• ice sheet
    Antarctica's ice sheet flows around mountains.

    Photograph by Jack Fletcher

    Martian Ice Caps
    Ice sheets are sometimes called polar ice caps. On Mars, the polar ice caps are called the Planum Australe (southern) and the Planum Boreum (northern). The Martian ice caps are made of water and carbon dioxide about 3 kilometers (1.9 miles) thick.

    Greenland's Warming Period
    In the 1980s, scientists drilled ice cores from the Greenland ice sheet and learned about a short warming period on the continent, called the Medieval Warming.

    About 1,000 years ago, Vikings settled on an island that had green, coastal meadows. They named it Greenland. The Vikings cattle had plenty of green grass to eat and the colony thrived there for 300 years. There were eventually 3,000 people in the Viking settlement.

    Then, Greenland suddenly got colder, during a period called the Little Ice Age. Ice blocked the Vikings ships from sailing. The summers got shorter, producing less vegetation for the dairy cattle during the long, cold winters. Eventually, Vikings left their colony.

    An ice sheet is a mass of glacial ice more than 50,000 square kilometers (19,000 square miles). Ice sheets contain about 99% of the freshwater on Earth, and are sometimes called continental glaciers. As ice sheets extend to the coast and over the ocean, they become ice shelves.

    A mass of glacial ice covering less area than an ice sheet is called an ice cap. A series of connected ice caps is called an ice field. Making up ice fields, ice caps, and eventually ice sheets are individual glaciers.

    Today, there are only two ice sheets in the world: the Antarctic ice sheet and the Greenland ice sheet. During the last glacial period, however, much of the Earth was covered by ice sheets.

    How Ice Sheets Form

    Ice sheets formed like other glaciers. Snow accumulates year after year, then melts. The slightly melted snow gets harder and compresses. It slowly changes texture from fluffy powder to a block of hard, round ice pellets. New snow falls and buries the grainy snow. The hard snow underneath gets even denser. It is known as firn.

    As years go by, layers of firn build on top of each other. When the ice grows thick enough—about 50 meters (165 feet)—the firn grains fuse into a huge mass of solid ice. At this point, the glacier begins to move under its own weight.

    Ice sheets tend to be slightly dome-shaped and spread out from their center. They behave plastically, or like a liquid. An ice sheet flows, oozes, and slides over uneven surfaces until it covers everything in its path, including entire valleys, mountains, and plains.

    Compression and geothermal energy sometimes cause the bottom of an ice sheet to be slightly warmer than the ice above it. The bottom of the ice sheet melts, causing the ice above it to move at a faster rate than the rest of the ice sheet. These fast-moving glaciers are called ice streams.

    Ice streams can move as quickly as 1,000 meters (.6 mile) every year. The slightly warmer, softer ice of the ice stream is where most of the ice sheet's crevasses are located.

    The largest glacier in the world is an ice stream, the Lambert Glacier in Antarctica. The Lambert Glacier moves as quickly as 1,200 meters (.7 mile) every year. It is more than 400 kilometers (249 miles) long and 2,500 meters (1.5 miles) thick.

    Continental Glaciers

    The Antarctic ice sheet is the largest block of ice on Earth. It covers more than 14 million square kilometers (5.4 million square miles) and contains about 30 million cubic kilometers (7.2 million cubic miles) of water.

    The Antarctic ice sheet is about 2 kilometers (2.6 miles) thick. If it melted, sea level would rise by about 60 meters (200 feet).

    The Greenland ice sheet is much smaller than the Antarctic Ice sheet, only about 1.7 million square kilometers (656,000 square miles). It is still the second-largest body of ice on the planet.

    The Greenland ice sheet interacts much more dynamically with the ocean than the Antarctic ice sheet. The annual snow accumulation rate is more than double that of Antarctica. Glacial melt happens across about half of the Greenland ice sheet, whereas it is much more isolated on the far western part of Antarctica. Greenland's ice shelves break up much faster than those surrounding Antarctica.

    Both the Antarctic and Greenland ice sheets have caused the land under them to sink. Eastern Antarctica is about 2.5 kilometers (1.6 miles) below sea level because of the colossal weight of the ice sheet above it.

