Title: Ice-Shelf Stability
Description: The Greenland and Antarctic ice sheets are losing mass and contributing to global sea-level rise. Ice shelves, floating ice attached to the margins of the ice sheets, modulate sea-level rise by restraining ice-sheet flow out towards the oceans, but are sensitive to surface melting. The formation of surface meltwater lakes on ice shelves can trigger rapid ice-shelf collapse. However, surface meltwater also flows atop ice shelves through rivers. The impact of rivers on ice-shelf stability is unknown. Previous studies of ice-shelf hydrology hypothesize that rivers mitigate the damage-potential of lakes by removing surface water off of the ice shelf, but also suggest that rivers enhance ice-shelf fracturing by incising into areas of already thin ice. This dissertation is focused on exploring the role of rivers on ice-shelf stability using remote sensing datasets, conceptual models, and Augmented Reality (AR). Focusing on ice shelves in Greenland, I present the discovery of a new ice-shelf surface hydrology feature, an ice-shelf estuary, and demonstrate its potential to weaken ice shelves.

I fully document this new process on the Petermann Ice Shelf, where flow reverses at the mouth of the Petermann Estuary. This study marks the first observation of ocean water atop an ice shelf. I also document the initiation and growth of fracturing along the estuary channel, and a history of rectilinear calving events, where icebergs calve along longitudinal rivers. Based on this analysis of the Petermann Estuary, I propose a new mechanism for damaging ice shelves: estuarine weakening. I present evidence that this process also occurs on the Ryder Ice Shelf in northwest Greenland. My analysis demonstrates that the role of rivers on ice-shelf stability depends on how the river mouth evolves. If ice-shelf waterfalls at the river mouth incise to sea level and form estuaries, flow reversal will modulate water export off the shelf and maintain the damage-potential of lakes, and estuarine weakening may lead to a new mode of ice-shelf calving. By analyzing the three-dimensional (3D) structure of the Petermann and Ryder Ice Shelves and Estuaries with remote sensing and radar data, I find that basal channels are an important driver of estuary development as they dictate the linearity of surface rivers.

Determining the role that basal channels play in estuary formation requires accurate and appropriate data visualization tools. I develop AR applications to visualize radar data on ice shelves, towards enabling more intuitive and sophisticated interpretation of the ice-shelf structure in 3D. Through simple conceptual modeling, I suggest that although basal channels precondition ice-shelf estuary formation, estuary formation is strongly controlled by river incision. Finally, I present a model of ice-shelf estuary formation as a function of surface and basal melting. Using this conceptual model, I predict that ice-shelf estuaries could form in Antarctica in the near future. Surface melting in Antarctica is predicted to increase in under half a century. Estuary formation in Antarctica will be accelerated by lengthening of the melt season, and estuaries may form far from the calving front if rivers intersect upstream rifts. I show that ice-shelf estuaries could evolve from ice-shelf rivers in a warming Antarctica, introducing new ice-shelf weakening mechanisms.

This increases the urgency to understand and include ice-shelf estuarine processes in ice-sheet models.