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Agriculture
inevitably makes a contribution to greenhouse gases, as does just about
every
human activity. Modern industrial agriculture is responsible for 25% of
the
world's carbon dioxide emissions, 60% of methane gas emissions and 80%
of
nitrous oxide.[i]
Nitrous oxide is generated through the action of denitrifying bacteria
in the
soil when land is converted to agriculture. When tropical rainforests
are
converted into a pasture, nitrous oxide emissions increase by three
times. Land
conversion is leading to the release of around half a million tonnes a
year of
nitrogen in the form of nitrous oxide.
Nitrous oxide is some 200 times more potent than carbon dioxide as a
greenhouse
gas, though fortunately atmospheric concentrations of nitrous oxide are
currently over 1,000 times lower than that of carbon dioxide - 0.31ppmv
compared with 365 ppmv. Nitrogenous fertilisers are another major
source of
nitrous oxide. Around 70 million tonnes a year of nitrogen are now
applied to
crops and are contributing as much as ten per cent of the total annual
nitrous
oxide emissions of 22 million tonnes. With fertiliser applications
increasing
substantially, especially in developing countries, nitrous oxide
emissions from
agriculture could double over the next 30 years.[ii]
Agricultural N2O emissions are projected to increase globally by 35-
60% up to
2030 due to increased nitrogen fertiliser use and increased animal
manure
production (FAO, 2002). Similarly, Mosier and Kroeze (2000) and US-EPA
(2006a)
estimated that N2O emissions will increase by about 50% by 2020
(relative to
1990). If demands for food increase, and diets shift as projected, then
annual
emissions of GHGs from agriculture may escalate further. But improved
management practices and emerging technologies may permit a reduction
in
emissions per unit of food (or protein) produced, and perhaps also a
reduction
in emissions per capita food consumption. [iii]
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