A brutal heat wave, intensified by accelerating global climate change, turned the shallow waters around the Florida Keys into the ecological equivalent of hot bathwater this summer—shocking, and possibly killing, entire swathes of the Keys’ fragile coral reefs.
The death toll startled marine biologists who were already struggling to protect the reefs, the countless sea creatures they shelter, and the coastal communities they support. The losses lit a fire under the small community of specialists who are racing against man-made climate change to help the reefs adapt to hotter waters.
“We need to give reefs the best chance to get over a decades-long period of climate stress,” Andrew Baker, director of the Coral Reef Futures Lab at the University of Miami, told The Daily Beast.
The strategy is simple: Find corals that have genes that make them resistant to the damaging effects of extreme heat—and help those corals breed.
But simple doesn’t mean easy. The same heat waves that make the selective breeding of coral so urgent also make this adaptation effort harder. That became painfully apparent this summer. The underwater nurseries where scientists raise many of their special corals themselves got too hot, necessitating an emergency evacuation.
The stakes are enormous—for the reefs and for us. Corals are an important link in ocean food chains that feed schools of fish that in turn feed millions of people. And the calcium structures that these invertebrate colonies produce—the “reef” in “coral reef”—break up waves and help prevent flooding in coastal communities.
But we haven’t been kind to our corals. Pollution, dredging, reckless navigation by ships and overfishing can damage reefs, but it’s the warming climate that has wreaked the most havoc. When a coral gets too hot, it can expel the symbiotic algae that lives inside it, and which gives it its color. Biologists call this “bleaching.”
If the heatwave is brief, the algae might return—and the coral might bounce back. But a bleached coral could need years to fully recover. The hotter the climate gets, the more frequent widespread bleaching events become. Some scientists fear it could become an annual occurrence for reefs in shallower, hotter waters.
A view of major bleaching on the coral reefs of the Society Islands on May 9, 2019 in Moorea, French Polynesia.
A coral that bleaches every year never gets a chance to heal either. As more and more coastal regions experience back-to-back heat waves and annual bleachings, extinction becomes a real possibility. “The worst-case scenario would be that we lose most of the corals and coral reefs we have today,” Madeleine van Oppen, a professor at the Australian Institute of Marine Science, told The Daily Beast.
The vast genetic diversity of reefs could save them—with a little human help. Among the countless billions of individual corals are a few individuals that, thanks to some lucky genes, are naturally a little bit tougher: less likely to bleach and more likely to recover when they do bleach.
If all corals possessed these genes, human-induced climate change might not bother them. The problem is that reefs have evolved across millions of years. It could take centuries, even millennia, for reef systems to select for heat-resistance and naturally breed themselves into a more resilient state.
But we don’t have that long. Thanks to our unabated burning of fossil fuels, the climate has warmed by nearly two degrees Fahrenheit since the 1950s—and could warm by another two degrees by 2050. Absent human emissions, it would take the Earth thousands of years to warm that much.
The challenge, for biologists, is to mash the genetic accelerator and hope that reefs can undergo millenia of evolution in just a couple of decades. “I am confident we will be able to enhance heat-tolerance in the field,” van Oppen said. “The question is by how much and whether this is enough to survive rising temperatures.”
Again, the process is simple if not easy. Researchers collect corals, test them for heat-resistance, breed them with other heat-resistant corals in tanks on land or in seafloor nurseries, then return these “supercorals” to the ocean and graft them onto natural reefs, where they should spread their genes the old-fashioned way.
But the nurseries themselves baked when the summer heatwave rolled over the Keys—and stayed. Spikes in heat aren’t uncommon in Florida. Spikes in heat that last months, like this summer’s did, are practically unheard-of in recorded history.
For weeks on end this year, shallower waters off of southern Florida were 100 degree Fahrenheit. That’s 15 degrees hotter than normal. “The total heat stress accumulation just grew and grew and grew,” Baker said.
The nurseries were in danger of being completely wiped out, a disaster that would’ve set back—potentially by decades—scientists’ effort to cultivate heat-resistant corals. So divers from conservation groups, universities and government labs organized an emergency rescue: They snatched up thousands of corals from coastal nurseries and returned them to tanks on land.
“Don’t judge the value of that effort based on the number of corals rescued,” Baker said. “The purpose was not to save the reefs, it was to save genetic diversity. Those few thousand corals are all independent genetic individuals—each can grow to large colonies. The coral cover can be brought back. But genetic diversity, if lost, is lost forever.”
Only now, months after the Florida heat wave ended, are the special baby corals going back into the water. Meanwhile, biologists are trying to assess the damage in the Keys and nearby coastal areas—and passing along a warning to their colleagues in the southern hemisphere as the brutal summer of 2023 shifts to the other half of the planet.
It’s clearer than ever that time is short. The planet is getting so hot, so fast, that efforts to breed heat-resistant corals might fall hopelessly behind.
If we fail and reefs can’t adapt fast enough, it’s possible the planet could lose most of its existing reefs—all in just a couple of decades. “We stand to lose coral reefs as we know them,” Ian Enochs, a coral expert with the National Oceanic and Atmospheric Administration, told The Daily Beast.
At that point, all those tough little corals biologists are breeding could become a kind of genetic reservoir. A Noah’s Ark for corals. The idea is that, if we ever manage to stabilize the climate—say, in the next 50 years or so—we might be able to seed dead reefs with the corals we’ve bred and kept safe, on land, for decades.
After that, we’d have to wait, potentially for centuries or even millenia, as corals slowly repopulated the skeletal remains of million-year-old reefs we let bake to death because we burned too much coal and oil.
Enochs said he doesn’t dwell on this worst-case scenario. “There's too much to lose,” he said, “and a lot that we can do now to solve the problem.”
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