The Great Barrier Reef Survived 5 'Death Events' By Physically Moving

In April, scientists revealed that two-thirds of the Great Barrier Reef’s coral died during a nine-month heat wave in 2016. This “mass mortality” was largely chalked up to the effect of greenhouse gas emissions, which drive up ocean temperatures. The reef has survived temperature-induced “death events” before, scientists write in a Nature Geoscience article published Monday, but this time around is different for one reason: Humans.

In the study, University of Sydney scientists show that the reef system has suffered five death events over the past 30,000 years, all of them driven by changes in temperature and sea level. It’s resilient enough to have survived to this day, but with the unique nature of the ongoing death event, the scientists warn, it might not be able to bounce back. “I have grave concerns about the ability of the reef in its current form to survive the pace of change caused by the many current stresses and those projected into the near future,” co-author and geoscientist Jody Webster, Ph.D. said in a statement on Monday.

Webster and his team dived into the reef’s history using underwater sonar to find submerged fossilized corals far away from its current location. Over the past ten years, they drilled at 16 sites, pulling up core deposits containing “comprehensive sedimentological, biological, and geochronological records” spanning the past 30,000 years. These cores showed that the reef is sensitive to temperature shifts, sea level changes, and sediment fluxes. It’s one thing when these changes occur naturally; it’s another when we cause them. Today, says Webster, “we need to understand how practices from primary industry area affecting sediment input and water quality on the reef.”

The first two death events happened 30,000 and 22,000 thousand years ago, when a receding ocean left the reef exposed to air. To survive, the reef moved seaward in search of protection. But rapid sea level rise after the Last Glacial Maximum triggered two other reef-death events — one 17,000 years ago and another 13,000 years ago. That time time, the reef moved landward, shifting once again, like a whack-a-mole escaping a hammer. The fifth death event, which happened around 10,000 years ago, was caused by a sudden increase in water sediments, reduced water quality, and a general rise in sea level.

The reef is alive today because of its ability to migrate laterally — sometimes up to 4.9 feet (1.5 meters) per year. However, the scientists write, even that might not be fast enough to escape the growing areas affected by rising surface temperature, decreasing water quality, increasing sediment flux, and continued coral bleaching. Back-to-back coral bleaching events, driven by climate change, have become the new normal, and even the historically hardy Great Barrier Reef — and all the life that calls it home — may not be able to deal with it.

But hope is not lost. The scientists who studied the 2016 heat wave argue that, if we can curb greenhouse gas emissions now, the surviving coral stands a chance. For its part, the Australian government is investing over $3 billion in an attempt to reverse the damage of coral bleaching via the gene editing technique CRISPR. The reef isn’t likely to make it past this death event by itself — whether or not it survives is on us.