As hot as a hot tub: how soaring ocean temperatures are affecting corals

Between 2014 and 2017, three quarters of the world’s coral reefs got sick. Many turned a bright white—a process scientists call coral bleaching—and thirty percent of those bleached corals died.

The culprit? Oceans warmed by climate change and an El Niño weather event turning up the heat on already hot water.

Record warming and the return of El Niño this summer is putting corals in danger once again.

This year is already “especially bad” says Doug Marcy, founder of the Caribbean Coral Restoration Center. Their reefs in Panama can tolerate waters in the high 70s°F but temperatures have already tipped over 90°F this year. Around the southwest coast of Florida, large swaths of the region’s vulnerable coral reef system are currently sitting in record-breaking 97°F waters and at risk of bleaching.

These extraordinarily high temperatures, which he expects will continue this year, could be “devastating” to corals, which “cannot adjust fast enough” to cope, and would struggle to breed if too many are lost.

With an entire ecosystem at stake, another El Niño event unfolding, and global temperatures rising, scientists are deploying experimental technology to help corals survive, restoring habitat by hand and tinkering with coral DNA. But it’s unclear whether it will be enough to help some of the world’s most vibrant coral survive.

What happens to corals in hot water? 

Corals are animals that live in mutually beneficial partnerships with a microalgae called zooxanthellae. This algae lives inside the coral’s skeleton and, in return for shelter, provides the corals with most of their nutrients and supplies their stunning colors.

When changing conditions—such as a couple of degrees of warming—put corals under stress, they throw out their house guest, turning white.

“It looks like somebody’s taken an eraser and taken all the colors out of the corals,” says Marcy.

Corals can survive mild or short-lived bleaching, but after too long without its zooxanthellae, the coral will starve. After bleaching, corals are more likely to catch diseases, can struggle to reproduce, and fish populations may leave or die off.

All these changes underwater also impact people around the world. Without coral reefs to provide safe nurseries where juveniles can shelter and find food, fish populations suffer and it’s harder for fishermen to provide for their communities.

“People depend on our ocean, and we can’t lose sight of that,” says Jake Edmiston, Projects Researcher at Blue Marine Foundation.

El Niño—weaker winds, warming waters

El Niños usually happen every two to seven years when the winds running across the equator weaken, increasing Pacific surface water temperatures.

Currently, the world has warmed by 1.2°C, but this year’s El Niño could cause the average global temperature to tip over the 1.5°C global warming threshold for the first time, exceeding a threshold scientists say will expose the world to serious environmental consequences. Even if the world limits warming to 1.5°C, 70–90 percent of tropical coral reefs could be lost.

Years ago, Marcy would have laughed at the idea he’d ever see waters “one step away from a hot tub”—which tend to bubble away at 90ºF to 104ºF. But despite this year already reaching “unimaginable” water temperatures of over 90°F, he doesn’t think we’ve seen the worst yet.

But all hope isn’t lost.

Sofia Regalado, Science and Research Coordinator at Blue Marine Foundation, says: “Corals are resilient. They’ve faced many threats over their 400 million years of existence” and can recover eventually.

Artificial reefs, hybrids, and underwater sound systems 

Where artificial reefs were once created from shipwrecks, sculptures, tires, and concrete, they are now being 3D printed with realistic nooks and crannies for corals to attach to and marine life to call home.

Biorock technology runs a light electrical current through metal structures, causing limestone to grow for coral to settle on. Cheaper than concrete, they can repair themselves, get stronger over time, and help damaged reefs recover more quickly.

Selective breeding may also help. Some researchers are cross breeding the most adaptable and resilient corals with others to create hybrids that have a better chance of survival. Others are seed banking: restoring and cloning corals that would die without intervention and storing the seeds for future.

The Caribbean Coral Restoration Center is trying to encourage their corals to “toughen up” by keeping their tanks at 85°F—normally too hot for coral—to see how readily corals can adapt to warmer temperatures.

Researchers are also tapping into new findings showing that “the underwater world isn’t as silent as Jacques Cousteau may have thought,” says Edmiston. A study published in Nature in 2019 showed that playing the sounds of a healthy reef through underwater speakers can bring fish back to degraded coral ecosystems. The study’s author and University of Bristol marine biologist Steve Simpson, explained in a press release that “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.”

Attracted by these sounds, fish larvae are more likely to settle on a noisy—and therefore healthy—reef.

Recognizing the warning signs of bleaching—which can include fluorescence, when reefs glow like a packet of highlighter pens—also helps conservationists react more quickly.

On a global scale, the Allen Coral Atlas is using hi-res satellite imagery to monitor reefs in exceptional detail, and processes regional data on a bi-weekly basis for areas that have been flagged by NOAA as having abnormally high temperatures.

Innovation alone won’t save reefs

But Regalado thinks we shouldn’t “bank on innovation” alone when the problem is so huge. Corals are being hammered by pollution, plastics, overfishing, coastal development, and more. Combined, these threats could be more than they can handle.

Edmiston agrees that science-backed innovation must sit alongside conservation measures like Marine Protected Areas (MPAs) which, when enforced, protect reefs from human-induced stressors. These don’t just benefit the protected areas, as marine life aren’t aware of MPA boundaries, and can “spill over from one area to another,” adds Regalado.

We shouldn’t “start writing the obituary of the ocean” just yet, says Edmiston. He believes we can reach the 30×30 target, a global effort to protect 30 percent of Earth’s land and sea by 2030, if everyone pulls together—but not without decarbonizing the economy and cutting the emissions that are driving rapid climate change.

But those cuts must come quickly.

“The only solution we have,” says Marcy, “is to buy time”.