The Great Barrier Reef is not doing so well. In 2016 it was hit with the worst coral bleaching event in its history, and it didn't help that another one struck the following year. As the fate of the reef remains uncertain, a new study has examined the health of the Great Barrier Reef over the last 30,000 years, and found that it has suffered five "death events" in the past – but its current woes could be the last straw.

The study was conducted over 10 years by an international team of scientists, who drilled fossil reef cores from 16 sites along the northeastern Australian coast, and analyzed the dating, geomorphic, sedimentological and biological data from them. In doing so, the team was able to piece together the evolution of the Great Barrier Reef over the last 30,000 years, and, in particular, how well it bounced back from past environmental upheavals.

The researchers found evidence of five distinct death events during that time, mostly as a result of changes in sea level. The first two large scale deaths occurred about 30,000 and 22,000 years ago, as the sea level drastically dropped, bottoming out at 118 m (387 ft) lower than it currently is. This left parts of the reef exposed to the open air.

As the last ice age ended, melting glaciers caused the sea level to rise again, triggering another two reef-death events, about 17,000 and 13,000 years ago. In these cases, large sections of coral effectively "drowned" as deeper waters deprived them of sunlight.

The final event occurred about 10,000 years ago, but wasn't accompanied by any known sea level change. Instead, the team found that the death may have been the result of a huge increase in sediment, which reduced the water quality.

The fact that the reef persisted through these events showed the researchers that it's more resilient in the face of danger than was previously thought. The scientists suggest that this is because the coral is able to spread at a rate of 0.2 to 1.5 m (0.7 to 4.9 ft) per year, meaning some parts of the reef are saved as they migrate into safer territory. During sea level drops, for instance, the Great Barrier Reef as a whole survived by migrating towards deeper waters, and marching back towards land as the level rose again.

As resilient as it is, unfortunately the researchers don't believe that defense mechanism will be enough to save the reef this time around. Those natural shifts occur slowly, over hundreds or thousands of years, giving the coral plenty of time to adapt and migrate. But it's now being destroyed it at an unprecedented rate, with a recent report concluding that human influences cut the coral cover in half between 1985 and 2012.

"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," says Jody Webster, lead researcher on the project. "Our study shows that as well as responding to sea-level changes, the reef has been particularly sensitive to sediment fluxes in the past and that means, in the current period, we need to understand how practices from primary industry are affecting sediment input and water quality on the reef."

The research was published in the journal Nature Geoscience.

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