Coherence resonance and ice ages

Abstract

The processes and feedbacks responsible for the 100-kyr cycle of LatePleistocene global climate change are still being debated. This paperpresents a numerical model that integrates (1) long-wavelength outgoingradiation, (2) the ice-albedo feedback, and (3) lithospheric deflectionwithin the simple conceptual framework of coherence resonance.Coherence resonance is a dynamical process that results in theamplification of internally generated variability at particular periodsin a system with bistability and delay feedback. In the LatePleistocene climate system, bistability results from a combination oflong-wavelength outgoing radiation and the ice-albedo feedback. Theseprocesses are in equilibrium at interglacial and full-glacialconditions. Delay feedback results from the influence of lithosphericdeflection on ice sheet advance and retreat. This process has commonlybeen represented in numerical climate models by complex models of icesheet dynamics. As an alternative, the present model incorporates icesheet dynamics implicitly by using the observed relationship betweenice coverage and global temperature. The result is a simple,well-constrained model for the Late Pleistocene global climate systemwith only one free parameter. The model accurately reproduces theclimate variability recorded in the Vostok ice core from timescales ofseveral thousand to one million years, including the histograms andpower-spectral behavior of the data. The 100-kyr cycle is a freeoscillation in the model, present even in the absence of externalforcing. The model also reproduces smaller-amplitude periodicities atodd harmonics of 100 kyr, suggesting that a significant portion of thespectral power at the Milankovitch bands of 41 and 29 kyr may beinternally generated. Finally, the development of 100-kyr oscillationsin the Mid-Pleistocene may be understood within this model framework asthe transition from a climate with one stable state to a system withtwo stable states brought about by the development of large continentalice sheets and the addition of the ice-albedo feedback to the climatesystem.