Can corals be saved? (2024)

As record ocean heat threatens corals off Florida and across the globe, conservationists are shifting their strategy

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In Florida, swaths of coral paint a colorful landscape across the ocean floor and serve a key role in its ecosystem.

But last summer, amid the longest marine heat wave in decades, many were scorched — drained of color and their survival left in question. It’s a scenario becoming much more common.

KEY LARGO, Fla.

With milk crates of corals in hand and scuba tanks strapped to their backs, Sam Burrell and his team disappeared under the water’s choppy surface. Heavy, breaking waves crashed against the charter boat anchored miles off the coast.

With each breath they let out, they descended beneath the surface and felt a sense of relief: On this November morning, they were finally returning hundreds of corals pulled out of the water earlier in the year after one of the hottest marine heat waves on record threatened to wipe them out. For months, the corals sat in temperature-controlled tanks in the shadow of the gulf’s bay until the waters were cool enough for them to go back — and though conditions weren’t ideal, this was that moment.

“Returning these corals felt a bit like a loved one leaving the hospital,” said Burrell, a senior reef restoration associate at the Coral Restoration Foundation, the largest nonprofit coral restoration group in the Florida Keys.

The afternoon sun was reflecting off the boat, but sand and other sediment made it look like the divers were swimming through a dark cloud. The underwater current pulled at them. Still, nothing could deter them from the day’s task.

Groups like Burrell’s had been prepared to do whatever it took to save the corals — even if that meant evacuating them each summer. But they now realize they need to radically shift their approach.

With record ocean temperatures threatening another dire summer, the National Oceanic and Atmospheric Administration and coral restoration groups are shifting their efforts to better keep up with the warming climate. The goal is clear: Find the survivors from last summer’s heat wave and focus on restoring areas where the species will have the best chance when heat strikes again.

How they execute the new plan could mean the difference between saving what’s left on Florida’s 360-mile-long coral reef and another summer of catastrophic loss. Already, some coral experts have questioned how far humans will go to keep corals alive in an environment that struggles to sustain them. At risk is the very future of these corals, a species that serves as an underwater city to more than 7,000 other marine plants and animals, survived prehistoric mass extinctions and outlived the dinosaurs. The collapse of coral in the Keys would also have an economic ripple effect: Thousands of people could lose jobs related to a key part of regional tourism.

Corals thrive in water temperatures between 73 and 84 degrees Fahrenheit, according to NOAA. But last summer, shallow-water temperatures in the lower Keys reached a walloping 101 degrees Fahrenheit. Other places simmered in the mid to high 90-degree range.

Florida is home to the third-largest reef in the world. Iconic sites along the reef have experienced a 90 percent decline in the past 40 years due to human-caused climate change, damaging hurricanes, disease and recreational activities.

As bleaching events increase in severity and frequency, it becomes harder for certain corals to bounce back. Last summer, corals in the Keys endured the hottest ocean temperatures on record, according to NOAA’s Mission: Iconic Reefs, a coalition of public and private groups. This heat streak has lasted more than a year: NOAA and the International Coral Reef Initiative just confirmed bleaching-level heat stress in every corner of the ocean.

Robyn Mast, a Key West reef restoration associate at CRF, recalled an emotional scene when she saw the havoc last year’s heat wrought on the species. Entire stretches looked like they were covered in a fresh layer of snow. The corals, full of color just days before, were bleached. As she dove closer, she realized many of the colorful coral left were burned to death: They had died so quickly that they didn’t even have the chance for color to drain. Temperatures were so hot that coral tissue melted from its skeleton.

“You spend so much time [in the ocean] that it kind of becomes a second home,” Mast said. “And when you go under the water and see your home like a house fire, and there’s just really nothing left … it’s hard to process.”

Many experts were stunned by the quick onslaught of the marine heat wave and how early in the season it occurred. Temperatures were so warm that NOAA added three new bleaching alert levels.

When corals are stressed, they release their algae and turn pale or white. A bleached coral doesn’t mean that the coral is dead, but that the coral is starting to starve — and last year, many corals in the Keys starved for three straight months.

NOAA’s restorative teams and coral groups across Florida felt then that they had no choice but to move thousands of corals to land. Now, they plan to focus on the heat-resilient coral survivors from last summer: boulder corals, a bumpy rocklike coral. Despite bleaching almost entirely in the U.S. Virgin Islands, boulder coral have made a full recovery, said Jennifer Koss, the director of NOAA’s coral reef conservation program.

