Watts Up With That? 
Tuesday, Sept 15th, 2009
Bill Illis writes about it:
Ice sheets formed in Antarctica about 35 million years ago when CO2 was about 1,200 ppm. Ice sheets also formed in Antarctica about 350 to 290 million years ago when CO2 was about 350 ppm. Ice sheets also formed in Antarctica about 450 to 430 million years ago when CO2 was about 4,500 ppm. The more common denominator is when continental drift places Antarctica at the south pole.
Below, Antarctica today.
New data illuminates Antarctic ice cap formation
From a Bristol University Press release  issued 13 September 2009
A paper published in Nature
New carbon dioxide data confirm that formation of the Antarctic ice-cap some 33.5 million years ago was due to declining carbon dioxide in the atmosphere.
A team of scientists from Bristol, Cardiff and Texas A&M universities braved the lions and hyenas of a small East African village to extract microfossils from rocks which have revealed the level of CO2 in the Earth’s atmosphere 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’s findings, published in Nature online , confirm that atmospheric CO2 started to decline about 34 million years ago, during the period known to geologists as the Eocene – Oligocene climate transition, and that the ice sheet began to form about 33.5 million years ago when CO2 in the atmosphere reached a tipping point of around 760 parts per million (by volume).
The new findings will add to the debate around rising CO2 levels in the Earth’s atmosphere as the world’s attention turns to the UN Climate Conference in Copenhagen which opens later this year.
Dr Gavin Foster from the University of Bristol and a co-author on the paper said: “By using a rather unique set of samples from Tanzania and a new analytical technique that I developed, we have, for the first time, been able to reconstruct the concentration of CO2 across the Eocene-Oligocene boundary – the time period about 33.5 million years ago when ice sheets first started to grow on Eastern Antarctica. “
Professor Paul Pearson from Cardiff University’s School of Earth and Ocean Sciences, who led the mission to the remote East Africa village of Stakishari 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 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.”
Co-author Dr Bridget Wade from Texas A&M University Department of Geology and Geophysics added: “This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago.
“Our study is the first to provide a direct link between the establishment of an ice sheet on Antarctica and atmospheric carbon dioxide levels and therefore confirms the relationship between carbon dioxide levels in the atmosphere and global climate.”
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. Eventually they discovered sediments of the right age near a traditional African village called Stakishari. By assembling a drilling rig and extracting hundreds of meters of samples from under the ground they were able to obtain exactly the piece of Earth’s history they had been searching for.
The paper:Atmospheric carbon dioxide through the Eocene–Oligocene climate transition . Paul N. Pearson, Gavin L. Foster & Bridget S. Wade. Nature online, Sunday 13th September.