The world’s coral reefs do more for the planet than provide underwater beauty.
They buffer shorelines from the effects of hurricanes. An estimated 500 million people earn their livelihoods from the fishing stocks and tourism opportunities reefs provide. The tiny animals that give rise to reefs are even offering hope for new drugs to treat cancer and other diseases.
Despite their importance, warming waters, pollution, ocean acidification, overfishing, and physical destruction are killing coral reefs around the world. Schemes to save those reefs are as creative as they are varied; most recently, scientists released data showing that marine protected areas can help save reefs if they are placed in just the right spots. Genetics is also becoming a larger area of coral research, giving scientists hope they might one day restore reefs with more heat tolerant coral.
But now, in the lead-up to World Oceans Day on June 8, scientists caution that these and other strategies may only buy reefs time until world leaders implement aggressive climate change action.
Without a mix of long-term cuts in emissions and short-term innovation, there’s a not-so-far-off future where coral reefs as we know them simply cease to exist, says Anne Cohen, a coral expert at the Woods Hole Oceanographic Institution in Massachusetts.
Scientists often compare coral reefs to underwater rainforests, yet unlike the leafy plant base of a forest, corals are animals. The soft polyps inside the hard parts of corals are naturally translucent and get their famously vibrant color from algae living inside them.
When corals experience stress from hot temperatures or pollution, they end their symbiotic relationship with this algae, typically expelling them and turning white, though one recent study indicates some coral turn a bright neon color when stressed. Coral are still alive when they bleach, but they’re at risk—essentially immunocompromised—and many eventually starve and die, turning a dark brown.
People first noticed coral bleaching events in the 1980s. The problem intensified in 2016, when an El Niño weather pattern, which causes warmer waters in the Pacific Ocean, mixed with an already unseasonably warm ocean and killed off a third of the corals on the Great Barrier Reef. Since then, roughly half the corals on Australia’s famous reef have died in subsequent bleaching events, jeopardizing an underwater landscape 1,500 miles long.
Scientists around the world are looking for all kinds of ways to protect and maybe even revive corals. One option is to create more marine protected areas—essentially national parks in the ocean. Scientists say creating marine refuges, where fishing, mining, and recreating are off limits, make the reefs healthier, and so more resilient.
An estimated 4,000 fish species, and some 25 percent of marine life, depend on coral reefs at some point in their existence. Fish keep the algae that grow on corals in check, allowing corals to breathe and access sunlight. While an MPA won’t protect corals from heat waves, these natural safe zones can keep fisheries more sustainable in the long term, and fishers around well-managed MPAs often benefit from the “spillover” of healthy fish stocks that populate surrounding waters.
At a talk hosted by the Woods Hole Oceanographic Institution on Wednesday, renowned marine biologist Sylvia Earle promoted the idea of using marine parks to protect coral, which she does through her organization Mission Blue.
“Reefs that have been protected or not yet exploited by fishing impacts survive when nearby places do not,” she says.
A recently published assessment of 1,800 reefs in 41 countries found that only 5 percent of reefs were able to provide all of their lucrative byproducts, such as healthy fish stocks and biodiversity. To increase that percentage, new marine reserves will need to be strategically placed in areas well away from humans, say experts. It wouldn’t save all reefs, but it would help ensure that more reefs function at 100 percent of their potential instead of just a fraction, says Alan Friedlander, the chief scientist for National Geographic’s Pristine Seas initiative and an ecologist at the University of Hawaii who helped author the reef assessment.
“Without this protection,” he says, “any technological enhancements will suffer the same fate as natural reefs, since the stresses have not abated.”
Beyond such nature preserves, some conservationists are looking to more hands-on methods. One research center in the Florida Keys is exploring a form of natural selection to keep corals afloat.
The reef system in the Keys has been hit hard by climate change and disease, which is especially tough, because corals there help support fisheries worth an estimated $100 million every year. In addition, corals off Florida’s coasts are polluted by agricultural and sewage runoff.
The additional stress from warming waters is like “the proverbial nail in the coffin,” says Erinn Muller, the science director at the Elizabeth Moore International Center for Coral Reef Research and Restoration at the Mote Marine Laboratory in Sarasota, Florida.
To keep the wild ecosystem alive, Muller and her team are harvesting samples of the corals that have survived the environmental stresses naturally, breeding them by hand, and reattaching them to the reef. At any given time, the center has 46,000 corals growing on underwater plastic lattices in its nursery. So far, the center has regrown over 70,000 corals from five different species on damaged reefs.
“The ultimate goal is we put ourselves out of a job,” says Muller.
In the Bahamas, Ross Cunning, a research biologist at Chicago’s Shedd Aquarium, is focusing on corals with robust genes that could make them natural candidates for restoration projects. He recently published a study of two Bahamian reefs, one that seemed to survive an intense 2015 heat wave, and one that didn’t.
“It sets the stage to find out which genes are responsible for thermal tolerance,” says Cunning, adding that he hopes identifying those genes will help scientists one day breed more heat-tolerant coral.
In Massachusetts, Cohen's research has found two key elements that seem to protect corals. The first: internal waves beneath the ocean’s surface that bring cooler currents to heat-struck corals, essentially air-conditioning them as temperatures rise. The second: adaptation, a trait that corals found in Palau’s warm lagoons seem to exhibit.
“What we’ve realized is these corals are sitting in naturally hot water all the time,” she says. On average, these lagoons submerge coral in water that is two degrees Celsius warmer than the water outside the lagoons. “We think the fact that they can deal with these higher temperatures is built into their genetics and allows them to deal with the heat waves.”
She’s also found evidence of corals evolving more quickly in the past two decades to withstand rapidly warming temperatures. The big question scientists are now investigating, says Cohen, is whether there’s a cap on how much more heat corals can adapt to.
Cohen calls these regions with heat-adapted corals as “super reefs,” and like Friendlander, advocates for using marine reserves to protect them.
Muller notes that their efforts on the Florida reefs can help keep them from what she describes as “functional extinction.” But she says the reefs ultimately won’t be restored to their potential until their environment becomes more hospitable to their survival.
All the scientists interviewed for this article noted that mitigating climate change is the only long-term, sustainable solution to conserve and restore coral reefs. Despite global lockdowns and sharply falling emissions, atmospheric carbon dioxide still reached a record high in May.
Global warming is “raising the background temperature,” compounding regular heat waves and making them even deadlier for corals, says Kristopher Karnauskas, an atmospheric scientist at the University of Colorado Boulder who recently published a study investigating the physical causes of the 2016 event.
The oceans absorb and store heat very efficiently; as Earth warms, the oceans take in over 90 percent of the planet’s heat trapped in the atmosphere by human-generated greenhouse gases. But their heat-storing capacity isn’t limitless, and excess heat over time takes its toll on ocean inhabitants.
In evolutionary history, corals date back 400 million years, and with each global temperature change Earth has undergone, corals have adapted—but never as quickly as they must today.
“We know that because there have been six major coral reef extinctions in the geologic past where they were basically wiped out. All those have been associated with excessive heat and ocean acidification,” Cohen says. “Coral reefs always come back, but it takes tens of thousands of years.”
Now, with climate change-driven temperatures rising at a rate higher than corals have ever had to naturally adapt to, Cohen says, “we don’t have that kind of time.”