This award supports a project to develop a better understanding of the processes and conditions that trigger ice shelf instability and explosive disintegration. A significant product of the proposed research will be the establishment of parameterizations of micro- and meso-scale ice-shelf surface processes needed in large scale ice-sheet models designed to predict future sea level rise. The proposed research represents a 3-year effort to conduct numerical model studies of 6 aspects of surface-water evolution on Antarctic ice shelves. These 6 model-study areas include energy balance models of melting ice-shelf surfaces, with treatment of surface ponds and water-filled crevasses, distributed, Darcian water flow modeling to simulate initial firn melting, brine infiltration, pond drainage and crevasse filling, ice-shelf surface topography evolution modeling by phase change (surface melting and freezing), surface-runoff driven erosion and seepage flows, mass loading and flexure effects of ice-shelf and iceberg surfaces; feedbacks between surface-water loads and flexure stresses; possible seiche phenomena of the surface water, ice and underlying ocean that constitute a mechanism for, inducing surface crevassing., surface pond and crevasse convection, and basal crevasse thermohaline convection (as a phenomena related to area 5 above). The broader impacts of the proposed work bears on the socio-environmental concerns of climate change and sea-level rise, and will contribute to the important goal of advising public policy. The project will form the basis of a dissertation project of a graduate student whose training will contribute to the scientific workforce of the nation and the PI and graduate student will additionally participate in a summer science-enrichment program for high-school teachers organized by colleagues at the University of Chicago.
In early March of 2002, the Larsen B Ice Shelf located in the Antarctic Peninsula region underwent a catastrophic, unexpected breakup. About 1600 km2 of the ice shelf exploded into thousands of small icebergs, many of which had capsized and broken into small pieces over the period of several days. This event is environmentally significant, because the loss of the ice shelf proffered a less resistant pathway for glaciers flowing off the Antarctic continent to flow into the ocean, thus allowing a rapid mechanical ice-discharge impact on sea level. Understanding the cause, and the environmental enabling conditions, of the Larsen B’s breakup is thus important for assessing possible future sea-level contributions associated with the Antarctic Ice Sheet’s other ice-shelf buttressed regions. Our research project addressed two unresolved questions of the Larsen B’s sudden breakup: (1) What was the process that fractured the ice shelf into fragments sufficiently small to be pre-disposed to capsize1? (Capsize-liberated potential energy drives ice-shelf disintegration.) (2) Did the conspicuous presence of thousands of supraglacial meltwater lakes, and the even more conspicuous synchronous drainage of these lakes in the days prior to the breakup, trigger the event? Our research showed that the necessary length scales of fractures within the ice shelf may have resulted from the ice shelf’s elastic-flexure response to surface loads introduced by > 2750 supraglacial lakes, which emerged as dominant surface features during the decade prior to the 2002 break-up. We found that drainage of these surface lakes, as was observed in the days prior to break-up, is self-perpetuating, by virtue of lake-to-lake flexure-stress interaction; and this is what synchronizes the break-up over a widespread area. Our research thus expands the previously articulated role of supraglacial lakes beyond mere water-reservoirs supporting "crevasse tip hydrofracture" (a process not entirely different from "hydrofracing used in the oil and natural gas industry), i.e., by including the effects of elastic flexure, we answer the two questions posed above, and suggest that environmental conditions leading to surface lake development on the remaining Antarctic ice shelves will constitute a major climate tipping-point leading to accelerated ice discharge and sea-level rise. The ultimate proof for the explanation for the Larsen B Ice Shelf breakup we have proposed here must be built upon through observations made in future research designed to confirm the flexural response of ice shelves in general to the presence or sudden absence of surface meltwater loads. The assessment of environmental enabling conditions for the possible future instability of other Antarctic ice shelves will additionally require more field work to assess energy balance conditions and solar forcing of surface lake development. Many of these elements of future work will be difficult, and thus underlines the importance of a healthy and vigorous Antarctic Research Program, which is hereby acknowledged as the principle supporter of the research reported here.
