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Sea level is the base level for measuring elevation and depth on Earth.
Because the ocean is one continuous body of water, its surface tends to seek the same level throughout the world. However, winds, currents, river discharges, and variations in gravity and temperature prevent the sea surface from being truly level.
So that the surface of the ocean can be used as a base for measuring elevations, the concept of "local mean sea level" has been developed. In the United States and its territories, local mean sea level is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements.
The 19-year period is called a Metonic cycle. It enables scientists to account for the long-term variations in the moon's orbit. It also averages out the effects of local weather and oceanographic conditions.
Sea level is measured in relation to the adjacent land. Just like the ocean, the elevation of land may rise and fall over time. For example, the tremendous weight of a glacier on land pushes the land down, closer to sea level. That same land bounces back (a process called post-glacial rebound) if the ice retreats, or melts, and its weight is removed.
Local mean sea level measurements are a combination of sea level variations and movement of the land.
Changes in Sea Level
Sea level may vary with changes in climate. During past ice ages, sea level was much lower because the climate was colder and more water was frozen in glaciers and ice sheets. At the peak of the most recent ice age, about 18,000 years ago, sea level was perhaps 100 meters (300 feet) lower than it is today.
Global warming, the current period of climate change on Earth, is causing glaciers and ice sheets to melt. Melting ice sheets cause an elevation in sea level. This phenomenon is called sea level rise.
Sea level rise threatens low-lying areas around the world. Island nations, such as Maldives and Comoros, are particularly at risk. Coastal cities, such as New York City, New York, and Mumbai, India, must also prepare for higher sea levels.
Although these bungalows are perched just above sea level, Bora Bora, French Polynesia, actually has a mountain that rises 727 meters (2,385 feet) higher.
Photograph by Greg Goin, MyShot
Highs and Lows
Straddling the border between Israel and Jordan, the Dead Sea is the lowest place on the Earth's surface. Its elevation is 400 meters (1,312 feet) below sea level. However, if depth were measured from the ocean floor, the Challenger Deep in the Pacific Ocean would be the lowest place on Earth. It measures 11,034 meters (36,200 feet) below sea level.
Conversely, the top of Mt. Everest in the Himalaya Mountains is the point with the highest elevation on Earth, at 8,848 meters (29,028 feet) above sea level. However, if elevation were measured from the floor of the ocean, the peak of the volcano Mauna Kea, in the U.S. state of Hawaii, would be higher than Everest. Mauna Kea stands 10,203 meters (33,476 feet) high when measured from the ocean floor, but rises only 4,207 meters (13,803 feet) above sea level.
adjacent
Adjective
next to.
Plural Noun
(singular: bacterium) single-celled organisms found in every ecosystem on Earth.
buoyant
Adjective
capable of floating.
climate
Noun
all weather conditions for a given location over a period of time.
conversely
Adverb
in opposition.
Dead Sea
Noun
body of water on the border between Israel and Jordan; the lowest point within land (400 meters, or 1,312 feet, below sea level).
dense
Adjective
having parts or molecules that are packed closely together.
depth
Noun
measure of how deep something is.
discharge
Verb
to eject or get rid of.
Noun
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
gravity
Noun
physical force by which objects attract, or pull toward, each other.
ice age
Noun
long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.
local mean sea level
Noun
average height of the ocean's surface at a specific place, measured over a certain period of time.
measurement
Noun
process of determining length, width, mass (weight), volume, distance or some other quality or size.
Metonic cycle
Noun
19-year period after which the Moon's phases occur on the same days of the year as in the preceding cycle. Scientists measure sea levels using the Metonic cycle.
Moon
Noun
Earth's only natural satellite.
Mount Everest
Noun
highest spot on Earth, approximately 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya and straddles the border of Nepal and China.
orbit
Noun
path of one object around a more massive object.
phenomenon
Noun
an unusual act or occurrence.
post-glacial rebound
Noun
process in which land that was crushed by a glacier regains its shape.
saline
Adjective
salty.
Noun
base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.
Noun
increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.
tremendous
Adjective
very large or important.
variation
Noun
difference.
Noun
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Noun
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
Noun
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.
