The end of coral reefs as we know them

Paige Vickers/Vox

Years ago, scientists made a devastating prediction about the ocean. Now it’s unfolding.

Several years ago, the world’s top climate scientists made a frightening prediction: If the planet warms by 1.5 degrees Celsius, relative to preindustrial times, 70 to 90 percent of coral reefs globally would die off. At an increase of 2°C, that number jumps to more than 99 percent.

These researchers were essentially describing the global collapse of an entire ecosystem driven by climate change. Warm ocean water causes corals — large colonies of tiny animals called polyps — to “bleach,” meaning they lose a kind of beneficial algae that lives within their bodies. That algae gives coral its color and much of its food, so bleached corals are not only white but also starving. And starved coral is more likely to die.

In not so great news, the planet is now approaching that 1.5°C mark. In 2023, the hottest year ever measured, the average global temperature was 1.52°C above the preindustrial average, as my colleague Umair Irfan reported. That doesn’t mean Earth has officially blown past this important threshold — typically, scientists measure these sorts of averages over decades, not years — but it’s a sign that we’re getting close.

David Gray/AFP via Getty Images

Marine biologist Anne Hoggett swims above bleached and dead coral on the Great Barrier Reef in April 2024.

David Gray/AFP via Getty Images

Tourists snorkel above a section of the Great Barrier Reef full of bleached and dead coral on April 5, 2024.

So, it’s no surprise, then, that coral reefs are, indeed, collapsing.

In April, the National Oceanic and Atmospheric Administration (NOAA) announced that the planet is experiencing its fourth global “bleaching” event on record. Since early 2023, an enormous amount of coral in the Atlantic, Pacific, and Indian oceans has turned white, including in places like the Great Barrier Reef and the Florida Keys. A record 60 percent of the planet’s coral reef habitat has experienced enough heat to cause bleaching in the last year, NOAA marine scientist Derek Manzello said in a press conference Thursday. In some regions, nearly all of the coral has died.

“What we are seeing now is essentially what scientists have been predicting was going to happen for more than 25 years,” Manzello, who leads the agency’s coral bleaching project, told Vox in April. A build-up of carbon emissions is the underlying cause of warming, which has also supercharged more temporary drivers of ocean heat, including El Niño (which has recently weakened).

Yet coral reefs were collapsing well before the current bleaching crisis. A study published in 2021 estimated that coral “has declined by half” since the mid-20th century. In some places, like the Florida Keys, nearly 90 percent of the live corals have been lost. Past bleaching events are one source of destruction, as are other threats linked to climate change, including ocean acidification.

The past and current state of corals raises an important but challenging question: If the planet continues to warm, is there a future for these iconic ecosystems? What’s become increasingly clear is that climate change doesn’t just deal a temporary blow to these animals — it will bring about the end of reefs as we know them.

Will there be coral reefs 100 years from now?

In the next few decades, a lot of coral will die. That’s pretty much a given. And to be clear, this reality is absolutely devastating. Regardless of whether snorkeling is your thing, reefs are essential to human well-being: Coral reefs dampen waves that hit the shore, support commercial fisheries, and drive coastal tourism around the world. They’re also home to an incredible diversity of life that inspires wonder.

“I’m pretty sure that we will not see the large surface area of current reefs surviving into the future,” said Hans-Otto Pörtner, who was involved in the landmark 2018 report, led by the Intergovernmental Panel on Climate Change (IPCC), that predicted the downfall of tropical reefs at 1.5°C warming. “Every year is going to be worse.”

NOAA

A map of coral bleaching “alerts,” which indicate where the ocean is unusually warm and bleaching is likely to occur. Red areas have a risk of reef-wide bleaching; magenta and purple regions are at risk of coral death.

But even as many corals die, reefs won’t exactly disappear.

The 3D formation of a typical reef is made of hard corals, such as brain corals, which comprise colonies of polyps that produce skeleton-like structures. When those polyps die, they leave their skeletons behind.

Animals that eat live coral, such as butterfly fish and certain marine snails, will likely vanish. Plenty of other fish and crabs, meanwhile, will stick around because they can hide among those skeletons. Algae will dominate on ailing reefs, as will “weedy” kinds of coral, like sea fans, that don’t typically build the reef’s structure.

Simply put, dead reefs aren’t so much lifeless as they are home to a new community of less sensitive (and often more common) species.

“Reefs in the future will look very different,” said Jean-Pierre Gattuso, a leading marine scientist who’s also involved with the IPCC. “Restoring coral reefs to what they were prior to mass bleaching events is impossible. That is a fact.”

On the timescale of decades, even much of the reef rubble will fade away, as there will be no (or few) live hard corals to build new skeletons and plenty of forces to erode the ones that remain. Remarkably, about 30 percent of the carbon dioxide that we pump into the atmosphere is absorbed by the oceans. When all that CO2 reacts with water, it makes the ocean more acidic, hastening the erosion of coral skeletons and other biological structures made of calcium carbonate, like snail shells.

Jennifer Adler for Vox

Bleached staghorn coral in a nursery run by the Coral Restoration Foundation off the coast of the Florida Keys in September 2023.

There’s only one real solution

For decades now, hard-working and passionate scientists have been trying to reverse this downward trend, largely through restoration. They’ve been “planting” pieces of coral on damaged reefs, much like you might plant saplings in a logged forest. In reef restoration, many scientists and environmental advocates see hope and a future for coral reefs.

But these efforts come with one major limitation: If the oceans continue to grow hotter, many of those planted corals will die too. Last fall, I dived a handful of reefs in the Florida Keys where thousands of pieces of elkhorn and staghorn — iconic, reef-building corals — had been planted. Nearly all of them were bleached, dead, or dying.

“When are [we] going to stop pretending that coral reefs can be restored when sea temperatures continue to rise and spike at lethal levels?” Terry Hughes, one of Australia’s leading coral reef ecologists, wrote on X in April.

Ultimately, the only real solution is reducing carbon emissions. Period. Pretty much every marine scientist I’ve talked to agrees. “Without international cooperation to break our dependence on fossil fuels, coral bleaching events are only going to continue to increase in severity and frequency,” Manzello told me. Echoing his concern, Pörtner said: “We really have no choice but to stop climate change.”

Jennifer Adler for Vox

A collection of bleached “planted” staghorn coral on a reef in Florida in September 2023.

In the meantime, other stuff might help. Planting pieces of coral may help bring back reefs, at least temporarily, if those corals are more tolerant to threats like extreme heat or disease. And researchers are trying to breed more heat-resistant individuals or identify those that are naturally more tolerant to stress — not only heat, but disease. Even after a summer of extreme bleaching in Florida, many corals survived, according to Jason Spadaro, a restoration expert at Florida’s Mote Marine Laboratory.

Some scientists also see an urgent need to curb other, non-climate related threats, such as intensive fishing. “To give corals the best possible chance, we need to reduce every other stressor impacting reefs that we can control,” Manzello told Vox. Not all marine biologists agree, however, that limiting fishing will do much to help reefs if the world’s oceans continue to warm.

The forecast for this summer is once again grim. The ocean is still unusually hot, and scientists fear that bleaching in the Caribbean could be severe for a second year in a row.

“This [bleaching] event is still growing in size,” Manzello said on a NOAA press call Thursday. “El Nino is dissipating, but the ocean is still so anomalously hot that it won’t take much additional seasonal warming to push temperatures past the bleaching threshold, particularly in the Caribbean.”

This story appeared originally in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.

Update, May 16, 12:10 pm ET: This piece, originally published April 26, has been updated with new information released by NOAA about the ongoing bleaching crisis.