Research will lead to better predictions about rising sea levels
The fastest ongoing rates of glacier retreat ever recorded have been documented by researchers at the University of California, Irvine (UCI) and NASA, in two new studies.
The results show how the frozen mass is influenced by the interaction between ocean conditions and the bedrock beneath a glacier. This offers an unprecedented peek into the melting ice on the floating undersides of glaciers, allowing scientists to better predict Antarctica’s future ice loss and the rise in global sea levels.
Three neighbouring glaciers – Smith, Pope and Kohler – that were melting and retreating at different rates were used in the studies. They flow into the Dotson and Crosson ice shelves in the Amundsen Sea embayment in West Antarctica, where the greatest ice loss on the continent can be found.
For UCI, their focus was to find out how the Amundsen Sea sector of West Antarctica would contribute to the rise in sea level in the future, after their observation of massive changes in the area in the previous two decades.
“Using satellite data, we continue to measure the evolution of the grounding line of these glaciers, which helps us determine their stability and how much mass the glacier is gaining or losing. Our results show that the observed glaciers continue to lose mass and thus contribute to global sea level rise,” said Bernd Scheuchl, lead author of one of the studies, which was published in the Geophysical Research Letters in August.
The grounding line is where the glacier loses contact with the bedrock and starts to float on the ocean. This detail is important because almost all glacier melting occurs on the underside of this floating portion, called the ice shelf. If too much mass is lost due to enhanced melting, the glacier may start to float farther inland away from its former grounding line. This situation is called a grounding line retreat.
It was discovered that since 1996, the grounding lines of the Smith and Pope Glacier had been retreating two kilometres and half a kilometre annually, respectively. For the Kohler Glacier, it actually readvanced two kilometres since 2011.
“Our work shows that the data collected is very well-suited for ice sheet science, and we can combine it with other satellite and airborne data sets to establish a more detailed record of these glaciers,” said Scheuchl.
In a separate study published recently in Nature Communications, Ala Khazendar from the NASA Jet Propulsion Laboratory and co-author of Scheuchl’s paper, measured ice loss at the bottom of the three glaciers. Suspecting that ice loss might influence changes in the glaciers’ grounding lines, he used radar and altimetry instruments to gauge the thickness and height of the ice between 2002 and 2014. Laser measurements of the surface elevation were used to infer changes in the thickness of the floating ice shelves.
Previous estimates of the average melting rates at the bottom of the Dotson and Crosson ice shelves, using other methods, were about 12 metres per year. This was much lower that the rates of ice loss from the glaciers’ undersides on the ocean sides of their grounding line, that were measured by Khazendar and his team. They discovered that Smith Glacier, the fastest-melting glacier, lost between 300 to 490 metres in thickness between 2002 and 2009 near its grounding line. This amounts up to 70 metres annually.
During 2002 and 2009, rapid mass ice loss was observed around the Amundsen Sea. Based on the regional scale of the decline, the scientists suspect that an increase in the influx of ocean heat beneath the ice shelves had taken place. “Our observations provide a crucial piece of evidence to support that suspicion, as they directly reveal the intensity of ice melting at the bottom of the glaciers during that period,” said Khazendar.
Two IceBridge instruments, which employ different techniques, both measured the same amount of rapid ice loss. “If I had been using data from only one instrument, I wouldn’t have believed what I was looking at, because the thinning was so large,” Khazendar said.
He said that Smith’s rapid retreat and thinning are likely to be due to the shape of the underlying bedrock over which it was retreating between 1996 and 2014, which sloped downward toward the continental interior. As the grounding line receded, warm and dense ocean water reached the newly uncovered deeper parts of this cavity, causing more melting.
As more sections of the glacier become thinner and float, the grounding line would continue to retreat. However, the retreat of Smith’s Glacier may slow down now that its grounding line has reached the bedrock that rises farther inland of the 2014 grounding line.
As for the Pope and Kohler Glaciers, they are on bedrock that slopes upward toward the interior.
For the other glaciers in West Antarctica, it is unknown whether they would behave more like the Smith Glacier or like Pope and Kohler Glaciers. Many in this sector of Antarctica are similar to the Smith Glacier, in that they are on beds that deepen farther inland.
Both Khazendar and Scheuchl said that more information is still needed about the shape of the bedrock, the seafloor beneath the ice, and ocean circulation and temperatures to better project how much ice the glaciers will contribute to the ocean.