The past and future impact of ice tongue loss on outlet glaciers in northern Greenland

Abstract

Ice discharge from fast-flowing outlet glaciers across the Greenland Ice Sheet (GrIS) has increased in response to 21st century climate warming. These outlets are sensitive to changes at their terminus, particularly iceberg calving from floating tongues. Many glaciers have accelerated and thinned in response to recent retreat, but the impact of major calving events and ice tongue loss on ice discharge and sea level rise remains poorly constrained. Northern Greenland is the last region with floating ice tongues, but remains understudied compared to other regions of the ice sheet. The aim of this thesis is to quantify outlet glacier change across northern Greenland and assess the role of ice tongues in modulating past and future glacier dynamics. To address this aim I use: i) remote sensing to assess past glacier change across northern Greenland, and ii) two sets of numerical modelling experiments to simulate future ice tongue loss at Petermann Glacier. The key findings are that outlet glacier retreat rates have increased in the last two decades, but the dynamic response to retreat was dependent on terminus type (grounded vs floating) and glacier geometry. Grounded outlet glaciers retreated, accelerated and thinned, while the response of glaciers with ice tongues was more varied, and dependent on tongue confinement and bed topography inland of the grounding line. Modelling experiments on Petermann Glacier further corroborate these findings, demonstrating that unconfined portions of the tongue had little impact on dynamics, but removing confined sections closer to the grounding line accelerated ice flow. However, the long-term response (up to 100-years) to ice tongue loss was muted by the absence of a retrograde bed-slope in the grounding zone, which limited grounding line retreat. Overall, this thesis highlights the complexity of outlet glacier behaviour in northern Greenland. It also notes the variability between individual glacier responses to ice tongue loss. These factors require careful consideration when assessing future glacier sensitivity to ice tongue/shelf loss in both Greenland and Antarctica in order to accurately project accelerated ice discharge and ultimately global sea level rise.