Thirteen thousand years ago, as the last ice age ended, entire stretches of Australia’s Great Barrier Reef perished. Rising sea levels blanketed the world’s largest collection of corals with sediment coming off the newly inundated land, blocking the sunlight corals need to grow. The reef eventually recovered, but it took hundreds to thousands of years. This near death and eventual resurrection wasn’t a one-off, according to a new study that reveals the reef’s shifting boundaries over geological time. It’s a tale that has played out five times over the past 30,000 years—and it may be happening again today.
The study “holds some really important lessons” for understanding how resilient corals are in the face of change, and how quickly they recover after catastrophic events, says Kim Cobb, a paleoclimatologist at the Georgia Institute of Technology in Atlanta, who wasn’t involved in the work. Today’s rate of sea level rise is moderate—about 10% of the rate 13,000 years ago—but going forward it may accelerate dramatically, she says.
To conduct the study, scientists used underwater sonar to locate places on the sea floor, beyond the current reef, where corals may have grown in the past. Then, they drilled 20 holes, extracting rock cores that contained fossil corals and sediments deposited over the past 30,000 years, spanning part of the last ice age and the warm millennia that followed.
The reef migrated up and down during that time, the team found, closely tracking changes in sea level at a rate of up to 20 vertical meters per thousand years. And when sea level reached its lowest point 21,000 years ago—118 meters below today’s level, its lost water locked up in massive ice caps—corals survived on the outer terraces of Australia’s continental shelf, the team reports today in Nature Geosciences.
Scientists have long wondered where the Great Barrier Reef went during the last ice age, says Jody Webster, a marine geologist at The University of Sydney in Australia and the lead author of the study. “We were able to find it.”
But the reef couldn’t always keep up with changing sea levels. The researchers identified five times when it appeared to die off—twice during the cool down of the last ice age, when falling sea levels exposed corals to air; and three times 10,000 to 17,000 years ago, when glacial melt caused sea levels to rise rapidly. “We haven’t drilled or sampled everything,” says Webster, so he and his colleagues can’t confirm how extensive the die-off was. But they think corals persisted in some places along the continental shelf during those times, allowing reefs in other locations to re-establish within 2000 years.
The historical die-offs are similar to “what we’re seeing right now on the Great Barrier Reef,” says Mark Eakin, a coral reef ecologist at the National Oceanic and Atmospheric Administration in College Park, Maryland, who wasn’t involved in the study. Sea level changes aren’t a huge problem at the moment, but temperatures are: Heat waves have triggered mass bleaching events, periods where heat-stressed corals expel symbiotic algae that live within their tissues. In 2016 alone—the hottest year on record globally—67% of corals died along the northernmost 700 kilometers of the reef.
The new research is “yet another reminder” that what we’re doing to the ocean is going to have dramatic consequences, Eakin says. “Don’t expect reefs to be able to bounce back quickly.”