The Great Barrier Reef could lose the ability to protect itself from future bleaching events if climate change continues unabated, a new report warns.
The study, published in the journal Science on Thursday, says that this would occur if ocean temperatures rise by just 0.5 degrees Celsius -- which scientists predict could happen in less than four decades if climate change continues.
The report points to coral's unique ability to cope with a stressful event, like a warmer ocean, in what researchers call a biological practice run. The weeks leading up to coral-damaging temperatures allow the creatures to prepare for the warmer water and become more tolerant to bleaching and death.
Study lead author Tracy Ainsworth, a biologist at the ARC Centre of Excellence for Coral Reef Studies, compared the process to marathon training.
"Training doesn't stop you from feeling a marathon, it just makes it possible or easier to get through it because your body's prepared," she said.
Over the past thirty years, 75 percent of stressful events have followed this pathway of slow warming and protective stress. But warmer oceans would cut that marathon training period drastically, with less than a quarter of future events allowing coral to prepare for bleaching.
That annual repeat, it doesn't allow corals to recover.
Co-author Scott Heron, a physical oceanographer with the National Oceanic and Atmospheric Administration's Coral Reef Watch, said without this pre-conditioning, past bleaching events would have been "much more severe."
"A healthy person is going to be better able to survive exposure to some illness than some person who's health level is reduced," he said, referring to coral's natural ability to prepare for bleaching. "The disheartening thing is that our model shows that in the future the proportion of events with that protective [warming period] will be reduced."
Coral becomes bleached due to environmental stress, caused primarily by warmer oceans. The colorful algae that feed the coral abandon the formations during these heat waves, leaving them ghostly white. If temperatures decrease, the algae can return and coral can recover. But during severe bleaching, the episodes can be fatal.
That's exactly what's happening on the Great Barrier Reef, which scientists say is suffering from the region's worst bleaching event in recorded history. Around 95 percent of individual reefs in the most pristine area of the ecosystem showed signs of severe bleaching during a recent National Coral Bleaching Taskforce survey.
Heron said despite the severity of the event, it has followed the pattern of pre-warmth that should allow the coral to resist the worst effects of bleaching.
But as oceans warm, bleaching events will become more frequent and severe, effectively forcing biomes like the Great Barrier Reef to effectively run the equivalent of a marathon without any training, again and again.
"We've seen places where they've had bleaching, back-to-back, year-after-year," Heron said, pointing to the ongoing global bleaching event threatening reefs in the Caribbean and Hawaii. "That annual repeat, it doesn't allow corals to recover."