Geoscience Reference
In-Depth Information
Table 3.3
Influence of greenhouse gases on atmospheric temperature.
Gas
Centres of
Temperature increase
Global warming potential
main absorption
(K) for
2 present
on a weight basis
bands (µm)
concentration
(kg
-1
of air)
†
Water vapour (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 6
0.1
3400
(CF
2
Cl
2
)
8.68, 9.13, 10.93
7100
Notes
:
*
Important in moist atmospheres.
†
Refers to direct annual radiative forcing for the surface-troposphere system.
Sources
: After Campbell; Ramanathan; Lashof and Ahuja; Luther and Ellingson; IPCC (1992).
box 3.1
topical issue
THE GREENHOUSE EFFECT
The
natural
greenhouse effect of the earth's atmosphere is attributable primarily to water vapour. It accounts for 21 K
of the 33 K difference between the effective temperature of a dry atmosphere and the real atmosphere through the
trapping of infra-red radiation. Water vapour is strongly absorptive around 2.4-3.1 µm, 4.5-6.5 µm and above 16 µm.
The concept of greenhouse gas-induced warming is commonly applied to the effects of the increases in atmospheric
carbon dioxide concentrations resulting from
anthropogenic
activities, principally the burning of fossil fuels. Sverre Arrhenius
in Sweden drew attention to this possibility in 1896, but observational evidence was forthcoming only some forty years
later (Callendar, 1938, 1959). However, a careful record of of atmospheric concentrations of carbon dioxide was lacking
until Charles Keeling installed calibrated instruments at the Mauna Loa Observatory, Hawaii, in 1957. Within a decade,
these observations became the global benchmark . They showed an annual cycle of about 5 ppm at the Observatory,
caused by the biospheric uptake and release, and the
c
. 0.5 per cent annual increase in CO
2,
from 315 ppm in 1957 to
370 ppm in 2001, due to fossil fuel burning. The annual increase is about half of the total emission due to CO
2
uptake
by the oceans and the land biosphere. The principal absorption band for radiation by carbon dioxide is around 14-16
µm, but there are others at 2.6 and 4.2 µm. Most of the effect of increasing CO
2
concentration is by enhanced absorption
in the latter, as the main band is almost saturated. The sensitivity of mean global air temperature to a doubling of CO
2
in the range 2 to 5°C, while a removal of all atmospheric CO
2
might lower the mean surface temperature by more than
10°C.
The important role of other trace greenhouse gases (methane and fluorocarbons) recognized in the 1980s and
many additional trace gases began to be monitored and their past histories reconstructed from ice core records. These
show that the pre-industrial level of CO
2
was 280 ppm and methane 750 ppb; these values decreased to about 180
ppm and 350 ppb, respectively, during the maximum phases of continental glaciation in the Ice Age.
The positive feedback effect of CO
2,
which involves greenhouse gas-induced warming leading to an enhanced
hydrological cycle with a larger atmospheric vapour content and therefore further warming, is still not well resolved
quantitatively.
