Geoscience Reference
In-Depth Information
circulation responsible for this transfer is not yet known
with certainty, although it does not seem to be a simple,
direct one.
The water vapour content of the atmosphere is
related closely to air temperature (see B.2, this chapter,
and Chapter 4B and C) and is therefore greatest in
summer and in low latitudes. There are, however,
obvious exceptions to this generalization, such as the
tropical desert areas of the world.
The carbon dioxide content of the air (currently aver-
aging 372 parts per million (ppm)) has a large seasonal
range in higher latitudes in the northern hemisphere
associated with photosynthesis and decay in the bio-
sphere. At 50°N, the concentration ranges from about
365 ppm in late summer to 378 ppm in spring. The
low summer values are related to the assimilation of
CO
2
by the cold polar seas. Over the year, a small
net transfer of CO
2
from low to high altitudes takes place
to maintain an equilibrium content in the air.
7 Variations with time
The quantities of carbon dioxide, other greenhouse
gases and particles in the atmosphere undergo long-term
variations that may play an important role in the earth's
radiation budget. Measurements of atmospheric trace
gases show increases in nearly all of them since the
Industrial Revolution began (Table 2.3). The burning
of fossil fuels is the primary source of these increasing
trace gas concentrations. Heating, transportation and
industrial activities generate almost 5
10
20
J/year
of energy. Oil and natural gas consumption account
for 60 per cent of global energy and coal about 25
per cent. Natural gas is almost 90 per cent methane
(CH
4
), whereas the burning of coal and oil releases
not only CO
2
but also odd nitrogen (NO
x
), sulphur
and carbon monoxide (CO). Other factors relating to
agricultural practices (land clearance, farming, paddy
cultivation and cattle raising) also contribute to modi-
fying the atmospheric composition. The concentrations
and sources of the most important greenhouse gases
are considered in turn.
Carbon dioxide
(CO
2
). The major reservoirs of
carbon are in limestone sediments and fossil fuels. The
atmosphere contains just over 775
10
12
kg of carbon
(C), corresponding to a CO
2
concentration of 370 ppm
(Figure 2.4). The major fluxes of CO
2
are a result of
solution/dissolution in the ocean and photosynthesis/
respiration and decomposition by biota. The average
Figure 2.3
Variation of total ozone with latitude and season
in Dobson units (milliatmosphere centimeters) for two time
intervals: (top) 1964-1980 and (bottom) 1984-1993. Values over
350 units are stippled.
Source
: From Bojkov and Fioletov (1995). From
Journal of Geophysical
Research
100 (D), Fig. 15, pp. 16, 548. Courtesy of American
Geophysical Union.
processes, the maximum would occur in June near the
equator, so the anomalous pattern must result from a
poleward transport of ozone. Apparently, ozone moves
from higher levels (30 to 40 km) in low latitudes towards
lower levels (20 to 25 km) in high latitudes during the
winter months. Here the ozone is stored during the
polar
night
, giving rise to an ozone-rich layer in early spring
under natural conditions. It is this feature that has been
disrupted by the stratospheric ozone 'hole' that now
forms each spring in the Antarctic and in some recent
years in the Arctic also (see Box 2.1). The type of
