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

alpine glaciers: important because they are sensitive indicators of climate
change over a wide range of latitudes, including even the tropics.

Scientists at NASA's Jet Propulsion Laboratory (JPL) 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 Beaufort 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.  Formation of sea ice in the Arctic Ocean affects the heat balance in the global atmosphere and ocean.
 

Sea ice

Sea ice consists of innumerable separate ice floes separated by leads 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.  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.

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'.  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).
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.

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.

http://antwrp.gsfc.nasa.gov/apod/ap991116.html

CAPTION: It's not easy to make a map of Antarctica. Earth's southern most
continent is so cold and inhospitable that much of it remains unexplored. From
space, though, it is possible to map this entire region by radar: by
systematically noting how long it takes for radio waves to reflect off the
terrain. The Canadian satellite RADARSAT has been orbiting the Earth for the
past five years making radar maps, and has recently released the most detailed
map of Antarctica ever created. Above is a computer-generated map of Antarctica
at relatively low resolution. From the RADARSAT map, scientists have been able
to better study this mysterious continent, including information about how
ancient ice-shelves are crumbling.
 

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.

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.

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.  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.

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.

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.

Plate Tectonics


The theory of plate tectonics and continental drift was proposed by Alfred Wegener
early in the 20th century but didn't become widely accepted until the 1960's.  It
is supported by a variety of evidence as summarized in the text and in the
presentation of Group X. It has important implications, not only for the movement
of the continents on time scales of tens to hundreds of millions of years, but
also for our understanding of chemical transformations that affect the composition
of the atmosphere.

The theory holds that the earth's crust is broken up into plates that float upon
the much thicker layer of porous but very viscous material that makes up the
earth's mantle.  Owing to slow convection within the mantle, the plates move at
speeds ranging up to a few centimeters per year (or meters per century; or tens of
km per million years).  Plates that lie above regions of upwelling in the mantle
are moving apart and plates that lie above regions of downwelling in the mantle
are being pushed together (Fig. 6-27).

Earthquakes are observed to be to be concentrated along plate boundaries
(Fig. 6-12).  Oceanic plates are thinner, but slightly denser than continental
plates, so that when the two collide, the oceanic plate is pushed (or 'subducted')
underneath the continental plate and disappears as it is incorporated into the
mantle (Fig.  6-15a, 6-21).  Rocks in the subducted oceanic crust are subjected to
increasingly higher temperatures and pressures as descend, causing physical and
chemical transformations (referred to by geologists as 'metamorphosis': change of
form) of certain rocks like carbonates (limestone).  Subducted ocean sediments
also carry with them hydrated rocks (i.e., minerals with water molecules
incorporated them).  As the temperature increases, the water molecules are
released as steam within the mantle.

One example of a region of subduction is the boundary of the Pacific and Juan de
Fuca plates off the Washington coast, the site of mega-earthquakes every few
hundred years.  Subduction zones correspond to trenches on the ocean floor.
Volatile substances subducted into the mantle are expelled in volcanoes which are
usually located nearby.

Collisions between plate boundaries are often associated with volcanic activity
(as in the Cascade range) and with the uplift of mountain ranges (Fig. 6-15).  The
most dramatic of the earth's mountain ranges, the Himalayas, has been created by
folding of the earth's crust following the collision of the Indian and Asian
plates, which is still going on today.  The Rockies, Cascades and Sierra ranges
have been created in a similar manner, by the collision of the Pacific and North
American plates.

Oceanic plates are continually being recycled.  The Pacific plate is being
subducted along much of its boundaries.  Meanwhile, along the mid-Atlantic ridge,
new oceanic crust is being formed as minerals upwelling in the mantle rise to the
surface and cool (Fig. 6-13a).  As this newly formed crust diverges away from the
mid-Atlantic ridge, the Atlantic plate widens, taking up the space lost by the
Pacific plate as portions of it are subducted.  As the Atlantic widens and the
Pacific shrinks, the continents may be viewed as drifting away from the Atlantic
on trajectories that will eventually (in another 100-200 million years) cause them
to converge over what is now the mid-Pacific.  A similar congregation of the
continental plates occurred about 200 million years ago when the continental
plates were clustered around what is now Africa.  The giant land mass that is
believed to have existed at that time is called Pangea (all earth).

The newly formed crust in the mid-Atlantic has a different chemical composition
from the oceanic crust in the Pacific that is being subducted.  In place of
limestone (CaCO3) is metamorphosed calcium silicate (CaSiO2) rocks in which the
limestone is combined with quartz (silicon dioxide, SiO2) and carbon dioxide is
released (CaCO3 + SiO2 -> CaSiO3 + CO2) , and hydrated minerals are absent.  The
gaseous by products of the reactions that take place within the earth's mantle
(CO2, H2) and H2) are released in volcanoes.  While these chemical transformations
are taking place in minerals processed within the mantle, the reverse reactions
are taking place due to weathering in the continental plates and deposition of
sediments in the oceans.  Weathering exposes calcium silicate rocks and decomposes
them into calcium ions that flow down the rivers into the oceans, and residual
quartz (silicon dioxide, SiO2) rocks.  Limestone is formed in the shallow seas by
the deposition of these same calcium ions after they are incorporated into
skeletons and shells.

On times scales of tens of millions of years carbon dioxide levels in the
atmosphere are determined by the comparative rates of continental drift (which
determines the rate of subduction, metamorphosis of limestone, and release of
carbon dioxide in volcanoes) and weathering (which determines the availability of
calcium ions for forming limestone.  If continental drift predominates over
weathering, atmospheric carbon dioxide levels should rise, and vice versa.  Over
the past 50 million years it is believed that weathering has predominated (due to
the uplift of the Himalayas and the slowing down of continental drift): hence, the
decrease in atmospheric carbon dioxide levels and the cooling of the atmosphere,
which set the stage for the current glacial period.