Geoscience Reference
In-Depth Information
Wavelength (µm)
25
20 18
15
14
13
12
11
10
9
8
7
140
300
120
280
100
260
80
240
60
220
200
40
20
400
600
800
1000
1200
1400
1600
Wavenumber (cm
-1
)
Figure 4.6 Emission spectrum of the earth observed at the top of the
atmosphere, with Planck curves for various temperatures drawn.
Adapted from Hanel et al. (1972).
shows Planck curves (emission spectra) for temperatures representative of so-
lar and terrestrial temperatures. Note the scaling of the axes needed to display
these two spectra on one plot. It is important to remember that molecular
absorption is irrelevant to the climate system if it occurs at wavelengths not
covered by these spectra.
Figure 4.7b
shows absorption spectra for the stratosphere (top) and the en-
tire atmosphere (bottom). The atmosphere is more transparent at solar wave-
lengths than at terrestrial wavelengths. For wavelengths shorter than about
0.3 mm and longer than about 22 mm, the atmosphere is opaque.
The panels of
Figure 4.7c
portray absorption spectra for individual mol-
ecules. The opacity of the atmosphere at long wavelengths (l
> 22 mm) is due to
absorption by H
2
O, and its opacity at short wavelengths (l
< 0.3 mm) is due to
O
3
, as discussed above. Strong absorption near the peak of the terrestrial spec-
trum is due to CO
2
. Methane absorbs approximately 90% of the energy at 3.3
mm and 7.7 mm. N
2
O absorption removes more than 90% of the radiation near
4.5 mm and 7.8 mm, and the O
3
absorption band at 9.6 mm is also powerful.
4.5 THE GREENHOUSE EFFECT
Absorption of longwave radiation by greenhouse gases in the atmosphere re-
distributes heat within the climate system, warming the surface and lower tro-
posphere by a process known as the
greenhouse effect
.
The
slab atmosphere model
is an idealized version of the earth/atmosphere
system that can be used to demonstrate the greenhouse effect, and allows us
to make quantitative estimates of the influence of the greenhouse effect on
atmospheric and surface temperatures. The spherical geometry of the earth is
removed from the problem, and the surface is modeled as a semi-ininite plane
