Climate changes during the course of long-term cycles which appear to reflect an astronomical forcing of the climatic system by changes in the seasonal patterns of insulation. These are driven by changes in the orbital parameters of the Earth, specifically due to variations in the tilt of the rotation of the axis and due to secular precession of the rotation axis. A complete precessional cycle is described every 26,000 years, and with the slow rotation of the orbit effectively changes the position of the Earth relative to the sun for any particular season. These changes in insulation play a critical role in determining whether ice sheets advance or retreat, as well as affecting the biological productivity of the oceans and the level of carbon dioxide in the atmosphere.
Major changes in climate occur approximately every 100,000 years, with associated changes of sea level of approximately 100 metres. Other changes occur in cycles of 41,000 years and 23,000 years, with subsidiary changes occurring in cycles of 59,000 years and 11,000 years. Changes in insulation of lesser significance are also expected, at 100,000 and 413,000 years due to changes in the ellipticity of the Earth's orbit. At present the Earth is relatively close to the sun during the northern winter, but the opposite situation will prevail in 11,000 years.
The great climatic changes of the past 2 million years created stress among living things. Forest, grassland and animal populations were repeatedly forced to change and migrate as glaciers waxed and waned. The most significant changes occurred in the sub-tropical deserts of the northern hemisphere. The Sahara was wetter (rock paintings there depict hunters of big game in extensive Savannah grasslands). The Indus Valley civilization of Mohenjo-Daro and Harappa arose during this wetter period, but subsequently faded because of climatic change. In the past 4000 years, this desiccation has predominated in much of the sub-tropical belt.