Through the Eocene, Antarctica was not yet today's ice-covered continent; it was a forested landmass with temperate north–south climate gradients. About 34 million years ago, over a geologically short interval — a few hundred thousand years — a permanent ice sheet formed across it. This marks the start of the Cenozoic Ice Age and sets the climatic stage of the modern world as we know it.

Two main mechanisms are recognised. The first is tectonic: the opening of the Drake Passage between South America and Antarctica (~34–30 Mya) and of the Tasmanian gateway established for the first time the Antarctic Circumpolar Current, an uninterrupted flow around the continent that thermally isolated it from warmer tropical waters. The second is atmospheric: silicate weathering of the rising Himalayas — and subsequently the developing Antarctic ice — drew down carbon dioxide, pushing atmospheric CO₂ below a critical threshold. Together, the two triggered persistent snow accumulation and the cooling feedback of the high reflectivity (albedo) of ice.

The consequences for marine life were profound. The contraction of shallow tropical seas, the reorganisation of plankton ecosystems, and changes in deep-water circulation drove the extinction of many marine invertebrates. On land, the faunas of Eurasia and North America — most strikingly in Europe — underwent the rapid turnover known as the 'Grande Coupure' (Great Break, Stehlin 1909): much of the Eocene forest-mammal fauna disappeared and was replaced by open-habitat forms migrating in from Asia. The freezing of Antarctica is now read as the first major trigger that shaped the climate and biogeography of the modern world.