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 CO2. 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 and orbital insolation changes. The lower CO2 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
CO2 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, CO2, and orbital insolation.
Figure 1. The...