Geoscience Reference
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1.2 Agriculture as a contributor
At the same time, agriculture has not only been shown to pro-
duce significant effects on climate change primarily through
the production and release of GHGs, such as carbon dioxide,
methane and nitrous oxide, but also by altering the Earth's land
cover that can change its ability to absorb or reflect heat and
light, thus contributing to radiative forcing. Land use change
such as deforestation and desertification, together with the use
of fossil fuels are the major anthropogenic sources of carbon
dioxide. Besides the problems associated with land use through
deforestation for example, can translate into increased ero-
sion. Agriculture itself is the major contributor to increasing
methane and nitrous oxide concentrations in the Earth's atmo-
sphere. Agricultural practices themselves have often added to
the water shortage problem as in Africa or other arid/semi-
arid areas more than anywhere else due to the differences in
property rights. More precisely, because farmers are often not
the owners of the land they work on, the preservation of natu-
ral resources is generally viewed as a secondary objective. In
addition, pressures represented by increasing populations and
changing technology add to the problem of land deterioration
related to agricultural practices (Drechsel et al., 2001). Another
illustration of environment-damaging agricultural practices
is the intense use of fertilisers in low-quality lands. As yields
increase, so will water consumption, thus creating a vicious
circle (Gommes and Petrassi, 1996).
Global trends
Agriculture releases into the atmosphere significant amounts
of GHGs, that is, CO 2 , CH 4 and N 2 O (Cole et  al., 1997;
Anonymous, 2001a,b; Paustian et  al., 2004). CO 2 is released
from microbial decay or burning plant litter and soil organic
matter (Janzen, 2004; Smith, 2004a-c); CH 4 from fermenta-
tive digestion by ruminants, stored manures, paddy cultivation
or decomposition of organic materials in anaerobic conditions
(Mosier et  al., 1998) and N 2 O from microbial transformation
of nitrogen in soils and manures especially under wet condi-
tions where available nitrogen exceeds plant requirements
(Smith and Conen, 2004; Oenema et  al., 2005). Direct agri-
cultural emissions were 10-12% of the total anthropogenic
GHG emissions in 2005, that is, 5.1-6.2 Pg CO 2 -eq. (Smith
et  al., 2007a). CH 4 contributes 3.3 GtCO 2 -eq. year −1 and N 2 O
2.8 GtCO 2 -eq. year −1 . Of the global anthropogenic emissions in
2005, agriculture accounted for about 60% of N 2 O and about
50% of CH 4 (Denman et  al.,  2007). Globally, agricultural
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