Background 



4. This section includes a brief summary of coastal and geological 

 processes that affect sea-level trends and the resulting effects on marine 

 coasts of the United States. References cited in the following text can be 

 found in the annotated bibliography section. In the literature, sea-level 

 variations are usually discussed in reference to one or more time scales, 

 decades, centuries, millennia, or over longer geological time scales. Deter- 

 mination of short-term sea-level trends are based on tide stations that mea- 

 sure relative sea-level changes. These instruments are primarily located at 

 the junction of river systems and ocean waters to determine local fluctuations 

 in water or land levels. To accurately measure the sea elevation, tide gages 

 are referenced to a defined datum (1929 National Geodetic Vertical Datum or 

 local mean sea surface) over a specific time period. Other types of measure- 

 ments such as oceanographic leveling and, more recently, satellite altimetry 

 are also employed to determine sea-level trends. 



Climatic Effects 



5. Global warming due to increases in COj is one of the most controver- 

 sial factors affecting sea-level change. Measuring and predicting the amount 

 of CO, gases that has been added to the atmosphere and its effects on sea 

 level is a complex problem due to measurement uncertainties and a general lack 

 of scientific knowledge needed to forecast climatic trends. A general rule of 

 thumb on global warming rates is that if the level of atmospheric CO2 doubles, 

 the Earth's global temperature increase is projected to be between 3° and 9° C 

 (Hoffman 1983) resulting in a rise of sea level between 70 and 330 cm. In 

 comparison, the rise of sea level was about 15 cm over the last century. 

 Several researchers (Barnett 1983, Revelle 1982, and Gornitz et al. 1982) 

 agree that steric or thermal expansion of the upper layers of ocean waters is 

 not the major contributing factor in sea-level rise. Out of a total projected 

 rate of 70 cm per century for sea-level rise, 30 cm is attributed to warming 

 and expansion of the upper ocean waters while the remaining 40 cm is attribut- 

 ed to glacial melting (National Research Council 1983). 



6. Variations in the Southern Oscillation and El Nino are also impor- 

 tant climatic factors that contribute to cyclic sea-level variation. Southern 



