Ice sheets are permanent layers of ice covering continental shelves, with only two in the world: the Antarctic and Greenland Ice Sheets. Ice caps form from compacted snow and have a jagged appearance due to glacial weathering. They are largely barren environments with only microbes living on rocks. Earth has not always had ice sheets, and the possibility of melting due to global warming is a concern.
An ice sheet is a large permanent layer of ice covering a continental shelf, defined as having an area greater than 50,000 km2 (19,305 mi2). An ice cap is larger than a glacier or an ice shelf. There are two ice sheets in the world today: the Antarctic Ice Sheet (containing 61% of the planet’s fresh water) and the Greenland Ice Sheet (containing 7%). Only about 32% of the world’s fresh water is found in streams, lakes and aquifers, the rest is found in ice sheets.
Ice caps form when snow falls on the ground in sub-zero temperatures and does not melt, even seasonally. Over thousands of years, the snow accumulates and compacts into ice, forming layers with an average thickness of 1 mi (1.6 km) or up to 2 mi (3.2 km) at most. In some areas of the West Antarctic ice sheet, the base is up to 1.5 km below sea level, comparable to the depth of some areas of the ocean. If the entire Antarctic ice sheet or the Greenland ice sheet were to melt, the world’s seas would rise by about 2.4 meters or 60 meters respectively. The possibility of ice sheets melting due to global warming has been a cause of concern around the world in recent decades.
The ground covered by ice caps takes on a characteristic jagged appearance due to extensive glacial weathering. Terrains of this type can be seen in Patagonia (southern extreme South America), Norway, northern Canada and Siberia. All of these areas were covered by ice sheets during the last ice age more than 10,000 years ago. Only when the ice caps retreated was humanity able to colonize these areas.
Ice caps are a largely barren environment for life. Despite being made of nothing but frozen water, ice sheets tend to be very dry, providing little moisture for life to thrive. They cover the earth, preventing the accumulation of nutrient-rich soil. The only life forms that actually live on ice caps are the microbes that live on rocks that pop out of the ice, blown by the wind. The tips of hills or mountains that protrude from the ice caps are called nunataks.
Earth hasn’t always had ice sheets around the poles. Indeed, such a circumstance is relatively atypical. For most of Earth’s history, the climate was warm enough to prevent ice sheets from forming around the poles, and forests stretched from pole to pole. Dinosaur fossils have also been found less than 10 degrees latitude from the South Pole.
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