The atmospheric concentration of CO2 has undergone significant and fairly regular changes on a time scale of 100 kyr during the at least last four glacial-interglacial cycles. Here we present a novel coupled physical-biogeochemical mechanism for these variations. Previous studies had to arbitrarily specify the behavior of the physical climate system in order to invoke a biogeochemical mechanism for the glacial CO2 changes, be it an arbitrarily specified change in the vertical ocean mixing [Toggweiler, 1999], or arbitrarily specified sea ice cover changes [Stephens and Keeling, 2000]. Instead, we present here a new, self-consistent, qualitative physical mechanism for both the vertical mixing and sea ice cover changes. In this mechanism, the cooling of North Atlantic Deep Water due to northern hemisphere glaciation is transported southward by the thermohaline circulation, and cools the deep water upwelling in the Southern Ocean. This, in turn, affects the Southern Ocean stratification, reduces the rate of vertical mixing of the surface water with the deep water and increases the sea ice cover. We also explain the continuous time evolution between glacial and interglacial states rather than treat them as two steady states, and are able to model explicitly for the first time the amplification of the glacial-interglacial variability of the physical climate system by the ocean biogeochemistry.