Antarctica was warmest during the late Cretaceous and early Eocene periods, roughly 90 to 50 million years ago. During peak warmth around 52 million years ago, coastal waters near the Antarctic Peninsula reached about 15°C (59°F), and the continent supported lush forests, diverse marine life, and even dinosaurs. It wasn’t quite the steamy equatorial jungle people sometimes picture, but it was dramatically warmer than today’s frozen desert.
Peak Warmth in the Early Eocene
The hottest stretch in Antarctica’s recent geological history came during what scientists call the Early Eocene Climate Optimum, roughly 52 million years ago. Shell fossils from Seymour Island, off the Antarctic Peninsula, show that ocean temperatures near the coast averaged around 15°C (59°F). Summer temperatures ranged from 12 to 15°C, while winters hovered near 10°C. That seasonal swing of about 8°C is comparable to what you’d experience along the coast of southern England or the Pacific Northwest today.
These weren’t truly tropical conditions. The tropics maintain year-round temperatures above 18°C. Antarctica during the Eocene was more accurately described as warm temperate, similar to modern-day New Zealand or Tasmania. Still, the contrast with today is staggering: the continent’s current average temperature is around minus 50°C in the interior.
Why Antarctica Was So Warm
Two factors kept Antarctica mild for tens of millions of years: its position on the map and the composition of the atmosphere.
During the Mesozoic Era (roughly 250 to 66 million years ago), Antarctica was part of the supercontinent Gondwana, physically connected to South America, Australia, Africa, and India. Although it sat near the South Pole for much of this time, those land bridges allowed warm ocean currents from lower latitudes to reach Antarctic shores. No cold circumpolar current existed to wall off the continent from warmer waters.
At the same time, atmospheric carbon dioxide levels were far higher than today. The warmest episodes of the Mesozoic were associated with CO2 concentrations around 1,000 parts per million, more than double today’s roughly 420 ppm. That elevated CO2 created a powerful greenhouse effect, warming the entire planet and flattening the temperature difference between the equator and the poles. During peak Eocene warmth, that pole-to-equator temperature gap was only about 15°C. By the late Eocene, it had widened to 25°C as the planet cooled.
Forests, Dinosaurs, and Diverse Seas
The fossil record paints a vivid picture of what lived on this warmer Antarctica. During the Cretaceous period (about 145 to 66 million years ago), dinosaurs roamed the continent. Fossils of a small plant-eating dinosaur called a hypsilophodontid, roughly 70 million years old, have been recovered from Antarctic rock. After the dinosaurs disappeared, marsupial mammals took their place.
The vegetation was dominated by southern beech trees (Nothofagus), the same genus that still grows in Patagonia and New Zealand. Leaf fossils show these trees persisted in West Antarctica until at least 22 to 20 million years ago, well after peak warmth had passed. By that point, the forests had thinned to a tundra-like scrubland, but woody plants still clung to parts of the continent. Sparse evidence suggests small pockets of tundra-like forest may have lasted in East Antarctica as recently as the Pliocene, roughly 3 to 5 million years ago.
The seas around Antarctica were equally rich. Fossil beds on Seymour Island have yielded at least 21 species of sharks and rays from the early Eocene, a diversity nearly equal to modern tropical shark communities. Ammonites, octopuses, barnacles, starfish, and beautifully preserved shrimp have all been found. Fish fossils show that cod-like species dominated the waters during the last temperate marine phase before glaciation set in.
How Antarctica Froze
The transition from mild forests to ice sheets happened in stages over tens of millions of years, driven by the breakup of Gondwana. Australia began separating from Antarctica between 90 and 50 million years ago. South America pulled away more gradually, with the Drake Passage between the two continents deepening to significant depths between 41 and 37 million years ago.
These openings were critical. Once the Tasmanian Gateway and Drake Passage were both deep enough, ocean currents could flow all the way around Antarctica without hitting a landmass. A proto-version of the Antarctic Circumpolar Current began around 36 million years ago, roughly 6 million years earlier than scientists previously estimated for the full modern current. This ring of cold water effectively isolated Antarctica from the warm currents that had kept it temperate for so long.
The temperature record from Seymour Island tracks this decline in detail. From a peak of about 15°C around 52 million years ago, coastal waters cooled to 10 to 11°C by the middle Eocene. A brief warm spike interrupted the trend, but by 41 million years ago temperatures had dropped sharply to around 7°C. By 37 million years ago, they reached just 5°C. Seasonal variation shrank as well: winters and summers both became uniformly cold rather than showing the wide swings of earlier periods.
The final blow came at the Eocene-Oligocene Transition, about 34 million years ago. The circumpolar current drove upwelling of nutrient-rich deep water, which fueled massive blooms of tiny marine organisms. As these organisms died and sank, they buried carbon on the ocean floor, pulling CO2 out of the atmosphere. That drop in greenhouse gases, combined with Antarctica’s increasing isolation, triggered the growth of a continental-scale ice sheet. Vegetation collapsed, and Antarctica entered the deep freeze it remains in today.
The Timeline at a Glance
- 90–66 million years ago (Cretaceous): Dinosaurs live on Antarctica. Forests cover the land. CO2 around 1,000 ppm keeps the planet warm.
- 56–48 million years ago (early Eocene): Peak warmth. Coastal temperatures near 15°C. Shark diversity rivals modern tropical waters.
- 48–34 million years ago (middle to late Eocene): Gradual cooling as Australia and South America separate. Temperatures fall from 15°C to 5°C.
- ~36 million years ago: Proto-Antarctic Circumpolar Current begins, isolating the continent.
- ~34 million years ago: Large-scale ice sheets form. Forests largely disappear.
- 22–20 million years ago: Last known woody vegetation in West Antarctica (southern beech trees in a tundra-like setting).