Sensitivity of glacial inception to orbital and greenhouse gas climate forcing

Abstract

Attempts to model climate system behaviour during the initiation of the last ice age provide a further framework in which to test the quality of the global climate models (GCMs) that are employed to predict future anthropogenic greenhouse-gas-induced climate change. The model employed in the study reported herein, which consists of the Canadian Climate Centre Atmospheric GCM2 model coupled to a mixed layer ocean model with oceanic heat transports tuned to reproduce modern SSTs, has been shown to initiate Northern Hemisphere (NH) glaciation solely in response to the reduced summer insolation forcing at 116 000 years before present. This paper, which constitutes a significant extension of work recently published elsewhere, describes analyses designed to investigate the model response to the changes in orbital forcing that were characteristic of the sequence of different inception events that occurred throughout the late Pleistocene. In order to assess the model sensitivity to NH glacial inception, a series of simulations have been performed in which the influence of the eccentricity-precession climatic factor and the obliquity factor are separated. The response of the climate system to changes in atmospheric carbon dioxide concentration during these same inception events has also been investigated to assess the sensitivity of the model to this additional forcing mechanism. A further ocean feedback experiment has also been conducted to assess the response of the mixed layer ocean to a combination of orbital and CO 2 induced radiative forcing. Our analyses demonstrate that while cool NH summers are a prerequisite for glacial inception, a low value of obliquity is most important in determining the strength of the inception process, followed in order of importance by the magnitude of the eccentricity-precession forcing, which dictates the timing and magnitude of the NH summer cooling through geologic time. The minimum and maximum values of carbon dioxide concentrations inferred from ice core records characteristic of the late-Pleistocene glacial inception periods also influence the strength of the inception phenomenon, in fact to the same degree as the eccentricity-precession forcing. Since changes in atmospheric carbon dioxide concentration have apparently been quite variable during interglacial periods, these fluctuations may contribute significantly to the instability that drives glacial advance.