SECT. 5] LONG-TERM VARIATIONS IN SEA-LEVEL 606 



5. Some Geophysical Implications of Long-Term Variations in Sea-Level 



Lack of space precludes anything other than a cursory discussion of the 

 geopliysical problems affected by long-term variations in sea-level. Nor is any 

 atteni})t made to give a comprehensive bibliography of papers dealing with 

 these problems. Such papers as are mentioned have been selected either because 

 tliey are of recent date and emphasize the contribution made to the subject by 

 sea-level changes, or because they aflFord useful guides to the subject by virtue 

 of their references. 



A. Eiistatic Changes, the Length of the Day and the Variation in Latitude 



Astronomical observations have long revealed the existence of irregularities 

 of an apparently random nature in the speed of angular rotation of the earth 

 (Spencer Jones, 1939). Their magnitude is such that the standard deviation of 

 the annual average of the length of the day during the past century is of the 

 order of 1.6 msec. 



One possible source of these anomalies is the change in the moment of 

 inertia of the earth about its axis due to the redistribution of matter on the 

 earth's surface following periods of glacial eustasy. A period of deglaciation 

 can have a three-fold effect : 



(i) by increasing the volume of water in the oceans, thus increasing the 

 moment of inertia and, in consequence of "the conservation of momentum, 

 increasing the length of the day, 



(ii) an effect opposite in sign to (i) due to isostatic compensation of the land 

 freed from the weight of ice, 



(iii) tilting of the instantaneous axis of rotation following the redistribution 

 of matter, resulting in a change of the observed latitude of a point on the earth's 

 surface. 



The most recent evidence appears to be that eustatic changes could only 

 account for a small fraction of the observed irregularities in the length of the 

 day (over the past century). It will be sufficient here to quote from three 

 papers on these subjects; by Munk and Revelle (1952 and 1952a) and by 

 Young (1953). Ignoring isostatic compensation, two of these papers present 

 independent calculations indicating that a rise in sea-level of 1 cm would in- 

 crease the length of the day by 0.06 msec and 0.1 msec respectively. Young's 

 estimate was made on the basis of a simple model of the ice-covered areas of 

 the globe, whilst Munk and Rcvelle's was from a rather more detailed model. 

 Estimates of the maximum possible eustatic changes in sea-level have been 

 referred to on page 596, and the value of 10 cm per century, to quote Munk and 

 Revelle, "cannot have changed the length of the day by more than 0.6 msec; 

 the observed decrease equals 3.8 msec, and would imply a lowering of sea-level 

 of 63 cm." This does not signify, of course, that the effect of eustatic changes on 

 a geological time scale can be neglected in their contribution to changes in 

 the length of the day. Indeed the same authors (1952a) have observed that 



