TABLE 4-8 



CHANGES IN WETLAND AREAS BETWEEN 1975 AND 2100 a/ 



(all areas in W hectares) 



aJ The projections are not interpretable as statistically valid estimates of regional tends. 



bl The number of cells in particular regions were not based on underlying population. Thus, the 

 percent reduction of sample does not necessarily reflect reductions in U.S. wetlands. 



When accretion rate is held constant and a subsidence rate of 2.9 mm/yr is assumed, condi- 

 tions are least favorable for maintenance of marshland (Table 4-7). Under the low scenario, total 

 wetland area is reduced to 22.7 percent by the year 2100, one-third less than without subsidence. 

 Inland marsh would have disappeared by 2100, but its area is unchanged from assumptions of 

 constant accretion without subsidence through the year 2075. Marsh areas react somewhat simi- 

 larly under both scenarios, but with subsidence, areas peak by 2075 instead of continuing to 

 expand, and then decline suddenly to the final level as inundation accelerates. Thus the 

 cumulative effect of subsidence becomes most apparent only late in the scenario period. 



The importance of accretion rate was examined in the Albemarle Sound East simulations by 

 comparing varying accretion rates with a constant accretion rate of 5 mm/yr (Figure 4-20). The 

 high accretion rate allows marshes to be maintained through the year 2050 rather than the year 

 2000 under a lower accretion rate. By 2050, despite the lower accretion rate, salt marsh initially 

 expands for the next 25 or 30 years. Later, rising waters rapidly inundate the salt marshes, 

 eliminating them completely by the year 2095. In contrast, the 10 mm/yr accretion rate allows 

 greater persistence of marshes through the year 2085. 



Shortly thereafter, however, the exponentially increasing rise in sea level drowns over 90 

 percent of the marshes, leaving a situation only marginally improved over conditions prevailing 

 under assumptions of a lower accretion rate. Although the importance of accretion in maintain- 

 ing marsh elevation against rising seas is seen, an accelerating rise in sea level allows accretion 

 rate to provide only a temporary means for maintaining coastal marshlands. 



121 



