During the middle Pleistocene, a change occurred in the climatic response of northern hemisphere ice sheets and the high-latitude North Atlantic Ocean to orbitally controlled variations in insolation. The dominant periodicity during the late Pliocene and early Pleistocene (2.47 Myr B.P. to about 0.735 Myr B.P.) was 41,000 years; thereafter, the dominance shifted to a period near 100,000 years. Although orbital forcing is the primary cause of each of these rhythmic responses, it does not explain the mid-Pleistocene shift of power. Changes in the solid boundary conditions of the ocean-air-ice system, and particularly in mountain elevation, are implicated.