Geoscience Reference
In-Depth Information
margin of the continent. An ice shelf is a
floating sheet of ice two to four hundred
meters thick; there are at least seventeen
in Antarctica. The largest ones are the
Filchner-Ronne ice shelf (430,000 km
2
,
or 166,000 square miles) and the Ross Ice
Shelf (487,000 km
2
, or 188,000 square
miles, the size of France). Most large West
Antarctic outlet glaciers terminate against
these ice shelves. When ice shelves break
up, the adjacent glaciers are “released” and
their flow to the sea is accelerated. A re-
cent, relatively minor example of such an
event was the much-studied breakup of
the Larsen B ice shelf (3,250 km
2
, or 1,250
square miles), on the Antarctic Peninsula in
February 2001.
A publication in 1968 by the late John
Mercer, an Ohio State professor once de-
scribed as a bold and eccentric glaciolo-
gist, may have been a very prophetic one,
especially since global warming was little
appreciated at that time. He argued that a
major deglaciation of the West Antarctic
Ice Sheet could happen within fifty years
as the ice shelves began to break up, lead-
ing to the retreat of grounding lines, which
in turn would lead to the disintegration of
the ice sheets by calving into the seawater.
He said that the breakup of the ice shelf on
the Antarctic Peninsula would be the first
step, and this first step has certainly hap-
pened, with the breakup of the Larsen and
Wilkins ice shelves.
Two large outlet glaciers, the Pine Island
Glacier and the Thwaites Glacier, empty
into Pine Island Bay along the Amundsen
Sea. These drain approximately 20% of
West Antarctica and are the only two
glaciers there that are not buttressed up
against an ice shelf. The surface on which
these are grounded is more or less a ridge,
which means they are likely to become un-
grounded as they shrink into the deeper
water behind the ridge. Once they are un-
grounded the calving is likely to increase,
causing the glacier to retreat at a very high
rate. This is why in 1981 Terry Hughes of
the University of Maine called the Pine Is-
land Glacier the “weak underbelly” of the
ice sheet.
A third mode of Antarctic ice sheet dis-
integration, observed by Doug Martinson
of Columbia University, is caused by the
incursion of warm water onto the shelf of
West Antarctica, which apparently only be-
gan in the last ten years or so. The water
arrives through upwelling of deep water in
response to winds that have changed direc-
tion because of global warming and possibly
also because of atmospheric changes caused
by the ozone hole. Warm water by Antarc-
tic standards would not likely attract swim-
mers, since it is only a few degrees above
freezing, but it is warm enough to increase
melting at the margin of the ice sheet.
The Disappearing Alpine Glaciers
One of the most compelling and irrefutable
lines of evidence of global warming is the
global retreat of alpine glaciers. These gla-
ciers, sometimes called mountain glaciers,
