For the first time, researchers have documented the Great Barrier Reef’s (GBR) evolutionary past going back 30,000 years. Not only does this 10-year multinational study suggest the reef system has nearly died on five separate occasions, it also unveils the surprising way the world’s largest reef has managed to stay alive. The findings are published in Nature Geoscience. 

Researchers drilled 20 holes at 16 separate sites to collect geomorphic, sedimentological, biological, and dating information from fossilized reef cores at the edge of the GBR shelf. Over 30 millennia, the reef has recovered from abrupt environmental changes – such as sea-level rise and sea-temperature change – in the most practical way. Simply put, the reef moves.

This ability to migrate laterally across the seafloor at distances between 0.2 and 1.5 meters (0.7-4.9 feet) each year means the reef is much more resilient to sudden changes in the environment than previously believed. 

In most cases, the coral was able to move habitats when the sea level rose or fell. During the Last Glacial Maximum 20,000 years ago, sea levels were 118 meters (385 feet) below current levels. Before that, there were two widespread death events (30,000 and 22,000 years ago) caused by subaerial exposure, the exposure of the reef to air. In these cases, the reef moved deeper into the sea to keep pace with falling levels.

Then again, 17,000 and 13,000 years ago rapid sea-level rise caused the reef to move landward. The final near-death event was about 10,000 years ago when the reef as we know it emerged and was associated with massive sediment increase and reduced water quality. 

But just because the GBR was able to survive these five potentially catastrophic events doesn’t mean it will adapt to current changes associated with global warming.

"Our study shows the reef has been able to bounce back from past death events during the last glaciation and deglaciation," said lead author Jody Webster in a statement. "However, we found it is also highly sensitive to increased sediment input, which is of concern given current land-use practices."

A report earlier this year mapped the impact of a “catastrophic” marine heatwave in 2016 that led to “mass mortality” – as much as half of the coral in the northern region is lost and the study authors say it’s unlikely to recover.

Webster agrees. It’s unlikely the reef’s rate of movement would be enough to survive current rates of sea-surface-temperature rise or coral bleaching. More importantly, coral is particularly sensitive to decreased water quality and increased sediment – something Australia has been experiencing at a rapid rate since European settlement.

"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," Webster said. He added that researchers need a better understanding of the GBR's sensitivity to sediment fluxes and more knowledge of how development and industry are affecting water quality around the reef.