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B. Geerts |
1/'99 |
The thickness of an ice sheet, such as the one on Greenland or Antarctica, is a function of the prevailing long-term climate. A balance is achieved when the accumulation of ice (through snowfall) equals the ablation (melting, sublimation, ice calving at the sheet's edge, and net wind transport). The balance is maintained by negative feedbacks: more ice formation raises the surface, and the extra elevation leads to colder air on top of the sheet, which reduces the saturation vapour pressure and therefore less snowfall. Also, the heavier weight of the ice may enhance ice divergence and the rate of calving at the edge.
The formation of an icecap, in reponse to global cooling, entails a positive feedback. As the icecap forms, a new 'mountain' arises. Therefore more precipitation is captured, more of it falls in the form of snow, and the warm-season melting is reduced. Therefore it is believed that the Laurentide and Scandinavian ice sheets build up rather quickly at the beginning of an Ice Age. In their formation the ice sheets covered or stifled significant sources of CO2, leading to less atmospheric CO2. The Ice Age cooling can only be explained by the lower concentration of greenhouse gases at that time.