The cryosphere
In the text the cryosphere is treated as a part of the hydrosphere. It consists of three major components:
- sea ice (also called 'pack ice') in the polar oceans: important because it covers a large area and is highly variable, so it exerts a strong influence on the earth's albedo
- continental ice sheets: important because it accounts for most of the mass of the cryosphere
- mountain glaciers: important because they are sensitive indicators of climate change over a wide range of latitudes, including even the tropics.
Sea ice consists of innumerable separate ice floes separated by leads (patches of open water that form between the ice floes) which open and close as the ice moves. The ice is dragged along in the direction of the prevailing wind, but it moves at only a percent or two of the speed of the wind. Like surface ocean currents it tends to be deflected somewhat to the right of the surface wind in the Northern Hemisphere and to the left in the Southern Hemisphere. Arctic pack ice is contained mainly within the Arctic basin, but is is continually exiting through the Fram Strait to the east of Greenland and flowing southward along the east coast of Greenland, melting by the time it reaches the southern tip.
Formation of sea ice
During wintertime, open leads and patches
of open water that form when the ice is blown away from the coastline by
an offshore wind freeze over very quickly, forming what is referred to
as 'first year ice' (see Fig. 1 for a picture of ice
floes and leads). This new ice thickens rapidly at first, and then
more slowly as it grows thick enough to insulate the water below it from
the cold air above. It also thickens whenever ice floes collide and
one 'rafts' above the other. Sometimes these collisions create 'pressure
ridges' (lines across the floes where the ice bulges upward into the atmosphere
by a meter or two and downward into the water below by a comparable amount).
Fig. 1.
Arctic Sea Ice Growth. Scientists
have used high resolution radar to see, for the first time ever, the development
of the Arctic sea ice cover. The images show a comparison of ice growth
during the Arctic winter. The two images are separated by nine days. Both
images represent an area located in the Baufort Sea, north of the Alaskan
coast. This radar view covers an area of 96 by 128 kilometers (60 by 80
miles). The brighter features are older thicker ice and the darker areas
show young, recently formed ice. The earlier image is shown on the left.
Within the nine-day span, large and extensive cracks in the ice cover
have formed due to ice movement. These cracks expose the open ocean to
the cold, frigid atmosphere where sea ice grows rapidly and thickens.
Evolution of sea ice
Ice that has survived through at least
one summer is referred to as 'multi-year ice'. Ice formed in the
Arctic Basin can survive for five years or longer before it is swept out
through the Fram Strait. The typical thickness of multi-year ice
on the order of a few meters but the ice can be much thicker in pressure
ridges. Ice thickness was routinely monitored by U.S. and Soviet/Russian
nuclear submarines while on missions in the Arctic. Much of these
data have become declassified and made available for scientific use during
the past few years. Dr. Rothrock of the University of Washington
and colleagues have found that the average draft of the sea ice (that is,
its thickness from the ocean surface to the bottom of the ice pack) has
declined by 4.3 feet (1.3 meters). This represents a reduction of about
40 percent as compared with 20-40 years ago.
Snow accumulates on top of sea ice during the colder months of the year and forms melt ponds on the ice surface during summer, when air temperatures are close to (but just above) freezing at the ice surface, but warmer aloft. Low stratus clouds formed by the cooling of the air near the surface are trapped within this low lying temperature inversion, making for a gloomy summer climate despite the midnight sun. Researchers much prefer being in the Arctic during winter night when skies are often clear.
Seasonality of sea ice
The edge of the pack ice advances and
retreats with the seasons. In winter the entire Arctic basin, the
Davis Strait to the west of Greenland and much of the Bering Sea are ice
covered, whereas during summer the ice pulls back from the Arctic coast
leaving stretches of ice free water and large patches of open water are
sometimes observed even near the North Pole. The Antarctic pack ice
expands and contracts in a similar manner. Sea ice typically covers
about 14 to 16 million square kilometers in late winter in the Arctic and
17 to 20 million square kilometers in the Antarctic's Southern Ocean. The
seasonal decrease is much larger in the Antarctic, with only about three
to four million square kilometers remaining at summer's end, compared to
approximately seven to nine million square kilometers in the Arctic. The
maps below (Fig. 2) provide examples of late winter
and late summer ice cover in the two hemispheres.
