Icebergs and Antarctica

About 98 percent of the surface of Antarctica—the coldest, driest, and southernmost continent on Earth—is covered in a thick, dense layer of ice. Some of this ice moves to the Southern Ocean surrounding the continent and breaks into icebergs. Antarctica is of profound interest to scientists because of its impact on the world’s oceans and climate and as a rich source of data about Earth’s history.

Antarctica, Ice Sheets, and Ice Cores

Antarctica is the southernmost of Earth’s seven continents. Surrounded by the Southern Ocean, Antarctica is situated largely within the Antarctic Circle, the parallel of latitude that runs 66.5622 degrees south of the equator.

Antarctica is the only continent with no permanent human residents. In 1959, an international treaty was signed to establish Antarctica as a scientific preserve to be used for peaceful purposes only. The treaty does not, however, recognize or dispute claims on any part of Antarctica as a national territory; it also does not allow the making of new territorial claims. About twenty-nine countries operate seventy scientific research stations on Antarctica, including the United States, Australia, India, France, Germany, and Japan.

Antarctica spans about 14 million square kilometers (about 5.4 million square miles) of land area, slightly less than 1.5 times the area of the United States. Antarctica is the continent with the lowest surface temperatures, the greatest amount of wind, and the least precipitation. Much of Antarctica is considered a desert because the continent receives, on average, only about 5 centimeters, or 2 inches, of precipitation per year. Whether the changing climate is altering the amount and type of precipitation Antarctica receives remains a subject of debate and, thus, ongoing research.

Almost the entire surface of Antarctica, about 97.6 percent, is covered in a thick and heavy layer of ice, which measures up to about 4.8 kilometers (3 miles) in depth in some places. This ice, which occupies about 29 million cubic kilometers of total volume (about 7 million cubic miles), represents approximately 90 percent of the world’s total ice and is known as the Antarctic polar ice sheet. (A second, smaller ice sheet covers most of the island of Greenland in the Arctic Circle surrounding the North Pole.) The Antarctic ice sheet contains enough frozen water that if it were to melt completely, it would cause a global rise in sea level of about 60 meters (200 feet).

Specific weather conditions are required for an ice sheet to form. Winter snowfall should not melt completely when summer temperatures rise. If snow persists throughout the year for thousands of years, each season’s snowfall forms a new layer on the previous season’s layer; these layers eventually freeze into ice. As new snow falls, its weight also compresses the existing ice, making the ice sheet even more dense and thick. Alternatively, katabatic winds can drive cold, dry inland air seaward and sublimate falling snow or transition it from a solid to a gas in mid-air. When this happens, lower amounts of snowfall reach the ground to form new layers of ice.

Because the ice sheet comprises thousands of layers of snow, it presents a detailed historical record of the environment and climate at the time of each snow deposit. Scientists can access that record by drilling into the sheet to collect samples known as ice cores. Ice cores have been used to determine, among other things, the temperature at which each snowfall occurred. The oxygen isotope ratio is used as a stand-in for air temperature. Other measurements that can be taken include dust levels, greenhouse gas levels (such as carbon dioxide and methane), hydrogen peroxide, and sulfate.

By analyzing these data, scientists have mapped a picture of climate change throughout Earth’s history, pinpointing when historic ice ages and interglacial, or warmer, periods occurred. Ice core data also have confirmed that the concentration of greenhouse gases in Earth’s atmosphere has increased significantly since the late eighteenth century, the beginning decades of the Industrial Revolution.

Ice Shelves, Ice Calving, and Icebergs

Because of the force gravity exerts on its own weight, the massive Antarctic ice sheet is in a state of extremely slow but constant movement. Glaciers and ice streams, both flowing masses of ice, ooze from the inland portions of the ice sheet into the Southern Ocean.

In the Southern Ocean, ice breaks off, forming many floating, frozen masses that hug the coastline. These masses are known as ice shelves. Ice shelves are dynamic, not static, in nature. They can gain more ice from the ice sheet but also lose ice to melting or calving.

Ice calving is a form of ablation, a term that describes several processes through which some icy material is removed from a larger whole. In calving, a large block of ice suddenly splits from the margin of another mass of ice, usually a glacier or an ice shelf, and breaks away. Calving occurs when fractures in the ice grow and spread toward the margin, causing the ice to become thinner and more brittle. Various factors contribute to the likelihood of a calving event, including the extent to which an ice shelf has melted below the water line, the speed at which glaciers and ice streams flow, and the temperature of the ice itself.

