News & Events

PhD thesis defense: Karen Alley

Thursday, May 11, 2017, 2:30PM - 4:30PM


Karen Alley


NSIDC (RL-2) room 155


Studies of Antarctic ice shelf stability: Surface melting, basal melting, and ice flow dynamics


Floating extensions of ice sheets, known as ice shelves, play a vital role in regulating the rate of ice flow into the Southern Ocean from the Antarctic Ice Sheet. Shear stresses imparted by contact with islands, embayment walls, and other obstructions transmit “backstress” to grounded ice. Ice shelf collapse reduces or eliminates this backstress, increasing mass flux to the ocean and therefore rates of sea level rise.

This dissertation presents studies that address three main factors that regulate ice shelf stability: surface melt, basal melt, and ice flow dynamics. The first factor, surface melt, is assessed using active microwave backscatter. Combined with measurements of annual melt, backscatter values provide insights into the state of the upper layers of the ice shelf. They can indicate whether melt ponds, which can destabilize ice shelves, are likely to form on the ice shelf surface. We present a map of the relative vulnerability of ice shelves to hydrofracture collapse caused by surface melt ponding.

As many authors have recently performed large-scale assessments of basal melt, the second factor is addressed at a smaller scale, through the study of channels that form on the undersides of ice shelves. These basal channels are mapped using visible-band imagery, and shown statistically to be related to the presence of warm ocean water. Landsat imagery and IceSAT laser altimetry provide evidence that basal channels can in some cases change very rapidly and cause weakening of ice shelf structures.

The final study addresses the calculation of surface strain rates from velocity fields. This common calculation, which is integral to the understanding of flow patterns and stresses on both grounded and floating ice, can be achieved using a variety of approaches. We examine two commonly used algorithms and the differences in results produced by the different methods. We also present a Matlab code for calculating strain rates and a data product of strain rates across the Antarctic continent. All three studies contribute to the knowledge needed to comprehensively assess ice shelf stability; proposed future studies that continue toward this goal are discussed in the final chapter.