Everything about Ice Sheets totally explained
An
ice sheet is a mass of
glacier ice that covers surrounding terrain and is greater than 50,000
km² (20,000
mile²). The only current ice sheets are in
Antarctica and
Greenland; during the
last glacial period at Last Glacial Maximum (
LGM) the
Laurentide ice sheet covered much of
Canada and
North America, the
Weichselian ice sheet covered northern
Europe and the
Patagonian Ice Sheet covered southern
South America.
Ice sheets are bigger than
ice shelves or
glaciers. Masses of ice covering less than 50,000 km² are termed an
ice cap. An ice cap will typically feed a series of glaciers around its periphery.
Although the surface is cold, the base of an ice sheet is generally warmer due to
geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet — these are
ice streams.
The present-day polar ice sheets are relatively young in geological terms. The Antarctic Ice Sheet first formed as a small
ice cap (maybe several) in the early
Oligocene, but retreating and advancing many times until the
Pliocene, when it came to occupy almost all of Antarctica. The Greenland ice sheet didn't develop at all until the late Pliocene, but apparently developed
very rapidly with the first continental
glaciation. This had the unusual effect of allowing
fossils of
plants that once grew on present-day Greenland to be much better preserved than with the slowly forming Antarctic ice sheet.
Antarctic ice sheet
The
Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km² and contains 30 million km³ of ice. Around 90% of the fresh water on the Earth's surface is held in the ice sheet, and, if melted, would cause sea levels to rise by 61.1 meters. Estimated changes in the mass of Greenland's ice sheet suggest it's melting at a rate of about 239 cubic kilometres (57.3 cubic miles) per year. These measurements came from
NASA's
Gravity Recovery and Climate Experiment (GRACE) satellite, launched in 2002, as reported by BBC News in August 2006 .
Ice sheet dynamics
Ice motion is dominated by the movement of
glaciers, whose activity is controlled by a number of processes. Their motion is dominated by cyclic surges interspersed with longer periods of inactivity, on both hourly and time scales.
Predicted effects of global warming
The Greenland, and probably the Antarctic, ice sheets have been losing mass recently, because losses due to melting and outlet glaciers have exceeded accumulation due to snowfall. According to the
IPCC, loss of Antarctic ice sheet mass and Greenland ice sheet mass contributed, respectively, about 0.21 ± 0.35 and 0.21 ± 0.07 mm/year to
sea level rise between 1993 and 2003.
The IPCC projects that ice mass loss from melting of the Greenland ice sheet will continue to outpace accumulation from snowfall. Accumulation from snowfall on the Antarctic ice sheet is projected to outpace losses from melting. However, loss of ice mass on the Antarctic ice sheet may continue, if there's sufficient loss of ice mass via outlet glaciers. According to the IPCC, scientific understanding of dynamical ice flow processes is currently "limited".
Further Information
Get more info on 'Ice Sheets'.
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