An Antarctic Glacier Is Thinning by 230 Feet Each Year
The glacier is retreating six times faster than previously thought, likely because of warming ocean water below.
Antarctica is frequently referred to as the frozen continent, for obvious and appropriate reasons. Some 90 percent of the planet's freshwater is locked up in the Antarctic ice sheet, which contains over 7 million cubic miles of ice. But at the continent's fringes, some of that ice is starting to melt, with potentially catastrophic future consequences; and a new paper in the journal Nature Communications has found that in at least one part of the continental coastline, that ice loss is happening at a faster rate than previously believed.
The Antarctic ice sheet is not static. Over the course of hundreds of thousands of years, the ice that forms from snowfall over the interior of the continent flows, under the influence of gravity, toward the perimeter. In a number of places, when it reaches the coastline, it extends out into the ocean and floats, even as it remains connected to the ice on the continent and is fed by relatively fast-flowing channels known as glaciers or ice streams. These floating areas of ice are called ice shelves, and they can, in the case of the Ross Ice Shelf, be almost as big as France, although most are substantially smaller.
"I like to describe ice shelves as both the gates of Antarctica and the gate keepers of Antarctica," Ala Khazendar of NASA's Jet Propulsion Laboratory, and the lead author of the new study, told Seeker. "The first concept refers to the fact that most ice in Antarctica that exits the continent does so through these ice shelves, because it either melts at the interface between the ice and the ocean beneath it, or breaks off from the front of the shelf and forms icebergs. Now the concept of gate keepers describes the fact that the presence of ice shelves to a large extent regulates and controls how much ice is getting out from the continent into the ocean; they control how fast ice leaves the continent. This is because they offer some resistance to the flow of ice from the interior of the continent. This resistance comes about because they often occur in embayments, and so there is friction between the ice shelf and the embayment walls, and often also with topographic features that rise from the seabed beneath them."
Because ice shelves are floating and thus already displacing water, their melting would not in itself add to sea level rise. However, added Khazendar, "if you destabilize these ice shelves - for example, by excessive thinning - the extent of the resistance that they are able to offer diminishes, and this allows the ice to flow even faster into the ice shelves and ultimately into the ocean."
As the climate warms, some areas of Antarctica appear to be suffering ice mass loss at a greater rate than others; ice loss is particularly high in two areas of West Antarctica - the Antarctic Peninsula and, above all, the Amundsen Sea. The latter contains two large glaciers called Pine Island Glacier and Thwaites Glacier; the U.S. National Science Foundation and the British Natural Environment Research Council recently announced a joint multi-million dollar research program to study Thwaites. Just to the west of those two glaciers sits a pair of ice shelves - Crosson and Dotson - and three glaciers that feed into them: Pope, Smith and Kohler. These three glaciers were the focus of the Nature Communications study.
Of particular interest to Khazendar and colleagues was the glaciers' grounding lines - the boundaries where they lose contact with bedrock and begin to float on the ocean. As glaciers undergo enhanced melting, the grounding line may retreat inland, especially if, as scientists increasingly suspect, the ice is being melted at the bottom by warmer ocean temperatures as well as at the surface.
Previous studies had estimated average melting rates at the bottom of Crosson and Dotson glaciers of around 40 feet per year; using direct radar and laser altimetry readings from NASA's Operation IceBridge and earlier NASA airborne studies, Khazendar and colleagues found the rate of loss was in fact far greater. The fastest-melting glacier, Smith, lost between 984 and 1,607 feet in thickness between 2002 and 2009 near its grounding line, or up to 230 feet per year. (A separate study in the journal Geophysical Research Letters found that, over the same period, Smith's grounding line had retreated 1.24 miles per year since 1996.)
Those years encompass a period when rapid mass loss was seen around the Amundsen Sea. The regional scale of the decline made scientists strongly suspect that an increase in the influx of ocean heat beneath the ice shelves must have taken place. Smith's grounding line retreat was and remains far more pronounced than that of Pope or Kohler, probably because the latter two have retreated to bedrock that slopes upward, causing them to become grounded.
In contrast, the seabed topography beneath Smith actually slopes downward as it heads inland, leaving more of the glacier exposed to warming water and depriving it of a place to become grounded, leading to further retreat. Khazendar and colleagues do predict that Smith's retreat "might slow a bit further [in the future], because it has really reached the bottom of the trench into which it was retreating."
This is a problem for more than just Smith Glacier. East Antarctica rests on a major land mass, but West Antarctica is, Khazendar explained, "what we call a marine ice sheet, which means it is grounded beneath sea level." That, combined with the downward slope of the bedrock away from the coastline is "in principle, not a stable configuration for a glacier," he observed laconically.
The study adds to an increasing sense of doom around the West Antarctic ice sheet. A 2014 study suggested that the ice sheet's ultimate collapse was now unstoppable, while research from earlier this year concluded that it could begin to disintegrate within decades, causing as much as 3 feet of sea level rise by the end of the century.
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