An ice sheet is coupled into the Earth system (Fig. 2.1a) across its interfaces with the atmosphere, the ocean and the lithosphere.
1 Ice sheet-atmosphere coupling. The atmospheric state (temperature, moisture content, energy transport) influences an ice sheet through:
• its impact on mass balance and ice temperature. An ice sheet influences the atmosphere through:
• the deflection of atmospheric flow over the ice sheet, which influences the distribution of temperature, pressure and precipitation and thereby the mass balance distribution.
2 Ice sheet-ocean coupling. An ice sheet influences the ocean through:
• the discharge of meltwater (whether or not the glacier reaches the sea), which influences the temperature, salinity and turbidity of nearby ocean water;
• the discharge of icebergs from marine margins, which influences the temperature and albedo of the nearby ocean.
The ocean influences an ice sheet at marine margins through:
• water temperature, which influences ablation from ice shelves and tidewater margins;
• water depth and depth variation, which influence the buoyancy of a marine margin and subglacial water pressure in the terminal zone, and thereby its susceptibility to calving and to fast flow (marine drawdown and streaming);
• wave action, which influences calving through its influence on the extent of sea-ice and the action of waves against the ice front.
3 Ice sheet-lithosphere coupling. An ice sheet influences the lithosphere through:
• its mass, which is able isostatically to depress and flex the lithosphere as a consequence of flow induced in the Earth's mantle by differential loading;
• meltwater, which is injected into the bed, so driving up pore fluid pressures;
• shear forces and flow that produce erosion of the bed, sediment transport over it, deposition on it, and deformation of bed materials;
• the effect on the thermal field in the lithosphere.
The lithosphere influences an ice sheet through:
• vertical movement of the ice-sheet mass through isostatic sinking or uplift with consequences for surface mass balance;
• the impact of water pressure (and therefore effective pressure) on friction, possible decoupling at the ice-bed interface and thereby on ice-sheet dynamics and form;
• the basal thermal regime, which influences the state of freezing or melting at the ice-bed interface and thereby basal friction and the nature of basal movement.
Ice-sheet surface processes, the transformation of snow to ice, the surface energy balance and mass balance in relation to surface meteorology are now well understood, quantified and used in a relatively sophisticated way in ice sheet models (e.g. Huybrechts, 1992; Payne, 1995) that have been successful in replicating current features of modern ice sheets and their future evolution (e.g. Huybrechts et al., 1991b), although full time-dependent coupling between an ice sheet and a general circulation model of the atmosphere remains computationally taxing. Outstanding prob lems at the marine boundary include the development of a theory of iceberg calving and of ice shelves in understanding ocean-ice-sheet coupling, crucial to understanding the powerful iceberg fluxes associated with Heinrich events of the past and of the future evolution of the marine margins of modern ice sheets (e.g. Hindmarsh & Le Meur, 2001). The major problem of the basal boundary is the nature and magnitude of the coupling at the ice-bed interface determined by interactions between subglacial drainage, thermodynamics and basal friction, and how these processes are reflected in the subglacial sedimentary record.
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