Are the 41 kyr glacial oscillations a linear response to Milankovitch forcing?

The characteristics of glacial oscillations changed drastically 0:8Ma ago, at the ‘‘mid-Pleistocene transition’’. During the past 0.8Ma the 100kyr glacial–interglacial oscillations were strongly asymmetric (i.e., long glacial intervals of growth followed by rapid intervals of deglaciation). The 40 kyr oscillations prior to the mid-Pleistocene transition were of a smaller amplitude and lessasymmetrical looking. The smaller amplitude, apparently symmetric form and period that matches that of obliquity, suggests that these oscillations were a linear response to Milankovitch forcing, while the 100 kyr oscillations are attributed either to some nonlinear self-sustained variability due to a mechanism internal to the climate system itself or to nonlinear amplification of the insolation forcing. The significant strengthening of the 100 kyr eccentricity power in the past 800 kyr is one of the intriguing questions of climate history. Here we show that glacial–interglacial oscillations pre-mid-Pleistocene transition are, in fact, significantly asymmetric. This asymmetry may contradict a straight forward linear Milankovitch explanation, and we therefore suggest that the glacial oscillations before and after the transition may both be explained as self-sustained variability (although the possibility of nonlinear response to insolation forcing still exists). The role of Milankovitch forcing is in setting the phase of the oscillations (e.g. time of terminations) and their period, rather in being the main driving force of the oscillations. This is demonstrated using a simple model based on the sea ice switch mechanism of Gildor and Tziperman (Paleoceanography 15 (2000) 605). r 2004 Elsevier Ltd. All rights reserved.