A team of scientists from Cardiff, Bristol and Texas A&M universities worked in Stakishari, a small East African village, to extract microfossils in rock samples which show the level of carbon dioxide at the time of the formation of the ice-cap.
Geologists have long speculated that the formation of the Antarctic ice-cap was caused by a gradually diminishing natural greenhouse effect.
The study establishes that atmospheric carbon dioxide declined during the Eocene-Oligocene climate transition and that the Antarctic ice sheet began to form when carbon dioxide in the atmosphere reached a tipping point of around 760 parts per million (by volume).
Paul Pearson, professor in Cardiff University's School of Earth and Ocean Sciences, who led the mission, said: "About 34 million years ago the earth experienced a mysterious cooling trend. Glaciers and small ice sheets developed in Antarctica, sea levels fell and temperate forests began to displace tropical-type vegetation in many areas."
"The period, known to geologists as the Eocene-Oligocene transition, culminated in the rapid development of a continental-scale ice sheet on Antarctica, which has been there ever since.
"We therefore set out to establish whether there was a substantial decline in atmospheric carbon dioxide levels as the Antarctic ice sheet began to grow."
The team mapped large expanses of bush and wilderness and pieced together the underlying local rock formations using occasional outcrops of rocks and stream beds, said a Cardiff release.
Eventually they discovered sediments of the right age near Stakishari. By assembling a drilling rig and extracting hundreds of metres of samples from under the ground, they were able to obtain exactly the piece of earth's history they had been searching for.
Co-author Bridget Wade, geologist from Texas A&M University, added: "This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago." These findings were published in Nature online.
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