SECT. 5] LONG-TERM VARIATIONS IN SEA-LEVEL 595 



Before leaving the subject of wind stress, mention must be made of the 

 effect of sea-ice as a mitigating factor. Lisitzin (1957) has shown from data for 

 the Gulf of Bothnia that this effect can have appreciable seasonal significance, 

 and, as the incidence of sea-ice can vary considerably from year to year, it 

 must also contribute to annual variations in sea-level. 



Time changes in the total water content of the oceans due to precipitation 

 and evaporation do not appear to have been seriously studied, presumably 

 because of lack of data. Our ignorance of the oceanic water budget has been 

 emphasized by Munk (1958), when expressing the hope that International 

 Geophysical Year observations will do much to improve the position. For a 

 partially enclosed sea, excess or defect of one of these variables over another 

 will, to a large extent, be balanced by flow in or out of the channel leading to 

 the ocean ; changes in sea-level will thus tend to be spread over the whole 

 surface of the oceans. Wyrtki (1954) has discussed variations in the water 

 content of the Baltic and the water exchange with the North Sea, taking these 

 and other factors into account. 



Variations in sea-level on a much longer time scale than those considered 

 hitherto are almost certainly dominated by changes in the ice coverage of the 

 Antarctic, Greenland and, to a much lesser extent, the Arctic. The phrase 

 "glacial eustasy" is used by geologists to describe changes in the total volume 

 of water on the earth's surface due to the grox^th and decline of glaciated 

 areas, the more general theory of eustasy embracing precipitation, evaporation, 

 movements of the ocean bed, the accumulation of sediment in the oceans, etc. 

 The main evidence for glacial eustasy comes from geological sources as, for 

 example, studies of old coastlines. Flint (1947) and Zeuner (1946) have provided 

 general reviews of the subject. Various estimates (see Young, 1953) suggest 

 that at the maximum of the fourth glaciation of the Pleistocene, sea-level was 

 of the order of 90 m lower than at the present time. It is possible that sea-level 

 everywhere could increase by something of the order of 30 m if all contemporary 

 ice and snow were to melt. This is necessarily a crude estimate as our know- 

 ledge of the thickness of the Antarctic ice-cap is meagre ; one of the projects 

 of the International Geophysical Year (1957-58) has been to form a clearer 

 picture of this ice-cap using seismic sounding techniques. Although a rise of 

 30 m is an improbable occurrence in the foreseeable future (unless nature is 

 assisted by man's surplus of hydrogen bombs), the rate of rise in some of the 

 more low-lying maritime countries must give rise to anxiety. It is estimated 

 that the probability of storm-surges exceeding danger level would be doubled 

 on the east coast of England and trebled on the west coast by a rise in mean 

 sea-level of 0.5 ft. Qualitative climatological evidence is able to indicate trends 

 which must have brought about changes in global sea-level in more recent 

 times (see, for example. Brooks, 1949), such as the "Little Ice Age" between 

 the 16th and 19th centuries, but only with the comparatively recent aid of 

 meteorological instrumentation have trends been observed with reasonable 

 accuracy. One example is the considerable increase in world temperatures 

 during the first 40 years of the present century. 



