Geoscience Reference
In-Depth Information
Table 12.2. Estimates of global N
2
O sources
Natural sources
(TgN/yr)
Soil
6.6
Oceans
3.8
Atmospheric chemistry/lightning
0.6
Total
11.0
Anthropogenic sources
(TgN/yr)
Fossil fuels
0.7
Agriculture
2.8
Biomass burning
0.7
Human waste
0.2
Rivers, estuaries, coastal waters
1.7
Atmospheric deposition
0.6
Total
6.7
Note
: Based on Table 7.7 in IPCC, 2007.
account for about 95% of the natural production of N
2
O; the remaining 5% is
produced by chemical reactions in the atmosphere (see
Table 12.2)
.
Natural nitrogen fixation in soils is insufficient to support modern agricul-
tural practice, so nitrogen fertilizer is added to agricultural soils. This addition
enhances the natural nitrogen cycle in soils and, consequently, N
2
O emissions
(
Table 12.2
). In addition, agricultural activity amplifies the natural source of
N
2
O from rivers, estuaries, and the coastal ocean when runoff from agricultural
land increases the nitrogen loading. Fossil fuel burning also produces atmo-
spheric N
2
O as a combustion product. Municipal waste, in the form of sludge
from waste management facilities, is another anthropogenic source of N
2
O.
As one can easily deduce from the numbers in
Table 12.2
, human activity
has significantly perturbed the global nitrogen cycle, and this is why atmo-
This overview provides only the briefest introduction to the biogeochemical
cycles of radiatively active components of the earth's atmosphere. The pro-
cesses are complex, and developing accurate and complete models will require
not only an accounting of the various sources and sinks but knowledge of the
nonlinear organic and inorganic interactions among natural and anthropogenic
systems. Global biogeochemical modeling is in the early stages of development.
It is an important area of research needed to complete our understanding of the
climate system and refine projections of the future.
12.6 EVALUATING MODEL UNCERTAINTY
Although climate models are multifaceted and sophisticated, the complexity of
the climate system may seem overwhelming, as
Figure 1.2
reminds us. Clearly,
