As Ross noted, the combination of melting glaciers and thermal expansion of the oceans is currently producing increasing sea level, as a global trend.  We are still in a glacial period, though a relatively milder part of it.  Melting of all the glaciers would produce extensive flooding of lowlands.  However, the last relatively mild interval had sea level only a little higher than today-enough to flood coastal real estate and the lowest islands, but not extensive flooding of the coastal plain.

Local factors may outweigh the global trend.  Again, several of these have been noted.  Perhaps the most famous examples of tectonically affected local change are mollusk-related.  In southern Italy, tectonic activity leads to areas rising and sinking faster than average.  Pillars of a Roman temple, originally built on land, can now be seen (or at least could be seen recently enough to get into science texts-I do not know what local development is like) with Lithophaga burrows well up the sides.  It was submerged to that depth and then emerged again during the past couple of millenia.  On the west coast of South America, large earthquakes often involve pushing up the land.  Darwin saw a newly exposed section of cliff just after an earthquake, with dying mussels, limpets, etc.  Looking further up the cliff, he saw a series of ledges, each with its pile of shells.

The glacial rebound in high latitudes can also provide good access to deeper water shells.  Glacial gravels often have Pleistocene fossil shells in them, stranded as the land rose when the weight of the glacier decreased.

Past sea level changes account for many of the fossil deposits that now provide us with all sorts of shells.  Higher sea level will provide new habitat for marine mollusks, while somewhat limiting land and freshwater species and making fosisl collection more challenging.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!