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Measurements and Sources of Error
Recent measurements show a sea-level rise of about 1 to 2 millimeters per
year or 100 millimeters in the last century. Sources of error in these
measurements come from intra-annual variations due to changing
meteorological conditions, such as persistence of wind from a particular
direction or at a particular speed. A stronger than normal wind from the
west will cause an apparent sea level rise on the west coast of continents
and lower sea level on the east coasts. A similar artificial change in sea
level could arise from persistently anomalous ocean circulation. By use of
historical records and numerical models, the contribution from these
sources can be removed to more accurately reveal actual sea-level rise. We
need significant amounts of data to do this, however, and there may be a
historical geographical bias that favors North America, Northern Europe and
Japan. Southern Hemisphere records are much less complete. See the Permanent Service for Mean Sea Level
site of the Proudman Oceanographic
Laboratory for information on how sea level is measured.