Ice sheet

An ice sheet is a huge mass of glacial ice that covers surrounding land and is larger than 50,000 km² (19,000 sq mi). Right now, there are two main ice sheets on Earth: the Antarctic ice sheet and the Greenland ice sheet. These ice sheets are the largest glacial bodies on our planet, much bigger than smaller ice caps or mountain glaciers. They can have several high points called ice domes from which the ice flows outward, and they are usually drained by fast-moving areas of ice known as ice streams and outlet glaciers near their edges.

Even though the surface of an ice sheet looks very cold, the bottom part is often warmer because of heat coming from inside the Earth, called geothermal heat. In some places, this causes melting, and the water helps the ice sheet move faster. This melting creates quick-moving channels inside the ice sheet.

Ice sheets are always moving slowly. The ice gradually flows from the highest central area outward toward the edges. Although the slope is gentle in the middle, it becomes steeper near the edges.

As the world gets warmer because of climate change, this can affect ice sheets. Melting on the surface can create lakes of water that flow to the base and help the ice move. The growth and shrinkage of ice sheets are closely tied to Earth's history of cold periods, called glacial periods, during ice ages. Huge ancient ice sheets once covered much of North America and Europe. Today, the future of the Greenland and Antarctic ice sheets is very important because their changes can affect global climate change and sea level rise.

Overview

An ice sheet is a very big piece of ice that covers lots of land, more than 50,000 km². Today, we have two big ice sheets on Earth: one in Greenland and one in Antarctica. These ice sheets are very thick, about 2 km (1 mile) deep on average. They grow because the snow that falls on them doesn’t melt; it gets squished by the weight of more snow on top.

Ice sheets are still growing today. For example, during World War II, a Lockheed P-38 Lightning fighter plane crashed in Greenland in 1942. When people found it 50 years later, it was buried under 81 meters (268 feet) of ice that had formed since the crash.

Dynamics

Ice sheets are always moving, even when they seem still. The ice flows slowly from the thick middle toward the edges. This happens because more ice builds up in the middle, while some melts at the edges. The slope and gravity help the ice move, but the speed changes with temperature and the ground below.

Ocean tides can also change how fast the ice moves. Sometimes, parts of the ice stop for hours and then move quickly after a high tide.

Warmer temperatures can create pools of water on top of the ice. These pools can reach the bottom and make the ice move faster.

When ice reaches the ocean, it can break off into the water as icebergs. Floating pieces of ice, called ice shelves, can break off too. When an ice shelf breaks up, the glaciers behind it can start moving faster. For example, when a large floating ice shelf called Larsen B broke apart, the glaciers behind it moved more quickly.

The strength of these ice shelves depends on the ocean temperature. Warmer water can melt the bottom of the ice shelves, making them thinner and less able to hold back the glaciers.

Some parts of Antarctica, like the West Antarctic Ice Sheet, sit below sea level and could lose ice quickly if the ocean warms. Two important glaciers there, Thwaites and Pine Island, have been thinning and moving faster in recent years. This could cause sea levels to rise.

Most of the East Antarctic Ice Sheet is safer. However, one large glacier there called Totten Glacier could also lose ice quickly if conditions change.

Earth's current two ice sheets

There are two huge masses of ice on Earth called ice sheets. These are the Antarctic ice sheet and the Greenland ice sheet. If all the ice sheets and glaciers in the world melted, it would take a very long time—thousands of years—but it would raise the level of the oceans. However, melting sea ice and ice shelves does not change the level of the sea.

Role in carbon cycle

Ice sheets used to be thought to have no role in the carbon cycle. But research in the 2010s showed that ice sheets host tiny life forms and store and move a lot of carbon.

The two ice sheets are different. Under the Greenland ice sheet, there is only a small amount of carbon. But under Antarctica, scientists think there is a huge amount of carbon. If this carbon is released as meltwater, it could make climate change worse by adding more carbon dioxide to the air.

For comparison, the Arctic permafrost holds a lot of carbon, and humans add carbon dioxide each year. In Greenland, meltwater releases methane, a gas that heats up the planet. But some bacteria there help reduce these emissions.

In geologic timescales

The Earth’s climate has changed over very long periods because of how much sunlight reaches our planet. This happens because of the shape of Earth’s orbit and its tilt, which change slowly.

During the past 100,000 years, large parts of the ice sheet that once covered much of North America broke apart. This sent many icebergs into the North Atlantic Ocean. When these icebergs melted, they left behind rocks and other materials.

Scientists also study sudden warmings in the northern hemisphere that happen about every 1,500 years. These warm periods often follow the large iceberg events. By studying ice from both Greenland and Antarctica, scientists have noticed that the ice sheets in these places did not warm up at the same time.