Geoscience Reference
In-Depth Information
Table 4.1. Absorption of
solar and terrestrial radiation by atmospheric molecules
Spectral regions ( mm)
a
Absorbing molecule
Solar
wavelengths
~0.1-2.5 mm
Terrestrial
wavelengths
~2.5-25 mm
Approximate
atmospheric
residence time
b
Carbon dioxide
(CO
2
)
1.4, 1.6, 2.0,
0.78-1.24
(weak)
13.5-16.5,
centered at 15
4.2-4.4, centered
at 4.3
2.7, 5.2, 9.4, 10.4
5-200 years
Water vapor (H
2
O)
0.72, 0.81, 0.94,
1.1, 1.4, 1.9
5.5-7.5, centered
at 6.3
2.6-3.3
Water vapor
continuum
c
10 days
Ozone (O
3
)
0.18-0.34,
centered at 0.26
0.32-0.36,
0.44-0.74
2.7, 3.27, 3.59,
4.75, 5.75, 9.0
9.6, 14.1
2 months
Methane (CH
4
)
3.3, 7.7
10 years
Nitrous oxide (N
2
O)
4.5, 7.8, 17.0
100 years
Carbon monoxide
(CO)
1.19, 1.57, 2.35
4.67
2.38-25.0
A few months
a
Only absorption at wavelengths relevant for the climate system are listed. In general, these molecules will
also have absorption lines and bands in other wavelengths, but this absorption does not directly influence the
flow of radiative energy through the atmosphere.
b
Residence times vary according to the atmospheric conditions. These values are estimates.
c
Water vapor absorption is complicated (see text) because the combined vibrational and rotational modes
give rise to tens of thousands of absorption lines.
As with the solar spectrum, energy is absorbed from the terrestrial spectrum
(see
Table 4.1)
. Molecules in the earth's atmosphere that absorb longwave ra-
diation are called
greenhouse gases
.
2
The most abundant greenhouse gas in the
earth's atmosphere is water vapor, and it is responsible for about 70% of the
atmospheric absorption of terrestrial radiation globally. Combined vibrational
and rotational modes give rise to tens of thousands of water vapor absorp-
tion lines. Gas, liquid, and solid states of water are all found in the atmo-
sphere and have different absorption characteristics.
Continuum absorption
occurs throughout the infrared and microwave portions of the spectra. This
2
Some greenhouse gases also absorb shortwave radiation. Ozone, for example, is radiatively
active in both solar and terrestrial wavelengths
