The most important climactic factor when talking in terms of tens of millions of years is not chemical change in the atmosphere, but continental drift. We currently live on a planet where the south pole is covered by a big land mass and the north pole is tightly surrounded by other land masses. As a result, there is no flow of warming ocean water over the south pole, and very little over the north pole. So, they're both frozen year-round. It hasn't always been like that. Antarctica moved over the south pole fairly recently, in geologic terms. Before that, ocean currents flowed freely over the pole and the climate was much warmer than today.