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. Here are five of them.

1. 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.

2. 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.

3. 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 — and collecting samples of what's left. These corals have the most resilient genes for the coming environmental stressors, so if humans can give a hard push to evolution by propagating these tough corals, it could save the species.

4. 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.

5. 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.

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