Date of Award

2008

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Terence J. Hughes

Second Committee Member

Gordon S. Hamilton

Third Committee Member

Peter O. Koons

Abstract

A problem ice sheet models using the shallow ice approximation have, is that they react slowly to a warmer atmosphere simply because it takes a long time for a large ice mass to warm up. Quite contrary to ice sheet model predictions, the Greenland Ice Sheet underwent dramatic changes in the last decade. Ice surfaces lowered in the lower parts of the ice sheet and some major outlet glaciers increased their outlet velocities dramatically. This was a incredibly quick reaction for such a big ice sheet to the present climate warming. Ice sheet models were lacking an important mechanism, a mechanism that enables ice sheets to react quickly to physical changes around the perimeter of the ice sheet. The most dramatic changes happened at three large ice streams in the southern part of Greenland. Ice streams are highly dynamic parts of an ice sheet where ice moves considerably faster that the surrounding ice sheet. They can accelerate, decelerate, shut off or start on short time intervals. Something makes the ice dynamics of ice streams very different from the major part of the ice sheet. In this thesis I develop a simple flow line model that simulates the ice dynamics of an ice stream. The model provides a gradual transition between sheet flow described by the shallow ice approximation to shelf flow. This is done by the introduction of the floating fraction (p that quantifies a flotation height of an ice column along the flow line. This flotation height gradually varies between zero at the ice sheet, to the height of the entire ice column at the ice shelf. This has consequences for the force balance the model uses. Because of the flotation height there is, next to the basal shear stress, a tensile stress and a water buttressing stress. This force balance enables the glacier to react quickly to small force perturbation changes around the marine perimeter of the ice sheet. The model is tested against field data of Jakobshavn Isbrae, a very fast ice stream on the west coast of Greenland. Around 1997 Jakobshavn Isbrae increased it's outlet velocity dramatically and in 2003 the velocity had almost doubled. Using ice surface data from 1993 and 2003, the flotation height is determined. Over that time interval the flotation height increased significantly and as a result the tensile stress increased considerably. Applying the observed tensile-stress increase to the force balance of 1993, the dynamic response of Jakobshavn Isbrae is modeled over a period of 10 years. As a result of the force perturbation, the modeled glacier reacts quickly by changing it's outlet velocity, thereby lowering it's surface. Though the modeled outlet velocity is smaller than the observed, the surface lowering of the perturbation closely resembles the observed ice surface change over that period. This suggests that indeed the force balance used is representative for ice-stream dynamics and that a small force perturbation such as the collapse of an ice shelf or increased basal water pressure can have a large impact on the stability of ice sheets.

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