What is an ice sheet?
An ice sheet is a mass of glacial land ice extending more than 50,000 square kilometers (20,000 square miles). Ice sheets once covered much of the Northern Hemisphere during a series of Pleistocene Ice Ages. Now, Earth has just two ice sheets: one covers most of Greenland, the largest island in the world, and the other spans across the Antarctic continent. Ice sheets are constantly in motion, spreading out under their own weight in broad domes.
Together, the Antarctic and Greenland Ice Sheets contain more than 99 percent of the land ice and over 68 percent of the fresh water on Earth. If global emissions continue at their current rate, Earth’s ice sheets are vulnerable to massive ice loss that could substantially raise sea levels.
For example, the Antarctic Ice Sheet covers almost 14 million square kilometers (5.4 million square miles), roughly the area of the contiguous United States and Mexico combined. The Antarctic Ice Sheet measures nearly 4.9 kilometers (3 miles) at its thickest point and contains about 30 million cubic kilometers (7.2 million cubic miles) of ice. If the entire Antarctic Ice Sheet melted, sea level would rise about 60 meters (200 feet).
The Greenland Ice Sheet covers about 80 percent of the world's largest island, stretching across 1.7 million square kilometers (656,000 square miles)—an area about three times the size of Texas. At its thickest point, the Greenland Ice Sheet measures over 3 kilometers (1.9 miles) thick and contains about 2.9 million cubic kilometers (696,000 cubic miles) of ice. If the entire Greenland Ice Sheet melted, sea level would rise about 7.4 meters (24 feet).
What is an ice cap?
Ice caps are essentially miniature ice sheets. An ice cap covers less than 50,000 square kilometers (19,300 square miles) and comprises several merged glaciers. Like ice sheets, ice caps tend to spread out in dome-like shapes as opposed to occupying a single valley or set of connected valleys.
Ice caps form in high-altitude polar and subpolar mountain regions. Northern Europe is home to many ice caps such as the Vatnajökull ice cap in Iceland and the Austfonna ice cap in the Svalbard archipelago of Norway, the latter of which is the largest ice cap (in area) in Scandinavia. The largest ice cap in the world is the Severny Island ice cap, part of the Novaya Zemlya archipelago in the Russian Arctic.
Ice caps are also found in mountain ranges like the Himalayas, Rockies, Andes, and the Southern Alps of New Zealand.
How do ice sheets form?
Like a glacier, an ice sheet forms through the accumulation of snowfall in areas where annual snowfall exceeds annual snowmelt. Over thousands of years, layers of snow build up, becoming more dense and transforming into glacial ice. The ice thickens to such a point that the pressure of the accumulated ice triggers movement, forming a flowing sheet of ice tens to thousands of meters thick and tens to thousands of kilometers across.
As long as an ice sheet accumulates the same mass of snow as it loses to melting or flowing into the sea, it remains “in balance.” Glaciologists refer to this measurement of stability as mass balance, or the net gain and loss of an ice sheet’s mass. Snowfall increases the ice sheet mass. Melting, calving, or evaporation of snow or ice decreases the ice sheet mass.
How do ice sheets move?
Unlike a glacier, which generally flows in one direction, an ice sheet flows outward in all directions from the center. The Greenland and East Antarctic Ice Sheets are roughly 3,000 to 4,000 meters (10,000 to 13,000 feet) high at their summits. The highest areas on the West Antarctic and the Antarctic Peninsula Ice Sheets are about 2,500 meters (8,200 feet).
Ice sheet flow is a function of surface slope, ice thickness, and whether the ice can slide on the bed or land beneath. Near the summit of an ice sheet, where the slope is the lowest, flow speeds are generally a few centimeters to a few meters per year. Near the coast, ice speeds can reach hundreds of meters or even several kilometers per year, as the ice flows into “outlet glaciers.” In some instances, fast ice motion can begin farther towards the ice sheet interior, creating “ice streams”: long ribbons of fast-flowing ice bounded by slower-moving areas on either side. As outlet glaciers move ice out to the ocean, that ice may not break off. Instead, the glacier ice can lose contact with the ground and become a floating ice shelf.
What is underneath a glacier also influences its speed. Subglacial surfaces can be quite variable, from mud-like till to bedrock to mixed environments.
Ice sheets flow outward from their dome-like centers, where they are generally thickest, and push ice outward towards the ocean. In areas where summer surface melt exceeds winter snowfall, old interior layers in the ice sheet are exposed. Ice sheet surface meltwater may refreeze before it leaves the ice sheet. In other cases, the meltwater can travel through various channels to run out from underneath the ice sheet or pour off its surface. When meltwater travels through the ice sheet, it can travel through moulins (vertical channels), crevasses (deep open cracks), and changing tunnels. If the ice sheet outlet glacier is marine terminating (ending at the ocean), then water will flow directly into the ocean. If, however, the ice sheet outlet is land terminating (ending before the ocean), then water will flow into a river.
For much of the fast-flowing ice in Greenland and Antarctica, ice flow terminates at the ocean, as a tidewater glacier (not fully afloat) or an ice tongue or ice shelf (fully floating thick sheets of ice on the ocean). In these areas, the location of the edge of the ice sheet is sensitive to both ocean conditions and the amount of ice fracturing (crevasses or rifts). With the ice in contact with the ocean, ocean heat may melt the floating ice from the underside, thinning the ice sheet and weakening it. Stresses from ice flowing over bedrock or around islands can also cause fracturing, and at the front edge of the ice, fracturing can lead to iceberg calving.