Ice in Antarctica

We know that ice started to form in Antarctica some 34 million years ago while that did not happen in the Northern Hemisphere until about 20 million years ago - why? Elevation! GHG concentrations and thus temperatures slowly declined over the last several 10s of millions of years. At the same time, Antarctica saw tectonic uplift and at some point the rising land and falling temperatures meant that the glaciation level dropped to below the land elevation and it became possible to snow to persist year-around - cool!