Causes of changes in sea level? It's a slippery topic, because the ocean
basins have such irregular shapes, and because the changes are slow
compared to human lifetimes. But here are some of the causes.
 
When snow accumulates on polar or nearly polar land (currently Antarctica
and Greenland) without melting or evaporating, it compacts into ice and
forms glaciers. This can lower sea level by hundreds of meters (yards).
When the ice melts again, sea level rises. Ice age = lower sea level;
interglacial (warm) period = higher sea level.
 
Another cause that is less well-known is the redirecting of freshwater by
people. People are currently withdrawing water from the ground at a much
faster rate than water is soaking into it, and this groundwater eventually
finds its way to the ocean. The effect is large enough to have a global
effect on sea level, though it is not as great as the rise from other
causes.
 
In the geologic past, the changing shape of the ocean basins themselves has
caused sea level to rise and fall. Imagine a big, plastic bowl of seawater.
Now imagine pressing your hand into the bottom of the bowl from outside.
This would make the water rise in the bowl. The same thing happens when
volcanism becomes more active in the mid-oceanic ridges. The ridges become
hot and therefore they swell, displacing water upward. As Archimedes
realized ("Eureka!"), water is not compressible, so it has to move out of
the way if it is displaced. This has probably been a major factor of
sealevel change in the past.
 
Heated water also swells--not much, but the ocean is deep enough that the
amount of swelling can be significant if the whole ocean is warmed. This
too has happened in the geologic past.
 
All of the preceding causes apply globally. The situation is vastly
complicated by the fact that the solid surface of the earth is not as rigid
as people like to think. When a kilometer (or mile) of ice is piled on top
of a continent, then the continent bows down under the weight. Likewise if
water covers part of a continent. Or if sediment is piled up, for instance,
at the end of the Mississippi River. Contrariwise, if erosion removes the
top of a mountain, then the remainder of the mountain tends to bob up; it
had been held down by the weight of the mountaintop before. The same for
broad areas that are being eroded but aren't mountainous. Also, areas where
a mile of ice has been melted. Sweden is still bobbing up since the ice
melted, and as a result the seas have been retreating throughout the
historic period. It's a matter of record.
 
Excessive removal of fluids from wells (oil or water), without injecting
other fluid to replace it, can lower the ground level in some cases,
because the fluid pressure helps to support the loose grains of sediment.
Parts of San Jose, California, are now below sea level because of excessive
withdrawals of ground water. Part of Baytown, on Galveston Bay near
Houston, Texas, was similarly affected by withdrawal of oil. Bad examples
such as these led to laws requiring that water be injected to replace the
oil as it is removed from danger spots such as these.
 
I could continue, but it would be a book, and I think you've got the idea.
This is a complex topic, the evidence is usually subtle, and the last word
hasn't been said yet.
 
Andrew K. Rindsberg
Geological Survey of Alabama