Glacial Inception in CCSM4

CCSM4 with ocean and land ecosystem components and freely evolving atmospheric carbondioxide is used to quantify the response of carbon fluxes and climate to changes in orbital forcing. Compared to the present-day simulation, the simulation with the Earth's orbital parameters from 115,000 years ago features significantly cooler northern high latitudes, but only moderately cooler southern high latitudes. This asymmetry is explained by the sea-ice/snow albedo feedback; the MOC is almost unchanged. Most importantly, there is a substantial build up of snow cover in Baffin Island and North Canada - the origins of the Laurentide Ice Sheet. The strong northern high-latitude cooling and the direct insolation induced tropical warming lead to global shifts in precipitation and winds to the order of 10-20%. These climate shifts lead to regional differences in air-sea carbon fluxes of the same order. However, the differences in global net air-sea carbon fluxes are small and provide no support for the hypothesis that the solubility pump is responsible for the initial drawdown of atmospheric CO2 during a glacial inception. This surprising result is due to several effects, two of which stand out: Firstly, colder sea surface temperature leads to a more effective solubility pump but also to increased sea-ice concentration which blocks air-sea exchange; and secondly, the weakening of Southern Ocean winds that is predicted by some idealized studies occurs only in part of the basin, and is compensated by stronger winds in other parts.