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Today the frozen Antarctic ice sheet borders the Southern Ocean. But tropical palm trees once flourished there.

An intense warming phase occurred 52 million years ago, leading tropical vegetation, including palms and relatives of today's tropical Baobab trees, to grow on the continent’s now frozen coasts.

The surprising discovery came from a study of drill cores obtained from the seafloor near Antarctica. The results, published in the journal Nature, show that warm ocean currents and high carbon dioxide (CO2) levels in the air boosted temperatures, allowing tropical vegetation to grow where visitors today meet only icebergs and freezing cold.

"The CO2 content of the atmosphere as assumed for that time interval is not enough on its own to explain the almost tropical conditions in the Antarctic," said Jörg Pross, a paleoclimatologist at the Goethe University and member of the Biodiversity and Climate Research Center in Frankfurt, Germany.

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"Another important factor was the transfer of heat via warm ocean currents that reached Antarctica."

When the warm ocean current collapsed and the Antarctic coast came under the influence of cooler ocean currents, the tropical rainforests, palm trees and Baobab relatives also disappeared.

The scientists used rock samples from drill cores on the seabed obtained off the coast of Wilkes Land, Antarctica, as part of the Integrated Ocean Drilling Program (IODP). The samples are between 53 and 46 million years old and contain fossil pollen and spores that are known to originate from the Antarctic coastal region.

The researchers were thus able to reconstruct the local vegetation on Antarctica and, accordingly, interpret the presence of tropical and subtropical rainforests covering the coastal region 52 million years ago.

The scientists' evaluations show that the winter temperatures on the Wilkes Land coast were warmer than 50 degrees Fahrenheit at that time, despite three months of polar night. The continental interior, however, was noticeably cooler, with the climate supporting the growth of temperate rainforests characterized by southern beech and Araucaria trees of the type common in New Zealand today.

Additional evidence of extremely mild temperatures was provided by analysis of organic compounds that were produced by soil bacteria populating the soils along the Antarctic coast.

"By studying naturally occurring climate warming periods in the geological past, our knowledge of the mechanisms and processes in the climate system increases," Pross said.