There hasn't been a worse time to be a coral in recent history.
Globally, record sea surface temperatures have been building since March 2023. During that time, more than three-quarters of the world's reefs have experienced heat stress intense enough to cause coral bleaching, said Derek Manzello, an ecologist who coordinates the National Oceanic and Atmospheric Administration's Coral Reef Watch Program.
"There was a kind of existential crisis a year ago where people were like, 'Oh my God, are we witnessing the end?'" Manzello said. "The oceans are just getting so warm that it's hard for them to survive."
In April, NOAA announced the world's fourth mass marine bleaching event, an event that continues and increases today.
"This is by far the worst bleaching event that has ever happened in the Caribbean, in Florida, but also in the South Atlantic and Brazil," Manzello said, adding that "99.9 percent of all reef areas in the Atlantic - the North and South Atlantic - have experienced thermal stress in the past year, which is insane. That has never happened before."
Coral reefs are home to about a quarter of all marine life and provide a natural barrier against storms. But they are sensitive to temperature and scientists have long worried that they could be among the first ecosystems to be lost to climate change.
So in hard-hit places like Florida and Puerto Rico, scientists are experimenting with new ways to restore reefs and make corals more resilient to warmer seas. These efforts could buy reefs time to recover and help humanity cut greenhouse gas emissions.
Some recent successes, including reports of corals resilient enough to survive the intense heat, have boosted researchers' spirits.
"We still have time to get the ship back on course," Manzello said.
In July, divers with a research team descended into tropical waters off southwestern Puerto Rico, along a reef in La Parguera Marine Preserve. Schools of barjack fish swam through, sunlight glinting off their silvery sides. A barracuda slunk past, threatening the smaller fish and surprising the divers, who were working with the Institute for Socio-Ecological Research (ISER Caribe).
The group was installing hanging houses for baby Diadema antillarum, a long-spined sea urchin that can promote coral regrowth by reducing harmful algae.
Nearby, a cluster of coral fragments had begun to take root; the researchers nursed them back to health on land before replanting them on the reef. Eventually, they plan to plant 22,000 such fragments.
The reef had a diverse array of corals, but showed signs of damage. The colors were muted and the "chattering" normally heard on a healthy reef-which sounds like noise to the human ear-was absent. Another troubling sign: The water was about 86 degrees Fahrenheit, just below the temperature range where scientists worry about bleaching.
Corals are sessile creatures, meaning they are rooted in one place. They depend on symbiotic, photosynthetic algae that live in their tissues, producing nutrients and giving them their characteristic color.
As temperatures rise, the symbiotic algae can go crazy, producing harmful chemicals and too little food, which in turn stresses the corals and forces them to let go of the algae. The process leaves the corals looking skeletal and white, and puts them at risk of dying.
"When the corals are bleached, they are under extreme stress. So any other stressors, like water quality or UV radiation or sedimentation from land, all that extra stress is going to kill these corals most likely," said Stacey Williams, the executive director of ISER Caribe.
The group is working to restore 2 hectares of coral reef in Puerto Rico by planting fragments on six reefs and reintroducing long-spined sea urchins into the ecosystem.
The sea urchins feed on harmful algae that thrive in warmer waters and can be harmful to coral.
"They are like the goats or the cows of the sea," Williams said.
When corals die or bleach, ecosystems can be overrun by such algae.
"If the ground is already covered with algae, the coral larvae will not settle there," says Juan Torres-Pérez, a coral expert and researcher at NASA who grew up and studied in Puerto Rico.
In the 1980s, long-spined sea urchins went extinct on Puerto Rico's reefs. Now they're struggling to survive beyond the early stages of their lives in La Parguera. To give the urchins a boost, ISER Caribe researchers have suspended lengths of AstroTurf-like material along several 25-foot-long lines, anchored to the seafloor with cement blocks.
The grass-like material provides a home for baby urchins to cling to. Divers collect the squares and transport the urchins to a land nursery to grow. Once the urchins reach the size of a young adult, researchers place them in a coral reef that needs additional support.
It is one of many ecosystem projects testing new ways to help coral survive.
In Florida, scientists from the University of Miami imported corals to the U.S. that evolved in the warmer waters of Honduras for the first time. The scientists hope to crossbreed the imported corals with native Florida species to produce a coral that is more heat-resistant.
Andrew Baker, director of the Coral Reef Futures Lab at the university's Rosenstiel School of Marine, Atmospheric, and Earth Science, flew with the corals in a cargo plane for 15 hours.
"We need a fail-fast approach and an openness to new ideas," Baker said, referring to a general mindset for technological development in engineering and business. "The natural state of affairs is rapidly going to waste because of climate change. If we do things to accelerate the response of these ecosystems to planetary change, the result of inaction will be much worse."
Some efforts are starting to show promise. In a study published Wednesday in the journal PLOS ONE, scientists reported that young, lab-grown corals raised for restoration projects in several parts of the Caribbean survived the worst of the ocean heat in 2023. The research suggests they fared better than wild adult corals in the same locations.
Scientists have warned about the fate of corals 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 at 2 degrees of warming.
Last year, the warmest year ever recorded on Earth, had temperatures about 1.48 degrees above those before the Industrial Revolution.
Manzello says scientists used to think coral had a longer lifespan - perhaps until 2040 or 2050 - before conditions became so bleak.
"Last year surprised everyone," Manzello said. "The Caribbean was just unreal last year and no one expected it to get this hot this fast."
Expensive, time-consuming coral restoration projects are unlikely to keep pace with losses from climate change. But creating healthy coral pockets can at least give reefs a chance to recover in the future.
"You're going to have to be very selective and critical about where you focus your efforts," Manzello said. "But the bottom line is, for some coral species, especially in places like Florida and the Caribbean, aggressive intervention and restoration are the only things that are going to keep those species from going extinct."
Baker compared Florida's reef systems to a jigsaw puzzle.
"We've probably lost 80, 90 percent of the corals. Despite all that, we haven't lost a single species of coral," Baker said. "We've messed up that jigsaw puzzle and broken it up, but we haven't lost the pieces."
Meanwhile, meteorologists say the natural El Niño climate pattern that has contributed to record ocean temperatures since spring 2023 has disappeared. The change could cool the seas a bit, at least temporarily.
Evan Bush reported from Seattle and Maura Barrett from La Parguera Marine Preserve, Puerto Rico.This article was originally published on NBCNews.com
