Antarctica is the coldest continent on Earth, almost entirely covered by a massive ice sheet. While its deep freeze seems permanent, the continent was not always a desolate polar wasteland. For most of its history, Antarctica enjoyed a temperate climate, transitioning to its current state in a relatively recent geological event. Understanding this transformation requires looking back at the planet’s tectonic shifts and oceanic circulation patterns. This history reveals that the Antarctic ice sheet is a feature of the last few tens of millions of years, not billions.
From Forest to Frost: Antarctica’s Warm Past
Millions of years ago, Antarctica was located further north and was part of the supercontinent Gondwana. During the Cretaceous Period (roughly 100 to 66 million years ago), the continent was a warm, lush landscape located much closer to the equator than its current position over the South Pole. Fossil evidence, particularly from the Antarctic Peninsula, confirms the presence of temperate or even subtropical rainforests. These forests, which included the southern beech genus Nothofagus, indicate a climate without long-lasting periods of freezing. The mean annual temperature during the Late Cretaceous may have been as high as 15°C, supporting a diverse ecosystem where dinosaurs and marsupials once roamed. This verdant past ended as the continent drifted southward and the global climate began a long-term cooling trend.
The Continental Separation that Caused Global Cooling
The process that led to Antarctica’s freezing began with the breakup of Gondwana. The subsequent separation of Antarctica from South America and Australia involved the gradual opening of two oceanic corridors: the Tasmanian Gateway and the Drake Passage. The Tasmanian Gateway began to widen around 40 to 30 million years ago, and the Drake Passage’s final deepening occurred between 31 and 26 million years ago. This separation allowed the formation of the Antarctic Circumpolar Current (ACC), a powerful ocean current that circles the globe unimpeded by continental landmasses. The ACC acts as a thermal barrier, trapping cold water around Antarctica and deflecting warmer currents away from the continent’s shores. This thermal isolation was a prerequisite for the growth of massive ice sheets.
The Initial Formation of the Ice Sheet
The permanent ice age began during the Eocene-Oligocene Transition (EOT), a major climate shift around 34 million years ago. This event marks the rapid, large-scale formation of the East Antarctic Ice Sheet (EAIS), the largest and most stable part of the continent’s current ice cover. Evidence from deep-sea sediment cores shows an abrupt increase in oxygen isotopes, signifying a major drop in ocean temperatures and the locking up of vast quantities of water in continental ice. This glaciation was triggered by the thermal isolation provided by the newly established ACC and a concurrent decline in atmospheric carbon dioxide levels. The EAIS quickly grew to near-modern size, establishing the continent’s permanent icy state. Subsequent climate fluctuations caused the ice sheet to expand and contract, but it never fully disappeared. The smaller, more dynamic West Antarctic Ice Sheet (WAIS) began to grow significantly later, primarily during the Pliocene and Pleistocene epochs.
How Scientists Read the Deep Past
Scientists reconstruct this ancient climate history through the analysis of geological and atmospheric archives. Marine sediment cores drilled from the ocean floor around Antarctica contain microscopic fossils and minerals that record changes in ocean temperature and circulation over millions of years. The presence of “dropstones”—rocks carried far out to sea by melting icebergs—provides direct evidence of past glacial activity. Ice cores, particularly those drilled deep into the East Antarctic Ice Sheet, offer a detailed record of the atmosphere and local conditions stretching back hundreds of thousands of years. These cores trap tiny air bubbles that preserve ancient atmospheric gases like carbon dioxide. The ratio of water isotopes within the ice layers indicates past temperatures and precipitation rates.