Sensitivity of late Quaternary climates to changes in northern hemisphere ice sheets: Experiments with a general circulation model

Sensitivity experiments with a general circulation model demonstrate the role of ice sheet size on the climate system and show how the climatic response to ice sheets differs from that to CO{dollar}sb2{dollar} and orbital insolation. Model experiments involving an LGM ice sheet, a flat LGM ice sheet, and a half-area LGM ice sheet isolate the effects of albedo, height, and area of the ice sheets. Other sensitivity experiments involving the 21, 16, 14, and 11 ka ice sheets are compared with the Cooperative Holocene Mapping Project (COHMAP) paleo-simulations to isolate the effects of the ice sheets from those of CO{dollar}sb2{dollar} and orbital insolation at these time periods. The climatic responses are discussed in terms of their local, downstream, and far-field effects.; The global mean temperature (GMT) is inversely proportional to the area of the ice sheets and independent of the height. High ice sheets maintain colder temperatures than lower ice sheets over the ice sheets themselves, but compensating warmer temperatures occur downstream from the high ice sheets. The warmer downstream temperatures are the result of (a) glacial anticyclones that cause subsidence and reduced cloud cover, as well as reduced soil moisture, during boreal summer and (b) increased southwesterly flow across the Atlantic Ocean that results in increased southerly advection of warm air during boreal winter. Weaker Asian monsoons develop as a result of this downstream cooling. At 21 ka both the LGM ice sheets and the lower CO{dollar}sb2{dollar} contribute equally to the lower GMTs and the weaker monsoon. The ice sheet retreat and CO{dollar}sb2{dollar} increase from 21 to 11 ka coincides with increasing orbital insolation during boreal summer, which reduces the global cooling and strengthens the monsoon, particularly at 14 and 11 ka. Orbital insolation has the most influence on the tropical monsoons through continental heating, whereas CO{dollar}sb2{dollar} and ice sheets have a larger impact on the GMT through high-latitude temperature changes related to changes in land and sea ice. The surface temperatures at 21, 16, 14, and 11 ka are, to the first order, a linear combination of the individual responses of ice sheets, CO{dollar}sb2{dollar}, and orbital insolation.