The influence of continental ice sheets on the climate of an ice age

The climate influence of the land ice that existed 18,000 years before present (18K B.P.) is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean. Simulated climates are obtained from two versions of the model: one with the land ice distribution of the present and the other with that of 18K B.P. In the northern hemisphere the tropospheric flow field is strongly influenced by the Laurentide ice sheet and features a split flow straddling the ice sheet, with a strong jet stream forming the southern branch. The northern branch of the flow brings very cold air over the North Atlantic Ocean, where thick sea ice is maintained. The distribution of sea surface temperature (SST) difference between the two experiments in the northern hemisphere resembles the difference between the SST at 18K B.P. and at present, as estimated by the CLIMAP Project (1981). The 18K B.P. ice sheets have very little influence upon atmospheric temperature and SST in the southern hemisphere. This is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation. Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North America and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.