Examining plant-leaf wax remnants in sediment core samples taken from under the Ross Ice Shelf of Antarctica, researchers found that summer temperatures along the Antarctic coast 15 to 20 million years ago were up to 11 degree Celsius warmer than today, with temperatures reaching as high as 7 degree Celsius.
That climate was suitable to support massive substantial vegetation, including stunted trees, along the edges of the frozen continent, the researchers said.
This vegetated period peaked during the middle Miocene, when atmospheric carbon dioxide levels were around 400 to 600 parts per million. As a result, global temperatures warmed, the researchers reported in the journal Nature Geoscience.
"When the planet heats up, the biggest changes are seen toward the poles," study researcher Jung-Eun Lee, a scientist at NASA's Jet Propulsion Laboratory, said.
"The southward movement of rain bands made the margins of Antarctica less like a polar desert and more like present-day Iceland." Lee was quoted as saying by LiveScience.
The NASA researchers along with a team from University of Southern California and Louisiana State University collected long cores of sediment from below Antarctica's Ross Ice Shelf.
Within the sediment, they found plant-leaf wax which is an indication of ancient vegetation. The cores also contained pollen and algae.
An analysis of the leaf-wax provided a record of the water taken up by the plants when they lived. The team could then track variations in the hydrogen molecules in the water, called isotopes. As isotopes vary over time and over certain environmental conditions, these variations allowed them to reconstruct what the climate would have looked like.
If current emissions continue as they are, atmospheric carbon is set to reach middle Miocene levels by the end of the century. The northern Antarctic Peninsula has already warmed by 2.5 degrees Celsius over the last 50 years, and satellite views reveal melting ice shelves.
The ancient Antarctic sediment could provide a vision of what is to come, said study leader Sarah Feakins, an earth scientist at the University of Southern California.
"Just as history has a lot to teach us about the future, so does past climate. What this record shows us is how much warmer and wetter it can get around the Antarctic ice sheet as the climate system heats up," he added.
