Mean sea level (MSL) is the average water level (height of the sea) for all stages of a tide. Locally, MSL is measured by tidal gages at one or more points over a given period of time. The resulting numbers average out wind waves and other periodic changes in sea level. The overall values of a MSL are measured with respect to level marks on land called benchmarks. Thus, scientists know a true change in MSL is either from a change in sea level from, for example, possible global warming effects, or changes in the gage’s height on land, such as in the case of local uplift.

There is also a more mathematically intensive way to determine the MSL. To a geodesist (a person who studies the shape of the Earth), MSL is determined by comparing measured heights of the global Mean Sea Surface (MSS) above a level reference surface called a geoid—a mathematical model of an ellipsoid shape that approximates Earth’s mean sea level. This comparison is done because the Earth does not have a geometrically perfect shape (for example, the Atlantic Ocean north of the Gulf Stream is about 3.3 feet (1 meter) lower than it is farther south). The MSS is not a “level” surface, thanks to such factors as currents created by wind, as well as atmospheric cooling and heating that cause differences in sea levels around the world. But interestingly enough, it never differs from the global geoid by more than about 6.56 feet (2 meters).