FEATURE ARTICLE
The Eurasian Arctic During the Last Ice Age
A vast ice sheet once covered the Barents Sea. Its sudden disappearance 100 centuries ago provides a lesson about western Antarctica today
Anders Elverhøi, Martin Siegert, Julian Dowdeswell, John-Inge Svendsen
Computing Collapse
In an effort to reconstruct more fully the history of this former ice sheet, we have conducted various numerical simulations. The principle behind such numerical modeling is that an ice sheet can be divided into a number of "ice columns." Each of these columns represents a "cell" within a two-dimensional grid. Ice-sheet models are usually arranged in a computational loop that begins by applying a series of algorithms that determine in each cell the flow of ice, mass balance and interaction with the Earth. The loop is completed by application of a final equation (the continuity equation) to the full grid to calculate the flow of ice between cells. To simulate the glacial history, one must specify sea level, air temperature and snowfall though time. By forcing the model to form an ice sheet compatible with the geological observations, we can assess the causes of ice-sheet growth and decay.
Both the PONAM and QUEEN programs employed ice-sheet modeling to provide quantitative details about the size and dynamics of the former ice sheet. We adjusted the model's environmental inputs until the size of the ice matched the ice margin determined from the geological data. The model was able to provide information on the size, thickness and flow velocity of the former ice sheet.
Having modeled the full-sized ice sheet, we next needed to make the simulated ice sheet decay in a manner consistent with the geological evidence for the real deglaciation. We found that this was actually quite difficult to achieve. To mimic the deglaciation on our computers, we had to enhance the rate of iceberg calving in the model rather strongly.
So why did the real ice sheet break up so quickly? The modeling work suggests that the answer lies with the mechanism responsible for iceberg production. As the world entered the first phase of deglaciation, the sea level rose, albeit gradually. Sea-level rise had two effects on the marine-based ice sheet covering the Barents Sea. First, the water depth increased, causing enhanced rates of calving, assuming the ice was grounded on the seafloor. Second, the effective weight of the ice sheet was reduced, leading to a reduction in basal drag, higher ice velocities and, thus, a more rapid transfer of mass from the interior of the ice sheet to the margin where calving takes place. These effects produced a positive feedback by which the decay of the ice sheet here and elsewhere led to an increase in sea level, which in turn led to further iceberg calving. Thus, a relatively small change in sea level at the onset of the last deglaciation was likely the trigger that caused the Eurasian ice sheet to break up.
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