The continent of Antarctica is currently a vast, cold expanse of ice that holds roughly 90% of the world’s freshwater. This image of a frozen, barren land is a relatively recent development in geologic time. For millions of years, this southernmost landmass was a radically different place, a world of warmth and life. Scientific evidence, primarily from fossil records and deep-sea sediment cores, reveals a deep past where Antarctica was a temperate, forested land. The story of its transformation from a green continent to a white one is a narrative of continental drift, shifting ocean currents, and dramatic climate change.
Antarctica’s Placement on the Gondwana Supercontinent
Antarctica was once the geographic anchor of Gondwana, a supercontinent that included South America, Africa, Australia, India, and New Zealand. This proximity allowed for the free exchange of flora and fauna across the landmass. Antarctica’s position meant its climate was heavily influenced by these land connections.
The fragmentation of Gondwana began about 180 million years ago in the Early Jurassic period. Africa and South America separated first, leaving Australia, India, and Antarctica connected. This slow process eventually set the stage for Antarctica’s deep freeze millions of years later.
For a long time, Antarctica remained linked to Australia and South America. This connection prevented the formation of a continuous, deep-water ocean current around the pole. This continental arrangement allowed warm ocean currents to flow down the coasts and penetrate the Antarctic region, keeping the climate mild. The final tectonic acts that isolated the continent were the opening of the Tasman Gateway and the Drake Passage.
Climate and Environment in the Warm Ages
The warmest period occurred during the Cretaceous and Paleogene, around 90 million years ago, often called a “Greenhouse Earth.” Evidence suggests that coastal regions of West Antarctica, even south of the Antarctic Circle, hosted temperate rainforests. Mean annual temperatures were likely 12 to 13 degrees Celsius, similar to those in parts of New Zealand today.
This warm climate was driven by significantly higher concentrations of carbon dioxide in the atmosphere compared to today. The warm air and high precipitation created a humid, mild environment that extended almost to the South Pole. Despite the warmth, the continent’s high latitude meant it experienced the polar night—a prolonged period of continuous darkness lasting up to four months each year.
This combination of mild temperatures and extended darkness required unique adaptations for life. Plant life had to thrive with no sunlight for a third of the year. The lack of extensive glaciation was also due to the absence of the Circum-Antarctic Current, which later blocked warm waters from reaching the polar coastlines.
The Ancient Forest Ecosystems
Fossil evidence shows the lush, complex plant life that covered the continent. During the Mesozoic Era, forests were dominated by southern conifers, such as podocarps and araucarias, which formed dense canopies. The undergrowth included a variety of ferns, indicating a humid, temperate rainforest environment.
Later, during the Cretaceous and Paleogene, angiosperm trees, including the southern beech (Nothofagus), became widespread. Fossilized leaves and pollen show that cool-to-warm temperate forests thrived at high latitudes. These Nothofagus forests, similar to those in modern South America and Australia, were a hallmark of the ancient Antarctic landscape.
The ability of these forests to survive the polar night suggests remarkable adaptations. Fossilized root networks and upright logs, some seven meters high, confirm the existence of standing forests. These findings demonstrate that the trees could switch their growth cycles on and off quickly to cope with seasonal light extremes.
Prehistoric Animal Life
The warm, forested continent supported a diverse array of animal life, both terrestrial and marine. Fossils of land-dwelling dinosaurs have been recovered near the Antarctic Peninsula, including mid-sized sauropods and ornithopods. The presence of these large reptiles confirms the environment was mild enough to support robust food chains.
Early mammals also inhabited the continent after the extinction event that ended the age of dinosaurs. Fossil remains on Seymour Island have yielded the earliest known marsupial fossils, which likely migrated from South America. Creatures like Lystrosaurus, a pig-sized herbivore, adapted to the polar light cycles, with some evidence suggesting they may have entered a hibernation-like state during the dark winter.
Warm coastal waters supported a rich marine ecosystem. Fossils of large marine reptiles like mosasaurs and plesiosaurs have been found in Antarctic sediments. Later, in the Cenozoic, the waters were home to early species of dolphins and whales.
The Global Cooling Event
The transition from a warm, forested continent to the frozen landmass of today was triggered by the Eocene-Oligocene Transition, a global cooling event about 34 million years ago. This shift was marked by a decline in atmospheric carbon dioxide levels, which dropped below a critical threshold. As the Earth’s main greenhouse gas diminished, global temperatures began to fall.
This climatic change coincided with the final geological separation of the landmasses, specifically the deepening of the Drake Passage and the Tasman Gateway. This allowed the unimpeded flow of ocean water around the continent, leading to the formation of the Circum-Antarctic Current (ACC). The ACC acted as a thermal barrier, isolating Antarctica from the warmer ocean currents that had previously moderated its climate.
Once the ACC was established, the continent rapidly cooled, forming the first continent-wide ice sheets. This created a positive feedback loop: as more ice formed, more sunlight was reflected back into space, causing further cooling and glaciation. Within a relatively short geological timeframe, the lush forests were replaced by a massive, permanent ice cap.