Abstract
The consequences of the hypothesis that Milankovitch forcing affects the phase (e.g., termination times) of the 100 kyr glacial cycles via a mechanism known as “nonlinear phase locking” are examined. Phase locking provides a mechanism by which Milankovitch forcing can act as the “pacemaker” of the glacial cycles. Nonlinear phase locking can determine the timing of the major deglaciations, nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear. A consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct. Phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing. Nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record. Nonlinear phase locking can also result in the observed “quantization” of the glacial period into multiples of the obliquity or precession periods.
Highlights
[1] The consequences of the hypothesis that Milankovitch forcing affects the phase of the 100 kyr glacial cycles via a mechanism known as ‘‘nonlinear phase locking’’ are examined
[3] Here we consider the possibility that Milankovitch forcing could affect the phase of the glacial cycles via a mechanism known as ‘‘nonlinear phase locking,’’ following the pioneering work on the role of phase locking to Milankovitch forcing by Saltzman and collaborators [e.g., Saltzman et al, 1984] and as proposed by Gildor and Tziperman [2000]
[4] Our main message, is that the timing of the major deglaciations can be set by Milankovitch forcing nearly independent of the specific mechanism that is responsible for these cycles
Summary
[2] Hays et al [1976] established that Milankovitch forcing (i.e., variations in orbital parameters and their effect on the insolation at the top of the atmosphere) plays a role in glacial cycle dynamics. We try to address this question by showing that in many models, even when the Milankovitch forcing completely determines the model termination times of the ice ages via nonlinear phase locking, the Milankovitch period bands of 19, 23 and 41 kyr are still responsible for only a small fraction of the spectral power, as observed. We show below that in one simple model used here, terminations may initiate at minimum highlatitude summer insolation forcing at one time, and at nonminimum insolation phase at other times This leaves open the possibility that nonlinear phase locking to Milankovitch forcing is responsible for the pacing of the glacial cycles even if a variable phase relation between them is observed. The highly simplified model used here is described in the appendix
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