    If the continental glaciers were to suddenly disappear, the landscape of Antarctica and Greenland would change drastically. Antarctica would shrink by more than a quarter as the western part shrunk into the sea. Greenland, the largest island in the world, would become an archipelago, a chain of small islands only connected by waterways.

    Ice Sheets in History

    Antarctica has been covered by an ice sheet for 40 million years. While the ice sheet has advanced and retreated with climate change, it has been a constant feature of the landscape the entire time.

    Huge ice sheets covered much of North America, Eurasia, and South America during the Pleistocene era. This was the last glacial period, or ice age. Ice sheets reached their greatest size about 18,000 years ago. During the Pleistocene Ice Age, nearly one-third of the Earth’s land was covered by glaciers. Today, about one-tenth of the Earth’s land is covered by glacial ice.

    The Laurentide Ice Sheet was almost 3 kilometers (2 miles) thick and covered North America from the Canadian Arctic all the way to the modern U.S. state of Missouri. Glacial retreat of the Laurentide Ice Sheet created such features as the Great Lakes. The glaciers on Baffin Island, Canada, are remnants of the Lauentide Ice Sheet.

    The Scandinavian Ice Sheet dominated Western Europe. It once spread as far west as the island of Great Britain and as far east as Moscow, Russia. It originated in the Jostedalsbreen area of southern Norway, where it eventually retreated. Jostedalsbreen remains the largest glacier in Europe today.

    The Barents-Kara Ice Sheet extended across Northern Asia. As it retreated to the Barents Sea, it created shallow lakes that still dot northern Russia.

    The Patagonian Ice Sheet spread out from the Andes Mountains to cover much of southern South America. Although most of what is today Chile was covered in ice, the Patagonian Ice Sheet did not spread as far west as the Argentine coast, or connect with the ice caps of the Altiplano. The dry climate of central Argentina and the arid Atacama Desert restricted the reach of the ice sheet. Today, the southern South American region of Patagonia is still marked by glaciers.

    Paleoclimatology

    Ice sheets, especially the enormous Antarctic ice sheet, are an important area for scientific research, including glaciology (the study of glaciers), meteorology (the study of weather patterns), and paleoclimatology.

    Paleoclimatology is the study of the Earth’s atmosphere in prehistoric times. Paleoclimatology relies on chemical analysis of ice. In particular, scientists study bubbles, or pockets of air, in glaciers and ice sheets.

    Paleoclimatologists do much of their research directly on ice sheets. They drill and extract long tubes of ice, called ice cores, from ice sheets, usually in the Antarctic. Ice cores are layered with successive deposits of snowfall and firn. By studying the chemicals present in each layer of the ice core, scientists can determine the climate during each time period. The deepest layers of the ice core can provide information on what the weather was like thousands of years ago.

    Ice cores can measure the state of the atmosphere as far back as 80,000 years. Wide bands of ice indicate a heavy snowfall. Darkly colored bands indicate smoke or other chemicals in the atmosphere. These chemicals may have been released by the eruption of a volcano, the impact of a meteor, or pollution.

    Melting Ice Sheets

    Scientists from around the world are concerned that the Earth’s temperature is increasing very rapidly, causing glaciers, ice caps, and ice sheets to melt. Glaciers melt when ice melts more quickly than firn can accumulate. Because they are so large, melting ice sheets can affect the climate of ecosystems in the entire world.

    Melting ice sheets contribute to rising sea levels. As ice sheets in Antarctica and Greenland melt, they raise the level of the ocean. Coastal habitats are put at risk of being flooded. In March 2008, a 400-square kilometer (160-square-mile) piece of the Wilkins Ice Shelf broke off of the Antarctic Peninsula. Large icebergs created by such an event create hazards for shipping.

    Scientists are particularly concerned about the effects that a melting ice sheet could have on some coastal U.S. cities, such as New York City, New York; Washington, D.C.; San Francisco, California; or New Orleans, Louisiana. These cities could become underwater cities if ice sheets melt enough to raise the sea level significantly.

    Melting ice sheets also reduce the ocean’s salinity, or salt content. Tons of freshwater are added to the ocean every day by melting ice sheets. Large additions of freshwater change the ocean ecosystems. Organisms, such as many types of corals, depend on saltwater for survival. Some corals may not be able to adjust to a more freshwater habitat.

