Modelling the ice flow
Ice is a quasi-viscous material, and it deforms under applied stress. In other words, if stress is applied to ice over long time periods, it behaves like a very tough fluid - it flows with a high viscosity. This is reflected in the shape of the ice sheet; the profile looks like bread dough that has been resting on the table for a while. The deformation of the ice is driven by gravity - the ice deforms as a response to the stress applied by the weight of the overlying ice. Ice deposited on the surface of the ice sheet is transported downwards and towards the ice margin.
The line that marks the border between ice that flows to the East and ice that flows to the West is called the ice divide. The central ice divide in Greenland runs almost North-South. The ice that is deposited on the ice divide does not move fast in the horizontal direction. It is transported almost straight down. The ice divide is an optimal location for drilling ice cores because it is easier to interpret the data from the ice core when the ice was deposited at the site and not transported from an upstream location by the ice flow. Furthermore, the ice is thickest at the ice divide making it likely to find the oldest ice here.
As the layers sink down, they get thinner and thinner. However, the amount of snow that falls on the surface each year changes with climate: the snow accumulation is higher in warm periods, because warm air can contain more moisture, and vice versa. Thus the initial thickness of the annual layers changes with time. Therefore, the thickness of annual layers found in the ice cap is determined by the surface accumulation history and the flow history combined.
Many different models of different complexity have been developed, and the choice of model for a study depends on the goal of the study. Large-scale ice sheet models are used to investigate the size and shape of the ice sheet under different climatic conditions and to predict its reaction to global warming, while simplified models tuned to the conditions at the drill sites are used to date the deep part of the ice cores.
Ice Core Drilling Projects