Modelling the ice flow

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 arrows indicate the paths the ice follows as it flows from the centre towards the margin. The ice divide (orange) is the optimal location for drilling deep ice cores.

Ice divide

The direction of the main ice divide in Greenland is almost North-South. In the northernmost part of the ice sheet it splits into two. The NEEM drill site is located on the western branch of the ice divide.

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.

Annual layer thickness

Annual layer thicknesses at NorthGRIP. At the depth of 1500 m the transition between glacial and interglacial climate is found. Much less snow fell on the ice cap each year during the glacial period compared to today. Therefore, we see a sharp change in the curve at this depth.

Read more about
- large-scale ice sheet models
- simplified ice flow models