Scientists studying the reef’s capacity to bounce back from those episodes detailed a disheartening set of findings in the journal Nature on Wednesday. Climate change, which has caused extreme heat stress on some reefs, has severely hindered the reef’s ability to heal, they found.
“The replenishment ability of the reef has been diminished,” Terry Hughes, the study’s lead author and director of the Australian Research Council Center of Excellence for Coral Reef Studies at James Cook University in Queensland, said in an interview. “Our study shows that [corals] are pretty much struggling to cope with rapid-fire bleaching events.”
Hughes said the researchers’ findings center on a key reality: Dead corals don’t make babies.
The massive bleaching events in 2016 and 2017 devastated nearly half of the Great Barrier Reef, which is a sprawling collection of nearly 3,000 individual reefs. The heat wave affected some parts of the reef more than others, and some species died off at a greater rate than others — an outcome that scientists said would forever alter its character.
Coral bleaching occurs when corals lose their color after the symbiotic algae that live in coral cells and provide them with nutrients are expelled because of heat stress. The longer this state of stress lasts, the less likely corals will recover. So scientists tend to distinguish between moderate bleaching, which can be managed, and severe bleaching, which can kill corals and leave surviving corals more vulnerable to disease and other threats.
Historically, after the damage from events such as bleaching or a hurricane, the remaining adult corals in the reef spawn trillions of larvae each year, which spread and slowly begin to revitalize the reef by replacing dead corals with new ones. But that’s not happening as it once did.
According to Wednesday’s study, the number of new corals settling on the Great Barrier Reef declined by 89 percent after the recent bleaching events. In addition, because it can take a decade or longer for even the fastest-growing corals to recover, a reef needs a long respite to return to its former state.
But climate change makes it less likely that the Great Barrier Reef will catch such a break. Already, it has suffered four mass bleaching events since 1998, and climate models project that the reef will bleach twice each decade by 2035 and annually after 2044 if the world does not sharply cut its greenhouse gas emissions, the study says.
“It’s highly unlikely that we could escape a fifth or sixth event in the coming decade,” Morgan Pratchett, a study co-author and professor at James Cook University, said in a statement. “We used to think that the Great Barrier Reef was too big to fail — until now.”
Kim Cobb, a coral reefs expert and climate scientist at Georgia Tech University who was not involved in Wednesday’s study, called the work of gathering the data behind it “painstaking” and its findings “devastating.”
“This is part of the ongoing train wreck that just never seems to stop,” Cobb said, adding, “We know that these reefs are going to be taking some very near-term hits with repeated heat waves.”
Even so, she said questions remain about whether the lack of coral replenishment in the Great Barrier Reef will prove to be a short-lived problem as reefs become more adaptable to the changing climate — or something that will become the new normal.
“The big question right now is, do they have enough time to recover the basic functions that will make them more resilient in the next heat wave?” Cobb said. “How much can they come back? How much time do they have?”
Unfortunately, they might not have long.
We’re “looking at 90 percent of reefs seeing the heat stress that causes severe bleaching on an annual basis by mid-century,” Mark Eakin, one of the study’s authors and coordinator of the National Oceanic and Atmospheric Administration’s Coral Reef Watch, said at the time.
The study surveyed 100 major coral reefs, from 1980 through 2016, and found that only a handful had not suffered severe bleachings during that period. It also found that the rate of severe bleaching is increasing over time. The average reef in the group bleached severely once every 25 or 30 years at the beginning of the 1980s, but by 2016, the recurrence time for severe bleaching was 5.9 years.
“As global temperatures continue to rise,” the authors of Wednesday’s study wrote, “the probability of avoiding further bleaching events on the Great Barrier Reef in the next decade or two is vanishingly small.”
Hughes said the damage to the Great Barrier Reef is about more than the corals. “It’s about the whole ecosystem that depends on them,” he said, noting that reefs help to protect coastlines from tropical storms and provide shelter and habitat for an array of marine organisms.
The world has warmed about one degree Celsius (1.8 degrees Fahrenheit) over preindustrial levels, but scientists project that warming to continue to increase unless nations drastically cut carbon dioxide emissions. Each bit of additional warming further threatens sensitive coral reefs, and a report backed by the United Nations found last fall that the vast majority of the world’s reefs could disappear if warming exceeds two degrees Celsius.
Still, Hughes said scientists shouldn’t assume that future bleaching events will affect the reef in quite the same way as past ones have. Recent research has found that corals that survived the 2016 bleaching were more resistant to a recurrence of the hot ocean conditions a year later. So there is hope that corals will adapt, even as world leaders try to keep global warming to no more than 1.5 degrees Celsius below preindustrial levels.
“I don’t think we’re going to lose coral reefs at 1.5 or even two degrees [Celsius], but we are certainly already changing the nature of reefs. That change is already underway,” Hughes said.
And there’s little doubt what is fueling the shift.
“We’ve always anticipated that climate change could affect reefs,” he said. But “it’s not something that might happen in the future. It’s unfolding right now.”