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Ice Sheets
The Greenland and Antarctica ice sheets cover 10% of the Earth’s land area and contain 77% of the world’s freshwater. The ice sheets comprise 99% of all the glacier ice on earth. The average thickness of both are approximately 2100 meters, but the Antarctic ice sheet (14x106km2 in area) contains about ten times the ice volume of the Greenland ice sheet(1.7x106km2 in area).
Ice sheets rest on land that is relatively flat in comparison to the ice thickness; glaciers are bound within channels that allow variation in thickness. Although the Earth’s crust is depressed hundreds of meters by the tremendous weight of the ice, the bases of the ice sheets are ,on average close to sea level. The exception is the West Antarctic ice sheet, comprising 12% of the total that is grounded as much as 2500 meters below sea level.
If all the fresh water ice locked in ice sheets and glaciers were to melt it would cause a sea level rise of nearly 80 meters. In the last few million years, ice sheets have waxed and waned with major climate changes, about every 100,000 years in response to changes in the Earth’s position relative to the Sun. During the last ice age, about 15,000 years ago, huge ice sheets covered parts of Eurasia and much of North America,extending as far south a Pennsylvania. As the climate has warmed up, sea level rose about 125 meters at an average rate 2.5cm/yr for roughly 5000 years. Although northern ice sheets melted,the Antarctica ice sheet decreased by only 10%.
In contrast to today’s climate the last interglacial warm period, 120,00 years ago was even warmer, sea level may have been 6 meters higher, and the West Antarctic ice sheet may have disintegrated.
Although the current rate of sea level rise is as much as 2mm/yr, it is not known whether the present ice sheets are shrinking or growing. Each year, about 8 mm of water from the entire surface of the Earth’s oceans accumulates as snow on Greenland and Antarctica. The average ice accumulation on Greenland is about 26cm/yr and Antarctica about 16cm/yr (please note accumulation is in water amounts, or about 5 times greater in snowfall). We do not know, however, whether the amount of water returned to the oceans in icebergs and melt water runoff balances the snow accumulation to within + 25%. This large uncertainty exists because there is little direct information on ice sheet volume change.
Mass balance and the Formation of Ice Sheets
The term mass balance refers to the difference between the mass input to a glacier or ice sheet and mass loss. Glacier formation occurs when, for a sustained period of time, the snow deposition in an area consistently exceeds the amount of snow that is lost, i.e., the mass balance is positive. In the case of ice sheets, this occurs over a prolonged cold periods on the order of tens of thousand years.
Snow falls on large areas of land, such as Greenland and Antarctica. During the summer months, some, but not all of this snow is removed by melt and evaporation (depending on local conditions). When snow remains beyond one melt season, it becomes denser as the grains become sintered and increase in size; such snow is referred to as firn. As the cycle is repeated year after year, the previously fallen snow becomes buried deeper and deeper relative to the surface. Under the increased pressure of the overlying snow, the grains become larger and more aggregated, and the density continues to increase. Eventually, the process reaches a point where the pressure and the sintering are so great that the air between particles is closed off, and impermeable ice is formed. As this process continues, the older ice is pushed deeper and outward, because of the overlying weight of snow and firn. As a result, the ice sheets are generally thicker, with higher surface elevations, near their centers, and thinner, with lower surface elevation, near their edges.
While the only mass gain to ice sheets, as a whole, comes from the accumulation of snow on the surface, the mechanisms of loss are somewhat more complicated.
Variable Parameters for Determining Mass Balance in Ice Sheets *
A. Ice Accumulation
B. Ice Melt
C. Distance of land mass from Earth’s Poles
D. Land Area
E. Average Temperature (Over 100 years)(Global Warming Factor)
F. Average Snowfall (Over 100 years)
*Note: these are only the parameters used in the activity for mass balance
at the 6-9 grade level. Additional parameters are discussed in lesson designed for determining melt sensitivity ratio, 9-12 grade level.
While the only mass gain to ice sheets, as a whole, comes from the accumulation of snow on the surface, the mechanisms of loss are somewhat more complicated.
Mechanisms of Loss
Altitude
Altitude is a factor that provides a number of variables considered in the loss of ice mass.
A. Gravity
B. Slope
C. Latitude
Use the site below for an view over of how altitude is measured on the ice sheets and the land masses that are covered by these ice sheets.
Click on the actual image of the land mass and the satellites for maps and graphs representing current data.
http://icesat4.gsfc.nasa
Gravity moves the ice from higher elevations to lower elevations and in most cases to lower latitudes, where temperatures are warmer. For example about half of the Greenland ice sheet is warm enough for surface melting to occur. The Antarctic land mass is much closer to the 90 degree latitude of the pole and typically less than 5% melts.
Slope will determine the rate of movement by gravity, the steeper slope will accumulate less snowfall and the ice that is on a steep slope will move more quickly once it begins to move.
Mass loss by discharge
Water that is formed by melting can be discharged as surface or sub-surface runoff or percolate into the firn where it refreezes at greater depths. The first two result in loss of mass to system the last, simply a redistribution. Mass can also be lost by direct transition to vapor from dry snow in a process known as sublimation.
The most dramatic and significant form of mass loss is known as calving . Ice flows down from higher elevations and follows the topography of the underlying land surface in the same manner of rivers forming drainage basins and streams. These ice flows are carried out onto the surrounding seas. Because ice has mechanical strength as well as buoyancy it can extend out beyond the water/land boundary as a solid unit.
In Greenland they are most often narrow,single streams and are called a
glacier tongue. In Antarctica there is more area over the which the streams can converge and form an ice shelf over water.
Eventually, the stresses on the floating ice that result from buoyancy and weight and the erosion form warm water contact cause it to break into icebergs that are carried out to sea.
Iceberg calving is believed to account for roughly 40% of mass loss in Greenland but 80 to 90 percent in Antarctica.
The melting that takes place on the underside of the ice where it contacts the water also contributes to the mass loss and affects ocean circulation by adding cold fresh water to the saline seas.
