healthy reefs. Efforts are underway to breed grazing animals that keep algae in check. In Florida, Caribbean king crabs are being released as part of coral restoration efforts, as well as sea urchins in Puerto Rico.
Scientists have also refined techniques at Palmyra Atoll to better target restoration work on reefs elsewhere. The Woods Hole Oceanographic Institution, Stanford University and The Nature Conservancy have developed a “Super Reefs” framework to identify reefs that stand the best chance at surviving a hotter future. They look at reef conditions, water currents, and genetically identify the most heat-tolerant corals. Working with local partners, they’re focusing on restoring those reefs, in places such as Maui and Belize.
Still, marine heat waves are becoming increasingly intense with climate change. At some point, most coral reefs won’t be able to keep up. “Restoration is not the long-term solution,” Vardi says. “There’s no world in which restoration can keep coral reefs around without changing the trajectory of our climate.”
If the world gets more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than pre-industrial temperatures, between 70-90% of coral reefs worldwide won’t survive, according to a report by the United Nations’ Intergovernmental Panel on Climate Change. Currently, the world will hit 2.6 degrees Celsius (4.7 degrees Fahrenheit) of warming by the end of the century, if countries follow through on their pledges to cut heat-trapping emissions from burning fossil fuels.
Coral reefs are biodiversity hotspots, but are also vital for many human communities. They provide food and livelihoods for hundreds of millions of people, as well as coastal protection against waves from stronger storms. “Given the value of coral reefs, given the fact that they support nearly a billion people’s lives and livelihoods around this Earth, it’s far too soon to give up,” Pollock says. “I honestly think we need to fight with everything we have to keep these reefs healthy.”
The NPR reporting on which this article is based was supported by the Pulitzer Center.