The impact of ice sheets, CO 2, and orbital insolation on late quaternary climates : sensitivity experiments with a general circulation model

Abstract

Sensitivity experiments with a general circulation model, the NCAR CCM1, are compared with simulations of late-Quaternary climates to separate out the effects of ice sheets from orbital insolation and CO 2. One goal is to determine whether the climatic responses to these three boundary conditions are additive. The northern hemisphere ice sheets at 11, 14, 16, and 21 ka are used in the sensitivity experiments, with all other boundary conditions held at modern values. These experiments are compared with the COHMAP simulations for these times that include the ice sheets and appropriate CO 2 and orbital insolation values for each time period. Both sets of experiments use NCAR CCM1 coupled to a slab ocean to calculate interactive sea-surface temperatures. The response of CCM1 to changes in ice sheets is largest over the ice sheets themselves and immediately downstream from the ice sheets. When this ice-sheet response pattern is subtracted from the 21 ka simulation, the residual pattern matches that obtained for CO 2 alone, with temperature changes 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 CO 2 patterns. Similarly at 11 ka, comparisons between the simulation and 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 monsoons at 21 ka are the result of both lower CO 2 levels and the large LGM ice sheets, whereas stronger monsoons at 11 ka are due to the increased orbital insolation during summer. At 16 and 14 ka, the simulation/sensitivity differences show the combined effects of CO 2 and orbital insolation changes. The lower CO 2 values contribute the maximum cooling to the oceans, while the enhanced orbital insolation during summer contributes the most warming to the land. During boreal winter both lower CO 2 and decreased orbital insolation contribute to the cooling of the northern hemisphere. Both the regional and global 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 2, and orbital insolation.