The Corals That Survive Climate Change Will Be Unrecognizable

Earth belonged to the corals first. And over hundreds of millions of years, they proved themselves remarkably good at adapting to each new version of the planet. As other groups of organisms dropped out of existence, corals endured so many catastrophes that their history reads like a biblical tale of resilience. Through extinctions mass and minor, through volcanic eruptions and asteroid strikes, the corals survived.  

And for tiny marine animals, they managed to exert tremendous force on the planet’s landscape. Corals have raised whole islands into existence. They are the natural guardians of coastlines; they sustain an estimated quarter of known marine life. If the reefs ringing the Maldives die, an entire nation could erode into the sea. Humans live in these places because corals exist.

The Earth that humans evolved on, in other words, is a coral planet. Today, the animals provide ecosystems that support the livelihoods of about 1 billion people. They are so fundamental to life as we know it that scientists wonder if one way humanity could discover alien life is by detecting the signature of fluorescent corals in the shallow waters of another planet. Corals are also, famously, being devastated by climate change. Even in a future where they survive in some form, their transformation could make our own experience of this planet profoundly different.

The earliest corals emerged about 500 million years ago, roughly alongside plant life on land. But the modern version of coral reefs appeared a short 4 million years ago, around the time our human ancestors began to walk upright (give or take a few million years). When researchers try to rescue suffering corals, carefully cutting pieces away and transporting them to aquariums, they’re visiting underwater metropolises that are thousands of years old. Despite all that corals have been through, given how fast conditions on Earth are changing, life has likely never been quite as stressful for them as it is now, according to the coral experts Bertrand Martin-Garin and Lucien Montaggioni in their book, Corals and Reefs.

Earlier this month, scientists reported that Australia’s Great Barrier Reef is sitting in water that, in one decade, has become hotter than at any other point in the past 400 years. Caribbean coral colonies are still reeling from the havoc of last year’s historic marine heat wave. Around the world, extraordinarily hot ocean temperatures have plunged corals into one of the worst bleaching events in recorded history—they’re expelling the algae that live in their tissues and turning a ghostly white. Corals can survive bleaching, if conditions improve. But the longer they remain without that algae, the more likely they are to die.

“These are strange days on planet Earth,” Derek Manzello, a coral-reef ecologist and the coordinator of the National Oceanic and Atmospheric Administration’s Coral Reef Watch, once told me. The planet used to give corals hundreds of thousands of years to adjust to a new reality; human activities—the burning of fossil fuels but also overfishing and pollution that have brought on global warming—have introduced a rate of change more dramatic than anything else in the geological record. “If we wanted to kill all reef-building corals on the planet, it would be hard to imagine a collection of activities quite as pointed and effective as what we’ve arrived at,” Stuart Sandin, a marine biologist at the Scripps Institution of Oceanography, told me.

Indeed, the Intergovernmental Panel on Climate Change, which presents science-driven predictions about the global effects of human-caused climate change, has said that if the world limits warming to 1.5 degrees Celsius above preindustrial levels—the current goal, though one we’re on a path to blast past—70 to 90 percent of tropical coral reefs will disappear. And if the world surpasses 2 degrees Celsius, virtually all of them will die off.

That would not necessarily mean that Earth would lose its corals entirely. Even as we draw closer to the worst-case scenarios for corals, Manzello believes that—with concerted human intervention—some of the destruction could be still counteracted or offset, at least in certain pockets of the globe. And as urgent as the IPCC warnings are, Sandin said, such estimates don’t take into account coral’s full potential for adaptation. “We still can’t say with any certainty if we will see coral species go extinct,” Manzello said when we spoke recently. Documenting extinctions is more difficult in the marine environment than on land, and like Sandin, Manzello thinks that coral refugia—places where species have historically persisted despite stressful conditions—could persist in even the gloomiest scenarios.

Sandin, for one, predicts a future split into thirds. One-third of coral reefs will certainly be devastated in the coming decades, mostly near urban areas. Another third is “going to scrabble along,” he said, echoes of the voluminous reefs that once thrived. And the final third is “going to look pretty darn nice,” having managed to handle the worst effects of warming and become nearly unrecognizable, unlike any corals that scientists are familiar with today. Although even corals known for their heat tolerance are succumbing in the Indian Ocean, some species in the Pacific Ocean have improved their capacity to withstand the stress by hosting a different kind of algae. Reefs have started cropping up in subtropical environments, too, where the water is cooler. “We’ve seen a lot of incredible locations where these reefs are rising from ashes, living in places that they shouldn’t,” Sandin said. “Those reefs are just fighting like hell.” Earth could keep its corals, long into the future.

Scientists, too, are fighting hard, but to save corals as we know them now. “The entire coral-reef-science community went through a huge, drastic shift in focus starting in the 1980s, when we first saw large-scale bleaching events emerge,” Manzello said. Before that, scientists studied corals out of pure curiosity about how these creatures came to be; now every aspect of coral research has turned to finding ways to preserve the animals. “If you’re a geologist and you want to study reef development 200 million years ago in Papua New Guinea, you’re going to have to tie that to, what is this going to tell us about the future of reefs?” Manzello said.

In a sense, the coral crisis is existential for humans, too. Even if coral cities persist in some fashion, what will ours be without those ecosystems? Fishing industries will suffer, and food supply with it. Familiar shorelines will slough off into the sea. Coastal communities will be at the mercy of powerful waves once slowed by reefs. A world with suffering corals leaves humans more exposed to the elements—and those elements are becoming more dangerous each year.

With every passing too-hot month, we turn more reefs into ruins, the remnants of another life form that existed alongside ours. Alive, some corals in shallow waters shimmer: They absorb ultraviolet radiation from the sun, which can prohibit growth in other organisms, and then emit it as visible light, in beautiful colors. That ability is what has made scientists imagine finding corals far beyond the solar system: Several years ago, Lisa Kaltenegger, an astronomer and the director of the Carl Sagan Institute at Cornell, suggested that scientists could search for signs of coral-like life forms on planets orbiting stars much smaller and dimmer than our own, that release ultraviolet flares. Perhaps life on those planets evolved to use that radiation, just as corals have. The glowing populations would have to be far more widespread than they are on Earth to give off a detectable signal; astronomers have already thought of the kind of telescope that could detect this glow, though it’s still many years away from creation.

Should alien astronomers ever look our way, they wouldn’t detect Earth’s fluorescent corals at the water’s surface, Kaltenegger told me: There aren’t enough of them. Any faraway civilizations are more likely to capture the perpetual illumination of human cities, or the radio waves from our inventions, flowing endlessly into space. But if corals were here long before us, they may also outlast us, despite the torment they’re experiencing now. Many, many years from now, “after humans have had their reign, corals will be fine if we give the ocean a break,” Sandin said. People may not bear witness, but corals’ dramatic resilience could survive us, too.