THE IMPACT OF ICE SHEETS, CO2, AND ORBITAL INSOLATION ON LATE QUATERNARY CLIMATES - SENSITIVITY EXPERIMENTS WITH A GENERAL-CIRCULATION MODEL

Sensitivity experiments with a general circulation model, the NC;AR CC M1, are compared with simulations of late-quaternary climates to separ ate out the effects of ice sheets from orbital insolation and CO2. One goal is to determine whether the climatic responses to these three bo undary conditions are additive. The northern hemisphere ice sheets at 11, 14, 16, and 21 ka are used in the sensitivity experiments, with al l other boundary conditions held at modern values. These experiments a re compared with the COHMAP simulations for these times that include t he ice sheets and appropriate CO2 and orbital insolation values for ea ch time period. Both sets of experiments use NCAR CCM1 coupled to a sl ab ocean to calculate interactive sea-surface temperatures. The respon se of CCM1 to changes in ice sheets is largess over the ice sheets the mselves and immediately downstream from the ice sheets. When this ice- sheet response pattern is subtracted from the 21 ka simulation, the re sidual pattern matches that obtained for CO2 alone, with temperature c hanges largest in high-latitude oceans of the winter hemisphere. This result implies that the simulated pattern of temperature change at 21 ka is, to the first order, a linear combination of the ice sheet and C O2 patterns. Similarly at 11 ka, comparisons between the simulation an d sensitivity experiments agree with the results obtained for orbital insolation alone, which shows that the response to changes in orbital insolation is largest over continental land masses in summer. Weaker m onsoons at 21 ka ale the result of both lower CO2 levels and the large LGM ice sheets, whereas stronger monsoons at 11 ka are due to the inc reased orbital insolation during summer. At 16 and 14 ka, the sirnulat ion/sensitivity differences show the combined effects of CO2 and orbit al insolation changes. The lower CO2 values contribute the maximum coo ling to the oceans, while the enhanced orbital insolation during summe r contributes the most warming to the land. During boreal winter both lower CO2 and decreased orbital insolation contribute to the cooling o f the northern hemisphere. Both the regional and global surface temper atures at 21, 16, 14, and 11 ka are, to the first order, a linear comb ination of the individual responses of ice sheets, CO2, and orbital in solation. (C) 1998 Elsevier Science Ltd. All rights reserved.