ICE-SHEET DYNAMICS

The behaviour of ice sheets, especially the ice-velocity characteristics, which govern the rate at which ice and sediment are transported from interior regions towards the margins and oceans, the location of preferred channels of ice transport and ice-mass evolution. Ice discharges or perimeter fluxes are now known around almost the entire peripheries of the Greenland and Antarctic Ice Sheets from satellite remote sensing of ice velocity and airborne measurements of ice thickness. In the antarctic, ice is lost primarily by basal melting and the calving of icebergs from ice shelves. In contrast, the loss of ice from the Greenland Ice Sheet is primarily by surface melting in addition to calving. The mass balances of the Antarctic and Greenland Ice Sheets have been largely controlled by the evolution of their outlet glaciers and ice streams. In Greenland, drainage of supraglacial lakes and surface melt to the bed of the ice sheet is thought to increase lubrication by weakening the contact between the ice and its bed leading to increased ice-flow velocity. However, the period of accelerated flow is reduced during years of rapid melting because, as the amount of meltwater increases, subglacial drainage takes less time to become more efficient and ice is once more frozen to its bed. In Antarctica, increased heat from the ocean in contact with the glacier groundinglines melts ice from underneath, disengages the glaciers from their bed, reduces buttressing of the inland ice and allows faster rates of ice discharge to the sea. Melt and increased ice discharge from the major outlet glaciers of both the Antarctic and Greenland ice sheets contribute directly to current sea-level rise. Large variations in sea level during the quaternary have been controlled by ice-sheet mass balance, with rates of sea-level rise at least one order of magnitude larger than at present during times of rapid deglaciation.

[See alsomarine-based ice sheet]