Agriculture Reference
In-Depth Information
3.2.6
Other Agriculture Sources
of Non-CO
2
Emissions
(CH
4
, N
2
O)
The combined contribution of deforestation
and forest degradation emissions to total
human-generated CO
2
emissions is about
12 % (with a range of 6-18 %) (Van der Werf
et al.
2009
).
Between 1980 and 2000, more than 55 % of
new agricultural land replaced intact forests;
another 28 % replaced degraded forests. Even
with agricultural yield increases and intensifi -
cation, net agricultural area expansion will
probably be needed to meet future demand
(Gibbs et al.
2010
).
Land-use activities, primarily the expansion
of agricultural land and the extraction of tim-
ber, have caused a net loss of ~7 to 11 million
km
2
of forest in the past 300 years (Foley et al.
2005
).
Forests cover about 3,952 million hectares of
the globe - about 30 % of the world's land
area. From 2000 to 2005, gross deforestation
continued at a rate of 12.9 million hectares per
year. Due to afforestation, landscape restora-
tion, and the natural expansion of forests, the
most recent estimate of net forest loss is 7.3
million hectares per year (IPCC
2007
).
Croplands and pastures have become one of
the largest terrestrial biomes on the planet,
occupying ~40 % of the land surface and rival-
ing forest cover in extent (Foley et al.
2005
).
Between 1963 and 2005, the global area of
cropland harvested increased 30 % from 8.4
million km
2
to 11.0 million km
2
.
Managed grazing occupies 25 % of the global
land surface (more than 33 million km
2
),
making it the planet's single most extensive
form of land use (Asner et al.
2004
).
Land use for the livestock sector spans more
than 39 million km
2
(~30 % of the world's sur-
face land area). Of this, 5 million km
2
is crops,
most of which are intensively managed; 14 mil-
lion km
2
is pasture with relatively high produc-
tivity; and 20 million km
2
is extensive pastures
with relatively low productivity (FAO
2006
).
Some irrigated lands have become heavily
salinized, causing a worldwide loss of ~1.5
million hectares of arable land per year and an
estimated US$ 11 billion in lost production
(Foley et al.
2005
).
This category includes emission sources from the
agricultural sector that are relatively small com-
pared to the sector overall. The data presented
include the following sources of CH
4
and N
2
O:
agricultural soils (CH
4
), fi eld burning of agricul-
tural residues (CH
4
, N
2
O), prescribed burning of
savannas (CH
4
, N
2
O), and open burning from for-
est clearing (CH
4
).
Between 1990 and 2005, total emissions from
other agricultural sources decreased from
1,283 to 1,164 MtCO
2
e (US-EPA
2011
), cor-
responding to 19 % of total agricultural emis-
sions (Table
3.2
) .
3.2.7
Deforestation Emissions
Agriculture is the leading cause of some 75 % of
global deforestation. If rates of deforestation con-
tinues as projected, forests will diminish dramati-
cally by 2100 (Strassburg et al.
2012
).
Deforestation and land-use change (the con-
version of forests into farmland) account for
2,200-6,600 million metric tons of carbon
dioxide equivalent (Mt CO
2
e) per year or
30-50 % of agricultural emissions and about
4-14 % of global emissions (Vermeulen et al.
2012
).
Since 1850, land-use change directly contrib-
uted some 35 % of human-generated CO
2
emissions (Foley et al.
2005
).
Past trends imply that ~10 million km
2
of land
will be cleared by 2050 to meet demand, lead-
ing to annual emissions of 3,000 MtCO
2
e per
year. A future course that spares more land
could reduce land clearing to ~2 million km
2
and GHG emissions to 1,000 MtCO
2
e/year per
year (Tilman et al.
2001
).
In the 1980s and 1990s, rainforests were the
primary source of new agricultural land in
the tropics. Future expansion of the global
agricultural land base will clear tropical for-
ests and shrub land ecosystems (Gibbs et al.
2010
).
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