Emissions Scenarios - IPCC

Emission Scénarios 257
is between 359 and 671 MtCH^, and the range increases further
to levels between 236 and 1069 MtCH^ by 2100. This wide
range in the SRES emissions reflects new information and
additional uncertainties conceming certain source categories
such as sewage systems. As for COj emissions related to land
use (see Section 5.3.2), at least one scenario from all four
SRES scenario families falls within the 25* and 75*
percentiles of the emission range. Thus, very different future
developments in energy and agricultural systems could lead to
similar outcomes in terms of global CH^ emissions, even if the
source categories and regional pattems of these emissions are
very different. At the same time, the uncertainty range for any
given scenario family is also substantial, as indicated by the
range of 2100 emissions for the Al, A2, and Bl scenario
families in Figure 5-5.
The subsequent sections discuss CH^ emission trajectories for
individual scenario families, with sectoral and regional patterns
described on the basis of the output of marker scenarios.
5.4.1.1. Al Scenario Family
The Al family of scenarios covers close to the full range of
CH4 emissions in all the SRES scenarios (Figure 5-5). AlC-
AIM shows the highest emissions before 2050 and AIG-
MiniCam scenario shows them after 2050 (Figure 5-6a). In
three Al fossil scenarios (AlG-MiniCAM, AlC-MiniCAM,
and AlC-MESSAGE) and in both alternative MiniCAM
scenarios (AlVl-MiniCAM and AIV2-MiniCAM) emissions
increase continuously through the 2U' century, while in the rest
of the Al scenarios emissions peak between 2030 and 2050
and decline thereafter.
In the AlB marker scenario (AlB-AIM) emissions increase
through 2030 and subsequently decline to levels similar to
those in 1990 (Figure 5-6a). Almost all of the emissions
dynamics in AlB-AIM are explained by a rise in emissions
from landfills, sewage, and fossil fuel production. At the same
time emissions from agriculture are relatively flat because
better management of animal wastes and high productivity are
assumed to offset the effect of increased food requirements
(Table 5-5). Growing population and per capita income
combined with increased use of landfills generates increasing
emissions from landfills and sewage in developing countries
through 2030. After 2030, declining population levels, the
introduction of modem management techniques, and increased
recycling reduces waste sent to landfills and thus emissions
from these wastes. Emissions from biomass buming in AlB-
AIM are assumed to decline steadily through the adoption of
bio-recycling and other "no-waste" agricultural practices.
Similarly, CH^ emissions from fossil fuel production and use
grow thi'ough 2030 and subsequently decline as fossil fuel
production falls.
5.4.1.2. A2 Scenario Family
The A2 family of scenarios contributes the upper half of the
full range of CH^ emissions in the SRES scenarios (Figure 5-
5). The global CH^ emissions in the A2 family scenarios grow
continuously thioughout the 2P' century and range from 550 to
1070 MtCH4 in 2100 (Figure 5-6b). The rate of growth
depends on the scenario-specific dynamics of major CH^
emission drivers. Emissions in the A2-ASF scenario, which are
close to the upper end of the range, are driven mainly by
increases in coal production, livestock population, and waste
management capacity to satisfy the needs of an expanding
population (Table 5-5). At the lower end of the emission range
are the two MiniCAM scenarios and A2-AIM. Relatively slow
emission growth in the MiniCAM scenarios is attributed
primarily to an increase in rice productivity (which offsets an
increase in the area of rice fields) and a shift in livestock
production from cattie to animal groups that have notably
lower emission factors per animal (e.g. poultry and swine). In
the A2-AIM scenario low CH^ emissions are caused by
relatively low CH^ emission factors for coal production and a
relatively slow increase in livestock production.
5.4.1.3. Bl Scenario Family
The В1 family of scenarios covers the lower half of the full
range of CH4 emissions in the SRES scenarios, with the Bl-
IMAGE marker scenario positioned at the low end of this range
(Figure 5-5). Global emissions in Bl-IMAGE grow through to
2030, driven primarily by increased emissions from landfills
and sewage (Figure 5-6c). This growth is partially offset by
declines in emissions from biomass burning (Table 5-5). After
2030, emissions level off and subsequently decline from 2050,
a reflection of reductions in fossil fuel production and use.
Emissions from other sources, which include enteric
femientation and rice production, also decline, primarily from
the combination of stabilizing and declining populations with
continued improvements in slaughter weight and off-take rate.
The В I-AIM scenario lies in the middle of the range for the Bl
family. CH4 emissions in this scenario increase through to
2050, with most of the increase associated with landfills and
sewage systems. After 2050, emissions from landfills decline
because of a combination of factors that include recycling and
a declining population. Emissions from sewage decrease after
2050 through the decline in population. Emissions from rice
production stay relatively flat because of increases in rice
productivity. Emissions from fossil fuel production increase
through to 2030 and subsequentiy decline, which mirrors the
increase in production of fossil fuels through to 2030 and the
increased use of renewable energy after 2030. Unlike other В1
scenarios, both MiniCAM cases have emissions that
continuously increase (Figure 5-6c). The reason is a slow rise
in non-energy system emissions for the В1-MiniCAM
scenario, which offsets the decrease in energy system
emissions and the nearly constant emissions in the agriculture
sector. In the BlHigh-MiniCAM scenario, the additional
energy demand designed into this scenario is met largely by an
increased use of natural gas, which thus explains the faster rise
in CH^ emissions in this scenario as compared to the base Bl
MiniCAM scenario.