Two doses of bad news for the world's coral reefs came in the last week. First, Australia's government confirmed that the Great Barrier Reef is in the midst of a second consecutive year of mass bleaching. It's the first time the reef has experienced back-to-back events, and it seems to be weakening many of the corals.
Then on Wednesday, leading scientists published a new study about last year's bleaching—the worst to date—suggesting that when the seas are hot enough for long enough, nothing can protect coral reefs. Their only hope is that we rapidly slow climate change.
The research, published in the journal Nature, looked at data from three bleaching events along the 1,400 mile-long Australian reef system dating back to 1998. By looking at factors including water temperature, water quality and fishing protections, the authors determined that last year's bleaching was linked almost exclusively to ocean warming.
Conservationists have long hoped that protecting corals from other threats, such as pollution and overfishing, might help shield at least some of them from bleaching, too. While the new paper doesn't entirely deflate that hope—such protections likely help reefs recover—it shows that such work provides little if any relief from severe bleaching.
"At the level of heat stress that was seen during this event, it just didn't matter," said C. Mark Eakin, coordinator of the Coral Reef Watch program at NOAA and a co-author of the paper.
Ilsa B. Kuffner, a marine biologist with the United States Geological Survey who was not involved in the research, said the new paper supports a solid body of evidence suggesting that disease and bleaching are driving coral mortality, while other factors play a more important role in the recovery from those threats. "It's a distinction that, while it's subtle, is also very important when you talk about what's actually causing coral reef decline," she said.
The paper also found that a reef's history made little difference. Some studies have suggested that previous bleaching may make reefs more resilient if they are given time to recover, perhaps by killing off weaker corals or driving some adaptive response.
Warmer-than-average temperatures can cause coral to expel the symbiotic algae that live on its surface, turning the reef white. Such bleaching stresses coral and can make it more susceptible to disease and death.
The world's reefs are in the midst of what scientists consider to be a single, mass bleaching event dating back to 2014. Climate models project that most of the world's reefs could experience annual bleaching by 2050 without rapid cuts in greenhouse gas emissions.
While coral can survive even extreme bleaching, surveys conducted this month along the Great Barrier Reef are showing evidence that successive hits take a toll. Eakin said the level of heat stress—a measurement of how hot the waters are for how long—is lower than last year, and yet the bleaching appears to be just as widespread.
"They haven't bounced back yet, so when you hit them with another event a year later, you can see more bleaching at a lower level of heat stress," he said. "A lot of the corals that have survived last year really are not ready for another event."
The bleaching has also spread to areas of the reef that escaped last year's event, according to the recent surveys.
Successive bleaching also appears to be reshaping the makeup of the reef system. Reefs are composed of a rich diversity of coral species, with some particularly sensitive to bleaching and some that recover much more quickly than others. With consecutive years of bleaching, and after four events over 20 years, the new paper said the composition of the reef is changing in areas that have seen recurrent bleaching, perhaps irreversibly.
"The good news is you've got some tough corals that are surviving," Eakin said. "The bad news is, one of the most important things about coral reefs is their diversity, and you're cutting out some of that diversity."
The paper's authors believe that protecting reefs from pollution and overfishing will help them recover from bleaching. But the most important action, they said, lies elsewhere.
"Securing a future for coral reefs, including intensively managed ones such as the Great Barrier Reef," they wrote, "ultimately requires urgent and rapid action to reduce global warming."