Geoscience Reference
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the changes in atmospheric circulation with more northerly winds bringing warmer,
moist air to the region, increasing snowfall. But this modest increase is more than
offset by the very rapid thinning (many metres per annum) of the glaciers feeding
into the Amundsen Sea Embayment.
Since the initial thinning observations were made from radar altimetry data
from space, there have been several coordinated international research activities to
identify the causes of the changes. These have involved geophysical surveys from the
air, on the ice and in the surrounding ocean to determine the present physical
environment and the palaeohistory of the ice sheet. It has now been established
that the recent changes of the WAIS result from changes in ocean currents.
There is an increased
flux of warm water being driven from the deep ocean up
glaciated marine trenches in the continental shelf into the cavity beneath the marine
glaciers of the WAIS. There is also compelling evidence that these ocean changes are
a further result of changing atmospheric circulation patterns of anthropogenic
origin.
Estimates of the current rate of mass loss from the Amundsen Sea Embayment
vary by a factor of three but their average is about 100 Gt per year, equivalent to
0.3mm per annum of sea-level rise or the current rate of mass loss from the
entire Greenland ice sheet, and represents about 10% of current sea-level rise.
The studies have been extended across the whole of the continent. Over 20 of
the 54 Antarctic glaciers studied, mostly those forming the WAIS, are now being
melted from below by warm ocean currents.
One further reason for the major focus on the WAIS is that over most of the
region the ice is resting on rock that is typically ~2000m below sea level. Thinning of
the ice near the marine edge causes the
flow of ice to accelerate under the effects of
gravity, leading to greater loss of ice and further thinning and hence contributes
more to sea-level rise. Essentially, the ice sheet may become unstable, and the
recent pattern of thinning could be a precursor to wholesale loss of this ice sheet,
leading to an even more rapid sea-level rise ~1.5m above current predictions. This is
often referred to as a
'
tipping point
'-
an abrupt, signi
cant and irreversible change
-
and is a focus of current research effort. Since the IPCC 2007 report, the best
estimate of sea-level rise by 2100 has been raised from 0.59m to 1m, but still with
considerable uncertainty.
As well as the melting of ice shelves the pattern of melting is extending inland.
Persistent melting
-
de
ned as melting that lasts for at least three daytime periods
-
has been steadily changing with not only the whole of the northern end of the
Peninsula being affected but areas now in East Antarctica.
Many factors affect sea-level rise in addition to the melting cryosphere. About
half the current rise results from thermal expansion as the oceans warm. Other
factors include changes in the freshness of the water, a further consequence of
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