We've heard plenty about the rapid and dramatic loss of coral reefs over the last decades — estimates are that about 20 percent of coral worldwide is already gone. Parts of Australia's Great Barrier Reef lost more than 70 percent 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 on 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 the above have weakened immune systems and are more easily killed by diseases.

Scientists are hoping to reverse the trend by helping healthy coral reproduce using a variety of techniques.

1. Cloud brightening

Great Barrier Reef aerial Cloud brightening could protect coral reefs by deflecting harmful solar radiation. (Photo: nicolas poizot/Shutterstock)

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 water sea water through a filter and using a fan to spray the water towards the clouds. The water would evaporate in the sky, but the salt particles in the water would stay behind and condenses 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. Assisted evolution

Coral reefs are home to a variety of marine creatures. Coral fragments that can survive in hotter, more acidic water are replanted on reefs in hopes they will propagate new coral. (Photo: Debra James/Shutterstock)

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 manage to 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.'"

3. 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.

4. Reskinning

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," says Bergh.

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.

5. Genetic selection

Red Sea coral reef and fish Scientists found that corals in the Red Sea might be naturally heat-resistant and able to survive temperatures several degrees higher than normal. (Photo: Svetlan/Shutterstock)

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 it's 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 and are found in algae. In 2017, researchers from King Abdullah University of Science and Technology in Saudi Arabia discovered that the genes could handle higher heat temperatures . The genes would make the algae that lives amongst coral reefs more heat tolerant. Therefore if the algae can survive higher temperatures, then the hope is that coral would follow suit.

6. Adding electricity

biorock Biorock reefs may offer a speedy solution to giving young coral reefs some much-needed protection against climate change. (Photo: Global Coral Reefs Alliance/Eunjae Im)

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.

7. Gene storage banks

The tragic beauty of bleached coral. This is what bleached coral looks like. (Photo: Ethan Daniels/Shutterstock)>

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 due to the above techniques and backstops, it would obviously be simpler (not to mention less costly — none of the research and work above is cheap), to mitigate global warming and water pollution now.

Editor's note: This article has been updated since it was originally published in Jan. 2018.

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.

7 creative techniques to keep coral reefs alive
Since the threats to coral reefs vary depending on location, scientists are customizing solutions to help them survive.