Sea level

Sea level is the height of the ocean’s surface, often averaged over time and used as a reference for measuring elevation and depth. It varies with tides, weather, currents, and longer-term changes in ocean volume and shape.

Tides: Periodic changes in sea-surface height caused mainly by the Moon’s and Sun’s gravitational forces. Tides produce short-term sea-level changes that averaging can smooth.
Mean sea level: The average sea-surface height over a specified period at a location or across a region. This averaging convention supplies many practical sea-level references.
Coastal flooding: The inundation of normally dry coastal land by seawater. Higher baseline water levels make some coastal floods reach farther inland.
Absolute sea-level change: A change in sea-surface height measured relative to Earth’s center or a geocentric reference frame. It differs from relative change, which also includes vertical land movement.
Thermal expansion: An increase in a substance’s volume as its temperature rises. Warming seawater expands and raises the ocean surface without adding water.
Vertical datum: A defined reference surface or system used to measure heights and depths. Elevation depends on the datum chosen to represent sea level.
Coastal erosion: The loss of coastal land through waves, currents, storms, or other processes. Rising water can shift erosion pressure onto beaches and shorelines.
Sea-surface height: The instantaneous or measured elevation of the ocean surface relative to a specified reference. A single surface-height measurement is not the same as a long-term sea-level average.
Glacial isostatic adjustment: The slow movement of Earth’s crust and mantle responding to past changes in ice-sheet loading. Postglacial land motion changes measured sea level relative to shorelines.
Geoid: An Earth-gravity equipotential surface that approximates global mean sea level beneath the continents. Geodesy uses the geoid to extend a sea-level-like reference across land.

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