According to the model of Hansen et al. (1981) for slow energy growth (1.5% annual 

 growth in energy consumption) one would expect an increase in global temperature of 

 about l.5°C at the end of the next century. Using the thermal expansion model (Gornitz 

 et al. 1982) for sea water, the steric effect alone would cause a corresponding increase in 

 global sea level (eustotic) of about 30 cm. If the steric effect has been responsible for 

 half of the observed sea-level rise over the last century and this same ratio should 

 continue under a regime of further global warming, then total eustatic sea-level increase 

 for the next century would be 60 cm. Eustatic sea-level rise over the last century was 

 only about 12 cm. This predicted five-fold increase in the rate of eustatic sea-level rise 

 should be attributed both to the increased atmospheric CO2 and the fact that for the 

 next 40 years the earth will experience the warming phase of the natural (Camp Century) 

 temperature cycles (Figure 5). Because of cyclicity of the natural temperature 

 variations, sea level is likely to increase in a step-wise rather than linear fashion over 

 the next century. The next 40 years (1980-2020) will probably be the period of the most 

 rapid rate of sea-level rise. The eustatic rate of rise could conceivably be as high as 

 I cm/yr during that time. That rate corresponds to the most rapid post-glacial rise some 

 I 1 ,000 to 1 2,000 years ago. 



Without intending to be alarmist, another consequence of the predicted global 

 warming must be mentioned for the sake of completeness. This concerns the West 

 Antarctic ice sheet. This ice sheet is grounded below sea level making it vulnerable to 

 rapid disintegration and melting in case of a general warming (Hughes 1973; Mercer 

 1978). Since the present summer temperature in its vicinity is about -5°C a global 

 warming of 2.5°C might seem insignificant. All global atmospheric models stress, 

 however, that the magnitude of polar temperature fluctuations exceed those of the 

 global mean because of albedo-related positive feedback. A global warming will reduce 

 high-latitude snow cover, reduce the surface albedo, and thus heat that region more 

 rapidly than low-latitude zones (Manabe and Stouffer 1980). A 2°C global warming may 

 cause a temperature rise of about 5°C in Antarctica and thus induce melting of the West 

 Antarctic ice sheet. The response to that event would be an increase in global sea level 

 of between 5 and 6 m (Mercer 1978). This rise would not be uniform across the globe, 

 however, because of changes in the gravitational attraction exerted by the ice sheet on 

 the surrounding ocean, the Earth's immediate elastic response to the unloading, and the 

 long-term response due to viscous flow within the mantle (Clark and Lingle 1977). 

 Furthermore, the time scale of ice sheet disintegration is presently unknown. 



SEA-LEVEL CHANGES IN LOUISIANA 



Local relative sea-level rise includes eustatic and local components. Prediction of 

 future sea-level changes along the Louisiana coast, therefore, requires knowledge about 

 land subsidence. In view of a "eustatic" sea-level rise of 1.2 mm/yr, it is clear that most 

 of the local sea-level rise observed on the Louisiana coast is due to subsidence (Swanson 

 and Thurlow 1973). 



Figure 6 presents three tide gauge records from the central Louisiana coast as well 

 as a longer time series from Galveston, Texas, all of which document a history of rapid 

 local relative sea-level rise. The longer Galveston record documents well the temporal 

 changes in observed rates of sea-level rise. For example, if the entire Galveston record 

 is averaged one finds a rate of rise of 5.5 mm/yr. If one only considers the 20-yr time 

 span from 1950 to 1970, the rate then was 2.5 mm/yr. The rapid local change in sea- 

 level at Galveston between 1940 and 1945 (Figure 6) might be due to man's activities in 



170 



