53 
December -February surface air warming (°F ) Doubled COe2 
90 
Latitude (degrees) 
-180 -120 -60 O 60 120 180 
Longitude (degrees) 
Fig. 4. Change in atmospheric surface air temperature during winter between the 
experiment with doubled COg and the control run. 
How does this result compare with land evidence of climate change at 11 to 10k B.P.? 
This time period is known as the Younger Dryas, for the widespread sudden cooling noted in land 
evidence (ice indications and pollen) over Western Europe. Fig. 2, prepared by Dr. Peteet, 
shows the palynological evidence for this cooling; it is extremely abundant in Western Europe, 
and less so as one progresses eastward. Fig. 3 shows that there is little or no evidence for 
simultaneous cooling over North America. Thus the temperature change produced in the model by 
cooler Atlantic Ocean temperatures, as in Fig. 1 is consistent with land evidence for the pat- 
terns of actual temperature change. As noted earlier, it is still uncertain whether the surface 
water cooling was associated with changes in NADW formation. 
What are the implications for the future? Fig. 4 shows the change in surface air tem- 
perature produced in the GISS GCM by doubling the atmospheric COg, for the months of 
December through February. Warming of 6-16°F occurs throughout the North Atlantic. Fig. 5 
shows the percent change in ocean ice for the same period. Reductions of up to 80% occur in 
the ice covered areas. The impact of such a large magnitude changes on NADW formation cannot 
be determined at this stage of our knowledge, but it conceivably could be large. Would this lead 
to cooler ocean surface temperatures, as a negative feedback, and result in temperature 
influences such as that shown in Fig. 1? If so, it would obviously modify the regional changes 
given in Fig. 4, and have further feedbacks on NADW formation. The potential for NADW for- 
mation to act as a feedback in a climate change scenario, and thus further influence the climate 
is a complex problem that needs further investigation. 
