For 20 years now, Ken Nedimyer has been strapping on his scuba gear and diving into the waters off the Florida coast in a desperate effort to restore coral reefs that have been decimated by climate change and pollution. In 2019, he founded his latest venture, Reef Renewal USA. The group’s YouTube channel shows Nedimyer and other members underwater, carefully attaching nursery-grown coral to structures designed to build healthy reefs.
“We’re working hard under pressure with innovation, speed, and efficiency to repopulate our coral reefs,” the narrator says.
Diver-conservationists like Nedimyer will lose the race against time, scientists say, unless humanity acts quickly to end emissions of climate-warming pollution. In the Southern Hemisphere’s Coral Sea, home of Australia’s Great Barrier Reef, extreme temperatures have recently hit their highest in 400 years, according to an article in the journal Nature.
“If we don’t divert from our current course, our generation will likely witness the demise of one of Earth’s great natural wonders, the Great Barrier Reef,” paleoclimatologist Ben Henley at the University of Melbourne told the New York Times.
‘Out of sight’
According to a 2023 Pew Research poll, a majority of Americans consider global warming to be a major threat. If you drill down a bit and ask this group which ecosystem most concerns them, odds are they’ll cite tropical rainforests, or maybe alpine areas or the Arctic tundra.
And they’re not wrong to be concerned about these important communities. But our terrestrial bias blinds us to what is arguably an even more endangered ecosystem lying beneath the ocean’s surface.
“Coral reefs suffer from an ‘out of sight, out of mind’ dilemma,” said Jessica Levy, a marine biologist working for the Florida-based Coral Restoration Foundation.
“What we’re looking at is the potential loss of an entire ecosystem, which we’ve never experienced in human history,” Levy said, “and I don’t think anyone wants to find out what that would mean if we had a complete collapse of our coral-reef ecosystems.”
The threat of coral bleaching
Marine biologists use the term “bleaching” to describe how corals respond when ocean waters get too hot and symbiotic algae living in their transparent tissue are expelled in response to stress, exposing the coral’s bone-white skeleton.
Bleaching has long been understood as a localized response, but that changed in 1998, Derek Manzello said.
“That summer was the first time on record that we experienced global coral bleaching,” he said.
Manzello coordinates the National Oceanic and Atmospheric Administration’s Coral Reef Watch, a federal program that integrates data from satellites and in situ devices to monitor ocean and weather conditions to warn of potential bleaching events.
Bleaching isn’t necessarily fatal. The algae may return to the coral tissue, but only if the water cools down fast enough.
“Whether corals survive depends on both how hot the water gets and how long temperatures remain elevated,” Manzello explained.
The summer of 1998 was brutal in both regards. The Florida reef tract experienced above-normal sea surface temperatures for 12 weeks. Ten thousand miles away, surface waters were even hotter on parts of Australia’s Great Barrier Reef, reaching nearly 90 degrees for 12 days. In the Indian Ocean, water temperatures surrounding the island nation of the Maldives remained far above normal from April through June – and down to a depth of 100 feet. These high temperatures combined with the duration of the heat anomalies were lethal in all three ocean basins. In that one catastrophic year, the planet lost 16% of all coral reef cover.
A report issued by the Australian Institute of Marine Science in late 1998 acknowledged that the reefs could regrow if “this is just a severe, one-off event.” The report ended on a cautionary note: “If, however, the recent bleaching is linked to global climate change and will be repeated regularly … the consequences would be serious for many coral reefs.”
Since the language of science is framed with caution, just how serious those consequences could be was left unstated.
Marine scientists later concluded that human-caused climate change was responsible for the carnage on coral reefs in 1998 and that the disaster was far from being a one-off event.
“It’s kind of crazy,” Manzello said. “It’s 26 years later and we’re in the midst of the fourth global bleaching event on record. This current one began in February 2023 and is so much worse than what happened in 1998. And it’s so much worse than the second bleaching global event [2010], and it’s looking like it’s going to be worse than the third global bleaching event [2014-2017].”
‘Rainforests of the sea’ in peril
The ocean is on the watery front line of the climate crisis. That’s because of all human-caused warming that’s occurred since the dawn of the Industrial Revolution, 90% has been absorbed by the ocean. Without this buffer, average land-based temperatures would be far higher than they already are.
“Coral reefs are paying the price for this buffering,” Manzello said.
This most biodiverse marine ecosystem covers less than 1% of the ocean floor but is associated with a quarter of all known marine organisms, or around 1.3 million species.
“Coral reefs are the rainforests of the sea,” Manzello explained.
Out of sight may be out of mind, but that doesn’t insulate us from what happens beneath the waves. The destruction of coral reefs has critical implications for humanity. Several species of fish that depend on reefs are the primary source of protein for millions of people and income for millions more.
Reefs also protect coastal communities from hurricanes and cyclones, absorbing 97% of wave energy during a storm. But that only happens if the reefs are healthy.
