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.

sea level
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.

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.

Noun

natural or artificial line separating two pieces of land.

buoyant
Adjective

capable of floating.

climate
Noun

all weather conditions for a given location over a period of time.

Noun

gradual changes in all the interconnected weather elements on our planet.

Noun

edge of land along the sea or other large body of water.

conversely
Adverb

in opposition.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

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.

Noun

height above or below sea level.

Noun

mass of ice that moves slowly over land.

Noun

increase in the average temperature of the Earth's air and oceans.

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.

Noun

thick layer of glacial ice that covers a large area of land.

Noun

body of land surrounded by water.

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.

Noun

political unit made of people who share a common territory.

Noun

large body of salt water that covers most of the Earth.

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.

Noun

large stream of flowing fresh water.

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.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

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.