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anthropogenic emission rate of the carbon in carbon
dioxide.
Permanent deforestation is one mechanism of deser-
tification ,theconversion of viable land into desert.
Desertification also results from overgrazing and is a
problem at the border of the Sahara Desert, which is
in continuous expansion. It increases the Earth's albedo
and decreases the specific heat of the ground. Desert
sand is more reflective than is forest or vegetation;
therefore, desertification tends to cool the ground when
only albedo change is considered. However, sand has
alower specific heat than do the trees that it replaces;
thus, expansion of deserts can also warm the ground
when only specific heat changes are considered. The net
effect of desertification on temperatures is uncertain.
Figure 12.11 shows the increase in anthropogenic
emissions of CH 4 (g) from different sources between
1860 and 1994. Total anthropogenic methane emissions
increased during that period from 29.3 to 371 Tg/yr. The
largest component of anthropogenic methane emissions
in 1994 was livestock farming, which overtook rice
farming as the leading methane emission source.
10 4
1000
Solid fuels
100
Gas fuels
Tot al
Liquid fuel s
10
Gas
flaring
Cement production
1
1750
1800
1850
1900
1950
2000
Year
Figure 12.9. Global anthropogenic emissions of
carbon dioxide from 1750 to 2007. 1 Tg (teragram)
=
10 6 metric tonnes
=
10 12 g. Data from Boden et al.
(2011).
10 6 g)
of CO 2 (g) were emitted. Half of the total emissions
occurred between 1982 and 2007. The 2007 emission
rate was 8,400 Tg/yr, an increase of 1.7 percent over
the 2006 emission rate and the largest yearly emis-
sion rate up to that time. Liquid and solid fuel combus-
tion accounted for 76 percent of anthropogenic CO 2 (g)
emissions in 2007. Cement production and gas flaring
(burning of natural gas emitted from coal mines and
oil wells) contributed to about 5 percent of the total
CO 2 (g) emissions. Rates of gas flaring have decreased
as a result of improved technologies enabling the cap-
ture of natural gas during coal and oil mining.
Amajor anthropogenic source of CO 2 (g) not
accounted for in Figure 12.9 is permanent deforesta-
tion. Deforestation is the clear-cutting of forests for
their wood and the burning of forests to make room
for farming and cattle grazing. Permanent deforestation
occurs when cut and burned forests are not permit-
ted to regrow, such as during conversion of a forest
to grazing land. The loss of trees resulting from defor-
estation prevents photosynthesis from converting atmo-
spheric CO 2 (g) to organic material. Deforestation rates
are high in tropical rainforests of South America, Africa
(Figure 12.10), and Indonesia. Clear-cutting of forests
for their wood in the Pacific Northwest of the United
States, in parts of Canada, and in other forested regions
of the world is also common. Permanent deforestation
results in a global emission rate of 1,500 to 2,700 Tg-
C/yr (Houghton, 1994; Lobert et al., 1999; Andreae
and Merlet, 2001), or one-fourth of the total annual
10 6 metric tonnes
337,000 teragrams (1 Tg
=
=
Figure 12.10. Fires in Africa, October 1994-March
1995. Fires are denoted by red; flares from oil and
gas exploration/extraction by green. The more
intense the color, the greater the frequency of fires.
Data collected by U.S. Air Force Weather Agency;
image and data processing by NOAA's National
Geophysical Data Center.
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