Coral Reefs Are Bleaching Too Frequently to Recover

Reefs rarely overheated and died in 1980. Now, they are bleaching too fast to recover.

Bleached corals underwater
The aftermath of a bleaching event off Caye Caulker, Belize (Susannah Sayler / Reuters)

From the Arctic to the Amazon, almost no part of the world has been left untouched by the human-caused warming of the Earth’s climate system. But one ecosystem seems to be disintegrating faster than almost anywhere else: coral reefs, the tropical rainforests of the undersea world.

A new study, published Thursday in the journal Science, quantifies this discipline-wide impression of a vast and fast-moving destruction. It brings together observations of 100 coral-reef sites around the world going back to 1980, and it finds that severe bleaching events are far more common now than they were 35 years ago.

“I do not like to say: Reefs will die. But we will not have reefs as we currently have them,” says Howard Lasker, a professor of ecology at the University at Buffalo, who was not connected to the study.

Lasker has spent three decades studying the biology of coral reefs. Specifically, he focuses on gorgonian corals: sea fans and sea plumes, “the stuff you see waving back and forth when you look at a TV special,” as he puts it. Through his career, he has witnessed an impoverishment of these natural treasures, and he predicts it will only continue.

“I personally believe that people will get in the water 20 years from now and they will go dive on things that people call ‘reefs,’” he told me. “And the reefs will still have some corals on them, still have lots of fishes on them, and they will still have gorgonian corals on them—and people will be impressed by them. But they will be dramatically, dramatically different than the reefs we have now, and certainly than the reefs that were present 30, 50, 100 years ago.”

A bleaching event is one of the fastest ways to kill a coral reef—and it’s the main way that corals react to the hotter oceans of climate change. Corals are tiny animals that live in huge, branching colonies of limestone. (Each “tree” of coral might contain hundreds of thousands of tiny, individual coral organisms.) Each coral polyp also contains a small amount of photosynthetic algae, which provides food to the coral and helps keep it healthy.

When the water gets just a few degrees warmer than usual, coral polyps expel their algae out of stress, and the entire branching colony turns white—that is, it bleaches. If the water does not cool down fast enough, the coral colony can then starve to death or get infected, and die. Even if the corals survive the episode, it takes about 10 years for them to fully recover.

“I studied coral reefs for 20 years before I first saw mass bleaching in 1998—the first global coral-bleaching event. It was also the first time that severe bleaching occurred along the Great Barrier Reef,” said Terry Hughes, an author of the new study and the director of the ARC Center of Excellence for Coral Reef Studies, the government agency that leads coral research in Australia.

“Since then then GBR has bleached three more times, in 2002, 2016 and 2017. We’ve gone from zero bleaching to back-to-back bleaching in just two decades,” he said.

The study finds that, in the early 1980s, bleaching events were rare, occurring at a rate of once every 25 to 30 years. By 2016, they had increased fivefold. Mass coral-bleaching events now strike about every six years, on average, far too fast for ecosystems to recover.

The region hardest hit, so far, are the reefs of the Caribbean and Western Atlantic, which include those in the U.S. Virgin Islands. The Western Atlantic began warming before other parts of the global ocean did, so more than half of the region’s reefs have bleached seven times since 1980. And the average reef in the Western Atlantic has bleached 10 times since 1980.

This devastation has targeted American reefs with particular ferocity. Half of all Caribbean coral reefs in the United States were lost in 2005, when sweltering waters swaddled Puerto Rico and the U.S. Virgin Islands.

Other parts of the world have so far avoided this blight. More than half of the reefs near Australia and in the Indian Ocean have bleached three times since 1980. But reefs in Australia seem to be getting worst fastest.

“The sense that everyone had, that, wow, this is a very different world than it was 30 years ago, was a correct impression,” said Lasker. “The nice thing about an analysis like this is it makes clear to people who make policy that this isn’t an impression, it’s actually occurring. It is important to do this.”

“Now, the impact it has on those officials ... that’s another issue,” he added.

Perhaps most worryingly, the study argues that soon it will not take a global heat wave—such as an El Niño event in the Pacific Ocean—to kill corals. “As global warming progresses, average tropical sea-surface temperatures are warmer today under La Niña conditions than they were under El Niño events only three decades ago,” says the paper. “We are already approaching a scenario in which every hot summer, with or without an El Niño event, has the potential to cause bleaching and mortality at a regional scale.”

This isn’t unexpected. Last year, a study conducted by Australian government scientists and published in Nature Climate Change found that even if the world warms by an average of only 1.5 degrees Celsius—the “reach” goal of the Paris Agreement—the Great Barrier Reef could suffer a major bleaching event every other year.

“By the time we’re seeing bleaching temperatures there every year, there probably will not be a reef anymore. There’s only five or six times bleaching can happen before a reef is essentially dead,” Ruth Gates told me at the time. Gates is a coral biologist at the University of Hawaii and the president of the International Society for Reef Studies; she wasn’t connected to either study.

“It will be a magnification each time a coral bleaches,” she said. “You will lose a portion of the reef each time, and there comes a point where it’s no longer functionally a reef.”

What Hurricanes Maria and Irma did to a coral reef: A vibrant and healthy section of coral reef in 2013 (at left) becomes a denuded and sparsely populated hilltop in 2017 (at right) after back-to-back hurricanes ripped apart corals or tore them up from their foundations. (Howard Lasker / University at Buffalo)

Bleaching is not the only way to kill a reef. One month after Hurricanes Irma and Maria struck, Lasker visited the coral reefs he studies near St. John, in U.S. Virgin Islands National Park. While not all areas were ravaged, the back-to-back hurricanes had effectively sandblasted many of the corals.

“Some colonies were severely damaged, and you could see organisms beginning to grow on parts of the colony. It will lead to the eventual death of the colony,” Lasker said. “Stony corals were toppled, sea fans, sea plumes were ripped up from the bottom, he said.

You could, in other words, see the degradation of another kind of climate-addled weather system, and the corals starting to adapt to that change. But he said that bleaching was the most dreaded risk. “Hurricanes have occurred over the millennia,” he told me. “Bleaching is a much more modern phenomenon and in some ways more insidious. While hurricanes are going to increase in intensity, we have to worry more about bleaching on reefs.”