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We Know: Underlying Processes |
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The balance between
energy absorbed by the earth and energy reflected
back into space is fundamental in determining how
warm or cool the planet becomes. The proportion
of radiation reflected away by a surface is called
its albedo. Albedo can range between 0 (no reflectance)
and 1 (complete reflectance—like a mirror).
The earth’s average albedo is .31, which means
that, overall, the planet reflects about 31% of
incoming solar radiation back into space. But forests
and deserts, oceans, clouds, snow, and ice all have
different albedos—and changes in these types
of ground cover can therefore affect how much solar
radiation the earth receives. For example, the albedos
of forests lie in the range 0.07–0.15, while
deserts have an albedo of around 0.3. |
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This satellite
image shows Antarctica’s Scott
Coastline on January 4, 2002. The
large, coke-bottle-shaped iceberg
in the lower right broke off the Ross
Ice Shelf in December 2001. (See “The
Collapse of the Larsen B Ice Shelf”
on this site for more information.)
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The
Dry Valleys are one of the few areas
of Antarctica not covered by ice.
Unlike much of the rest of the earth,
the Dry Valleys have cooled over the
last 100 years. (See “The
Collapse of the Larsen B Ice Shelf”
on this site for more information
on the implications of this cooling
trend.)
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| The albedo of the earth’s surface
varies from about 0.1 for the oceans to 0.6–0.9
for ice and clouds, which means that clouds, snow,
and ice are good radiation reflectors while liquid
water is not. (This is because clouds, snow, and
ice have multiple layers that reflect radiation,
whereas a body of water reflects only from its surface.
A calm ocean is a poor reflector, but when it foams
up in the surfline, producing many reflecting surfaces,
it becomes white— reflecting most of the light
hitting it.) In fact, snow and ice have the highest
albedos of any parts of the earth’s surface:
Some parts of the Antarctic reflect up to 90% of
incoming solar radiation. |
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| Continued global warming will have one obvious
effect on the world’s polar ice, sea ice, glaciers, and
permanent snow cover: Warmer temperatures will melt some of
this frozen water. Melting of land-based ice sheets and glaciers
could contribute to sea-level changes. (Melting sea ice would
not contribute to rising sea levels: When ice floating in water
melts, the level of the water doesn’t change. You can prove
this to yourself by watching the ice melt in a glass of water.)
(See “Global Glacier
Volume Change” on this site for more on melting glacier.) |
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These photographs,
taken in 1928 and 2000, show how South Cascade
Glacier in the Washington Cascade Mountains
has retreated over time.
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| Evidences
and Uncertainties |
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Melting ice could change ocean temperatures.
This, in turn, could change the course and speed of ocean currents,
significantly change the habitats of sea organisms, and affect
rainfall by altering the rate of evaporation of seawater.
Increases in sea levels and temperatures are not the only possible
outcomes. When ice and snow melt, they generally expose a much
darker underlying surface. Dark surfaces absorb more heat (have
a lower albedo) than light surfaces. This suggests the possibility
that a small amount of melting could lead to a warmer surface,
which could melt more ice, warming the surface still further—initiating
the positive feedback loop of a “runaway” warming
trend. There is some evidence of such an albedo-reducing effect
in the Cretaceous Period (120–65 million years ago): Fossil
and other evidence suggests that there was little or no snow
and ice cover during this time, and global temperatures then
were at least 8° to 10°C higher than they are now. (See
“Northern Hemisphere
Snow and Ice Chart” and “South
Pole/Ice Concentration” on this site to see the extent
of current snow and ice cover.)
The cryosphere also provides a way to study past climatic conditions.
If snow falls in a region of the earth where melting rarely
occurs, it leaves a layered record as it deposits contemporary
molecules and aerosols. As each layer is pushed deeper and deeper
under increasing pressure, the snow turns to ice, capturing
small bubbles of air. By examining ice cores taken from these
areas, we can determine associations between past temperature
and carbon dioxide levels. But one of the biggest problems in
any ice core study is determining the age-depth relationship.
Many different approaches have been used, and it’s now
clear that fairly accurate time scales can be developed for
at least the last 10,000 years. (See “Climate
records from the Vostok Ice Core Covering the Last 420,000 years”
on this site to learn more about the Antartica's Vostok ice
core.)
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glossary
terms
