The summer air temperature that regulates the ice margin covaries with
the sea surface temperature but precedes the ice volume during glacial
cycles, suggesting that the ocean is the intermediary of the orbital
forcing of the ice sheet. We formulate a minimal model to elucidate the
ocean role in the genesis of glacial cycles. We show that, because of
the atmospheric coupling, an eddying ocean exhibits bistates of maximum
entropy production, which would translate to ice bistates of polar ice
cap and Laurentide ice sheet, enabling large ice-volume signal when the
bistable interval is crossed by the forcing. Since this bistable
interval is set by the global convective flux, it is lowered during
Pleistocene cooling, whose interplay with the ice-albedo feedback may
account for the mid-Pleistocene transition from 41-ky obliquity cyles to
100-ky ice-age cycles paced by eccentricity. Quantitative tests of the
theory and its parsimony in resolving myriad glacial puzzles suggest
that the theory has captured the governing physics of the Pleistocene
glacial cycles.