ON THE STRUCTURE AND ORIGIN OF MAJOR GLACIATION CYCLES .2. THE 100,000-YEAR CYCLE

Climate over the past million years has been dominated by glaciation c ycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be exp lained as responses to insolation cycles driven by precession and obli quity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to pr oduce the corresponding climate cycle by direct forcing. We present ph ase observations showing that the geographic progression of local resp onses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic me chanisms operates in all three. But the phase sequence in the 100,000- year cycle requires a source of climatic inertia having a time constan t (similar to 15,000 years) much larger than the other cycles (similar to 5,000 years). Our conceptual model identifies massive northern hem isphere ice sheets as this larger inertial source. When these ice shee ts, forced by precession and obliquity, exceed a critical size, they c ease responding as linear Milankovitch slaves and drive atmospheric an d oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is parti cularly sensitive to eccentricity-driven modulations in the 23,000-yea r sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wast ing accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations. Whether a glacier or i ce sheet influences the climate depends very much on the scale....The interesting aspect is that an effect on the local climate can still ma ke an ice mass grow larger and larger, thereby gradually increasing it s radius of influence.