“If corals die off, Manzello said, “the reefs will eventually flatten from erosion. That’s going to mean more damage to coastal communities during storms, a problem that will only be exacerbated by sea level rise.”
Other dangers to coral reefs
Climate change isn’t the only threat to reefs.
“Reefs have already been battered by so many human activities,” said Jessica Levy, who listed a variety of scourges including overfishing, damaging fishing practices like trawling and blast fishing – using dynamite to stun and kill fish – and pollution that flows from rivers into the ocean.
Karen Neely’s work as a coral ecologist at Nova Southeastern University in Florida focuses on another major threat: disease.
“We talk a lot about coral reefs seeing death by 1,000 cuts,” Neely said. “Climate change and coral disease are the two biggest threats to emerge in the last couple of decades.”
Though disease is a natural phenomenon, Neely pointed out that climate change exacerbates the problem.
“Corals get stressed when it’s really hot,” she said. “Just like with humans, stress can weaken their resistance to disease. And a lot of coral pathogens actually do better in warmer waters than they do in colder temperatures.”
An improved environment for disease-causing microorganisms coupled with lower resistance in corals is having a devastating effect.
A race to save corals
Nedimyer understands the urgency of the situation all too well. In the 1970s, he ran a small business in the Florida Keys collecting and selling tropical reef fish and live rock for the aquarium trade.
“I have a memory of what it looked like 50 years ago seared into my brain,” he said. At the time, coral cover on the major Florida reefs was about 45%. Today, it’s down to just 2%. “After a while, I was basically just watching the reefs dying around me, and I thought, somebody needs to do something. I need to do something.”
Nedimyer used his extensive experience diving on reefs and a degree in marine biology to become a pioneer in the field of coral propagation and reef restoration, growing corals from small living fragments taken from healthy corals. When they’ve grown large enough, the corals are moved from nurseries in 20-30 feet of water and outplanted back on the reefs in places that had died.
His operation, Reef Renewal USA, has grown from a small-scale experiment to a far larger and more technologically advanced project, with hundreds of thousands of corals outplanted in the Keys alone.
“Ken Nedimyer is basically the godfather of coral gardening,” said Reef Watch’s Manzello. “His work has been the foundation for almost all the coral restoration work that’s taking place in the Caribbean.”
Originally, when selecting which corals to grow, Nedimyer could only choose between coral species. But advances in genetic testing in recent years have allowed him to focus on growing subsets of coral species known as genotypes. Since some genotypes are more tolerant of warmer water, those became the ones Nedimyer sought for propagation.
“We went to areas that had gotten extremely hot in the past,” he explained. “We were looking for the winners, the survivors in these extreme environments.”
But in 2023, the ocean reached record-high temperatures that lasted longer than ever before. Reef Watch began issuing warnings in mid-May, three months ahead of what was once the norm. Previous bleaching events in the Keys did not occur until mid-August, and the extreme conditions put Nedimyer’s new techniques to the test.
“I’ve been diving down here for 50 years,” he said, “and I’ve never seen what I saw last summer. It was just crazy.”
As the summer progressed, coral restorationists hoped that their nurseries would survive the heat. Under Nedimyer’s direction, Reef Renewal USA worked at breakneck speed to create a nursery at a depth of 70 feet and moved 1,200 corals there.
“We had 120 different genotypes of elkhorn coral, and we moved 10 of every genotype to the deeper water,” he said.
The nursery remained there until the ocean started cooling off in September, when Nedimyer began raising the corals in the water column, a few feet at a time.
“Most of the corals in the nurseries that weren’t moved didn’t fare too well,” he said. In fact, few of them made it past July, regardless of genotype.
“The water went from hot to deadly hot in a single week,” Nedimyer said. “Most of them didn’t even have time to bleach. They just cooked and died. I don’t think anybody had ever seen that happen before.”
But a majority of the 1,200 coral fragments Nedimyer had moved to deeper water survived, providing a new potential path forward for reef restoration.
Given how dire the situation has become, even with these innovations, the question remains: Can coral reefs be saved?
“That’s the question that keeps me up at night,” Levy said. “It all depends on how you define ‘coral reefs’ and ‘saved.’”
She is convinced that saving the reefs is still possible, given advances in genetics and changes in how the entire field is being reorganized from heavy reliance on university-based research to a multidisciplinary endeavor with programs constructed on a much larger scale.
“It’s not just your average coral reef biologists working on these problems,” she said. “Now we’re seeing folks from engineering, materials science, and other disciplines all coming together for the common good. That’s where I think we’re going to see some really exciting and positive stories down the line.”
Still, she worries that reefs, like the one in Florida, may not get back to their historical baselines of biodiversity and coverage: “I think we’re just way past that point. Restoration needs to look at preserving what we have. In places like the Keys, we’re looking at just preventing extinction.”
We help millions of people understand climate change and what to do about it. Help us reach even more people like you.