An Introduction to Environmental Chemistry

is cleared and in part burned, but also by bacterially aided decomposition of dead

plant matter, including the soil litter (see Section 4.6.5). None of the new uses

for the land store carbon as effectively as the original forest. Even cultivated land,

which might appear to be a good store of carbon, contains approximately 20 times

less fixed carbon per hectare than a typical mature forest.

Humans have been converting virgin forest and other well-vegetated areas into

carbon-poor states for many hundreds of years. This process must therefore be

a substantial source of CO 2 to the atmosphere, both in the past and today. It has,

however, been difficult to quantify the size of this source. Several attempts have

been made to assess how its magnitude has varied over the last century (arguably

the period of most rapid change in land use the world has ever experienced). The

results from three studies, published in 1983, 1990 and 1993, are shown in Fig.

7.3. There are large discrepancies between the three results and it appears that

the earliest attempt overestimated the source compared with the more recent

studies. The best estimate of the flux for the 1980s is 1.7 GtC yr-^1 , with a range

from 0.6 to 2.5. The large range confirms the considerable difficulty in trying to

quantify CO 2 emissions due to changes in land use.

244 Chapter Seven

Annual CO

flux (GtC yr 2

–1)

1850 1870 1890 1910 1930 1950 1970

1990

1983

1993

1990

Year

0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 7.3Estimates of CO 2 flux to the atmosphere from land-use changes made in 1983, 1990 and 1993. After

Houghton (2000). With kind permission of Cambridge University Press.