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A detailed climate model is used to investigate the factors contributing to differences between the climate 9000 years ago and that at present. The climate model comprises an atmospheric general circulation model coupled to a mixed layer ocean in which heat convergence due to oceanic advection is prescribed. In the first experiment, only the Earth's orbital parameters are changed. As in earlier work, it is found that monsoon circulations are enhanced during northern summer and weakened during the northern winter and that mid‐latitudes become drier throughout the year. The simulated changes are in general agreement with paleoclimatic evidence. Various feedback mechanisms modify the local response to the changes in insolation at the top of the atmosphere. In the northern hemisphere the warming in high latitudes is greatest in autumn and winter as a result of changes in sea ice, the mid‐latitude oceans are warmer throughout the year because of changes in cloud, and the surface temperature change over land in summer is reduced in low latitudes and enhanced in mid‐latitudes because of changes in cloud and soil moisture. Thus the climatic response to a simple change in boundary conditions may be complex, a fact which should be borne in mind when interpreting paleoclimatic data. In a second experiment a representation of the Laurentide ice sheet is also included. The warming in high latitudes is substantially reduced, especially just downstream of the ice sheets. This suggests that there may have been a substantial warming in high latitudes just after the Laurentide ice sheet collapsed.
Mitchell et al. (Wed,) studied this question.