The Antarctic Ice sheet has been routinely observed over the past three decades and is known to be decaying progressively, due to accelerated rates of ice thinning, retreat and flow. Although this phenomenon has been ascribed to warming atmospheric and oceanic factors, there has been an implicit assumption that there are no seasonal variations in the rate of ice flow. A new study now finds evidence that this assumption is erroneous and that past predictions of ice loss from the Antarctic Ice Sheet, and its contribution to sea-level rise, will be inaccurate as a consequence.
The researchers, from the University of Cambridge and Austrian engineering company ENVEO, identified distinct, seasonal changes in the velocity of land-based ice from 21 glaciers that drain into the George VI Ice Shelf – a floating platform of ice roughly the size of Wales – on the Antarctic Peninsula. They say there has been a shortage of high temporal resolution observations of the Antarctic Ice sheet in the past, in contrast to the situation for Greenlandic and other Arctic and ice masses where observational data is far more available. The timing and magnitude of seasonal ice-flow variation in Arctic ice sheets is now well observed and understood.
The assumption that there is no seasonal effect on ice flow rates in Antarctica was based partly on the fact that the ice sheets are very large on Antarctica, and the temperatures remain below zero degrees all year round. But it was also fueled by a lack of imagery collected over the icy continent in the past.
“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,” said Karla Boxall from Cambridge’s Scott Polar Research Institute (SPRI), the study’s first author.
“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 co-author Dr. Frazer Christie, also from SPRI.
In the past, scientists wishing to study variations in Antarctic ice flow had to rely on information collected by optical satellites such as NASA’s Landsat 8 Operational Land Imager. These observations were not frequent or detailed enough to follow the seasonal patterns. In the current study however, the researchers used 6/12-day repeat-pass radar imagery from the Copernicus Sentinel-1 satellites, in addition to 16-day repeat-pass observations made by the Landsat 8 satellites, to determine rates of ice movement. They analyzed images from these two sources acquired between 2014 and 2020.
“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 analysis, published in The Cryosphere, showed that the glaciers feeding the George VI Ice Shelf speed up by approximately 15 percent during the summer months. This is the first time that such seasonal cycles have been detected for land ice flowing into ice shelves in Antarctica.
Although the researchers are not exactly sure what mechanism is causing this seasonal increase in flow rate, they suggest two possible alternatives. It could be caused by surface meltwater reaching the base of the ice and acting like a lubricant, as is the case in Arctic and Alpine regions, or it could be due to relatively warm ocean water melting the ice from below, thinning the floating shelf ice and allowing upstream glaciers to move faster.
“These seasonal cycles could be due to either mechanism, or a mixture of the two,” said Christie. “Detailed ocean and surface measurements will be required to understand fully why this seasonal change is occurring.”
The results imply that other glaciers in Antarctica may also undergo ice flow seasonality, including the highly vulnerable and rapidly retreating 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,” said Boxall.
“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,” said co-author Professor Ian Willis, also from SPRI. “We look forward to taking a closer look at, and shedding light on, these important questions.”
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