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 fresh water 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 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 (0.6 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 (0.7 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 two kilometers (1.2 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 Earth’s land was covered by glaciers. Today, about one-tenth of Earth’s land is covered by glacial ice.

The Laurentide ice sheet was almost three kilometers (two 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 Laurentide 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 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 east 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 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 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 fresh water 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.