19 



coastline to establish a more stable value. This averaging length could 

 encompass, for example, the North American or North and South American 

 shoreline(s) . Thus, if a wave with length exceeding the expanse of the 

 stations encompassed were contributing to the "noise," this process would 

 tend to reduce or (in the very fortuitous cases) eliminate its 

 contribution. By first averaging over long segments of the shoreline, 

 weighting each station by its alongshore influence length, then combining 

 appropriately the results for various such shoreline segments, a much more 

 stable year by year value could be obtained (i.e., Eqs . 2.3 and 2.4). This 

 would allow effective use of such data as are available for the east coast 

 of South America where eight of the twelve available gages are less than 

 30 years in duration. As is evident from Fig. 2.6 which presents the mean 

 annual sea level variation of Pensacola, Florida, 30 years is not adequate 

 to obtain a stable estimate from an individual gage. 



2.4.2 Need for New Data 



There are two types of new data that would contribute to improved 

 estimates of ESLR: those that contribute immediately and those that would 

 require a data base of at least several years before meaningful results 

 could be obtained. It is anticipated that even with the potential benefits 

 of satellite altimetry, at least one decade and possibly two decades will 

 be required before adequate confidence will be placed in these data to 

 yield accepted reliable estimates of eustatic sea level rise. Three 

 research needs in the category of "new data" are described below. 



Compaction Gages - As is well -documented by a number of studies, 

 withdrawal of ground water and hydrocarbons can contribute to substantial 

 subsidence and thereby a "relative sea level rise" (see also section 2 for 

 a discussion of compaction effects) . It is worth noting that this is 

 probably the only component that realistically can be controlled by humans. 

 The obvious general but not universal correlation of areas of tide gage 

 locations and ground fluid extraction near population concentrations 

 justifies a possible concern over this activity. Also the fact that these 

 are the areas that continued RSL rise may contribute most to the ultimate 

 response cost (relocation, defense, repair, etc.) makes it important that 



