We begin by first increasing the Earth's albedo, causing a loss of insolation. This increase in albedo began about 15 million years ago, when the Antarctic continent moved over the South Pole which led to the development of a permanent polar ice cap. To supplement this change in albedo in the Southern Hemisphere, continental rearrangements about 3-5 million years ago placed the northern regions of Greenland, Canada, Russia, and Alaska as a fringe around the Arctic Ocean, cutting off warm ocean circulation to the North Pole. The stage was set for development of an Ice Age. The most acceptable theory for the development of and ice age was put forward in the 1930s by the Yugoslav Milutin Milankovitch , who used a sequence of cyclic variations in the Earth itself.
Circular orbits mean more uniform receipt of energy from season to season and from year to year.
A more severe tilt means a greater contrast in seasonal distribution of energy. A lower tilt means more uniform receipt of energy between the seasons. Also, the polar regions will receive more energy during periods with low tilt angles. For example, what was once the spring equinox will be the fall equinox 10,500-11,000 years later. This phenomenon affects the amount of incoming energy.