present cycle of ice ages has come to an end. There could be 

 another advance of ice over the land with the consequent reduction 

 of sea level by about 300 feet. On the other hand, the ever-present 

 threat of the Antarctic and Greenland ice melting, with a consequent 

 rise of 300 feet in sea level, is still with man. 



Although we do not know just what controls the delicate balance, 

 it seems probable that both the major icecaps provide some sort of 

 stability. They appear to be held in deep basins by rings of mountains, 

 whereas the ice sheets which covered Canada, part of the United 

 States, Asia, and northern Europe were not held and could spread 

 out thinly and so be more readily dispersed. It is important, then, 

 to measure the present-day rate of snow accumulation on the ice- 

 caps and compare it with the rate of evaporation, and to measure 

 the escape of ice that plunges over the rims of the containing 

 mountains and tumbles into the sea. 



It is an easy matter to explain the rise and fall in sea level: 

 glacial ice simply melts and becomes part of the sea or water freezes. 

 But the mechanisms that touch off these processes are much more 

 difficult to explain. We have already seen that the radioactivity of the 

 mantle rocks may provide a periodic heating up of the Earth's crust 

 and so alter world climate from time to time, and there are other 

 mechanisms that may be associated with warming our planet. 

 Changes in the carbon dioxide content of the atmosphere, for 

 instance, may affect world climate. An atmospheric blanket of 

 carbon dioxide would produce a greenhouse effect and trap the 

 reflected long-wave (heat) radiations of the Earth and so trigger 

 off a warming up period which would melt the remaining ice and 

 drown all the low-lying land. However, carbon dioxide is absorbed in 

 sea water, and circulation of the large volume of water in the oceans 

 may be adequate to preserve a fairly constant balance of carbon 

 dioxide between air and water and thus provide a steadying influence. 



But there are other ways of shielding the Earth from the Sun's 

 rays and thus upsetting the delicate balance which controls climate. 

 The Earth could become enveloped in a cloud of meteoric dust; 

 continuous volcanic activity over a long period of time could throw 

 up a screen of fine ash in the sky; or there could be a long-term 

 change in the intensity of solar radiation. Also, a change in incli- 

 nation of the Earth's axis could cause the regions of ice to shift 

 location. It would seem that any number of these conditions could 

 conspire and touch off another period of cooling. A cooler Earth 

 would have longer and more severe winters. Snow rather than rain 

 would fall and would tend to remain rather than be melted and 

 washed away. Thick accumulations of snow would form ice patches 

 and in turn these would grow into icecaps or glaciers, which creep 

 downhill forcing a retreat of life before them. 



But the slow march of glaciers does not constitute the entire 

 invasion of ice. The extension of winter conditions far away from 

 the polar regions, plus the slow accumulation of snow, are the real 

 mechanisms that advance an ice age. And the sea is the provider of 

 the snow. During an ice age it may give up as much as two to five 

 per cent of its volume in the form of ice and snow, but when the 

 mechanisms that touch off the ice age reverse, the sea reclaims its 

 loss as the ice melts. And so the cycle goes on, apparently without 

 end, from water to rain and snow and ice and then back to water 

 again. 



// the Green/and and Antarctic icecaps stiould 

 melt suddenly, tfie levei of ttie oceans wouid 

 rise about 300 feet. Ttiis would turn New 

 York, London, Paris, and Tokyo into submerged 

 cities, leaving only the tops of the highest 

 buildings rising out of the water. 



15 



