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FIGURE 2 Change in percentage annual snow cover due to colder North Pacific sea surface temperatures (SST —2°C) at 11,000 B.P. compared to a model run with today's SST. Grid-point standard deviations are about 0.3%. (From Peteet et al., 1997. With permission.)

perature response is additive for the three parameters. At 16,000 B.P. (13,500 14C B.P.) and 14,000 B.P. (12,000 14C B.P.), the simulation differences showed that lower CO2 (210 ppm at 16,000 B.P., 230 ppm at 14,000 B.P.) contributed to maximum cooling in the oceans, while enhanced orbital insolation during the summer contributed primarily to warming over land. The lower CO2 concentrations caused the most cooling in regions of sea ice growth in the winter hemisphere, which added to the regional ice sheet cooling during boreal winter, and created large cold anomalies in the Southern Ocean during boreal summer (Fig. 3).

22.3.2.8. N. AGCM GENESIS Younger Dryas Termination Experiment

Two GCM experiments using GENESIS (modified CCM1) were completed for the Younger Dryas time (Fawcett et al. 1997). This model used the boundary conditions of 12,000 B.P. orbital conditions, Peltier's (1994) land ice extent, and 250 ppm CO2. In the first experiment, the North Atlantic Ocean heat flux was reduced to 10 W/m2 (watts per square meter heat off), simulating a shutdown in the NADW formation. For the second experiment, the North Atlantic Ocean heat flux was turned on (300 W/ m2, active deepwater formation). Model results over Greenland showed an annual average temperature increase from heat off to heat on of 2.80C, which is about half of the temperature indicated by the Greenland ice core oxygen isotope profile (e.g., Alley, 2000). The warmer climate experiment showed a shift in storm tracks toward Greenland, but the annual average precipitation rate increased by a few tens of percent, as compared with the 100% accumulation increase documented in the ice core. Thus, the authors concluded that either the model is not fully sensitive to the ocean heat flux forcing or some other factor(s) are active.

FIGURE 3 Surface temperature difference for winter (December-January-February, DJF) between (a) the 14,000 cal. B.P. (14 ka) sensitivity experiment (330 ppm) and (b) the 14 ka simulation experiment (230 ppm) and the pre-industrial control (267 ppm) (Kutzbach et al., 1998). Contour interval is 6°C. The DJF lower orbital insolation in the Northern Hemisphere contributes to the ice sheet cooling effects over the Northern Hemisphere landmasses, while the decreased CO2 of 37 ppm contributes to the ice sheet cooling of the high-latitude oceans in the Southern Hemisphere. (From Felzer et al., 1998. With permission.)

FIGURE 3 Surface temperature difference for winter (December-January-February, DJF) between (a) the 14,000 cal. B.P. (14 ka) sensitivity experiment (330 ppm) and (b) the 14 ka simulation experiment (230 ppm) and the pre-industrial control (267 ppm) (Kutzbach et al., 1998). Contour interval is 6°C. The DJF lower orbital insolation in the Northern Hemisphere contributes to the ice sheet cooling effects over the Northern Hemisphere landmasses, while the decreased CO2 of 37 ppm contributes to the ice sheet cooling of the high-latitude oceans in the Southern Hemisphere. (From Felzer et al., 1998. With permission.)

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