A decade ago, the Spectral Analysis, Mapping and Prediction Project (SPECMAP) analyzed the relationship between changes in Earth's orbit produced by variations in obliquity precession, and eccentricity and all available long-term responses observed within the climate system [Imbrie et al., 1992, 1993]. In the intervening decade, improvements to the gas time scale for the Vostok ice core from Antarctica have provided age constraints on greenhouse gas changes that are comparable to those for climatic responses based on marine δ18O signals; these prompted a new investigation of the orbital-scale climatic changes.
Milutin Milankovitch proposed that variations in northern hemisphere summer radiation at the 22,000-yr precession cycle and 41,000-yr obliquity cycle drive northern ice sheets by summer ablation control of ice mass balance, and he inferred that ice sheet responses should lag 5000 years behind summer radiation forcing. In confirmation of Milankovitch's hypothesis, Hays et al.  found that coherent changes in δ18O lag several thousand years behind the forcing at those two orbital cycles. Imbrie et al.  determined that δ18O lags of 5000 years behind precession and 8000 years behind obliquity simultaneously satisfied a 17,000-yr time constant of ice response (Figure 1a). These studies also detected a dominant δ18O response at 100,000 years that Milankovitch had not anticipated because direct solar forcing at that period is negligible. Because the 100,000-yr δ18O signal is phased with orbital eccentricity, Hays et al.  called it a “paced” response but left its origin unresolved.
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