Worth a peek: Larger view of the crack can be seen here.
NASA has advised that a floating shelf of ice off the Antarctica coast of Victorialand, Antarctica is ready to calve an enormous iceberg into the Southern Ocean. This shelf buttresses two valley glaciers (Reeves and Priestly) from emptying into Terra Nova Bay. Scientists Christine Dow and Ryan Walker noted that in just two years, a small crack grew large enough to spread across nearly the entire width of the Nansen Ice Shelf:
In early March 2016, with southern winter soon to set in, satellite imagery indicated that the cracking ice front was still attached to the shelf. Even in winter, strong winds can prevent the water beyond the shelf from freezing, so it is unclear whether the front will separate soon or hang on like a loose tooth.
Dow is talking with researchers about examining the crack during the coming summer field season. “I’m really interested to see whether this feature is occurring because of the topography around the ice shelf, or whether it was initially created by surface water flowing into a small ice surface crack,” she said. “We’re planning an intensive survey of this feature in the coming years and will hopefully get a handle on the causes.”
National Snow and Ice Data Center describes what an ice shelf is, and the role it plays on marine terminating glaciers.
An ice shelf is a thick slab of ice, attached to a coastline and extending out over the ocean as a seaward extension of the grounded ice sheet. Ice shelves range in thickness from about 50 to 600 meters, and some shelves persist for thousands of years. They fringe the continent of Antarctica, and occupy a few fjords and bays along the Greenland and Ellesmere Island coasts. (An ice shelf occupying a fjord is sometimes called an ice tongue.) At their seaward edge, ice shelves periodically calve icebergs, some the size of a small U.S. state or European country. Because they are exposed to both warming air above and warming ocean below, ice shelves and ice tongues respond more quickly than ice sheets or glaciers to rising temperatures.
Most ice shelves are fed by inland glaciers. Together, an ice shelf and the glaciers feeding it can form a stable system, with the forces of outflow and back pressure balanced. Warmer temperatures can destabilize this system by increasing glacier flow speed and—more dramatically—by disintegrating the ice shelf. Without a shelf to slow its speed, the glacier accelerates.
Remember that ice shelf collapse does not directly contribute to sea level rise. The ice shelves are already floating on the surface. But the calving event indirectly speeds up the sea level process, because ice streams and glaciers behind them are left with a uninterrupted path to slide into the sea.
NASA reports on tracking the influence of tides on the Nansen ice shelf.
"Ice shelves are very important for holding back ice flow behind them because what they're essentially doing is acting as a plug; as soon as you remove them, there's nothing there preventing the ice mass from moving quickly down," Dow said. "It's a particular worry at the moment that the ice shelves around Antarctica are going to break up, and we're going to see an unprecedented speed-up in the ice coming from the center of the ice sheet."
Walker and Dow's instruments consisted of five GPS stations to measure the vertical and horizontal motion of the ice shelf, and two tilt meters, which are sensors that record subtle changes in the angle of the ice shelf as it bends with the ocean tides. They installed the GPS stations in the middle section of the ice shelf, where ice floats freely on ocean water, whereas the tilt meters were mounted in the grounding line area, or the boundary between the ice resting on land and the floating ice.
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"Examining how the ice shelf responds to tides helps us get at the dynamics of how the ice flows and we're hoping will help with future computer simulations, in particular of where the grounding zone is," Walker said. Pinpointing grounding lines is key to being able to observe how glaciers evolve, because changes in the grounding line can lead to rapid changes in ice flow.
"If you're going to be making a model of the system, you want to know everything that you can about it," Walker said. "Actually seeing it gives you a bit more of a feeling for what sort of assumptions you can make, which things are essential and which things are oversimplifications."
Wilkins Ice Shelf: