The Antarctic Ice Sheet, like every other land ice mass on Earth, is in constant motion. Until now, however, researchers have assumed that the Antarctic Ice Sheet is not subject to the same seasonal differences in flow velocity as its Arctic and alpine counterparts. Using optical and radar satellite data, a research team has now been able to show for the first time, using the George VI Ice Shelf as an example, that ice movements in the Antarctic towards the ocean are subject to seasonal fluctuations just like other land ice masses. Thus, the overall contribution to sea level rise may have been over- or underestimated. The study appeared in the journal The Cryosphere.
In particular, where Antarctic ice masses flow into large ice shelves, large seasonal variations in flow velocity had not been expected, since here the temperature is below freezing most of the year. In part, this assumption was also fueled by the comparatively poor availability of satellite imagery from the region.
“Unlike the Greenland Ice Sheet, where a high quantity of data has allowed us to understand how the ice moves from season to season and year to year, we haven’t had comparable data coverage to look for such changes over Antarctica until recently,” explains Karla Boxall of the Scott Polar Research Institute (SPRI) in Cambridge, lead author of the study.
Using images from the Copernicus/European Space Agency Sentinel-1 satellites, the research team, consisting of scientists from the University of Cambridge and the Austrian engineering firm ENVEO, determined for the first time that there are distinct seasonal differences in the flow of land ice draining into the George VI Ice Shelf on the Antarctic Peninsula. They found that the glaciers feeding the ice shelf accelerate by about 15 percent in summer.
“Observations of ice-speed change in the Antarctic Peninsula have typically been measured over successive years, so we’ve been missing a lot of the finer detail about how flow varies from month to month throughout the year,” said Dr. Frazer Christie of SPRI, co-author of the study.
Before detailed ice velocity records were available from the Sentinel-1 satellites, researchers could only rely on information collected by optical satellites such as NASA’s Landsat 8.
“Optical measurements can only observe the Earth’s surface on cloud-free days during summer months,” said co-author Dr. Thomas Nagler, ENVEO’s CEO. “But by using Sentinel-1 radar imagery, we were able to discover seasonal ice-flow change thanks to the ability of these satellites to monitor year-round and in all-weather conditions.”
The causes behind the seasonal fluctuations are not yet clear. The research team suspects that meltwater from the surface reaches the base of the ice and acts like a lubricant, as it does in Arctic and alpine regions. However, it could also be that relatively warm ocean water is melting the ice shelf from below, thinning the floating ice and allowing the glaciers to move faster.
“These seasonal cycles could be due to either mechanism, or a mixture of the two,” Christie said. “Detailed ocean and surface measurements will be required to understand fully why this seasonal change is occurring.”
It is possible that these seasonal variations also occur at other, more sensitive locations in Antarctica, such as Pine Island and Thwaites glaciers in West Antarctica. “If true, these seasonal signatures may be uncaptured in some measurements of Antarctic ice-mass loss, with potentially important implications for global sea-level rise estimates,” Boxall said.
“It’s the first time this seasonal signal has been found on the Antarctic Ice Sheet, so the questions it raises regarding the possible presence and causes of seasonality elsewhere in Antarctica are really interesting,” says co-author Professor Ian Willis, also of SPRI. “We look forward to taking a closer look at, and shedding light on, these important questions.”
Julia Hager, PolarJournal
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