Mean sea level

Mean sea level is the average height of the sea surface over a specified period and location. It provides a reference surface for measuring elevation and changes in ocean height.

Vertical datum: A reference surface used to define elevations or depths. Mean sea level can serve as the reference surface for elevations, but its definition depends on the chosen datum.
Tide: The periodic rise and fall of sea level caused mainly by the gravitational forces of the Moon and Sun. Averaging over tidal cycles suppresses short-period rises and falls in sea level.
Sea-level rise: A long-term increase in the average height of the sea surface. Changes in mean sea level are the basis for measuring persistent ocean rise.
Global mean sea level: The average sea-surface height over the global ocean, usually estimated over a specified period. It aggregates regional conditions, unlike a mean tied to one location.
Tide gauge: An instrument that records changes in water level relative to a fixed point on land. Long tide-gauge records provide local observations used to estimate mean sea level.
Ocean circulation: The large-scale movement of seawater driven by winds, density differences, and Earth's rotation. Circulation redistributes ocean water and heat, producing regional differences in mean sea level.
Coastal flooding: The inundation of normally dry coastal land by seawater. Higher mean sea level raises the baseline on which tides and storm surges act.
Local mean sea level: The average sea-surface height measured at a particular location over a specified period. Local averages can differ from the global mean because of currents, gravity, and land motion.
Satellite altimetry: A method that measures surface height from the travel time of radar or laser pulses sent from satellites. Satellite altimetry estimates sea-surface height across broad ocean areas.
Steric sea-level change: A change in sea level caused by changes in seawater density, especially from warming or freshening. Thermal expansion and density shifts alter ocean height without adding water mass.

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