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2) The Continental
Ice Sheets
Just as an ice cube in a glass of water
is almost totally immersed so that the level of the water in the glass
is almost unchanged when it melts, sea ice is almost totally immersed in
the ocean so the global sea-level doesn't change appreciably when it freezes
or melts. In contrast, if an ice cube is dropped into a glass of
water, the level of the water in the glass rises. In a similar manner,
when chunks of continental ice sheets or glaciers break off to form icebergs
and subsequently melt (a process called 'calving') their volume represents
an addition to the volume of the oceans, and sea level rises just as soon
as they become free floating. Hence, the concerns about the possibility
of catastrophic sea-level rises in connection with global warming relate
to the calving or melting of large segments of the existing continental
ice sheets: not to the melting of pack ice in the polar oceans. The
Antarctic ice sheet accounts for about 90% of the mass of the cryosphere
and the Greenland ice sheet for most of the remaining 10%. If both
were to melt completely, global sea level would rise by 70 to 80 meters;
enough to submerge roughly 20% of the continental land areas including
entire countries like Holland and Bangladesh.
Continental Ice sheets in the ice
ages
The mass of the continental ice sheets
was much larger during the ice ages than it is today, and global sea level
was correspondingly lower, exposing large areas of the shallow seas.
The continental ice sheets were so heavy that they depressed the earth's
crust beneath them into the underlying mantle. Parts of Scandinavia
are still rebounding following the melting of the ice sheet that covered
the region 10,000 years ago.
Formation
The continental ice sheets are formed
from the accumulation of snow that forms a succession of annual layers
that gradually get compressed and turned into ice as more layers pile on
top of them. As the snow is compressed, the air within it is trapped
and preserved in small bubbles and retains the chemical properties that
it had when it was deposited. As a newly formed ice sheet grows to
maturity, its surface forms a dome centered over the interior of the continent.
The domed shape is a result of the action of gravity that causes the ice
to flow downhill toward the edges of the ice sheet and finally fall (or
calve) off the edge. Look at Figure 3 for a picture
of this calving process. Ice under high pressure from the weight
of the overlying ice behaves as a plastic medium, so ice from the interior
flows outward toward the edge of continent to replace the ice lost to calving,
while more snow and ice piles on top. When the ice sheet reaches
equilibrium, the outflow of ice that is lost in calving along the edge
of the ice sheet exactly balances the accumulation of snow and ice in the
interior. It's a messy experiment, but one can create a domed surface
that resembles the shape of an ice sheet by pouring a thin stream of a
viscous liquid like honey uniformly across the top of an inverted drinking
glass until a balance is reached between the honey being poured on top
and the amount dripping over the edge of the glass.
Fig. 3.
Calving of an iceberg in Antarctica. A massive iceberg (known
as B-15) broke off the Ross Ice Shelf near Roosevelt Island in Antarctica
in mid-March 2000. Top Left image: March 3, before the calving; Top Right
image: March 17, just after the calving; Bottom Left image: March 20; Bottom
Right image: March 28, just after the calving of B-17, a comparatively
small chunk to the left of B-15. Among the largest ever observed, the B-15
iceberg is approximately 300 km long and 40 km widean area about twice
the size of the state of Delaware. The iceberg was formed from glacier
ice moving off the Antarctic continent.
The Vostok and Greenland ice cores are near the interior of their respective continents where the outflow is very small and the accumulating annual layers of snow / ice are relatively undisturbed so that they remain intact over tens and hundreds of thousands of years. The Antarctic ice sheet is much thicker, so the Vostok core traces the history of the ice much farther back in time.
3) Mountain glaciers
Mountain glaciers account for a small
fraction of the mass of the cryosphere and of the contribution of the cryosphere
to the earth's albedo. They are marked by a similar balance between
accumulating snow and ice in a dome in the upper part of the glacier and
'calving' of pieces of ice or summertime melting at the snout, which is
usually located at much lower elevation, where air temperatures are much
warmer. Many mountain glaciers exhibit a recurrent pattern of sudden
surges that last for a few years, alternating with much longer periods
of slow retreat. Such glaciers are retreating most of the time, but
they have to be observed over an interval long enough to include one or
more of the surges in order to get a clear indication of what's really
happening to them. The Carbon glacier on the west side of Mt. Rainier
surged back in the 1960's but has retreated considerably since that time.
Last
Updated:
10/29/2001