A longstanding question in paleoclimatology is how the periodic changes in
the Earth's orbit aroung the sun, which are known to be related to glacial
cycles, actually cause the onset of ice ages. Shackleton (2000) reports new
evidence showing that the orbital changes somehow lead to changes in atmospheric
CO2 which in turn modifies the greenhouse effect which then regulates the
expansion and contraction of ice sheets. He has found that orbital variations
themselves cannot directly lead to variations in ice volume, but that
intermediate mechanism of sufficient magnitude to control global temperatures
and thereby the equatorward advance and poleward retreat of ice sheets. Kerr
(200) provides a summary of the Shakleton (2000) paper.
References
Kerr, R.A., 2000: Ice, mud point to CO2 role in glacial cycle.
Science, 289, 1868.
Shackleton, N.J., 2000: The 100,000-year ice age cycle identified and found
to lag temperature, carbon dioxide, and orbital eccentricity. Science,
289, 1897-1902