Emissions Scenarios - IPCC

Scenario Driving Forces 145
the extent of rice ai'ea), but also by changes in the productivity
of animals as these alter the CH^ emission factor.
Some authors doubt that assumed increases in meat production
and animal productivity can be sustained indefinitely. For
example, Brown and Kane (1995) argue that livestock
production cannot be increased greatly because nearly all of
the world's suitable rangelands are intensively exploited
already. They claim that the rapidly growing demand for meat
and dairy products can only be met by livestock production in
feedlots, which would result in a rising demand for feed that
requires further development of agricultural land and further
GHG emissions.
an emission factor by the sum of mineral and organic nitrogen
applied as fertilizer. The emission factor depends on the
fertilizer type and local environmental circumstances, and
those used in IPCC (1996) resuh in an assumed loss of 1.25%
(range 0.25 to 2.25%) of nitrogen as N2O per year.
To estimate the trend in fertilizer use, difterent references
employ different approaches. For example, Leggett et al.
(1992) directiy estimate the amount of feitilizer used, whereas
Alcamo et al. (1996) back-calculate fertilizer use from the
future amount of agricultural land. Despite these difterent
approaches, estimates of future fertilizer use are quite
consistently given as an increase by about a factor 1.4 to 2.8
between 1990 and 2100.
3.5.5. Nitrous Oxide Emissions from Agriculture
budgets are associated with considerable uncertainties.
Agricultural activities and animal production systems are the
largest anthropogenic sources of these emissions. Recent
calculations using IPCC 1996 revised guidelines indicate that
emission from agriculture is 6.2 MtN as N^O per year
(IPCC, 1996; Mosier et al., 1998). About one-third is related to
direct emissions from the soil, another third is related to N^O
emission from animal waste management, and the final third
originates from indirect emissions through ammonia
(NH3), nitrogen oxides (N0,,), and nitrate losses. This compares
to earlier estimates of total anthropogenic emissions that range
between 3.7 and 7.7 MtN (Houghton et al., 1995). Industrial
sources contribute between 0.7 to 1.8 MtN (Houghton et al.,
1995; see also Chapter 5, Table 5-3 and Section 3.6.2).
Total natural emissions amount to 9.0 ± 3.0 MtN as N2O, so
oceans, tropical, and temperate soils are together the most
important source of N2O today. Atmospheric concentrations of
N2O in 1992 were 311 parts per billion ( 10') by volume (ppbv)
(Houghton et ai, 1995); the 1993 rate of increase was 0.5 ppbv,
somewhat lower than that in the previous decade of
approximately 0.8 ppbv per year (Houghton et al., 1996).
Among the anthropogenic sources, cultivated soils are the most
important, contributing 50 to 70% of the anthropogenic total
(see Chapter 5, Table 5-3). This source of N2O is particularly
uncertain as the emission level is a complex function of soil
type, soil humidity, species grown, amount and type of
fertilizer applied, etc. The second largest anthropogenic source
of N2O is industry; two processes account for the bulk of
industrial emissions - nitric acid (HNO3) and adipic acid
production. In both cases N2O is released with the off-gases
from the production facilities. Recently, N2O release from
animal manure was identified as another significant source of
N,0 emissions.
N^O emissions from agricultural soils occur through the
nitrification and denitrification of nitrogen in soils, particularly
that from mineral or organic fertilizers. Emissions are very
dependent on local management practices, fertilizer types, and
climatic and soil conditions, and are calculated by multiplying
Although the different references are consistent in their
findings about future global fertilizer use, the question arises
whether these are at all reasonable guesses. Some researchers
assume that fertilizer use will increase even more. For
example, Kendall and Pimentel (1994) in their "business-asusual"
scenario assume a 300% increase in the use of nitrogen
and other fertilizers by 2050. Moreover, most studies of future
worid food production assume improvements in crop yield.
These yield improvements may imply higher overall rates of
fertilizer use because many high-yielding crop varieties depend
on large amounts of fertilizer.
However, some authors question whether global average
fertilizer use will grow. For example. Brown and Kane (1995)
note that world fertilizer use has actually fallen in recent years
and Kroeze (1993) assumes that per capita N2O emissions from
fertilizer consumption decrease by 50% in 2100 relative to 1990
tlnough policies that promote the more efficient use of synthetic
fertilizers. Future fertilizer use may also be lower than in the
"business-as-usual" scenarios because farmers have other
incentives to reduce nitrogen fertilizer use, such as to reduce
farming costs and avoid nitrate contamination of groundwater.
This brief review of the literature on prognoses of fertilizer use
indicates that the N^O emission scenarios depicted in Figure 3-
16 do not take into account the full range of views about future
trends in fertilizer use. Additional uncertainty in future
emissions occurs because changes in the number of livestock,
as discussed above for CH^ emissions, and animal husbandry
practices will also affect N2O emissions.
3.5.6. Findings Regarding Driving Forces
Herein, some of the many specific factors that affect scenarios
of land-use emissions have been discussed. From a correlation
analysis that compared the influence of changing population,
economic activity, and technological change on land-use
emission scenarios, Alcamo and Swart (1998) concluded that
population was the most influential driving force. The reason is
the relationship between population and increasing food
demand, which leads to more cows that produce CH^ and more
extensive fertilized croplands that release N^O. Although most