• 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.

    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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    adjacent Adjective

    next to.

    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    border Noun

    natural or artificial line separating two pieces of land.

    Encyclopedic Entry: border
    buoyant Adjective

    capable of floating.

    climate Noun

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

    Encyclopedic Entry: climate
    climate change Noun

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

    Encyclopedic Entry: climate change
    coast Noun

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

    Encyclopedic Entry: coast
    conversely Adverb

    in opposition.

    current Noun

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

    Encyclopedic Entry: current
    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.

    Earth Noun

    our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

    Encyclopedic Entry: Earth
    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    global warming Noun

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

    Encyclopedic Entry: global warming
    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.

    ice sheet Noun

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

    Encyclopedic Entry: ice sheet
    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    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, 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya range, in Nepal and China.

    nation Noun

    political unit made of people who share a common territory.

    Encyclopedic Entry: nation
    ocean Noun

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

    Encyclopedic Entry: ocean
    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.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    saline Adjective

    salty.

    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    sea level rise Noun

    increase in the average reach of the ocean. The current sea level rise is 1.8 millimeters (.07 inch) per year.

    temperature Noun

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

    Encyclopedic Entry: temperature
    tremendous Adjective

    very large or important.

    variation Noun

    difference.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano
    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    wind 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.

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