The modern continent of Antarctica is the coldest, driest, and most isolated landmass on Earth, locked beneath an immense permanent ice sheet. The journey from a temperate, green landscape to a polar desert took place over millions of years, driven by massive geological and atmospheric shifts. This transition ushered in the current “icehouse” climate.
Antarctica Before the Ice Age
During the mid-Late Cretaceous period, approximately 85 million years ago, Antarctica was situated near its current polar location but experienced a climate far warmer than today. Fossil evidence reveals a landscape supporting dense vegetation, including ancient ferns, flowering plants, and southern beech (Nothofagus) forests. Summer temperatures in the high latitudes averaged around 20°C. Although this period followed the breakup of Gondwana, the continent’s warm conditions persisted well into the Paleogene period. As the planet began a long-term cooling trend, the warm-loving flora gradually gave way to more cold-tolerant species like araucarian conifers.
The Geological Separation
The tectonic movement of continental plates drove Antarctica’s final separation from the remaining fragments of Gondwana, creating two oceanic gateways, a process that occurred primarily during the Eocene epoch and set the stage for a shift in global ocean circulation. The Tasmanian Gateway opened, separating Antarctica from Australia. Concurrently, the Drake Passage began to open between the Antarctic Peninsula and South America. While a shallow connection may have formed as early as 41 million years ago, the complete and deep separation allowing for unrestricted water flow took longer. A deep-water connection in the Drake Passage developed between 34 and 30 million years ago, coinciding with seafloor spreading, which was a prerequisite for the formation of the largest ocean current on the planet.
Birth of the Antarctic Circumpolar Current
The opening of the Tasmanian Gateway and the deepening of the Drake Passage allowed a continuous, unobstructed flow of water to circle the entire continent. This flow established the Antarctic Circumpolar Current (ACC), the dominant circulation feature of the Southern Ocean. The ACC acts as a thermal barrier, effectively isolating Antarctica from warmer ocean waters. Before the ACC was fully established, warm equatorial waters transferred heat to the Antarctic coast. Once the current flowed unimpeded, it diverted these warm waters northward, trapping cold water around the pole in a zonal flow pattern. This thermal isolation caused cooling of the surrounding sea surface temperatures. The onset of the ACC is closely linked to the Eocene-Oligocene Transition (EOT), a major climate shift that occurred around 34 million years ago. Although the current’s formation was once thought to be the sole trigger for glaciation, models suggest it reinforced the cooling already underway. The volume of the ACC, estimated to transport 137 million cubic meters of water per second, made it a powerful driver of the new polar climate.
The Final Climate Shift
While the ACC provided oceanic isolation, a simultaneous global decline in atmospheric carbon dioxide (\(\text{CO}_2\)) pushed the planet past the threshold for continental glaciation. Over the Paleogene period, the global climate was cooling due to the long-term drawdown of \(\text{CO}_2\), which gradually reduced the greenhouse effect. The drop in \(\text{CO}_2\) concentration around 34 million years ago provided the final atmospheric trigger for the abrupt formation of the ice sheet, leading to cooling where high-latitude surface-water temperatures dropped by an estimated 5°C to 10°C. This temperature change, combined with the oceanographic isolation from the ACC, allowed ice to accumulate rapidly. The initial expansion of the Antarctic Ice Sheet, unfolding over approximately 100,000 years, marked the end of the “greenhouse” Earth and the beginning of the modern “icehouse” world.