Ice sheet

An ice sheet is a massive body of glacial ice covering a continental land area. To be classified as an ice sheet, the ice must cover more than 50,000 square kilometers.

These structures are much larger than alpine glaciers or smaller ice caps. Currently, only two ice sheets exist on Earth: the Greenland ice sheet and the Antarctic ice sheet. The Antarctic ice sheet is the largest single mass of ice on the planet. It covers approximately 98% of the Antarctic continent and reaches an average thickness of over 2 kilometers.

Ice sheets grow through a continuous process of accumulation and compression. Most snow that falls onto the surface of an ice sheet never melts. Instead, the weight of newer snow layers presses down on the older layers below. This immense pressure compresses the snow into thick, solid ice. This process is so steady that it can bury objects quite deeply over time. For example, a Lockheed P-38 Lightning fighter plane crashed in Greenland in 1942. When it was recovered 50 years later, it was buried under 81 meters of ice.

Even though the surface is freezing, the base of an ice sheet is often warmer. This warmth is caused by geothermal heat rising from inside the Earth. This heat can cause melting at the bottom of the ice. When meltwater forms at the base, it acts as a lubricant. This lubrication allows the ice to slide more easily, creating fast-flowing channels called ice streams.

Gravity also drives the movement of the ice. The ice flows outward from a central plateau, which is the highest point. The slope is gentle at the plateau but becomes very steep at the margins. This movement is a constant cycle of ice accumulating at the center and flowing toward the edges.

Ocean activity can also influence how ice moves. Tidal oscillations can be felt by ice even 100 kilometers away from the sea. During large spring tides, an ice stream might stay still for hours. Then, it might surge by about a foot in less than an hour following a high tide.

Surface melting also plays a role in ice dynamics. Increasing air temperatures can create supraglacial lakes on the surface. These lakes can form cracks that allow warm water to reach the base of the glacier. This water can lubricate the bed and cause the glacier to surge forward rapidly.

In the 1970s, scientist Johannes Weertman proposed a theory about ice stability. He noted that seawater is denser than ice. This leads to a process called Marine Ice Sheet Instability, or MISI. This occurs when ice sheets are grounded below sea level. As the ice melts and becomes thinner, it loses the mass needed to displace seawater. Seawater can then push into the gaps at the base of the ice. This creates a self-reinforcing cycle where the grounding line moves backward, causing even more ice loss.

Ice shelves help stabilize the glaciers behind them by providing a buttressing effect. When an ice shelf collapses, the glaciers behind it often accelerate. A notable example occurred in February 2002 with the collapse of the Larsen B ice shelf. Following this collapse, four glaciers—Crane, Green, Hektoria, and Jorum—began flowing at much faster rates.

Some scientists also study Marine Ice Cliff Instability, or MICI. This theory suggests that tall ice cliffs might collapse under their own weight once the surrounding ice is gone. This could lead to rapid sea level rise by the year 2100.

Ice sheets are vital indicators of Earth's climate history. During the Last Glacial Period, massive ice sheets covered much of the northern and southern hemispheres. The Laurentide and Cordilleran ice sheets merged with the Greenland ice sheet to cover much of North America. Meanwhile, the Weichselian Ice Sheet covered Northern Europe, and the Patagonian Ice Sheet covered southern South America.

Today, the melting of the Greenland and Antarctic ice sheets is a major factor in climate change. The rapid thinning of glaciers like Thwaites and Pine Island is a significant concern for future sea level rise.

Scientists continue to monitor these systems to understand how our planet is changing.