226 



potential sea level rise of -0.75 mm/yr (Newman and Fairbridge , 1986). 

 Ground-water mining, which increases runoff to the oceans and thereby tends 

 to raise sea level, could counteract a large fraction of this effect 

 (Korzun, 1978). 



Subsurface fluid withdrawal (oil, gas, water) can locally exacerbate 

 the eustatic sea level rise. For example, subsidence rates of around 4.2 

 mm/yr have been measured between 1906 and 1978, at the Galveston Island 

 tide gage (Gabrysch, 1984). Here, compaction due to groundwater removal 

 could account for as much as 2/3 of the relative sea level rise. On the 

 other hand, at Savannah, Georgia, where land subsidence of up to 4 mm/yr 

 has been reported inland between 1955 and 1975, benchmarks near the tide 

 gage at Fort Pulaski (on the shore) show no subsidence during this period 

 (Davis et al . , 1977). These subsidence effects are thus highly variable 

 and site-specific. This may necessitate the establishment of the local 

 subsidence history for each tide station. Independent means of estimating 

 land subsidence will be reviewed in the discussion of Sect. 3. 



Atmospheric and oceanographic effects (winds , waves , currents , ocean 

 temperatures, and salinity) 



Atmospheric and oceanographic processes generate considerable 

 interannual sea level variability. However, monthly and annual averages of 

 sea level records greatly reduce the <l-year sources of variability (due to 

 tides, storms and seasonality). Longer fluctuations, up to a decade in 

 duration and coherent over long distances , are produced by steric changes 

 (temperature or salinity), ocean currents, and the El Nino-Southern 

 Oscillation. 



Multivariate regression can be used to minimize large-scale, low- 

 frequency atmospheric forcing (winds, currents). Large-scale spatially 

 coherent signals (such as those associated with the El Nino phenomenon) can 

 be identified and quantified using eigenanalysis (Ghelton and Enfield, 

 1986). 



SECT. 3. COMPACTION EFFECTS 



As noted above , compaction due to ground fluid withdrawal can affect 

 relative sea level curves. This impact should be greater for cities 



