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  • Periodic Report Summary 2 - CRAG (A coupled ice sheet - ocean model for calibrated prediction of the future contribution to sea level change from the Pine Island Glacier, Antarctica)

Periodic Report Summary 2 - CRAG (A coupled ice sheet - ocean model for calibrated prediction of the future contribution to sea level change from the Pine Island Glacier, Antarctica)

The potential of marine ice sheets to undergo rapid irreversible retreat has been considered in the glaciological community since the 1970s. One of the world’s major marine ice sheets, the West Antarctic Ice Sheet (WAIS), contains enough ice to raise global mean sea level by approximately 5 metres. However, of immediate relevance to policy makers, the likely rate of WAIS contribution to sea level change on century timescales is not known.

The underlying mechanism for marine ice sheet instability is the positive feedback that occurs between retreat of the grounding line (the divide between grounded and floating ice) and ice flux across the grounding line if the grounding line lies on bedrock that slopes up towards the ocean. Since ice flux across the grounding line is a function of ice thickness, grounding line retreat leads to increased discharge of grounded ice, leading to further thinning of the ice sheet and hence further grounding line retreat. This conceptual model is complicated in the real world by the stabilising impact of back stress from ice shelf buttressing (such as occurs if the ice shelf is in an embayment or partially resting on an island) and by irregular bedrock geometry.

Ice sheet models have only recently reached sufficient maturity to be applied to predictive studies of marine ice sheet behaviour, partly due to recent and ongoing advances by the Marie Curie research fellow in modelling the grounding line (the line dividing ice grounded on bedrock from the floating part of the ice sheet).

A driver for the onset of marine ice sheet instability in a changing climate is melt occurring under the Antarctic ice shelves (the floating portion of the Antarctic Ice Sheet) due to ocean circulation bringing warmer waters in contact with the ice. In order to use Ice Sheet Models (ISMs) to predict how marine ice sheets will respond to future evolution of ice shelf melting, the ISMs will need to interactively learn about evolving melt rates.

Ocean models incorporating ice shelf cavity processes are also emerging, and the candidate’s collaborators at the outgoing host institution in Hobart are leading development in this area. Ice sheet – ocean model coupling is essential in predictive studies due to the feedback between evolving ice shelf cavity geometry/grounding line location and ice shelf cavity circulation via sub ice shelf melt rates.

For this project the Marie Curie research fellow has developed a coupling framework enabling state of the art ice sheet and ocean models to be run interactively: The Framework for Ice Sheet – Ocean Coupling (FISOC). The coupling framework has been developed to be flexible, and currently couples the Elmer/Ice model used to simulate ice sheets to the Regional Ocean Modelling System (ROMS) used to simulate the ocean circulation under ice shelf cavities and near the ice front.

FISOC is currently being used to carry out investigations into ice-ocean processes, subglacial feedbacks, and melt channels that form underneath ice shelves. A regional study will focus on the Totten Glacier in East Antarctica, and the setup for the ice and ocean components for this study is nearing completion. Over the next two years it will be used to project future changes over the Antarctic Ice Sheet and its contribution to sea level change.

International collaborators from Nasa’s Jet Propulsion Laboratory have made plans to follow up the initial FISOC development by coupling in the Ice Sheet System Model (ISSM) and the Massachusetts Institute of Technology Ocean General Circulation Model (MIT-GCM). This will increase flexibility of FISOC, and will help in quantifications of uncertainty.

It is anticipated that the results from these studies will feed in to the next assessment report from the Intergovernmental Panel for Climate Change (IPCC), informing policy makers of global sea level risk.

The project has also contributed to an assessment of the state of Antarctic Ice Shelves in which actual fluxes of mass loss due to iceberg calving were calculated for the first time. The study showed that calving fluxes are higher than had been realized for regions of the Antarctic Ice Sheet undergoing ocean melt induced retreat. This work was disseminated to the public through a press release when the research was published in the Proceedings of the National Academy of Sciences, and shortly afterwards in news items on Australian Television and radio.

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