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Earth's Energy Budget
Part 2
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Principle
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Absorption and re-emission of radiation
at the earth's surface is only one part of an intricate web
of heat transfer in the earth's planetary domain. Equally
important are selective absorption and emission of radiation
from molecules in the atmosphere. If the earth did not have
an atmosphere, surface temperatures would be too cold to
sustain life. If too many gases which absorb and emit
infrared radiation were present in the atmosphere, surface
temperatures would be too hot to sustain life.
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Figure 2 - Globally Averaged Energy Budget
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Outgoing Terrestrial Radiation
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- The earth's surface, atmosphere, and clouds emit
radiation in the infrared band and near-infrared band.
- Outgoing infrared (IR) radiation from the earth's
surface (also called terrestrial radiation) is
selectively absorbed by certain molecules, particularly
water vapor and carbon dioxide.
Gases which absorb IR radiation
are termed collectively as "greenhouse gases".
- Water vapor and carbon dioxide
emit infrared radiation.
Infrared radiation from
greenhouse gases in the atmosphere is emitted in all
directions, including back to the earth's surface. It is
this re-emission to the earth's surface that maintains a
higher temperature on our planet than what would be
possible without the atmosphere.
- Condensed water is also an efficient absorber and
emitter of IR radiation. Thus, clouds act in a manner
similar to greenhouse gases.
- Satellite infrared imagery detects infrared emission
from clouds and the earth's surface and can be used
during both day and night.
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Global Energy Balance
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- When averaged over a year, the incoming energy in
both the earth and its atmosphere equals the outgoing
energy.
If we consider the entire
Earth-atmosphere system, then the amount of radiation
entering the system must equal to the amount leaving, or
the system would continually heat or cool. Not all of
this energy is radiative energy; some is sensible and
latent heat.
- If we consider the atmosphere
alone, we find that the atmosphere experiences
radiative cooling.
The atmosphere is kept from a
net cooling by the addition of energy by latent and
sensible heating.
- The atmosphere has a warming
effect on Earth's surface
-- the "atmospheric greenhouse effect".
If Earth had no atmosphere, the
globally averaged surface temperature would be -18
degrees Celsius. Because Earth does have an atmosphere,
the average surface temperature actually is 15 degrees
Celsius.
The atmosphere acts as a
greenhouse because of gases that selectively allow solar
radiation to pass through but absorb and then re-emit
terrestrial radiation. These gases are collectively
called "greenhouse gases" and include water vapor, carbon
dioxide, ozone, molecular oxygen, methane and nitrous
oxide. These gases are selective as to which wavelengths
they will absorb. For example, ozone absorbs shortwave
ultraviolet radiation whereas water vapor absorbs
infrared radiation more readily.
- Most of the sun's radiation
that passes through the atmosphere to hit the earth is in
the visible part of the spectrum.
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