Geoscience Reference
In-Depth Information
(CH
4
), nitrous oxide (N
2
O) and tropospheric
ozone (O
3
). These gases absorb strongly at
wavelengths within the atmospheric window
region, in addition to their other absorbing bands
(see
Figure 3.1
and
Table 3.3
). Moreover, because
concentrations of these trace gases are low, their
radiative effects increase approximately linearly
with concentration, whereas the effect of CO
2
is
related to the logarithm of the concentration. In
addition, owing to the long atmospheric residence
time of nitrous oxide (132 years) and CFCs
(65-140 years), the cumulative effects of human
activities will be substantial. It is estimated that
between 1765 and 2000, the radiative effect of
increased CO
2
concentration was 1.5W m
-2
, and
of all trace gases about 2.5W m
-2
considerably warmer during the early evolution of
the earth, when the atmosphere contained large
quantities of methane, water vapor and ammonia.
The largely carbon dioxide atmosphere of Venus
creates a 500K greenhouse effect on that planet.
Stratospheric ozone absorbs significant
amounts of both incoming ultraviolet radiation,
harmful to life, and outgoing terrestrial longwave
re-radiation, so that its overall thermal role is
a complex one. Its net effect on earth surface
temperatures depends on the elevation at which
the absorption occurs, being to some extent a
trade-off between shortwave and longwave
absorption in that:
(
cf
. the solar
1
An increase of ozone above about 30km absorbs
relatively more incoming shortwave radiation,
causing a net
decrease
of surface temperatures.
2
An increase of ozone below about 25km
absorbs relatively more outgoing longwave
radiation, causing a net
increase
of surface
temperatures.
constant value of 1366W m
-2
).
The net warming contribution of the natural
(non-anthropogenic) greenhouse gases to the
mean 'effective' planetary temperature of 255K
(corresponding to the emitted infrared radiation)
is approximately 33K. Water vapor accounts for
21K of this amount, carbon dioxide 7K, ozone
2K, and other trace gases (nitrous oxide, methane)
about 3K. The present global mean surface
temperature is 288K, but the surface was
Longwave radiation is not merely terrestrial in
the narrow sense. The atmosphere radiates to
space, and clouds are particularly effective since
Table 3.3
Influence of greenhouse gases on atmospheric temperature
Gas
Centres of
Temperature increase
Global warming
main absorption
(K) for
×
2 present
potential on a
μ
bands (
m)
concentration
weight basis
(kg
-1
of air)†
Water vapor (H
2
O)
6.3-8.0, >15
(8.3-12.5)*
Carbon dioxide (CO
2
)
(5.2), (10), 14.7
3.0 ± 1.5
1
Methane (CH
4
)
6.52, 7.66
0.3-0.4
11
Ozone (O
3
)
4.7, 9.6, (14.3)
0.9
Nitrous oxide (N
2
O)
7.78, 8.56, 17.0
0.3
270
Chlorofluoromethanes
(CFCl
3
)
4.66, 9.22, 11.82
0.1
3400
(CF
2
Cl
2
)
8.68, 9.13, 10.93
7100
Sources: After Campbell; Ramanathan; Lashof and Ahuja; Luther and Ellingson; IPCC (1992).
Notes: *Important in moist atmospheres.
†Refers to direct annual radiative forcing for the surface-troposphere system.
