Emissions Scenarios - IPCC

Scenario Driving Forces 153
Nakicenovic, 1996; McDonald, 1999; Riahi and RoeM, 2000),
which are linked with the acidification model RAINS for
Europe and Asia, or the AIM (Morita et al., 1994) model for
Asia. These models extend earlier energy sector models that
dealt with a comparative costs assessment of isolated sulfur
and carbon reductions and joint mitigation, such as the OECD
GREEN model (Complainville and Martins, 1994) or the
IIASA MESSAGE model (Grübler, 1998b; Nakicenovic et al,
1998a). The state of knowledge and availability of models to
study the joint benefits of sulfur and carbon emission
reductions was reviewed in the 1995 IPCC SAR WGIII Report
(Bruce et ai, 1996) and is expanding rapidly (CIRED et al,
1997; Nakicenovic et al, 1997; Grübler, 1998c).
3.6.5. Ozone Precursors
IPCC Working Group I (WGI) SAR (Houghton et al, 1996)
confirmed the importance of tropospheric ozone as a
greenhouse gas. Ozone is produced in the troposphere in a
complex chain of reactions that involve the ozone precursors
nitrogen oxides (NO^,), non-CH^ hydrocarbons or volatile
organic compounds (NMVOCs), and CO. Therefore, it is
important to explore possible future developments of emissions
of these substances to analyze the evolution of tropospheric
ozone levels.
3.6.5.1. Nitrogen Oxides
N0^ are released through fossil fuel combustion (24 MtN per
year around 1990), natural and anthropogenic soil release (12
MtN per year), biomass burning (8 MtN per year), lightning (5
MtN per year), NH3 oxidation (3 MtN per year), aircraft (0.4
MtN/year), and transport from the stratosphere (0.1 MtN per
year). These figures are mean estimates within a range; for fossil
fuel combustion, aircraft emissions, and stratospheric input the
ranges may be as narrow as 30%, but for natural sources the
ranges may be up to a factor of 2 (Prather et ai, 1995). The
uncertainties in the estimates are illustrated by comparison of
the detailed emissions inventory of Olivier et al. (1996) with the
1994 IPCC estimates - while global total emissions estimates by
source are very similar, at the regional level emissions estimates
show pronounced differences, particularly in Asia.
Fossil fuel combustion in the electric power and transport
sectors is the largest source. Emissions from fossil fuel use in
North America and Europe have barely increased since 1979
because fossil fuel consumption leveled off and air quality
abatement was enacted, but in Asia emissions are believed to
increase by 4% annually (Prather et ai, 1995). As a result of the
first NOjj Protocol in Europe, N0^^ emissions in Europe had
decreased from 1987 levels by 13% in 1994, but the European
Union is unlikely to meet its target of the 5* European Action
Plan of a 30% reduction (EEA, 1999). An important reason is
that it is difficult to abate N0^ emissions in the growing
transport sector. Perhaps critically, there are significant
differences between the characteristics of abatement of SOj and
NOj^ emissions. While both substances have regional
acidification effects, a priority for SO2 abatement is induced by
its important local health effects. Also, whereas SO^ emissions
relate closely to the type of fuel, NO^^ emissions are more
dependent on the combustion technology and condifions.
Few scenarios for N0^, emissions exist beyond the studies for
Europe, North America, and Japan (IS92 scenarios are a notable
exception). New scenarios, such as those by Bouwman and van
Vuuren (1999) and Collins et al. (1999) often still use IS92a as
a "loose" baseline, with new abatement policies added as they
were introduced in the OECD countries after 1992, according to
current reduction plans (CRP). Collins et al. (1999) also explore
a maximum feasible reduction scenaiio, in which European N0^,
emissions decrease by 60% by 2015 and North American
emissions by 5%. In the related CRP scenaiio of Bouwman and
van Vuuren (1999), N0^, emissions in the developing countries
are assumed to decrease also (by more than 10%) by 2015.
These studies, however, should be used with care as the authors
developed their somewhat arbitrary scenarios primarily for
atmospheric chemistry analysis; they are not based on an indepth
analysis of the characteristics of the emissions sources and
potential policies in the various regions outside the OECD.
3.6.5.2. Carbon Monoxide and Non-Methane Hydrocarbons
Prather et al. (1995) estimates the total global emissions of CO
at 1800 to 2700 MtC per year in the decade before 1994. The
most important of the approximately 1000 TgC anthropogenic
sources are technological (300 to 550 MtC per year) and
biomass burning (300 to 700 MtC per year). Technological
sources dominate in the northern hemisphere, and include
transport, combustion, industrial processes, and refuse
incineration. Biomass burning dominates in the southern
hemisphere, and includes burning of agricultural waste,
savanna burning, and deforestation. The detailed,
geographically explicit EDGAR database (Olivier et al., 1996)
has similar emissions estimates for CO. Other sources are
biogenics (60 to 160 MtC per year), oceans (20 to 200 MtC per
year), and oxidation of CH^ (400 to 1000 MtC per year) and of
NMVOCs (200 to 600 MtC per year). To a large extent, this
oxidation nray be considered anthropogenic in origin because
many emissions sources of CH4 and NMVOCs are of an
anthropogenic nature.
Global emissions estimates of NMVOCs are also very
uncertain. Prather et al. (1995) indicate a global total for
anthropogenic NMVOCs of about 140 MtC per year, from road
transport (25%), solvent use (14%), fuel production and
distribution (13%), fuel consumption (34%), and the rest from
uncontrolled burning and other sources. The EDGAR
inventory by Olivier et al. (1996) suggests that global
emissions may be higher (178 MtC per year) because of higher
estimates of emissions from energy production and use. As
with CO, emissions in the northern hemisphere are dominated
by transport and industry, while in the southern hemisphere
biomass and biofue! burning is often the dominant source. In
Europe, emissions of NMVOCs are controlled under the 5*
Environmental Action Programme of the European Union and