Calving occurs mainly where ice meets or stands in water. The resulting calved blocks become known as icebergs, important in the Antarctic ecosystem. Many seabirds live on their surfaces and krill, fish, and phytoplankton form communities below the icebergs. (Ships traveling in the Southern Ocean and the North Atlantic are at risk of being nipped, or struck, by floating icebergs.)

One study has found that when icebergs melt far from the coastline, they introduce trapped nutrients, including iron and krill, to the water. This process speeds the growth of algae and other photosynthesizing phytoplankton, which take up carbon dioxide from the atmosphere. The effects of global warming have been causing ice shelves to disintegrate and to calve icebergs more frequently, and scientists predict that this trend will continue. If icebergs do contribute to the removal of carbon dioxide, they may add an important additional feedback mechanism to the complex, dynamic global climate system.

Sea Ice and the Climate

The seawater surrounding Antarctica freezes at a slightly lower temperature than the freshwater that makes up its ice sheet, glaciers, ice shelves, and icebergs. Typically, seawater freezes at –1.9 degrees Celsius (28.58 degrees Fahrenheit); normally, water freezes at 0 degrees C (32 degrees F).

The ice crystals that form when seawater freezes do not contain salt, so they are lighter and less dense than the unfrozen water. This ice floats on the surface, acting as a barrier that prevents heat and gases from traveling between the atmosphere and the ocean. Even when sea ice breaks into large pieces, known as pack ice, the ice tends to be driven together into a nearly continuous mass that provides excellent cover. This prevents the atmosphere above the Southern Ocean from cooling drastically, making sea ice a critical control factor in stabilizing the global climate.

Sea ice is important for another reason. It has a bright, light surface that reflects about 80 percent of the sunlight that strikes it into space. This process is known as the albedo effect. (A surface’s albedo measures how reflective it is.) The ocean's surface is much darker, and it has an albedo that is closer to about 10 percent. As a result, when sea ice melts, a dramatically greater amount of sunlight and radiation is absorbed by the ocean, which heats up, creating a cycle that leads to even more melting.

Decades of scientific data have shown that sea ice in the Arctic north has become thinner and has shrunk more each summer since about 1980 as temperatures there slowly warm. Antarctic sea ice is reacting somewhat differently. Around the Antarctic Peninsula, a long, thin stretch of land that extends outside the Antarctic Circle, higher temperatures have reduced the extent of sea ice cover. This is also the region where two major ice shelves, known as the Larsen B and the Ross Ice Shelves, disintegrated (completely calved into icebergs) in the 1990s and early 2000s, and the Larsen C Shelf calved a 5,800-square-kilometer iceberg in 2017; however, it remains unclear whether and how climate change factored into that calving event. In the rest of the continent, sea ice has increased slightly since the start of the twenty-first century, partly because the circumpolar current buffers warm water from the tropics from reaching it. Still, as the Antarctic sea ice reached its annual minimum in February 2023, it was the lowest extent ever recorded. In 2024, scientists once again announced that Antarctic sea ice had hit near-historical lows for the third consecutive year. Though concerning to scientists, several more years of study are required to understand if these changes were related to global climate change.

Principal Terms

ablation: the removal of material from a glacier, ice shelf, or other mass of ice through evaporation, melting, or splitting

albedo: a measure of the proportion of incoming light or radiation that is reflected from a surface, such as snow, ice, or water; also known as reflectivity

calving: the breaking away of a smaller piece of ice from a larger one

glacier: a river of freshwater ice that is massive enough to be put into motion by gravity; usually contains ice, air, rock, and some water

iceberg: a large mass of freshwater ice that has broken from an ice shelf or a glacier; floats in a body of water

ice core: a cylinder-shaped piece of ice that is collected by drilling into a glacier; can be used to analyze the history of Antarctica’s climate

ice shelf: a large, flat sheet of freshwater ice formed from a glacier or an ice sheet; floats in a body of water

ice stream: a rapidly moving current of freshwater ice flowing from an ice sheet and moving more quickly than the ice that surrounds it; carries ice from the ice sheet

nipping: process in which ice pushes forcibly against the edge of a ship

pack ice: large, mobile masses of frozen, floating seawater that are not attached to a landform; also known as sea ice

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