A predicted rise in sea level of one foot within the next 30 to 40 years will drive much of the Atlantic and Gulf shoreline inward by 100 feet and some of it by more than 1,000 feet, according to marine geologists. The environmental and economic consequences will be felt much farther inland.

For the United States, those studying the threat describe it as more like an expensive irritant than an impending disaster. But in densely populated lowlands of some poor nations, they say, the effects could be catastrophic. Hardest hit may be Bangladesh, where the combined effects of rising seas and subsiding land over the coming century could obliterate as much as 10 percent of the crowded country, inundating vast tracts of fertile land and extending the deadly sweep of tidal waves.

Most of the world's shorelines are already receding, according to geologists who estimate that the seas worldwide have climbed an average of about 4 inches over the last century. This is attributed to the slow melting of midlatitude mountain glaciers, still retreating since the last ice age. But the total change in sea level around the United States during the last hundred years has been greater, about one foot, because much of the country is gradually sinking even as the ocean rises.