During a glacial circumstances favor stacking ice in the Greenland basin of attraction. During an inter glacial the opposite occurs. Also consider what the massive weight of the ice does to the shape of the basin over time. Slope or curve changes of the basin probably help determine when a transition occurs. Over time the Greenland basin will tend to sink during a Glacial under the added weight. Ice at lower elevations may more easily melt and sea water may undermine the ice if the land is pushed down far enough.