PhD defense by Signe Hillerup Larsen

Title: "Dynamics of Upernavik Isstrøm - Controlling mechanisms of flow".

Abstract: Fast flowing ice streams are responsible for draining the vast majority of the Greenland ice sheet. During the past few decades, the ice streams have undergone rapid acceleration and retreat Greenland wide. However, the controlling mechanism of the dynamic changes are still not well understood. Due to the ice streams' importance for the drainage of the entire ice sheet, the fifth assessment report (AR5) of the Intergovernmental Climate Panel (IPCC) deemed uncertainties in the flow of ice streams one of the major uncertainties in predicting future changes of the Greenland ice sheet. In this thesis, the dynamical changes at Upernavik Isstrøm (UI), Northwest Greenland, are analysed and an ice flow model is used to study specific controlling mechanisms of the ice stream flow. The analysis of observations of velocity, thickness and calving front position changes, reveals asynchronous behaviour of the neighboring ice streams at UI. However, overall dynamical changes at UI are in line with general trends in the region. Thus, establishing UI as an optimal study site for detailed process studies of controlling mechanisms of ice stream flow. A model study of velocity changes at the end of the melt season in 2014, reveals that the ice streams are increasingly sensitive to melt water changes towards the front. Part of the spatial trend in sensitivity is attributed to the softening effect of water entering the shear margins. A second model study focussing specifically on reproducing the observed flow, establish that including softer shear margins in ice flow models will improve the models ability to reproduce fast flow. Thus, a method for defining softer shear margins in ice flow models, without knowing details about the mechanisms behind the softening, is suggested to be included in future model studies of ice stream flow. The thesis establish the importance of understanding the inhomogeneity of the ice viscosity to be able to correctly model the dynamics of ice streams, and thus to be able to predict future changes of the Greenland ice sheet.

Christine Hvidberg, Centre for Ice and Climate, Niels Bohr Institute
Andreas Peter Ahlstrøm, Glaciologi og Klima, GEUS

Assessment Committee
Thomas Blunier, Centre for Ice and Climate, Niels Bohr Institute
Peter Nienow, School of Geosciences, University of Edinburgh
Guðfinna Th. Aðalgeirsdóttir, Institute of Earth Sciences, University of Iceland

If interested please contact Signe Hillerup Larsen ( for a copy of the thesis.