An ice sheet is a mass of glacial ice more than 50,000 square kilometers (19,000 square miles). Ice sheets—sometimes called continental glaciers—contain about 99% of Earth’s ice and about 70% of its freshwater. When 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 can be an ice cap or an ice field. Ice caps completely cover the land. Ice fields have exposed features like mountains and ridges that affect the flow of the ice. Ice fields, ice caps and ice sheets are made up of 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 form like other glaciers. Snow accumulates year after year, compressing the deeper layers of snow. It slowly changes texture from fluffy powder to a block of small, hard, round, ice-like grains. New snow falls and buries the grainy snow. The hard snow underneath—known as firn—gets even denser.

As years go by, layers of firn build on top of each other. Snow is mostly air, which makes it fluffy, but firn is at most 30% air. When the ice grows thick enough and is less than 20% air, firn grains fuse into a huge mass of solid glacial ice.

Ice sheets tend to be slightly dome-shaped and spread out from their center. Though frozen, ice sheets deform under their own weight and actually flow. An ice sheet flows, oozes and slides over uneven surfaces until it covers everything in its path, including entire valleys, mountains and plains.

Ice sheets can produce fast-moving glaciers, called ice streams, that flow from interior land to the ocean, much like rivers. These ice streams have a steady, consistent flow. Ice streams can move as quickly as 1,000 meters (0.6 miles) every year. Ice streams are often bordered by harder, slower-moving ice.

Because of their remote location, ice sheets and streams are difficult to study, making it tricky for scientists to definitively say how they form. One way ice streams can form is from the land beneath the ice warming due to heat from inside Earth. This causes the ice to slide. Friction from the sliding creates more heat, leading to faster sliding.

One of the largest glaciers in the world, the Lambert Glacier in Antarctica, is an ice stream. The Lambert Glacier moves as quickly as 1,200 meters (0.7 miles) every year. It is more than 400 kilometers (249 miles) long and over 2,500 meters (1.5 miles) thick in some places.

Continental Glaciers

Antarctica is home to the largest ice sheet on Earth, which covers around 14 million square kilometers (5.4 million square miles) and contains about 30 million cubic kilometers (7.2 million cubic miles) of ice. This ice sheet can be divided into the West Antarctic Ice Sheet and the East Antarctic Ice Sheet, which are separated by the Transantarctic Mountains. There is also a significantly smaller portion, known as the Antarctic Peninsula Ice Sheet, which is about 1,300 kilometers (800 miles) long.

On average, the Antarctic Ice Sheet is about 2 kilometers (1.2 miles) thick. The thickest point is around 4.8 kilometers (3 miles) thick. If it melted, the sea level would rise by about 60 meters (200 feet). Beneath the ice there is a diversity of geographic features, including mountain ranges, canyons and what were once riverbeds.

At only about 1.7 million square kilometers (656,000 square miles), the Greenland Ice Sheet is much smaller than the Antarctic Ice Sheet, though it is still the second-largest body of ice on the planet. If melted, the ice from Greenland would cause sea levels to rise by about 7 meters (24 feet).

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.

Despite the freezing temperatures, the ice sheets of Antarctica and Greenland are home to plant and animal life, which have adapted to live in these harsh, cold conditions. However, most of the lifeforms in Antarctica are microorganisms.

Some groups of Indigenous peoples have lived alongside the ice sheets for many generations. While there are no people native to Antarctica, there are three Indigenous groups in Greenland: the Iivit, the Inughuit (or Avanersuarmiut) and the Kalaallit, who make up the majority of people in Greenland. These groups are collectively known as Inuit.

The Indigenous people of Greenland have a longstanding connection to the ecosystems on and around the ice sheet, which has affected their hunting, navigation, medicine and culture. For centuries, the Greenlandic Inuit people have managed, adapted to and stewarded this land. In fact, people have used part of Greenland’s interior as a hunting ground for over 4,200 years.

Ice Sheets in History

Antarctica’s ice sheet began forming around 34 million years ago. While the ice sheet has advanced and retreated with climate changes throughout Earth’s history, it remains a defining feature of the landscape. Although Earth has only two ice sheets today, ice sheets have existed around the globe at different times in Earth’s history.

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 20,000 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.

During the Ice Age, an ice sheet called the Laurentide Ice Sheet was a maximum of about 3 kilometers (2 miles) thick and covered North America from the Canadian Arctic all the way to the modern northeastern United States. Glacial retreat of the Laurentide Ice Sheet helped create such features as the Great Lakes. The glaciers on Baffin Island, Canada, are remnants of the Laurentide Ice Sheet.

The Scandinavian Ice Sheet dominated Northern Europe. It once spread as far west as the island of Great Britain and as far east as Russia. It eventually retreated to the Jostedalsbreen area of Norway. Jostedalsbreen remains the largest glacier in Europe today.

