Geoscience Reference
In-Depth Information
are perennial masses of ice that occupy the
valleys of high mountain ranges and slowly
flow downhill toward lower elevations or
to the sea, where ice can no longer survive.
Almost all the world's alpine glaciers are
currently shrinking, with the exception of
glaciers at high elevations. The survival of
an alpine glacier, just like the survival of
the outlet glaciers of the ice sheets, depends
on a so-called mass balance. The balance is
the difference between the loss of ice in the
summer and gain of ice through snowfall in
the winter. By some estimates, if all alpine
glaciers melt and disappear, sea level will
rise perhaps one-half to two-thirds of a me-
ter (one and a half to two feet).
As alpine glaciers begin to melt, the first
thing that happens is thinning of the ice,
followed by the more obvious retreat of the
end of the ice lobe. The fastest shrinking
by far occurs when the nose of the glacier
calves in a body of water, although on land
shrinking can be significant as well. The
Khumba Glacier in the Himalayas has re-
treated more than 5 km (3.1 miles) since
Edmund Hillary and Tenzing Norgay be-
gan their climb up the glacier on the way
to Mount Everest in 1953. The Mendenhall
Glacier near Juneau, Alaska, has retreated
the same amount since 1760. But when a
lake formed in front of the glacier and calv-
ing began, the ice retreated a full kilometer
(0.6 mile) since 2000. Another impact of
glacial retreat is illustrated by the dropping
sea level at Juneau caused by the removal
of the weight of the Mendenhall Glacier,
allowing the land to rebound. The rate of
movement of glaciers ranges from nil to
several meters a day. The Jakobshavn Is-
brae outlet glacier in Greenland is currently
whizzing along toward the sea at 14 km (8.7
miles) a year.
The islands of the Canadian Arctic have a
number of ice caps, all of which are shrink-
ing and thinning. Ice caps are miniature
ice sheets that form a central ice mass or
dome from which smaller glaciers flow. Ca-
nadian examples are the Bylot ice cap on
Bylot Island, the Barnes and Penny ice cap
on Baffin Island, and the Devon ice cap on
Devon Island. Other ice caps include the
Vatnajökull of Iceland and the southern
Patagonia ice field in Argentina and Chile.
A few floating ice shelves are associated
with the Canadian alpine glaciers and ice
caps. Although much smaller than their
Antarctic counterparts, they have the same
impact on glacier movement. When they
break up, the glaciers formerly buttressed
up against them immediately increase the
rate at which they move toward the sea.
Most of the shelves, including the Ward
Hunt and Serson ice shelves on Ellesmere
Island, are breaking up. The Markham ice
shelf, also on Ellesmere Island and twenty
square miles in area, broke away in its en-
tirety almost overnight in 2008. What
makes this event particularly interesting
is that the ice shelf is known to be 4,500
years old, an indication that today's warm-
ing conditions in the Canadian Arctic may
be quite extraordinary. The Ward Hunt ice
shelf is estimated to be 3,000 years old.
One of the most worrisome aspects of the
