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ICE SHEETS AND ICE VOLUME Robert Thomas Sigma Space, Gorzow Wlkp, Poland

Definition and introduction An ice sheet is a large mass of ice resting on land that is continental or subcontinental in extent, with the ice thick enough to cover most of the underlying bedrock topography. Its shape is mainly determined by the dynamics of its gravity-driven outward flow. There are only two ice sheets in the modern world, in Greenland and Antarctica, but during glacial periods there were others. At the beginning of the Cenozoic, some 65 million years (Ma) ago, neither Greenland nor Antarctica supported an ice sheet. Then, as greenhouse gases in the atmosphere decreased, slow, irregular cooling allowed ice masses to accumulate and survive, first as mountain glaciers and then with the first continental ice sheet forming over Antarctica as early as 33 Ma ago. Further cooling led to extensive ice formation on Arctic land areas about 2.6 Ma ago, initiating the Quaternary series of ice ages with warmer interglacials at roughly 0.12 Ma intervals. Cycling between cold glacial and warmer interglacial periods is driven by periodic features of Earth’s orbit, with ice sheets growing when sunshine shifts away from the Northern Hemisphere and melting when northern sunshine returns. These changes are amplified by feedbacks, such as greenhouse gas concentrations that rise and fall as the ice cover shrinks and grows, and greater reflection of sunshine caused by more extensive ice (Figure 1). Human civilization developed during the most recent (Holocene) interglacial, extending over the past 11,000– 12,000 years (ka). Holocene warming was interrupted during the Little Ice Age (about 1250–1850 AD), when the Greenland ice sheet and most Arctic glaciers reached

their maximum Holocene extent. Since then, warming has resulted in Arctic-wide glacier recession, with similar responses becoming apparent more recently in Antarctica. These trends will continue if greenhouse gas concentrations continue to increase into the future (Table 1). During the previous interglacial, 130–120 ka ago, Arctic summers were about 5
C warmer than now and the Greenland ice sheet was considerably smaller than at present, with probable losses also from Antarctica, resulting in global sea level some 4–6 m above present values. An ice sheet forms by the continual accumulation of snow on its surface. Over parts of an ice sheet, summers are warm enough to melt some of this snow, with some of the melt water percolating into the snow and refreezing and some running off the ice sheet into the ocean. The difference between local precipitation and melt-water runoff is the surface mass balance. As successive layers of snow build up, the layers beneath are gradually compressed into solid ice, at depths up to several tens of meters. The snow input is approximately balanced by glacial outflow, so the height of the ice sheet stays roughly constant through time. The difference between the mass of ice added by snowfall and that lost by runoff and ice motion is the total mass balance. Th