What is mean sea level?
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 com-
paring measured heights of the global
Mean Sea Surface (MSS) above a level ref-
erence surface called a geoid—a mathe-
matical 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 differ-
ences 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).
How do scientists use mean sea levelin connection with global climate change?
Many scientists are interested in the long-term mean sea level change, especially in
connection with global climate change. By taking such long-term measurements,
these scientists are hoping to confirm the predictions of several climate models,
including the idea that global warming is a result of the “greenhouse” gases from
either human or natural sources.
There are two major ways to determine such sea level variations. The first esti-
mates sea level changes using tide gauge measurements, mathematically averaging
the numbers. Graphs of the most recent estimates using this method show a 0.669 to
0.960 inch (1.7 to 2.44 millimeter) rise in sea level per year. The second method uses
global positioning system (GPS) devices and satellite altimeter measurements, both of 303
MATH IN THE NATURAL SCIENCES
A forester uses a global positioning system (GPS) to
accurately pinpoint his location within an undevel-
oped landscape. Using satellite technology, a GPS
can not only determine latitude and longitude, but
also heights above sea level. Stone/Getty Images.