We've heard plenty about the rapid and dramatic loss of coral reefs over the last decades. According to some estimates, as much as half of the world's coral has already disappeared in the last 30 years alone. Parts of Australia's Great Barrier Reef had lost more than 70% of their coral by 2016, and some scientists are calling coral loss an extinction-level event.
Coral reefs are important not just because they're incredibly beautiful places for humans to snorkel and dive in; they are also biodiversity hot spots: "A quarter of all marine fish species reside in coral reefs and 500 million people depend on these 'underwater rain forests' for their livelihood," according to the Worldwatch Institute.
The disturbing combination of rapid coral loss and the importance of reefs mean that researchers are hustling to figure out a way to keep corals alive — which is tricky, because corals face different threats depending where they're located. In some places, the threat is water pollution, in others it's heat stress from warming waters caused by climate change, while ocean acidification kills off coral in other spots. Corals stressed by any or all of these problems have weakened immune systems and are more easily killed by diseases.
Scientists are now scrambling for ways to save what's left of Earth's coral reefs, brainstorming and testing a wide range of strategies. Many of those ideas are collected in a pair of reports from the U.S. National Academies of Sciences, Engineering and Medicine, released in November 2018 and June 2019. As the reports point out, human help may "buy time" for coral reefs, but truly saving them will require addressing the main existential threat they face: climate change.
In the meantime, here are a few of the tactics scientists are considering to help coral survive and reproduce for as long as possible.
1. Cloud brightening
To save coral reefs in the ocean, one oceanographer is looking to the skies for help. Dr. Daniel Harrison is working on a method called "cloud brightening," which would use clouds to essentially provide a protective barrier over coral reefs.
Cloud brightening involves pumping sea water through a filter and using a fan to spray the water toward the clouds. The water would evaporate in the sky, but the salt particles in the water would stay behind and condense with other water — therefore making clouds appear brighter. The theory is that brighter clouds would deflect the sun's rays, which would keep the ocean water temperature cooler and in turn hopefully prevent coral reefs from bleaching and dying.
"That one droplet creates an aerosol particle that then grows 15 million times in size into a cloud droplet," Harrison told the Sydney Morning Herald.
2. Using the power of sound
The latest unorthodox solution involves loudspeakers and a bit of deception. Healthy reefs are relatively noisy places, and these scientists used that to their advantage, playing the sounds of a healthy reef in an unhealthy reef environment to see how the ecosystem would respond.
An international team of researchers from University of Exeter, University of Bristol, and Australia’s James Cook University and Australian Institute of Marine Science, used this "acoustic enrichment" in a six-week experiment in the Great Barrier Reef. They published their work in Nature Communications.
To their surprise, it increased the number of fish that not only came back, but stayed, by roughly 50%.
"Healthy coral reefs are remarkably noisy places — the crackle of snapping shrimp and the whoops and grunts of fish combine to form a dazzling biological soundscape," Stephen D. Simpson, a marine biology professor at the University of Exeter and a senior author of the study, said in a university press release. "Reefs become ghostly quiet when they are degraded, as the shrimps and fish disappear, but by using loudspeakers to restore this lost soundscape, we can attract young fish back again."
3. Assisted evolution
This method uses coral fragments and relies on a survival of the fittest approach, focusing on those that can tolerate, survive and even thrive in hotter, more acidic water.
A team of scientists in the Florida Keys break off pieces of coral and submerge them in hot, acidic water tanks. The fragments that survive are attached to artificial "trees" underwater so they can continue to grow before being transplanted back to the reef from which they were taken. It's a painstaking process that involves replanting each individual fragment one-by-one.
Their efforts appear to be paying off.
"It has already made a difference," Mark Eakin, coordinator of the Coral Reef Watch project at the U.S. National Oceanographic and Atmospheric Administration, told the Los Angeles Times. "There are places that have not had branching corals in 30 years and now you go out and look at the bottom and say, 'Wow, this is starting to look like it used to.'"
4. In-water propagation
In-water propagation has been used since the early 2000s and works well for fast-growing corals like staghorn and elkhorn corals, which naturally fragment and drift in the water column, taking root in new areas as a floating dandelion seed might on land. But instead of waiting for this to happen naturally, humans do the fragmentation and replanting by hand as you can see in the above video.
"There's a good record of success for this technique," says Chris Bergh, the South Florida program manager for The Nature Conservancy.
Some corals, like brain coral, can take 100 years to grow to a square meter. A new technique involves growing a small part of one of the larger boulder corals and then attaching it to an old, bleached-out base.
"Scientists glue them all over the coral head, not touching, sort of like hair plugs, or grass plugs. There's no need to plant on every square inch," Bergh says.
Due to a natural stress response, the baby corals grow and cover the surface of the old coral more quickly than starting from scratch. Because corals depend on size rather than age to reproduce, the young corals reach maturity in less time and start reproducing.
6. Genetic selection
Corals have been around for 500 million years (for comparison, humans have been here for only 2 million). So within their DNA they have the tools to deal with change — just not as quickly as change is happening now. So scientists have been going to areas where corals have been subjected to abuse — like Oahu, where some corals survived raw sewage being dumped on them, or Australia, with the aforementioned mass-bleaching events, or even the Red Sea, where corals survive high temperatures — and collecting samples of what's left.
Retrotransposons, also known as "jumping genes," are genes that replicate and mutate. In 2017, researchers from King Abdullah University of Science and Technology in Saudi Arabia discovered genes associated with heat resistance in the symbiotic algae that live within corals. In theory, the genes would make the algae more heat-tolerant, and if the algae can survive higher temperatures, the hope is that corals would, too.
7. Adding electricity
In some areas of the planet, scientists are rebuilding coral reefs with Biorocks, which are steel-framed structures with a low voltage of electricity pulsing through the frame. The electric current passes through sea water and creates a chemical reaction that coats the coral with limestone minerals similar to the natural coating created by young coral.
"These currents are safe to humans and all marine organisms," explains the Gili Eco Trust, a nonprofit that has set up more than 100 Biorock structures around islands in Indonesia. "There is no limit in principle to the size or shape of Biorock structures, they could be grown hundreds of miles long if funding allowed. The limestone is the best substrate for hard coral."
According to the nonprofit Global Coral Reef Alliance, Biorock reefs help speed the growth of coral and make them more resistant to increases in temperature and acidity.
8. Gene storage banks
The worst-case scenario is that we lose many or all corals in the next 50 to 100 years. We need to have a repository of their genetic information so there's still a possibility of restoration even if they disappear from the wild.
"We need to be gathering this genetic diversity and trying to help while there's still a lot of diversity in place in the ocean," Mary Hagedorn, a senior research scientist at the Smithsonian Conservation Biology Institute, who's based in Kaneohe, Hawaii, told NPR's Science Friday.
It's encouraging that all this work is ongoing. "We created these problems," Michael P. Crosby, president of the Mote Marine Laboratory & Aquarium in Sarasota, Florida, told The New York Times. "We have to get actively involved in helping the corals come back."
But while there is renewed hope and action for corals, it would obviously be simpler (not to mention less costly), to mitigate global warming and water pollution now.
Editor's note: This article has been updated with new information since it was published in January 2018.