The surviving glacial ice of the Northern Hemisphere lies under the eastern limbs of the troughs of the bipolar wave in the circumpolar westerlies. Other distributions such as forest-tundra and the arctic tree line are closely related to the Arctic front, which is also deformed by the bipolar wave. There is some evidence that in the post-Wisconsin warming the phase of these waves was essentially the same as today. and that quite small changes in amplitude are sufficient to account for most climatic variation since that time.