Geoscience Reference
In-Depth Information
3.1 The greenhouse effect
The natural greenhouse effect of the earth's atmosphere is attributable primarily to water vapor. It
accounts for 21K of the 33K difference between the effective temperature of a dry atmosphere and
the real atmosphere through the trapping of infrared radiation. Water vapor is strongly absorptive
around 2.4-3.1
m. 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 only
forthcoming some 40 years later (Callendar, 1938, 1961). 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 5ppm at the Observatory, caused by the
biospheric uptake and release, and the ca. 0.4 percent annual increase in CO
2,
from 315ppm in 1957
to 383ppm in 2007, 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
μ
m, but there are others at 2.6 and 4.2
μ
m. 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
is in the range 2-5
°
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, nitrous oxide, fluorocarbons) was
recognized in the 1980s and many additional trace gases began to be monitored. The latest is
nitrogen trifluoride used during the manufacture of liquid crystal flat-panel displays, thin-film solar
cells and microcircuits. Although concentrations of the gas are currently only 0.454 parts per
trillion, it is 17,000 times more potent as a global warming agent than a similar mass of carbon
dioxide.
The past histories of greenhouse gases, reconstructed from ice core records, show that the pre-
industrial level of CO
2
was 280ppm and methane 750ppb compared with 383ppm and 1790ppb,
respectively, today. Their concentrations decreased to about 180 ppm and 350ppb, respectively,
during the maximum phases of continental glaciation in the Pleistocene 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 vapor content and therefore further
warming, is still not well resolved quantitatively.
μ
m, 4.5-6.5
μ
m and above 16
μ
they act as black bodies. For this reason, cloudi-
ness and cloud-top temperature can be mapped
from satellites by day and night using infrared
sensors. Radiative cooling of cloud layers averages
about 1.5
mean absorbed solar radiation is approximately
285W m
-2
, whereas the emitted terrestrial radia-
tion is 265Wm
-2
. Including cloud-covered areas,
the corresponding global values are 235W m
-2
for
both terms. Clouds reduce the absorbed solar
radiation by 50W m
-2
, but reduce the emitted
radiation by only 30W m
-2
. Hence, global cloud
C per day.
For the globe as a whole, satellite measure-
ments show that in cloud-free conditions the
°
