There is recent research that supports the assertion that the start of an ice age epoch is initiated by an increase in Galactic Cosmic Rays (GCR) and that the ice age glacial/interglacial cycle is controlled by processes that periodically modulate the amount of GCR that penetrates the solar large scale magnetic field and the geomagnetic field, and hence penetrates the earth's atmosphere. The following is a link to a paper that provides data and an explanation for the mechanisms, to support that GCR hypothesis. From Kirby, Mangini, and Muller's 2004 Paper "The Glacial Cycles and Cosmic Rays" "The cause of the glacial cycles remains a mystery. The origins is widely accepted to be astronomical ...(however) ... high precision paleoclimatic data revealed serious discrepancies with the Milankovitch model that fundamental challenges its validity and re-open the question of what causes the glacial cycles. We propose that the ice ages are not driven by insolation cycles but by cosmic ray changes ..." The Glacial Cycles and Cosmic Rays http://arxiv.org/pdf/physics/0407005 Comment: GCR creates ions in the atmosphere which recent papers show affects the amount of low level cloud cover. The data shows a decrease in GCR results in a decrease in low level clouds which will warm the planet and an increase in GCR results in an increase in low level clouds. See the thread Clouds and Albedo for details.