Recent studies have shown that the sea levels are rapidly increasing, probably at the fastest rate in 28 centuries, and its accompanying tidal flooding, increasingly frequent, is already causing headaches in low-lying places, especially in the South. Though sea levels have risen and fallen significantly in the past, scientists say they have been fairly constant for the last several thousand years.

Mr. Hauer and his co-authors also found a highly regional effect of sea-level rise. Of the projected population at risk, nearly 50 percent will be in Florida, and an additional 20 percent in other parts of the Southeastern United States. In 30 counties, more than 100,000 people would be at risk if the sea level rose by about six feet.

None of the 22 coastal states in the continental United States, as well as Washington, D.C., will be immune from the effects of sea-level rise, the authors predict. If the seas were to rise by about six feet by 2100, more than one million people in California, and almost as many in New York and New Jersey, would be affected, the study shows.