Coral groups outplant coral to help rebuild reef coverage by taking small bits of coral pieces from living coral grown in nurseries and reattaching it to the reef.

Previously, restoration groups chose not to prioritize outplanting boulder coral because it took them longer to grow, and they were more susceptible to diseases. Instead, they prioritized staghorn coral, which resemble skinny deer antlers, and elkhorn coral, which look like thick moose antlers. These corals can grow quickly and can asexually form new colonies if broken up. But they are very heat sensitive.

According to recent NOAA data, less than 22 percent of 1,500 outplanted staghorn coral across five restoration sites survived the marine heat wave. Staghorn coral outplants only survived in the two most northern sites where temperatures remained the coolest. Nearly 4.9 percent of outplanted elkhorn coral remain alive. Researchers found no living staghorn or elkhorn corals in one of the southernmost reefs, where temperatures were the warmest.

Now, it will be up to the boulder and other heat-resilient individual corals to be the parents of the next generation, Koss said.

“This has been sort of a Darwinian event in terms of the corals that are left out there,” Koss said.

As of November, there were similar levels of coral mortality at Pickle’s Reef, an in-water restoration site off the coast of Key Largo. A dive under the water’s surface revealed scars from the heat wave still mark the bottom of the ocean. Miles of the seafloor — lined with thickets of dull staghorn and elkhorn corals — resembled brown, dirty carpeting. Schools of fish with stunning patterns and vivid colors stood in stark contrast against lifeless coral.

“It’s pretty sad because you put all this time and effort into planting these corals and then you show up the next year, and they’re no longer alive,” Burrell said.

CRF outplanted over 800 staghorn and elkhorn corals at Pickles Reef in 2023. After last year’s heat wave, more than 90 percent of the outplanted corals at Pickles Reef had died.

Restoration groups don’t plan to completely abandon staghorn and elkhorn corals, they are just taking a pause after losing so many last summer. CRF also plans to move nurseries to deeper, cooler water and is experimenting with different shading techniques.

Not all locations along the Florida coral reef track warmed equally. While some areas simmered, others experienced moderate impacts. NOAA and coral restoration groups plan to take advantage of the temperature patchwork to identify habitats that can withstand heat stress for future restoration efforts.

“We don’t want to go through all the effort of creating new corals and new coral tissue, and put it right into a spot where we know it’s likely to get hammered again,” Koss said.

If corals are abandoned, some groups worry, it could hurt the livelihoods of the people who rely on them. For Steven Campbell, a diving boat captain in Key Largo, news of severe marine bleaching took a toll. Nearly every time the phone rang after the heat wave began, he said it was a group canceling a dive trip.

Campbell, who’s been a dive instructor and boat captain there for 22 years, questions whether last year’s bleaching was an anomaly or a sign of climate change’s rapid intensification. But, pockets of resilience on outer reefs leave him optimistic. “I’m the kind of guy who, I live my life with my cup half full,” he said.

Still, some coral experts question whether current restoration practices are feasible long-term.

Terry Hughes, a professor of marine biology at James Cook University, thinks restoration practices only produce small scale interventions that fall flat, and may even be counterproductive.

“If we look at the evidence, from 50 years or so of coral restoration projects throughout the tropics, they have made very little difference. You could argue that they offer false hope and distract attention from addressing the root causes of coral reef decline,” Hughes said. “It’s an appealing message — that clever coral reef scientists can climate-proof reefs. Unfortunately, we cannot.”

It’s “logical” to outplant adapting coral, Hughes added, “since the alternative is to outplant dead corals that didn’t survive.”

Other coral experts think surviving corals should be allowed to adapt with less human intervention.

“We can’t shield corals from the changing conditions in the water,” said Michael Webster, a professor in New York University’s department of environmental studies who has studied coral reefs for nearly 30 years.

Researchers at CRF believe corals should be in the ocean as much as possible, Burrell said, but say it’s their duty to protect corals from conditions that are too extreme. “We’re their guardians essentially.”

It’s still too early to predict what this summer will hold, but the threat of another devastating marine heat wave looms. Leaders at NOAA have already observed premature heat beginning to accumulate in the ocean.