In a perfect, non-moving, homogeneous sphere, the elevation of the Earth's liquid shell would be distributed equally about the center of gravity, and sea levels would be the same everywhere. However, the Earth is a heterogeneous, oblate spheroid that rotates on an axis and experiences gravitational influences from other planets and the sun. These factors, together with geographic variations of continents and submerged terrains, climate systems, water volume, tectonics, etc., the surface of the ocean, and hence sea level, change on various time scales, ranging from minutes to millennia. Therefore, it is a challenge to determine the exact sea level of the Earth, but it is done.
As a result of these complications when referring to sea level, geoscientists have to be a little bit more specific when they discuss "sea level." Hence, there are a number of different definitions for "sea level" that need to be understood.
- Global Sea Level - the average height of the Earth's oceans combined (relative to the Earth's center). Influenced primarily by the factors that influence the volume of seawater, and size of ocean basins, etc. Often referred to as "Eustatic Sea Level"
- Local (or regional) Sea Level - the height of seawater relative to a fixed point on land that is used as a continuous reference. Influenced by meteorological factors, tidal range, ocean currents, rates of subsidence/uplift. Also referred to a "Relative Sea Level"
- Mean Sea Level (MSL) - the average height of seawater relative to a fixed datum established by a statistical average of water heights over a period of time. This is the most functional definition for sea level because it helps establish the elevation of all points on Earth (topographic elevation, and bathymetric elevation). In the U.S., MSL is often reported relative to the 1983-2001 NTDE (National Tidal Datum Epoch). Tidal datums need to be updated every couple of decades because sea level is not stable, and a new datum is likely to be announced.