Coherence resonance and ice ages

Abstract

The processes and feedbacks responsible for the 100‐kyr cycle of Late Pleistocene global climate change are still being debated. This paper presents a numerical model that integrates (1) long‐wavelength outgoing radiation, (2) the ice‐albedo feedback, and (3) lithospheric deflection within the simple conceptual framework of coherence resonance. Coherence resonance is a dynamical process that results in the amplification of internally generated variability at particular periods in a system with bistability and delay feedback. In the Late Pleistocene climate system, bistability results from a combination of long‐wavelength outgoing radiation and the ice‐albedo feedback. These processes are in equilibrium at interglacial and full‐glacial conditions. Delay feedback results from the influence of lithospheric deflection on ice sheet advance and retreat. This process has commonly been represented in numerical climate models by complex models of ice sheet dynamics. As an alternative, the present model incorporates ice sheet dynamics implicitly by using the observed relationship between ice coverage and global temperature. The result is a simple, well‐constrained model for the Late Pleistocene global climate system with only one free parameter. The model accurately reproduces the climate variability recorded in the Vostok ice core from timescales of several thousand to one million years, including the histograms and power‐spectral behavior of the data. The 100‐kyr cycle is a free oscillation in the model, present even in the absence of external forcing. The model also reproduces smaller‐amplitude periodicities at odd harmonics of 100 kyr, suggesting that a significant portion of the spectral power at the Milankovitch bands of 41 and 29 kyr may be internally generated. Finally, the development of 100‐kyr oscillations in the Mid‐Pleistocene may be understood within this model framework as the transition from a climate with one stable state to a system with two stable states brought about by the development of large continental ice sheets and the addition of the ice‐albedo feedback to the climate system.