Emissions Scenarios - IPCC

270 Emission Scenarios
GHGs to date (Houghton et al., 1996). As interest in the role of
sulfur has increased the since the previous IPCC assessment,
and to encapsulate recent trends and expectations, sulfur
emissions are discussed here in substantial detail. Nitrates,
ammonia, organic compounds, and black carbon also
contribute to the formation of atmospheric aerosols.
Carbonaceous aerosols exert a small positive forcing effect,
while the effects of other compounds and aerosols are less
clear. Tropospheric ozone is a GHG also, with a small net
positive forcing effect. Future tropospheric ozone levels will be
determined by emissions of CH^, CO, N0^,, and NMVOCs.
The last three groups are repotted and discussed here in a more
aggregated and stylized form only, because these gases are
short lived, their potential to form ozone is highly non-linear;
NMVOCs are not distinguished by their reactivity, and data
problems associated with including key sources in aggregated
long-term models are large.
5.5.1. Ozone Precursors: Nitrogen Oxides, Non-Methane
Volatile Organic Compounds, and Carbon
Monoxide
5.5.1.1. Nitrogen Oxides
Emissions of NO^, primarily result from the combustion of
fossil fuels. The N0^^ concentration in exhaust gases depends
on combustion conditions (temperature, residence time, air-tofuel
ratio, mixing) and varies widely across different
applications. In particular, internal combustion engines used in
road vehicles and ships have very high emissions, although
new designs and exhaust-gas treatment offer much lower
specific emission levels. Recent research (Davidson and
Kingerlee, 1997; Delmas et al., 1997; Mosier et ai, 1998)
indicates that soil may be a significant source of N0^^ emissions
also. This source, however, is not included in the models used
in the current report.
The 1990 NOjj emissions in the six SRES models range
between 26.5 and 34.2 MtN, but not all the models provide a
comprehensive description of N0^^ emissions. Some models do
not estimate N0^ emissions at all (MARIA, MiniCAM^),
whereas others only include energy-related sources
(MESSAGE) and have adopted other source categories from
corresponding model runs derived from other models (i.e.
AIM). Standardized (see Box 5-1 on Standardization) 1990
NO^ emissions in the SRES scenarios, measured as nitrogen,
amount to 31 MtN (Figure 5-9).
As mentioned in Chapter 4, the volume of fossil fuels used for
various energy purposes varies widely in the SRES scenario
families. In addition, the level and timing of emission controls,
inspired by local air quality concems, is assumed to differ. As
a result the spread is largest within the Al scenario family, in
* For the AlG-MiniCAM scenario emissions from congruent model
runs derived from other models have been estimated.
which it is almost as large as the range across all 40 SRES
scenarios. Up to the 2020s, all scenarios project rising N0^
emissions (Figure 5-9). The 25* and 75"^ percentile emissions
corridor spans between 40 and 60 MtN by the 2020s, which
can be inteipreted as a "central tendency" among the entire
spectrum of the 40 SRES scenarios. Beyond 2030,
uncertainties in emission levels increase significantly. By 2100,
the SRES range is between 16 and 150 MtN (i.e. emissions
decrease by a factor of two or increase by a factor of five
compaied with 1990 levels). The median and mean emissions
are tracked by a number of scenarios, most notably by B2-
MESSAGE (B2 family marker) and AlB-IMAGE. In these
scenarios, N0^^ emissions tend to increase up to 2050 and
stabilize thereafter, the result of a gradual substitution of fossil
fuels by altematives as well as by the increasing diffusion of
NOj^ control technologies. Low emission futures are described
by various Bl family scenarios, whereas the upper bound for
future NOj^ emissions is represented by scenarios of the fossil
fuel intensive Al scenario groups (e.g. AlC- and AIG-
MESSAGE) and the high population, high fossil energy A2
scenario family (A2-ASF, A2-MESSAGE, or A2G-IMAGE)
(Figure 5-9).
The fossil fuel dominated A2-ASF (A2 family marker) with
limited environmental concern has a rapidly increasing N0^,
üajectory (Figure 5-9). Emissions in other A2 scenarios also
continue to grow, except in A2-AIM for which emissions level
off by the last decades of the 2Г^ century. In the AlB marker
(AlB-AIM), the emissions growth is initially about as strong
as in A2-ASF, but emissions peak in 2030, and decline as the
fossil fuel share of total primary energy falls and the remaining
fossil fuel technologies become more advanced (Figure 5-9).
Scenarios from other Al family groups that assume a much
larger and continued role of fossil fuels yield much higher N0^,
emissions, which reach 150 MtN by 2100 in the coal-based
AlC-MESSAGE scenario. Emission growth in the B2 family
scenarios is less steep than in the Al family, but persists
throughout the entire period, albeit at a declining rate. By 2100,
emissions in the B2-MESSAGE scenario (B2 family marker)
are about twice as large as in 1990 (Figure 5-9). B2-ASF has a
similar trajectory, while the B2-AIM scenario has essentially
constant NO^ emissions over the entire period. Emissions in
the В1 marker (B1-IMAGE) are among the lowest of all the 40
scenarios (Figure 5-9). In this scenario, emissions increase
stops around 2050 and subsequently declines toward the end of
the 2U' century to 60% of the current level. Other scenarios
withm the ВI group coincide well with the В1 marker in 2100,
although the maximum emission levels in these scenarios are
much higher than in Bl-IMAGE (Figure 5-9).
5.5.1.2. Non-Methane Volatile Organic Compounds
NMVOCs arise from fossil fuel combustion (as with N0^,
wide ranges of emission factors are typical for internal
combustion engines), and also from industrial processes, fuel
storage (fugitive emissions), use of solvents (e.g., in paint and
cleaners), and a variety of other activities. As the chemical
reactivities of the various substances grouped under the