Ocean photo

What’s really striking about coral reefs isn’t their color—though they’re as vibrant as every nature documentary has led you to believe—but how loud they are. They literally crackle and buzz with life. And as many as five hundred million people depend upon living coral reefs for their lives—either in the form of coastal protection, food, or tourism income. But coral reefs around the globe are under threat due to climate change. Coral reefs like warm water, but not too warm. And as the world’s ocean’s heat up because of climate change, these slow growing, stationary creatures can’t do a thing about it. Like the frog in that apocryphal, slowly warming sauce pan, they die.

Coral have a symbiotic relationship with microscopic algae that live in their tissues, and gives them their color. When coral get stressed—either because of pollution, extreme low tides, or increase in temperature—the algae leave. Without them, these bleached coral are susceptible to disease and basically doomed.

In 2015 the National Oceanographic Atmospheric Administration issued a warning stating that the world was undergoing a global reef bleaching event. A study released today in Nature details just how badly Australia’s Great Barrier Reef, the world’s largest coral reef system, was harmed. And fair warning, the results were pretty grim.

Coral Bleaching guide
Why corals bleach in hot water. NOAA

The researchers combined data on coral reefs from aerial surveys—because bleached corals are white, you can see them from the air if you’re flying low enough—with onsite data taken by scientists on boats.

“We compared the measure of bleaching severity from the aerial scores with the measure of amount of bleaching that had come in water surveys,” said Sean Connolly, an associate professor at Australia’s James Cook University and a part of the ARC Center of Excellence Coral for Reef Studies located at the university. They ranked the bleaching from zero—no bleaching—to four for severe bleaching. Because the same methodology was also used for the 1998 and 2002 bleach events, they were able to compare the effects of the bleaching.

“It’s a comprehensive regional snapshot that we don’t ordinarily get,” said Carrie Manfrino, the president and director of research of the Central Caribbean Marine Institute in the Cayman Islands, who was not involved in the new study. “Usually a scientist will go out and do a small study of a site. In this instance, they assembled a large group of people so they really could cover the entire Great Barrier Reef region, plus they had a helicopter so they could do a snapshot over the region during this critical period.”

dead coral
Staghorn coral killed by bleaching on the northern Great Barrier Reef Greg Torda, ARC Centre of Excellence for Coral Reef Studies

In 1998 and 2002, roughly 10 percent of the reef surveyed had severe bleaching. But in 2016, only 10 percent of the reef remained unbleached. In 1998 and 2002, roughly 45 percent of the reef remained totally untouched by bleaching, while in 2016 at least 50 percent of the reef experienced severe bleaching.

The Great Barrier Reef is one of the world’s better studied coral reef ecosystems. “But coral bleaching is a global phenomenon,” said Collins. “It’s happening in somewhat different severities and frequencies in different parts of the world, but what’s happening here—the same basic story line will be playing all over the world.”

“1998 the event that happened here was massive. Over all three islands, the reefs bleached white,” said CCMI’s Manfrino. In 2016, the Cayman’s experienced some bleaching, but nothing on par with the Great Barrier Reef. But in the longterm, there’s not much hope.

“Globally, reefs as we knew them in the 1990s, I don’t think those will come back,” said Connolly. “I don’t think we’ll see reefs with that very high coral cover and complexity in the future. The question is how degraded they’ll be.”

a coral graveyard
Graveyard of Staghorn coral, Yonge reef, Northern Great Barrier Reef Greg Torda ARC Centre of Excellence for Coral Reef Studies

The study also ruled out the idea that the reefs are somehow adapting to the warming waters. They looked for evidence that reefs that had bleached in prior events didn’t bleach as badly this time around, but they bleached just the same. It also noted a quirk—the southern reefs were somewhat protected by the wind, rain, and cloud cover of an ex-tropical cyclone named Winston, which cooled the waters there.

“You can actually see it in the temperature record, the whole Great Barrier Reef was warming up, and as Winston started to affect the Southern Great Barrier Reef the temperature trajectory started to diverge,” said Connolly. “The north continued to get hotter.”

If there had been no cyclone, there might have been an extreme bleaching event along the whole length of the Great Barrier Reef.

The researchers were able to get such good data because NOAA’s warning gave them time to prepare.

“We had prepared a plan to go collect data on the bleaching if it did happen,” said Connolly. “Terry Hughes, the center and lead author of the study, had assembled something called a National Coral Bleaching task force which involved researchers at several different institutions. So when things did heat up we were ready to go out and really document what was happening at a scale that had never been done before.”

Hughes was unavailable to comment on the study—he was out preparing more aerial surveys. The Great Barrier Reef is bleaching yet again, marking the first time the reef has experienced two bleachings in a row. How severe the event will be—and what this means for the future of the reef—is still unknown.

aerial view of reef
Aerial view of widespread coral bleaching, northern Great Barrier Reef Terry Hughes, ARC Centre of Excellence for Coral Reef Studies

But even the best observational data can’t help scientists stop a bleaching event. And even marine parks and protected areas aren’t safe. The study found that marine parks experienced bleaching events all the same—but they were quicker to bounce back than unprotected areas. The question is, how many times can the reefs recover?

“The thing you have to bear in mind is, prior to human induced warming of the Earth’s climate, bleaching was only ever a very localized and short term event,” said Connolly. “You might get a bit of localized bleaching in a lagoon if there was an extreme low tide, and the water was sitting there for a long time, and it was hot and the sun was shining. Mass bleaching, bleaching on very large spatial scales, was unprecedented until it was first detected in the second half of the 20th century. There’s no evidence that mass bleaching was happening before detectable warming of the Earth’s climate that the models attribute to greenhouse gas production. Mass bleaching is a consequence of climate change.”