Orbital forcing of the inception of the Laurentide ice sheet?

Abstract

According to the astronomical theory of palaeoclimates, the glacial–interglacial oscillations of the Pleistocene have their origin in the insolation forcing caused by the orbital variations of the Earth1. The potential response of the climatic system to the resulting insolation perturbations has been investigated mainly using zonally-averaged energy balance models (EBM)2–6. Atmospheric general circulation models (GCM) include many essential processes (such as the hydrological cycle) neglected in EBMs, but, because of their high computational cost, can be used only for ‘snapshot’ reconstructions of climate at certain times in the past. We show here that the climatic transition at the end of the last interglacial is a particularly relevant period for GCM studies of the sensitivity of climate to the astronomical variations of insolation. We have used a low-resolution spectral GCM to simulate two annual cycles with the insolation conditions of 125,000 yr BP and 115,000 yr BP. For the 115-kyr simulation we find an annual mean cooling over Canada of more than 2 K and increased precipitation. We suggest that such a pattern of response to the insolation changes would favour the extension of permanent snow cover over the Labrador area and could have triggered formation of the Laurentide ice sheet.