Greenland lost its entire ice sheet millions of years ago, and it could happen again

In order to predict what will happen to the Greenland ice sheet as a result of climate change in the years ahead, two teams of researchers have used new techniques to peer into its past farther than was previously possible – but their studies produced contrasting results.

Four times larger than the state of California, the Greenland ice sheet holds so much water that, were it to melt, the global sea level would rise by more than 20 feet, the researchers explained in a statement. Such an event could be catastrophic for those living in coastal areas, so experts have been working to determine if such melting could occur in the near future.

Now, two separate groups of scientists have attempted to look into how the ice sheet behaved in the distant past – several million years ago, in fact, when global temperatures were three or more degrees hotter than they currently are. However, each of these groundbreaking studies painted a much different picture of how the ice sheet responded to past climate change.

Both teams used “a powerful new tool for Earth scientists,” explained Imperial College London scientist Dylan Rood, co-author of one of the newly-published papers. Using isotopes contained within grains of quartz produced during a time when Earth’s bedrock was being bombarded by a steady stream of cosmic rays. By determining the ratio of two elements made by cosmic rays, they could determine how long the quartz was exposed and how long it was covered by ice.

Findings appear to be contradictory… but are they?

Specifically, the researchers looked at the ratio of aluminum-26 and beryllium-10 (two elements produced by cosmic rays) using an accelerator mass spectrometer – a method that has long been used to study land-based erosion, but which had not previously been used to investigate samples collected from the ocean. Both studies have been published in the journal Nature.

In one of the studies, a team led by University of Vermont geologist Paul Bierman investigated deep cores of ocean-bottom mud containing bedrock that eroded Greenland’s eastern site. They found that the ice sheet in this region had not completely melted for an extended period of time at any point in the past 7.5 million years, and that it has actively been covered by glacial ice for the majority of that span – a discovery said to be consistent with current computer models.

Furthermore, their research found that during cooler periods in the past, the ice sheet expanded into areas that were previously ice-free, demonstrating that the ice sheet “responds to and tracks global climate change,” according to Bierman. “The melting we are seeing today may be out of the bounds of how the Greenland ice sheet has behaved for many millions of years.”

While that study found that eastern Greenland has been pretty much consistently covered by an ice sheet for the past seven million years, the other study took a look at the middle of the current ice sheet and found that the ice sheet was nearly ice-free for at least 280,000 years in the middle Pleistocene era (about 1.1 million years ago), in contrast to most existing computer models.

While the studies appear to blatantly contradict one another, Bierman emphasizes that this is not necessarily the case. He explained that both of the studies have “some blurriness” regarding what they were able to determine about short-term changes to the ice sheet’s size, since the studies use data from different locations. “It’s quite possible that both of these records are right for different places,” the geologist concluded. “Both of these studies apply a similar innovative technique and let us look much farther into the past than we have been able to before.”

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Image credit: Joshua Brown/UVM

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