Solar heat is the acknowledged driving force for climatic change. However, ice sheets are also capable of causing climatic change. This property of ice sheets derives from the facts that ice and rock are crystalline whereas the oceans and atmosphere are fluids and that ice sheets are massive enough to depress the earth's crust well below sea level. These features allow time constants for glacial flow and isostatic compensation to be much larger than those for ocean and atmospheric circulation and therefore somewhat independent of the solar variations that control this circulation. This review examines the nature of dynamic processes in ice streams that give ice sheets their degree of independent behavior and emphasizes the consequences of viscoplastic instability inherent in anisotropic polycrystalline solids such as glacial ice. Viscoplastic instability and subglacial topography are responsible for the formation of ice streams near ice sheet margins grounded below sea level. As a resuR the West Antarctic marine ice sheet is inherently unstable and can be rapidly carved away by calving bays which migrate up surging ice streams. Analyses of tidal flexure along floating ice stream margins, stress and velocity fields in ice streams, and ice stream boundary conditions are presented and used to interpret ERTS 1 photomosaics for West Antarctica in terms of characteristic ice sheet crevasse patterns that can be used to monitor ice stream surges and to study calving bay dynamics.Paper number 6R0700. rized in Figure 1 and consists 6f 10 postulates. (1) During global ice ages the Antarctic ice sheet expands to the edge of the Antarctic continental shelf, shown by the dashed 500-m bathymetry contour in Figure 1. (2) Glacial flow near the ice sheet margin becomes concentrated in fjords and channels between mountains and islands along the coast to form outlet glaciers and ice streams, shown by the dotted areas in Figure 1. (3) At the end of an ice age these outlet glaciers and ice streams punch through the ablating ice shelves fringing Antarctica and surge along glacially eroded bedrock troughs, as shown by the arrows in Figure 1. (4) The Ross and Filchner-Ronne ice shelves were created because ice shelf grounding lines retreated up surging West Antarctic ice streams faster than the ice shelf margins retreated by iceberg calving. (5) Retreat of an ice shelf grounding line is slowed when the surging ice streams and outlet glaciers can no longer effectively punch through the ice shelf. (6) Retreat of the ice shelf grounding line stops when it reaches high bedrock up-down sills in ice stream channels and upward steps in glacier fjords. (7) Foredeepening by glacial erosion creates low sills at the front of ice stream channels ending at the edge of the continental shelf and high steps at the rear of outlet glacier fjords beginning at the edge of the continental shield. (8) The Antarctic ice sheet is primarily grounded over a continental shield in its eastern hemisphere portion and a continental shelf in its western hemisphere port...

Data pertaining to the dynamics and history of the west antarctic ice cover are reviewed and interpreted in terms of a possible inherent instability of the ice cover. A study of published data concerning the past and present ice cover of West Antarctica indicates that during the last few million years the ice sheet has been retreating in stages, each retreat stage being preceded by an advance of comparable duration. Thus disintegration of the west antarctic ice sheet seems to follow the disintegration pattern of other continental ice sheets and may be the last phase of the worldwide Late Cenozoic ice age. At least some of the retreat stages seem to have been rapid enough to be called surges. Stages of advance seem to have temporarily introduced equilibrium conditions, since equilibrium ice sheet surface profiles can be reconstructed from the moraines, etc., and thus mark the stable limits of each advance. Present ice sheet surface profiles along flowlines entering both the Ronne and the Ross ice shelves from Marie Byrd Land are not equilibrium profiles, suggesting that the west antarctic ice sheet is unstable. An analysis of the grounded portion of the west antarctic ice cover indicates that data relating to the surface profile, ice velocity, and the mass balance are all incompatible with an equilibrium ice sheet. Instability seems to be centered in the major ice streams that drain Marie Byrd Land. An analysis of the floating portion of the west antarctic ice cover indicates that basal melting is most pronounced along the Siple Coast of the Ross ice shelf and causes retreat of the grounding line into Marie Byrd Land. Instability seems to be related to sudden retreats of the grounding line.

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An ice age model is proposed in which glacial-interglacial global climatic cycles are controlled by interactions between the cryosphere, hydrosphere, and atmosphere in the Atlantic environment. In the model, climatic change results from instabilities which develop in the snowfields or ice sheets of North America, Europe, and Antarctica. Disintegration of the West Antarctic ice sheet (that portion of the Antarctic ice sheet lying in the western hemisphere) initiates a chain of events which culminates in a global ice age. Ten independent bodies of data can be interpreted as evidence that the West Antarctic ice sheet has been and is disintegrating. The dynamics of the Ross Sea ice drainage system of Antarctica is examined to determine what controls disintegration and recovery of the West Antarctic ice sheet. It is concluded that disintegration is controlled by ice streams which drain the inherently unstable West Antarctic ice sheet and recovery is controlled by outlet glaciers which drain the inherently stable East Antarctic ice sheet. Glacial stability in both cases is determined by the degree of coupling between the ice sheet and its bed. Ice drainage channels develop when this coupling is weakened normal to the margin of an ice sheet and can lead to surges in ice streams or outlet glaciers. Ice shelves develop when this coupling is weakened parallel to the margin of an ice sheet and can lead t,o a rapid grounding line retreat of floating ice tongues or ice shelves. An inflection maximum on the ice sheet surface migrates inland during a surge and migrates seaward after the surge is spent. A transition zone between the ice sheet and the ice shelf widens during a grounding line retreat inland and narrows during a grounding line advance seaward. Inflection line and grounding line migrations combine to give the ice sheet a concave surface during retreat and a convex surface during advance.A train of surging segments in an ice stream lowers the ice sheet in stages, creating a terraced ice stream surface which causes rapid discontinuous retreats of the grounding line. Rapid glacial recovery following a surge can truncate the advancing ice sheet-ice shelf boundary. Today at least one West Antarctic ice stream is terraced and at least one East Antarctic outlet glacier is truncated in the Ross Sea ice drainage system. If this condition is general, the West Antarctic ice sheet is disintegrating along the Siple Coast as a result of surging ice streams and is recovering along the Transantarctic Mountains as a result of thickening outlet glaciers. The competition between these processes will provide a critical test of the ice age model, which predicts that progressive disintegration of the West Antarctic ice sheet results in progressive growth of adjacent parts of the East Antarctic ice sheet.