|Upsala glacier photographed from the International Space Station. Image credit: NASA.|
The astronomical theory of the Ice Age is based on cyclical changes in the earth’s orbital geometry. It is also known as the Milankovitch mechanism after the Serbian scientist who refined the theory. The theory is composed of an eccentricity cycle of about 400 ka (thousand years) and 100 ka, a tilt cycle of 41 ka, and a precessional cycle of about 21 ka. These cycles generally redistribute the solar radiation by latitude (tilt cycle) and by season (the precessional cycle) but barely change the total amount of radiation hitting the earth.You can read the rest by clicking on "Phase problems with the astronomical theory".
When summers have less solar radiation at 65°N latitude, a glacial or stadial climate occurs and for summers with more solar radiation, an interglacial or interstadial climate results. Stadials are short cold intervals of a few tens of thousands of years (assuming the secular timescale) within warmer interglacials, while interstadials are short warmer intervals within glacial periods.