The change from a warm, ice-free greenhouse world to the glacial Antarctic icehouse occurred during the latest Eocene-earliest Oligocene. Prior to this, during the Early-Middle Eocene, Antarctica experienced warm climates, at least on the margins of the continent where geological evidence is present. Climates appear to have been warm and wet, the seas were warm and plants flourished in a frost-free environment, although there is some suggestion of valley glaciers on King George Island. Climate signals in the geological record show that the climate then cooled but not enough to allow the existence of significant ice until the latest Eocene.

Glacial deposits on Seymour Island indicate that ice was present there at Eocene/Oligocene boundary times. Further south, ice-rafted clasts in drill cores

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from the Ross Sea region and deposits of tidewater glacier origin in Prydz Bay confirm the presence of ice at the continental shelf by the earliest Oligocene. This matches the major Oi-1 oxygen isotope event in the marine record. On land, vegetation was able to persist but the warmth-loving plants of the Eocene were replaced by shrubby vegetation with the southern beech Nothofagus, mosses and ferns, which survived in tundra-like conditions. Throughout the Oligocene, glaciation waxed and waned until a major glacial phase in the Miocene.

Coupled climate-ice sheet modelling indicates that changing levels of atmospheric CO2 controlled Antarctica's climate. Factors such as mountain uplift, vegetation changes and orbital forcing all played a part in cooling the polar climate, but only when CO2 levels reached critical thresholds was Antarctica tipped into its icy glacial world.

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