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

The Patagonian Ice Sheet spread out from the Andes Mountains to cover much of southern South America. Most of what is now Chile was covered in ice. 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 climate throughout its history. One paleoclimatology research method relies on chemical analysis of ice. In particular, scientists study bubbles, or pockets of air, in samples from 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. Layers in these ice cores show successive deposits of snowfall and firn. The deepest layers of the ice core can provide information on what the weather was like from thousands of years ago to up to 3 million years ago.

By studying the chemicals present in each layer of the ice core, scientists can determine the climate during each time period, including the state of the atmosphere and seasonal fluctuations. Ice core bands may even have visually apparent differences, ranging in clarity, which can tell scientists about the conditions of the world at that time. For example, dark bands may indicate smoke or other particles released from a wildfire or volcanic eruption.

Melting Ice Sheets

Scientists from around the world are concerned that Earth’s temperature is increasing very rapidly due to climate change from burning fossil fuels. Warming temperatures are causing glaciers, ice caps and ice sheets to melt. Glaciers melt when ice melts more quickly than firn can accumulate. Ice sheets also lose significant amounts of ice when glaciers break off into the ocean, an event known as calving.

Antarctica is losing about 150 billion tons of ice per year. Since 2002, the Greenland Ice Sheet has been losing 270 billion metric tons of ice per year. If the world’s governments, companies and people cannot reduce greenhouse gas emissions enough, Greenland could be ice-free by the year 3000.

Because they are so large, melting ice sheets can affect climates, ecosystems and societies around 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. This puts coastal habitats at risk of flooding.

The effects of sea level rise are expected to displace millions of people. Scientists are particularly concerned about the effects that a melting ice sheet could have on some coastal cities around the world, such as Dhaka, Bangladesh; Bangkok, Thailand; Kolkata, India; and Lagos, Nigeria. These cities could become submerged by the ocean if ice sheets melt enough to raise the sea level significantly.

Often, communities who have contributed the least to climate change are the most vulnerable to its impacts, including sea level rise. For example, Pacific Island nations, such as Fiji, are expected to receive at least 15 centimeters (6 inches) of sea level rise over the next 30 years, whereas the sea level is rising slower in other places. Governments are preparing the residents of these island nations to take refuge elsewhere. In response to worsening conditions, the island nation of Tuvalu is working with the Australian government on a treaty that would create a pathway for migration and Australian citizenship for the people of Tuvalu. This is the first treaty between governments facilitating migration of an entire country due to climate change impacts.

The Greenlandic Inuit are already experiencing the impacts of climate change. The warming climate is threating their traditional practices, including hunting and fishing, by melting ice cover. It has also contributed to loss of culture and mental health issues, such as anxiety.

Indigenous communities are informing solutions to adapt to climate change. The Inuit Circumpolar Council’s Pikialasorsuaq Commission leads efforts using traditional knowledge to support and manage the Pikialasorsuaq, or North Water Polynya, an ecologically important area that is threatened by climate change.

Melting ice sheets also introduce freshwater into the ocean, impacting the ocean’s salinity, or salt content. Changes in salinity can alter the dynamics and movement of the ocean. This is because the temperature and salinity of water drive thermohaline circulation of the "ocean conveyor belt."

Melting ice sheets could radically alter this system. The ocean conveyor belt circulates nutrient-rich water from polar regions throughout the world's oceans in a long, slow, continual loop. This circulation relies on the way water temperature and salinity influence water density. Cold, saline water from polar regions sinks to deeper levels as it travels away from the poles and then gradually rises to the surface in the tropics. Meanwhile, warm water at the surface travels toward the poles and cools again. Melting ice sheets would increase the amount of warm, freshwater in polar marine ecosystems. This would slow deep water formation, which is the development of cold, saline, nutrient-rich water on which entire marine ecosystems depend. Weakening thermohaline circulation could have widespread, devastating impacts to weather patterns, such as rainfall, which would in turn affect crop growth and food security.

According to the United Nations, sea ice will continue to decrease in coming years. This may lead to changes in phytoplankton (a type of algae) blooms, the basis of the Antarctic marine food web, which could ripple throughout other ocean ecosystems.

Governments around the world have worked together to protect ice sheets. The Antarctic Treaty reserved the continent of Antarctica for peaceful, scientific endeavors. Twelve countries signed this treaty in 1959. The Madrid Protocol in 1991 offered further protection, designating Antarctica a natural reserve. It prohibits mining and requires environmental assessments for any new activities on the continent. Today, countries around the world, including Chile, Argentina, China and India, are contributing to the expanding knowledge of ice sheets. With better understanding, we can be better stewards of the environment.