The discovery provides a startling glimpse of what might be in store for the world in centuries to come if global warming continues unchecked.
If Antarctica ever became as warm again, sea levels could rise 60 metres (197 feet), swamping major coastal cities such as New York, Sydney and Hong Kong.
Scientists drilled a kilometre into the ocean floor to collect samples of fossilised pollen that have lain undisturbed for millions of years.
They revealed a vastly different version of Antarctica than exists today.
During the Eocene epoch, between 48 and 55 million years ago, high levels of greenhouse gas in the atmosphere forced up temperatures. The UK was, on average, 15C warmer than it is today and a steaming rainforest covered the site of London, according to scientists with the 2010 Integrated Ocean Drilling Research Programme.