Agriculture Reference
In-Depth Information
Figure 7.4
Adsorptive
potentials of several important
gases in the atmosphere. Also
shown are spectra for the
incoming solar energy and the
outgoing thermal energy from
Earth. Note that the wavelength
scale changes at 4
µ
m.
From Gilbert Masters,
Introduction to
Environmental Engineering and
Science
, Prentice Hall, Upper Saddle
River, NJ, 1998.
1
H
2
O
0
1
CO
2
0
1
CH
4
0
1
O
2
O
3
0
1
0
N
2
O
1
Total
0
Incoming
short
wavelengths
5800 K
Transmitted
Outgoing long
wavelengths
288 K
Absorbed
0
1
2
3
4
10
20
30
40
50
Wavelength (
µ
m)
The absorptive potential of several important gases is shown in Figure 7.4,
along with the spectra for the incoming light (short-wavelength) radiation and
outgoing heat (long-wavelength) radiation. Incoming radiation is impeded by
water vapor and oxygen and ozone. However, most of the light energy comes
through unimpeded.
The heat energy reradiated from the Earth's surface, however, encounters sev-
eral potential impediments. As it is trying to reach outer space, it finds that water
vapor, CO
2
,CH
4
,O
3
, and nitrous oxide (N
2
O) all have absorptive wavelengths
right in the middle of the heat spectrum. Quite obviously, an increase in the con-
centration of any of these will greatly limit the amount of heat transmitted into
space. Appropriately, these gases are called
greenhouse gases
because their presence
will limit the heat escaping into space, much as the glass of a greenhouse or the
glass in your car limits the amount of heat that can escape, thus building up the
temperature under the glass cover.
The effectiveness of a particular gas to promote global warming (or cooling,
as is the case with aerosols) is known as
forcing
. The gases of most importance
in forcing are listed in Table 7.3. Climate change results from natural internal
processes and from external forcings. Both are affected by persistent changes
in the composition of the atmosphere brought about by changes in land use,
release of contaminants, and other human activities.
Radiative forcing
, the change
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