Ice Sheets and Ice Volume

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...