The Great Barrier Reef off the northeast coast of Australia in December 2017. (Kyodo)
The damage caused in recent years to the Great Barrier Reef by ocean heat waves has compromised the resilience of the mbadive reef, and climate change could aggravate the problem in the future, according to research published on Wednesday.
The world's largest coral reef, which extends more than 1,400 miles off the coast of Australia, has suffered four mbad "bleaching" events driven by above-average sea temperatures over the past two decades, including consecutive episodes in 2016 and 2017.
Scientists studying the reef's ability to recover from those episodes detailed a series of discouraging findings in the journal Nature on Wednesday. They found that climate change, which has caused extreme heat stress in some reefs, has severely hampered the reef's healing capacity
"Reef replenishment capacity has decreased," Terry Hughes, lead author of the study and director of the Center for Excellence at the Australian Research Council for Coral Reef Studies at James Cook University in Queensland, said in an interview. "Our study shows that [corals] they are struggling to cope with rapid fire-bleaching events. "
Hughes said the researchers' findings focus on a key reality: dead corals do not produce babies.
Mbadive whitening events in 2016 and 2017 devastated almost half of the Great Barrier Reef, which is an extensive collection of almost 3,000 individual reefs. The heat wave affected some parts of the reef more than others, and some species died at a higher rate than others, a result that, according to scientists, would alter their character forever.
Coral bleaching occurs when corals lose their color after the symbiotic algae that live in coral cells and provide them with nutrients are expelled due to heat stress. The longer this state of stress lasts, the less likely it is that the corals will recover. Therefore, scientists tend to distinguish between moderate discoloration, which can be managed, and severe discoloration, which can kill corals and leave surviving corals more vulnerable to disease and other threats.
Historically, after damage from events such as discoloration or a hurricane, adult corals that remain on the reef generate billions of larvae each year, which spread and slowly begin to revitalize the reef by replacing the dead coral with new ones. But that is not happening as before.
According to the study on Wednesday, the number of new corals that settled on the Great Barrier Reef decreased by 89 percent after the recent whitening events. In addition, as the recovery of the fastest growing corals can take a decade or more, a reef needs a long respite to return to its previous state.
But climate change makes it less likely that the Great Barrier Reef would catch such a break. It has already suffered four mbad bleaching events since 1998, and climate models project that the reef will be bleached twice each decade by 2035 and annually after 2044 if the world does not drastically reduce its greenhouse gas emissions, the study notes.
"It's very unlikely we can escape a fifth or sixth event in the next decade," said Morgan Pratchett, co-author of the study and a professor at James Cook University, in a statement. "We used to think that the Great Barrier Reef was too big to fail, until now."
Kim Cobb, a coral reef expert and climate scientist at the University of Georgia Tech who was not involved in the study on Wednesday, called the work of collecting the data behind her as "thorough" and her findings "devastating."
"This is part of the ongoing disaster that never seems to stop," Cobb said, adding, "We know that these reefs will receive some very short-term impacts with repeated heat waves."
Still, he said there are still questions about whether the lack of coral replacement in the Great Barrier Reef will prove to be a short-lived problem as the reefs become more adaptable to the changing climate, or something that will become the new normal.
"The big question at the moment is whether they have enough time to regain the basic functions that will make them more resilient in the next heat wave," Cobb said. "How much can they return? How much time do they have?
Unfortunately, they may not have much time.
A study in the journal Science last year found that coral reefs around the world are being bleached four to five times more frequently than in 1980.
"We are observing that 90 percent of reefs see the heat stress that causes severe discoloration annually by the middle of the century," said Mark Eakin, one of the authors of the study and coordinator of Coral Reef Watch of the National Oceanic Administration. Atmospheric, he said at the time.
The study examined 100 major coral reefs, from 1980 to 2016, and found that only a few had not suffered severe discoloration during that period. He also found that the rate of severe bleaching is increasing over time. The average reef in the group decolorized severely once every 25 or 30 years in the early 1980s, but by 2016, the recurrence time of severe discoloration was 5.9 years.
"As global temperatures continue to rise," the authors of the study wrote Wednesday, "the likelihood of avoiding further discoloration events on the Great Barrier Reef in the next decade or two is very small."
Hughes said the damage to the Great Barrier Reef is more than the corals. "It's about the entire ecosystem that depends on them," he said, noting that the reefs help protect the coasts from tropical storms and provide shelter and habitat to a wide variety of marine organisms.
The world has warmed around a degree Celsius (1.8 degrees Fahrenheit) at pre-industrial levels, but scientists project that warming will continue to increase unless nations drastically reduce carbon dioxide emissions. Every bit of additional warming threatens even more sensitive coral reefs, and a report endorsed by the United Nations discovered last fall that the vast majority of the world's reefs could disappear if the warming exceeds two degrees Celsius.
Still, Hughes said scientists should not badume that future discoloration events will affect the reef in the same way as previous ones. Recent research has found that the corals that survived the 2016 discoloration were more resistant to a recurrence of warm ocean conditions a year later. So there is hope that corals adapt, even when world leaders try to keep global warming to no more than 1.5 degrees Celsius below pre-industrial levels.
"I do not think we're going to lose coral reefs to 1.5 or even two degrees [Celsius], but we are certainly already changing the nature of the reefs. That change is already underway, "Hughes said.
And there is little doubt of what is fueling the change.
"We have always anticipated that climate change could affect the reefs," he said. But "it's not something that can happen in the future, it's developing right now."