There hasn’t been a worse time to be a coral in recent history.
Record-breaking sea surface temperatures have persisted globally since March 2023. In that time, more than three-quarters of the world’s reefs have experienced heat stress intense enough to bleach corals, according to Derek Manzello, an ecologist who coordinates the National Oceanic and Atmospheric Administration’s Coral Reef Watch Program.
“There was this kind of existential crisis a year ago where people were like, ‘Oh, my God, are we witnessing the end right now?’” Manzello said. “The oceans are just basically getting so warm that it’s hard for them to keep surviving.”
In April, NOAA declared the world’s fourth mass bleaching event — one that continues today and is growing.
“This is by far the worst bleaching event that’s ever hit the Caribbean, in Florida as well as the South Atlantic and Brazil,” Manzello said, adding that “99.9% of all the reef areas in the Atlantic Ocean — the North and South Atlantic — experienced thermal stress within the last year, which is crazy. That’s never happened before.”
Coral reefs are home to roughly a quarter of all marine life, and they provide a natural barrier against storms. But they’re sensitive to temperature, and scientists have long worried they’d be among the first ecosystems lost to climate change.
So in hard-hit places like Florida and Puerto Rico, scientists are experimenting with new methods of restoring reefs and making corals more resilient to warmer seas. Those efforts could buy time for reefs to recover and for humanity to ratchet down greenhouse gas emissions.
Some recent successes, including reports of corals resilient enough to survive the intense heat, have buoyed researchers’ moods.
“We still have time to right the ship,” Manzello said.
In July, divers with a research team descended into tropical waters off southwest Puerto Rico, along a reef in La Parguera Marine Preserve. Schools of bar jack fish swam through, rays of sun bouncing off their silvery sides. A barracuda slinked past, menacing the smaller fish and surprising the divers, who were working with the Institute for Socio-Ecological Research (ISER Caribe).
The group was installing suspended homes for baby Diadema antillarum — a long-spined sea urchin — a creature that can aid coral’s regrowth by reducing harmful algae.
Nearby, a group of coral fragments was starting to take root; the researchers had nursed them back to health on land before they replanted them on the reef. Eventually, they plan to place 22,000 such fragments.
The reef boasted a diverse layout of corals but was showing damage. Colors were muted, and the “chatter” normally heard on a healthy reef — which sounds like static to the human ear — was missing. Another troubling sign: The water was about 86 degrees Fahrenheit, just shy of the temperature range where scientists worry about bleaching.
Corals are sessile creatures, which means they’re rooted in one place. They depend on symbiotic, photosynthetic algae that live within their tissues, produce nutrients and give them their trademark color.
When temperatures rise, the symbiotic algae can go haywire, producing harmful chemicals and too little food, which in turn stresses corals and forces them to release the algae. The process leaves the corals looking skeletal and white and puts them at risk of death.
“When the corals are bleached, they’re under extreme stress. So any other impacts, like water quality or UV radiation or sedimentation from land, all that additional stress will most likely kill these corals,” said Stacey Williams, the executive director of ISER Caribe.
The group is working to restore 5 acres of coral reef in Puerto Rico by planting fragments across six reefs and returning long-spined sea urchins to the ecosystem.
The urchins feed on harmful algae that thrive in warmer waters and can harm coral.
“They’re like the goats or the cows of the sea,” Williams said.
When corals die or become bleached, ecosystems can be overrun by such algae.
“If the ground is already covered by algae, the coral larvae will not settle there,” said Juan Torres-Pérez, a coral expert and a NASA research scientist, who grew up and studied in Puerto Rico.
In the 1980s, long-spined sea urchins died off across Puerto Rican reefs. Now, they struggle to survive past the early stages of life in La Parguera. So to give the urchins a boost, the ISER Caribe researchers have suspended pieces of AstroTurf-like material along several 25-foot-long lines, which are anchored to the sea floor by cement blocks.
The grasslike material offers a home for baby urchins to cling to. Divers collect the squares and bring the urchins to an on-land nursery to grow. Then, once the urchins reach young adult size, the researchers place them in a coral reef in need of extra support.
It’s one of many ecosystem projects testing new ways to help corals survive.
In Florida, University of Miami scientists for the first time imported corals to the U.S. that evolved in Honduras’ warmer waters. The scientists hope to breed the imported corals with Florida’s natives to produce a more heat-tolerant coral.
Andrew Baker, who directs the Coral Reef Futures Lab at the university’s Rosenstiel School of Marine, Atmospheric, and Earth Science, spent 15 hours flying with the corals in a cargo plane.
“We need a fast-fail approach and to stay open to new ideas,” Baker said, referring to a common mentality for technology development in engineering and business. “The natural state of things is fast going down the toilet because of climate change. As we do things to accelerate the response of these ecosystems to planetary change, the result of inaction is going to be much worse.”
Some efforts are beginning to show promise. In a study published Wednesday in the journal PLOS ONE, scientists reported that young, lab-reared corals bred for restoration projects in several parts of the Caribbean had survived the worst of the marine heat in 2023. The research suggests they fared better than wild adult corals in the same locations.
Scientists have warned about corals’ fate for years. In 2018, the Intergovernmental Panel on Climate Change estimated that 70% to 90% were at risk of “long-term degradation” if global temperatures rose by 1.5 degrees Celsius and that 99% would be at risk with 2 degrees of warming.
Last year, Earth’s hottest recorded year ever, had temperatures about 1.48 degrees above those of pre-industrial times.
Manzello said scientists used to think coral had a longer runway — perhaps until 2040 or 2050 — before conditions became so grim.
“Last year caught everyone off guard,” Manzello said. “The Caribbean last year was just unreal, and nobody expected things to get that hot that fast.”
Expensive, time-consuming coral restoration projects are unlikely to keep pace with losses because of climate change. But creating healthy pockets of coral can at least give reefs a chance to rebound in the future.
“You’re going to have to be very choosy and picky on where you put your efforts,” Manzello said. “But the bottom line is: For some species of coral, especially in places like Florida and the Caribbean, aggressive interventions and restoration are going to be the only things standing between those species’ eventually going extinct.”
Baker compared Florida’s reef systems to a jigsaw puzzle.
“We’ve probably lost 80, 90% of corals. Despite all of that, we haven’t lost any coral species yet,” Baker said. “We have messed that jigsaw puzzle up and broken it into parts, but we haven’t lost the pieces yet.”
Meanwhile, forecasters say the natural El Niño climate pattern that contributed to record ocean heat since spring 2023 has dissipated. The change could help cool the seas a bit — at least temporarily.
Evan Bush reported from Seattle and Maura Barrett from La Parguera Marine Preserve, Puerto Rico.