Palm trees swayed on the green shores of Antarctica 50 million years ago while temperatures soared above 20C, a study has shown.
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. But the most dramatic effect of the Eocene "greenhouse world" was seen at the poles, according to the new evidence.
Detailed examination of fossil-rich Antarctic sediments has not been possible before, since any remaining on land have been destroyed by glaciation or buried under thousands of metres of ice. The new research involved an expedition to drill off Wilkes Land on the east coast of Antarctica. Members of the 2010 Integrated Ocean Drilling Research Programme dropped a string of drill pipes through four kilometres of water to bore into the ocean floor.
Dr James Bendle, from the University of Glasgow, said: "The Eocene sediment samples are the first detailed evidence we have of what was happening on the Antarctic during this vitally important time.
"We conducted the drilling expedition against a backdrop of freezing temperatures, huge ocean swells, calving glaciers, snow-covered mountains and icebergs. It's amazing to imagine a time-traveller, arriving at the same coastline in the early Eocene, could paddle in pleasantly warm waters lapping at a lush forest."
Pollen from plants living in two different environments were found in the sediment cores. The pollen shows that average Eocene temperatures on the Antarctic coast were around 16C and summers reached a balmy 21C, the scientists reported in the journal Nature. Winters were warmer than 10C even during the coldest and darkest months of the year.
At this time in history Antarctica was almost in the same position it occupies today, covering the South Pole. The winter months would have been dark, as they are today, but the weather was far warmer. Organic molecules preserved from Eocene soil bacteria confirmed the temperature readings derived from pollen.