trast, in autumn and winter, when the processes of respiration and decomposi-
tion of plants remains dominate over photosynthesis (see Section 5.5), the net
flux is into the air. Averaged over the whole yearly cycle there is no net flux in
either direction. In the tropics, where there is less seasonality in biological
processes, the up and down fluxes are in approximate balance throughout the
year. However, it should be noted that in the tropics, as at higher latitudes,
the fluxes show considerable spatial variability (patchiness).
The seasonal asymmetry in the up and down CO 2 fluxes at middle and high
latitudes provides the explanation for the seasonal cycle of atmospheric CO 2
shown in Fig. 7.1. The decreasing values found in spring and summer result from
net plant uptake of CO 2 from the air during photosynthesis and the rising limb
is due to net release of CO 2 during the rest of the year when respiration and
decomposition are dominant. The amplitude of this seasonal pattern varies with
latitude, being least at the poles (see CO 2 record for the South Pole in Fig. 7.1)
and equator due to lack of biological activity and seasonality respectively. At mid-
and sub-polar latitudes the amplitude (peak to peak) is 10–15 ppm, i.e. consider-
ably greater than the average yearly increase (1–2 ppm). The amplitude tends to
be greater in the northern compared with the southern hemisphere because of
the greater land area in the former compared with the latter. With the uptake
and release of CO 2 during photosynthesis and respiration/decomposition there
is a concomitant release and absorption of atmospheric oxygen, as indicated in
equations 5.19 and 5.20 (see Section 5.5). It has recently become possible to
measure these changes in atmospheric oxygen and the data are shown in the inset
to Fig.7.1. The oxygen record is much shorter than that for CO 2 due to the very
considerable analytical difficulties of measuring the small percentage changes in
oxygen compared with CO 2 , due to the former being about 550 times more abun-
dant. The seasonality discussed above for CO 2 is observed for oxygen at both
Cape Grim in Tasmania (southern hemisphere) and Barrow in Alaska (northern
hemisphere), but with the opposite sign (i.e. when atmospheric CO 2 is falling due
to plant uptake during photosynthesis, oxygen is rising, and vice versa). It is also
clear that the seasonality for atmospheric oxygen is displaced by 6 months
between the Barrow and Cape Grim measurement sites, due to their location in
the northern and southern hemispheres, respectively.
From the above discussion it is apparent that, while human activities in
burning fossil fuel are the primary control on the year-to-year increase in atmos-
pheric CO 2 , it is biologically induced exchanges that determine the observed sea-
sonal pattern. Thus, it is clear that the land biota can strongly affect the levels of
atmospheric CO 2. This raises the question of whether human activities, for
example through change in land use (e.g. clearing of virgin forest), or through
enhanced photosynthesis arising from the increasing concentration of atmos-
pheric CO 2 , can have produced significant net transfers of carbon into, or out of,
the atmosphere.
Turning first to changes in land use, it is clear that when areas formerly storing
large amounts of carbon fixed in plant material, for example forests, are converted
to urban, industrial or even agricultural use, a large percentage of the fixed carbon
is released to the atmosphere as CO 2 quite rapidly. This occurs when the forest
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