Environmental Engineering Reference
In-Depth Information
Figure 7.5
Net regional release of carbon to the atmosphere as a result of deforestation during the 1980s
(teragrams of carbon)
Source:
After Mintzer (1992)
vegetation which causes the level of atmospheric
CO
2
to increase by reducing the amount recycled
during photosynthesis. Photosynthesis is a
process, shared by all green plants, by which solar
energy is converted into chemical energy. It
involves gaseous exchange. During the process,
CO
2
taken in through the plant leaves is broken
down into carbon and oxygen. The carbon is
retained by the plant while the oxygen is released
into the atmosphere. The role of vegetation in
controlling CO
2
through photosynthesis is clearly
indicated by variations in the levels of the gas
during the growing season. Measurements at
Mauna Loa Observatory in Hawaii show
patterns in which CO
2
concentrations are lower
during the northern summer and higher during
the northern winter (see Figure 7.4). These
variations reflect the effects of photosynthesis in
the northern hemisphere, which contains the bulk
of the world's vegetation (Bolin 1986). Plants
absorb CO
2
during their summer growing phase,
but not during their winter dormant period, and
the difference is sufficient to cause semi-annual
fluctuations in global CO
2
levels.
The clearing of vegetation raises CO
2
levels
indirectly through reduced photosynthesis, but
CO
2
is also added directly to the atmosphere by
burning, by the decay of biomass and by the
increased oxidation of carbon from the newly
exposed soil. Such processes are estimated to be
responsible for 5-20 per cent of current
anthropogenic CO
2
emissions (Waterstone 1993).
This is usually considered a modern
phenomenon, particularly prevalent in the
tropical rainforests of South America and South-
East Asia (Gribbin 1981) (see Figure 7.5), but
Wilson (1978) has suggested that the pioneer
agricultural settlement of North America,
Australasia and South Africa in the second half
of the nineteenth century made an important
contribution to rising CO
2
levels. This is
supported to some extent by the observation that
between 1850 and 1950 some 120 billion tonnes
of carbon were released into the atmosphere as
