56 
Similarly, the model can be used to investigate the seasonal cycle tens of millions of years 
ago. This study suggests that the configuration of land-sea distribution was such as to not 
favor ice sheets until about 30 or 40 million years ago, at about which time Greenland and 
Antarctica became glaciated, although other factors clearly were involved (Crowley et al., 1984). 
The underlying reason for the transition from an ice-free to an ice-covered Greenland was the 
warmer summer temperatures caused by the greater continentality in the North Atlantic 50 or 60 
million years ago. As the Norwegian Sea opened and widened just after this time, combined with 
the separation of Greenland from North America and the simultaneous movement of the North pole 
towards Greenland, conditions began to favor a continental ice sheet as follows. The presence 
of more open water in the far North Atlantic moderated the summers enough for snow deposited 
over the winter to last over the summer months and build up an ice sheet. Fig. 1 (from North 
and Crowley, 1984) shows the late summer surface temperature distribution computed by the 
model. Of particular interest is the minimum located in the Norwegian Sea and covering 
Greenland. Model runs show that this minimum develops as the Norwegian Sea opens. The pre- 
sence of such a minimum whose minimum value is near freezing can be shown theoretically to 
induce a transition to a large ice sheet (North, 1984). 
The model is so crude, not even including a moisture budget, that it would be unwise to 
consider the theory as conclusive but rather as suggestive of experiments to be done with more 
comprehensive climate models of the future. This is a formidable task since general circulation 
models do not yet model the moisture/precipitation cycle with any degree of reliability. 
