(CN) - Massive glaciers in West Antarctica are shrinking at the fastest rates ever recorded, presenting both a challenge and insight for researchers who are monitoring progress of an ice sheet collapse that could raise global sea levels by several feet.
The unsettling findings come from two studies, which used satellite data to track the melting patterns of three glaciers that flow into the Dotson and Crosson ice shelves in the Amundsen Sea embayment in West Antarctica.
The authors analyzed the Smith, Pope and Kohler glaciers, which are melting at different rates.
"Our primary question is how the Amundsen Sea sector of West Antarctica will contribute to sea level rise in the future, particularly following our observations of massive changes in the area over the last two decades," said University of California, Irvine scientist Bernd Scheuchl.
Scheuchl's team compared radar measurements from the European Space Agency's Sentinel-1 mission and data from older satellites to identify changes in each glacier's grounding line — the boundary where it loses contact with bedrock and begins to float on the ocean. Their findings were published in the journal Geophysical Research Letters this past August.
"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," Scheuchl said. "Our results show that the observed glaciers continue to lose mass and thus contribute to global sea level rise."
Measuring changes in a glacier's grounding line is important because most melting takes place on the underside of the floating portion, called the ice shelf. As a glacier loses mass from enhanced melting, it may begin floating farther inland from its original grounding line - known as grounding line retreat - a process similar to how a boat stuck on a sandbar may be able to float again if heavy cargo is removed.
The team found that the Smith Glacier's grounding line had retreated 1.24 miles per year since 1996, while the Pope Glacier's line had receded more slowly, at 0.31 miles per year during the same time frame. The Kohler Glacier's grounding line actually advanced 1.24 miles since 2011, after initially retreating.
"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," Scheuchl said, crediting the Sentinel-1 radar mission with adjusting how scientists look at polar ice sheets.
In a separate study, the NASA Jet Propulsion Laboratory's Ala Khazendar measured ice loss at the bottom of the three glaciers, which he thought might be affecting the changes in their grounding lines. His findings were published Tuesday in the journal Nature Communications.
Khazendar used radar and laser altimetry — the measurement of height or altitude — to gauge the thickness and height of the ice, which allowed him to assess how the bottom profiles of the three glaciers at their grounding lines differed between 2002 and 2014.
While previous studies using other techniques estimated the average melting rates at the bottom of the Dotson and Crosson ice shelves to be about 40 feet, Khazendar's team analyzed direct radar measurements and found more drastic rates of ice loss from the glaciers' undersides on the ocean side of their grounding lines. According to the team, the fastest-melting glacier, Smith, lost about 984 to 1,607 feet in thickness between 2002 and 2009 near its grounding line — or up to 230 feet per year.
That time span coincides with a period when rapid loss of mass was seen around the Amundsen Sea. The regional scale of the decline led scientists to suspect that an influx of ocean heat beneath the ice shelves must have occurred.
"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," Khazendar said.
Khazendar said Smith's fast retreat and thinning are likely due to the shape of the underlying bedrock over which the glacier was retreating between 1996 and 2004, which sloped downward toward the continental interior, as well as oceanic conditions beneath the glacier.
Scientists must determine whether other glaciers in West Antarctica will behave more like Pope and Kohler or more like Smith, which Khazendar and Scheuchl said requires more information on the shape of the bedrock and seafloor beneath the ice, as well as more data on ocean circulation and temperatures.
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