Scientists investigate what nearly destroyed the Great Barrier Reef 125,000 years ago

A new study on the Great Barrier Reef's past may provide an important glimpse into its future.

The largest reef on Earth nearly drowned at the beginning of the Last Interglacial period more than 125,000 years ago, according to researchers from the University of Sydney. Their report, published in the journal Global and Planetary Change, explains how the Great Barrier Reef was almost destroyed because of rapidly rising seas from melting glaciers and polar ice sheets. 

While the research offers an important analysis of the reef's past, it also carries valuable implications going forward in an era of climate change. The study shows the resilience of the Great Barrier Reef, as shallow reef growth started up again once the rapid sea-level rise had stabilized. But pressures on the reef today, including warming sea temperatures, pesticide run-off, dredging from mining operations, and predicted sea-level rises, could put that resiliency to the test and threaten the survival of the World Heritage Listed "wonder."

"This provides the first snapshot of this paleo-reef against a background of rapid environmental change, including possible mass ice-sheet collapse," said lead author Belinda Dechnik, from the Geocoastal Research Group in the University of Sydney's School of Geosciences, in a news release. "The findings highlight the importance of increasing the reef’s resilience now." 

Temperatures and sea levels were higher 125,000 years ago, during the Last Interglacial period, than they were today. But if CO2 emissions are left unchecked, geologists say, we could be headed for similar levels in the future. 

"In combination with climate change predictions by the Intergovernmental Panel on Climate Change and in the absence of improvements to reef management and human impacts, sea-level pressures could tip the reef over the edge, potentially drowning it for good," Dr. Dechnik explained. 

The dangerous effects of climate change on the Great Barrier Reef have long worried scientists. In April, a team of researchers reported that coral bleaching of the reef in 2016 was the worst that had ever been observed. The scientists found bleaching in almost 1,100 kilometers of northern barrier reef, and estimated that 30 to 50 percent of the corals there were already dead, as Lonnie Shekhtman reported for The Christian Science Monitor at the time: 

Coral bleaching happens when ocean temperatures rise to a point that zooxanthellae – tiny algae that live on corals and provide them with nutrients and their radiant colors – leave their coral homes, thereby rendering coral white or "bleached." When corals go without zooxanthellae for too long, they die. This affects about a quarter of marine species that depend on coral reefs for shelter, and the humans who depend on those species for their livelihoods....

It is possible that barrier reef corals will adapt to warmer waters over many generations, but no one knows how long that could take or if there will even be any coral left to do the adapting.

The most viable immediate remedy, say paper authors, is to reduce the carbon emissions that cause warming and restrict other human activities near the reefs that add more stress, including runoff from agriculture, unsustainable fishing practices, and physical damage to the reef from ship groundings.

The publication of the University of Sydney researchers' findings coincides with the release of another recent study on coral reefs by scientists at the University of Miami. Their research, published in the journal Scientific Reports, combined climate projection models with various scenarios, such as the impact of the Paris Climate Agreement, to predict the fate of coral reefs over the coming century. They found that the world’s reefs will begin experiencing annual bleaching in 2043, on average. 

"Bleaching that takes place every year will invariably cause major changes in the ecological function of coral reef ecosystems," said study leader Ruben van Hooidonk in a news release last week. "Further, annual bleaching will greatly reduce the capacity of coral reefs to provide goods and services, such as fisheries and coastal protection, to human communities." 

If current trends continue, the researchers learned, 99 percent of all reefs on Earth will experience severe bleaching every year within the next 100 years.

"It is imperative that we take these predictions seriously and that, at the very minimum, we meet the targets of the Paris Agreement," said Erik Solheim, head of UN Environment, in a statement. "Doing so will buy time for coral reefs and allow us to plan for the future and adapt to the present."