Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
“Eustatic” change (as opposed to local change) results in an alteration to the ‘global’ sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Sea level changes on a range of temporal and spatial-scales. The total volume of the ocean can change as a result of changes in ocean mass (addition of water to the ocean from the land) or expansion/contraction of the ocean water as it warms/cools.
In addition, the sea level does not change uniformly as water is added or taken away. There can be large regions of ocean with decreasing sea level even when the overall Global Mean Sea Level (GMSL) is increasing. Obviously there must be regions of ocean with trends correspondingly greater than the mean to balance out the regions with trends less than the mean. There are many processes that drive these changes.