About every 100,000 years for the last 700,000 years, the Earth enters a period of glaciation. During these times, the northern ice sheets extend much further south than during interglacial periods. As a result of the increased ice coverage, more solar radition is reflected off the surface, thereby creating a positive feedback loop.

The map to the right illustrates the extent of the ice coverage during the glacials. Compared to interglacial periods, ice sheets cover much more of North America and Northern Europe.