The West Antarctic Ice Sheet is undergoing a rapid retreat, with numerous glaciers across the region melting at an alarming rate. However, the situation was not always this way, according to new research published last month in The Cryosphere. A team of scientists from the International Thwaites Glacier Collaboration (ITGC), including two researchers from British Antarctic Survey (BAS), discovered that the ice sheet near Thwaites Glacier was thinner in the last few thousand years than it is today. This unexpected finding shows that glaciers in the region were able to regrow following earlier shrinkage. This discovery reveals that ice sheet retreat in the Thwaites Glacier region can be reversed, but the challenge for scientists now is to understand the conditions required to make that possible.
Sea level rise is presently putting millions of people in low-lying coastal communities around the world at risk from flooding. The contribution from melting Antarctic ice is currently the greatest source of uncertainty in predictions of how much and how quickly sea level will rise in the coming decades and centuries. Thwaites Glacier, together with its immediate neighbor, currently dominates the Antarctic contribution to sea level rise. To understand how this important glacier will respond to climate changes expected in the coming century, scientists need to know how it behaves under a wide range of climatic conditions and over long timescales. Given that satellite observations only go back a few decades in time, scientists need to look at the geological record to find this information.
A team of scientists from the ITGC, including two researchers from BAS, used drills specially designed to cut through both ice and the underlying rock to recover rock samples from deep beneath the ice sheet next to Thwaites Glacier. They then measured specific atoms in those rock samples that are made when rocks are exposed at the surface of the Earth to radiation coming from outer space. If ice covers those rocks, these specific atoms are no longer made. Their presence can, therefore, reveal periods in the past when the ice sheet was smaller than present.
Keir Nichols, a glacial geologist from Imperial College London and the lead author of the study, said, “This was a huge team effort: several of us spent weeks away from home doing fieldwork in an extremely remote part of Antarctica, while others endured literally thousands of hours in the lab analyzing the rocks we collected. The atoms we measured exist only in tiny amounts in these rocks, so we were pushing right to the limit of what is currently possible, and there was no guarantee it would work. We are excited that this is the first study to reveal the recent history of an ice sheet using bedrock collected from directly beneath it.”
The team discovered that the rocks they collected were not always covered by ice. Their measurements showed that, during the past 5,000 years, ice near Thwaites Glacier was at least 35 meters thinner than it is now. Furthermore, their models demonstrated that its growth since then, making the ice sheet the size it is today, took at least 3,000 years.
Jonathan Adams, co-author and Ph.D. student at BAS, said, “By studying the history of glaciers like Thwaites, we can gain valuable insight into how the Antarctic Ice Sheet may evolve in the future. Records of ice sheet change from rocks that are presently exposed above the ice sheet surface end around 5,000 years ago, so to find out what happened since then, we need to study rock presently buried beneath the ice sheet.”
Joanne Johnson, a geologist at BAS and co-author of the study, said, “On the face of it, these results seem like good news—Thwaites Glacier was able to regrow from a smaller configuration in the geologically-recent past. However, our study shows that this recovery took more than 3,000 years, in a climate that was likely not as warm as what we expect for the coming centuries. If we want to avoid the impacts of sea level rise on our world that will result from continued retreat of the West Antarctic Ice Sheet, that timescale is far longer than we can afford to wait.”
In summary, the study reveals that the West Antarctic Ice Sheet can regrow, but the process is slow and takes thousands of years. Further research is needed to understand the conditions required to make this regrowth possible and to prevent the impacts of sea level rise that will result from continued retreat of the West Antarctic Ice Sheet.