    Thermohaline circulation, the so-called "ocean conveyor belt," would be radically altered by melting ice sheets. The ocean conveyor belt circulates nutrient-rich water from polar regions throughout the world's oceans in a long, slow, continual loop. Circulation relies on the relationship between water with different densities. Cold, saline water from polar regions gradually rises to the surface in the tropics.

    Melting ice sheets would increase the amount of warm and freshwater in polar marine ecosystems. This would slow "deep water formation," the development of cold, saline, nutrient-rich water on which entire marine ecosystems depend.

    According to the United Nations, sea ice may decrease by 25% within the next century. This may lead to a reduction in krill, the basis of the Antarctic marine food web. Animals from crustaceans to penguins will face much greater competition for fewer food resources.

  • Term Part of Speech Definition Encyclopedic Entry
    accumulate Verb

    to gather or collect.

    Altiplano Noun

    high plateau in the Andes Mountains of South America.

    archipelago Noun

    a group of closely scattered islands in a large body of water.

    Encyclopedic Entry: archipelago
    arid Adjective

    dry.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    chemical analysis Noun

    isolation of atoms and molecules of a substance.

    climate Noun

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

    Encyclopedic Entry: climate
    compress Verb

    to press together in a small space.

    continental glacier Noun

    ice sheet that covers an enormous area.

    crevasse Noun

    deep crack in a glacier.

    Encyclopedic Entry: crevasse
    dense Adjective

    having parts or molecules that are packed closely together.

    dome Noun

    shape that is half of a sphere.

    Encyclopedic Entry: dome
    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    firn Noun

    grainy ice that forms glaciers. Also called neve.

    flood Verb

    to overflow or cover in water or another liquid.

    food web Noun

    all related food chains in an ecosystem. Also called a food cycle.

    Encyclopedic Entry: food web
    freshwater Noun

    water that is not salty.

    fuse Verb

    to combine or meld together.

    geothermal energy Noun

    heat energy generated within the Earth.

    glacial period Noun

    time of long-term lowering of temperatures on Earth. Also known as an ice age.

    glacial retreat Noun

    process by which glaciers melt faster than precipitation can replace the ice.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    glaciology Noun

    study of glaciers and ice sheets.

    Great Lakes Noun

    largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.

    iceberg Noun

    large chunks of ice that break off from glaciers and float in the ocean.

    Encyclopedic Entry: iceberg
    ice cap Noun

    area of fewer than 50,000 square kilometers (19,000 square miles) covered by ice.

    Encyclopedic Entry: ice cap
    ice core Noun

    sample of ice taken to demonstrate changes in climate over many years.

    ice field Noun

    area of fewer than 50,000 square kilometers (19,000 square miles) covered by interconnected ice caps.

    ice pellet Noun

    rain that freezes as it falls to Earth. Also called sleet.

    ice sheet Noun

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

    Encyclopedic Entry: ice sheet
    ice shelf Noun

    mass of ice that floats on the ocean but remains attached to the coast.

    ice stream Noun

    part of an ice sheet that moves faster than the ice around it.

    krill Noun

    small marine crustacean, similar to shrimp.

    landscape Noun

    the geographic features of a region.

    Encyclopedic Entry: landscape
    meteor Noun

    rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.

    Encyclopedic Entry: meteor
    meteorology Noun

    study of weather and atmosphere.

    Encyclopedic Entry: meteorology
    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    paleoclimatology Noun

    study of the atmosphere of prehistoric Earth.

    Patagonia Noun

    large plateau in southern South America, stretching from the Andes Mountains to the Atlantic Ocean.

    plastically Adverb

    in a moldable, alterable manner.

    Pleistocene Noun

    epoch lasting from about 2 million years ago to 12,000 years ago.

    pollution Noun

    introduction of harmful materials into the environment.

    Encyclopedic Entry: pollution
    salinity Noun

    saltiness.

    sea level Noun

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

    Encyclopedic Entry: sea level
    shipping Noun

    transportation of goods, usually by large boat.

    temperature Noun

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

    Encyclopedic Entry: temperature
    thermohaline circulation Noun

    ocean conveyor belt system in which water moves between the cold depths and warm surface in oceans throughout the world.

    tropics Noun

    region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

    Encyclopedic Entry: tropics
    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
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