13. Glacial periods

Return to History 8 Human Origins Concepts During interglacials, the Earth has normal climates-warm at the equator, cold at the poles. Interglacial periods gradually grow colder, and the ice spreads south towards the Equator, covering large parts of Europe and North America. Asia becomes bitterly cold and in some cases will not freeze if there is not a sufficient amount of rain. After a certain number of years, it reverses back to an interglacial period and the cycle happens again. This cycle is caused by variations in the Earth's orbit through time that change the amounts of solar radiation that hits the Earth in each season.

National Oceanic and Atmospheric Administration, "Glacial-Interglacial Periods" http://www.ncdc.noaa.gov/paleo/abrupt/data2.html

Manual Chapter 3, page 23