Geoscience Reference
In-Depth Information
Greenland would be an island archipelago
(until the land rebounded from the weight
of the ice and recovered its elevation).
Antarctica is 98% covered with ice, which
on average is about 1.6 kilometers (one
mile) in thickness. The southern Indian,
Atlantic, and Pacific oceans, known col-
lectively as the Southern Ocean, surround
it. The Antarctic continent is divided into
West and East Antarctica, separated by the
Transantarctic Mountains. East Antarctica
is about the size of the continental United
States and West Antarctica about the size
of Texas.
The margins of the ice sheets are always
melting as the marginal glaciers are pushed
out to sea. It is clear now that the rate of ice
loss at the outer edges of the ice sheets is
greater than the rate of annual addition of
ice by snowfall, so these ice sheets are expe-
riencing a net loss of water volume.
The Greenland Ice Sheet has been melt-
ing for decades, but toward the end of the
twentieth century the rate of melting in-
creased. Probably until 2000 the West Ant-
arctic Ice Sheet was essentially not losing
mass at all and perhaps was even growing
slightly (or so it was assumed by glaciolo-
gists). In the first decade of the twenty-irst
century this changed dramatically. Some
researchers believe that the melting rate of
the Antarctic Ice Sheet will eventually ex-
ceed Greenland's.
Atmospheric warming, at least at this
point, does not affect the interior of the
ice sheets, which are always well below
freezing. Ice loss from ice sheet interiors is
mostly by sublimation, a process by which
dry snow or ice changes to water vapor.
Satellite observations are critical to mon-
itoring the progress of the world's great ice
sheets. Satellites make three types of mea-
surements. The first is satellite altimetry,
which simply measures the elevation of
the ice surface. Satellite gravity measure-
ments measure the mass of the ice cover.
Changes in the mass are the all-important
measure of loss or gain of ice. Satellite ra-
dar interferometry measures the velocity of
moving ice and can also pinpoint the loca-
tion of grounding sites for individual outlet
glaciers. A grounding site is where the nose
of a glacier is jammed up against an island
or some sort of rise on the sea floor of the
continental shelf.
Based on all of these types of satellite
observations, the latest measurement of
the rate of ice loss is 247 billion metric tons
(273 billion short tons) of ice each year for
Greenland and 120 billion metric tons (132
billion short tons) for West Antarctica.
Moreover, in 2009 a report from J. L. Chen
and associates at the University of Texas
indicated that for the first time, the Ant-
arctic ice sheet in the east experienced a net
loss of ice, at a relatively small estimated
annual rate of 52 billion metric tons (57 bil-
lion short tons). This ice loss is believed to
have begun in 2006. This first report of a
significant ice loss from the East Antarc-
tic Ice Sheet is very likely a harbinger of a
sea level rise larger than that indicated by
current estimates. The combined melting
ice contributions from the three ice sheets
