The zero level varies in different countries due to different reference points and historic measurement periods.
Climate change and other forces can cause sea levels and elevations to vary over time.
Common techniques are:[8][9] Accurate measurement of historical mean sea levels is complex.
Land mass subsidence (as occurs naturally in some regions) can give the appearance of rising sea levels.
Conversely, markings on land masses that are uplifted (due to geological processes) can suggest a relative lowering of mean sea level.