A Porites coral colony on the Great Barrier Reef .
Photo: Justin Marshall

In 2016 and 2017, disaster struck the world’s largest living structure: back-to-back heat waves caused half of all corals on the 1,400-mile-long Great Barrier Reef to die. But there’s a glass-half-full perspective on this grim news, which is that half of the reef’s corals—approximately 10 billion of them—are still alive. And they just went through one hell of a natural selection event.

Research published today in Nature Climate Change puts a new spin on the worst die-off to roil the Great Barrier Reef in recent memory, by showing that the ecosystem is exhibiting an emerging resilience to rising temperatures. Reefs that cooked and bleached during the summer of 2016 could tolerate much more heat in 2017 without experiencing the same effects. The study’s authors believe we’re witnessing the emergence of an “ecological memory,” wherein response to one climate shock dampens the effect of the next one.

Advertisement

When ocean temperatures crank up too high, it causes corals to jettison the algae that live inside them and provide them with food, turning their jewel-toned exoskeletons a sickly white. This is known as bleaching, and if a coral stays bleached for too long, it begins to starve. Alternatively, if water temperatures are way too hot, corals will simply cook.

Both of these processes ravaged the Great Barrier Reef in 2016 and 2017. But by carefully examining patterns of heat stress via satellite-derived temperature data, along with bleaching patterns determined through aerial surveys, the researchers at James Cook University in Queensland, Australia and elsewhere teased out some key differences between these two years.

Photo: Tane Sinclair-Taylor

Advertisement

Chief among them: While the first heat wave took a disastrous toll on the northern third of the Great Barrier reef, causing more than 80 percent of individual reefs to experience severe bleaching, the second heat wave resulted in far less bleaching, even on reefs that experienced the same heat stress.

Lead study author Terry Hughes told Earther this is likely because the most temperature-sensitive species, including branching and table-shaped Acropora corals, were pretty much wiped out by the first event. “In the north, it took an enormously large amount of heat to produce the same amount of bleaching in 2017 because the heat-sensitive corals were gone,” Hughes said.

The southern third of the reef, meanwhile, only saw a slight amount of bleaching in 2016 because the heat wave was cut short by a cyclone. But even there, the researchers saw less bleaching than they expected in 2017, indicating the 2016 exposure had had some protective effect. In fact, the only area where bleaching was worse in 2017 than 2016 was the reef’s middle section. There, temperatures shot up in 2017 after relatively mild bleaching in the previous year that left most of the sensitive corals intact.

Advertisement

Overall, Hughes said, the results suggest “that the behavior of the corals, their response to repeated bouts of bleaching, changes through time.” That’s a critical insight, and one that calls into question one of the most dire projections laid out in a recent UN climate report: that just 1.5 degrees of warming could cause 70 to 90 percent of the world’s reefs to die.

“The IPCC report that makes projections of horrendous decreases of reefs globally is based on climate modeling with the assumption that bleaching events in 10 to 20 years will have the same impact as bleaching today,” Hughes said. “Our study shows that’s not necessarily correct... the mix of species is shifting very quickly, and that’s clearly having an effect.”

While it’s heartening that not all corals are equally doomed in our warming world, it’s also crucial to note that not all corals play the same ecological role. As Hughes and his colleagues noted in a paper out earlier this year, the Great Barrier Reef is fast becoming a “highly altered, degraded system” as the corals that provide the most nooks and crannies to shelter reef fish vanish.

Advertisement

Hughes predicts that the corals that will be the ultimate winners in our hotter future are slow-growing, hemispherical brain corals and Porites, which he described as good at protecting shorelines but not so great at supporting biodiversity.

And eventually, if emissions continue unchecked, we may reach a point where even the toughest corals succumb to heat stress, ocean acidification (also associated with rising CO2), or some combination of stressors.

The Great Barrier Reef’s newfound resilience may be tested again all too soon. Australia’s summer kicked off early last month with a record-smashing heatwave that brought unprecedented fire conditions to Queensland in what is normally the wet season. While Hughes was reluctant to make predictions, noting that the Great Barrier Reef doesn’t start bleaching until February or March when water temperatures are at their hottest for the year, he did say he’s “concerned” about what this summer will hold.

Advertisement

After all, nobody really wants to know what happens when the world’s largest reef roasts three times in four years.