  The cause of the glacial cycles remains a mystery. The origin is widely
accepted to be astronomical since paleoclimatic archives contain strong
spectral components that match the frequencies of Earth's orbital modulation.
Milankovitch insolation theory contains similar frequencies and has become
established as the standard model of the glacial cycles. However, high
precision paleoclimatic data have revealed serious discrepancies with the
Milankovitch model that fundamentally challenge its validity and re-open the
question of what causes the glacial cycles. We propose here that the ice ages
are initially driven not by insolation cycles but by cosmic ray changes,
probably through their effect on clouds. This conclusion is based on a wide
range of evidence, including results presented here on speleothem growth in
caves in Austria and Oman, and on a record of cosmic ray flux over the past 220
kyr obtained from the 10Be composition of deep-ocean sediments.
