| Summary: | Ice sheets are integral to the earth's climate system, both modulating and responding to climatic change. Iceberg calving fronts are the only dynamic interface at which the atmosphere, oceans and ice sheets directly interact. Calving introduces mechanical instability to glacier systems such that the response of calving glaciers to climatic forcing is commonly nonlinear. The interaction between calving dynamics and the ice-marginal environment, notably the topographic geometry of glacier troughs, can partially or totally decouple glacier fluctuations from climate for periods of several centuries. In West Greenland these instability mechanisms appear to have been important both during deglaciation and recently. In the Late Glacial/early Holocene, trough geometry controlled the retreat stages of the ice sheet margin in the Ilulissat (Jakobshavn) area of central west Greenland. During the second half of the twentieth century, the oscillations of 72 outlet glaciers between 61 °N and 72°N show that land- terminating glaciers respond directly to climate change (albeit with variable time lags) but that calving glaciers behave non- linearly. Freshwater calving glaciers have lower calving fluxes and calving rates than tidewater glaciers, and may be the first to respond to climatic cooling. It is not clear whether ice sheet outlet glaciers oscillate cyclically as do calving mountain glaciers, but the instabilities introduced by calving cause many glaciers to respond more directly to topographic than climatic factors. It is therefore hazardous to attach palaeoclimatic significance to the glacial geomorphological record of the fluctuations of former calving margins, or to regard the behaviour of contemporary calving outlets as indicative of climative trends. Factors affecting the stability of ice margins have a fundamental impact on the dynamics of ice sheets, and are important controls on the timing and patterns of ice sheet response to climate change.
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