The ocean rises and falls with the coming and going of the ice ages. Currently the polar caps are still melting, raising sea level against shores around the world. The rise appears to have speeded up this century to a rate at least one foot per century, and possibly as high as three feet per century.
Scientists suspect, but remain uncertain, that growing carbon dioxide and other gases in the atmosphere from human industry are speeding the polar meltdown through the ``greenhouse effect.'' Sea levels also rise because the water expands as the Earth's average temperature increases.
Rising ocean means different things on different coasts. Because of tides and currents, sea level varies from coastal region to region - it is higher in New England, for example, than in the Gulf of Mexico.
The shoreline does not provide a fixed target either.
Maine is still springing skyward in relief from the glacial weight of the last ice age. South Louisiana is sinking under the always-growing weight of the Mississippi Delta.
Further, a rising ocean against the rocky cliffs of northern California or upper New England changes the shoreline only slightly compared to the effect on the flat coastal plains of the Gulf and Atlantic seaboards. There, an inch of vertical rise can move the beach from 8 to 80 feet landward.