Emissions Scenarios - IPCC

42 Technical Summary
family). Chapter 5 gives further detail about the full range of
GHG emissions across the SRES scenarios. The emissions of
other gases follow dynamic patterns much like those shown in
Figures TS-7 and TS-8 for carbon dioxide emissions. A
summary of GHG emissions is given in Chapter 6 and further
details in Chapter 5.
9.2.1. Methane Emissions
Anthropogenic CH^ emissions arise from a variety of activities,
dominated by biologic processes, each associated with
considerable uncertainty. The future CH^ emissions in the
scenarios depend in part on the consumption of fossil fuels,
adjusted for assumed changes in technology and operational
practices, but more strongly on scenario-specific, regional
demographic and affluence developments, together with
assumptions on preferred diets and agricultural practices. The
writing team recommends further research into the sources and
modeling approaches to capture large uncertainties
surrounding future CH^ emissions.
The resuhant CH^ emission trajectories for the four SRES
scenario families portray complex patterns (as displayed in
Figure 5-5 in Chapter 5). For example, the emissions in A2 and
B2 marker scenarios increase throughout the whole time
horizon to the year 2100. Increases are most pronounced in the
high population A2 scenarios where emissions rise to between
549 and 1069 (A2 marker: 900) MtCH^ by 2100, compared to
310 MtCH4 in 1990. The emissions range by 2100 in the B2
scenarios is between 465 and 613 (B2 marker: 600) MtCH^. In
the AIB and Bl marker scenarios, the CH^emissions level off
and subsequently decline sooner or later in the 2P' century.
This phenomenon is most pronounced in the AIB marker, in
which the fastest growth in the first few decades is followed by
the steepest decline; the 2100 level ends up slightly below the
current emission of 310 MtCH^. The range of emissions in
Table TS-4 indicates that alternative developments in energy
technologies and resources could yield a higher range in CH^
emissions compared to the "balanced" technology AIB
scenario group that includes the AIB marker scenario
discussed above. In the fossil-intensive AlFI group (combined
from AIC and AIG groups, as in the SPM), CH^ emissions
could reach some 735 MtCH^ by 2100, whereas in the postfossil
AIT scenario group emissions are con'espondingly lower
(some 300 MtCH4 2100). Interestingly, the Al scenarios
generally have comparatively low CH^ emissions from nonenergy
sources because of a combination of low population
growth and rapid advances in agricultural productivity. Hence
the SRES scenarios extend the uncertainty range of the IS92
scenario series somewhat toward lower emissions. However,
both scenario sets indicate an upper bound of emissions of
some 1000 MtCH^ by 2100.
9.2.2. Nitrous Oxide Emissions
Even more than for CH^, the assumed future food supply will
be a key determinant of future emissions. Size, age
structure, and regional spread of the global population will be
reflected in the emission trajectories, together with
assumptions on diets and improvements in agricultural
practices. Other things being equal, N2O emissions are
generally highest in the high population scenario family A2.
Importantly, as the largest anthropogenic source of NjO
(cultivated soils) is already very uncertain in the base year, all
future emission trajectories are affected by large uncertainties,
especially if calculated with different models as is the case in
this SRES report. Therefore, the writing team recommends
further research into the sources and modeling of long-term
NjO emissions. Uncertainty ranges are correspondingly large,
and are sometimes asymmetric. For example, while the range in
2100 reported in all Al scenarios is between 5 and 10 MtN (7
MtN in the AIB marker), the A2 marker reports 17 MtN in
2100. Other A2 scenarios report emissions that fall within the
range reported for Al (from 8 to 19 MtN in 2100). Thus,
different model representations of processes that lead to N2O
emissions and uncertainties in source strength can outweigh
easily any underlying differences between individual scenarios
in terms of population growth, economic development, etc.
Different assumptions with respect to future crop productivity,
agricultural practices, and associated emission factors,
especially in the very populous regions of the world, explain the
very different global emission levels even for otherwise shared
main scenario drivers. Hence, the SRES scenarios extend the
uncertainty range of future emissions significantly toward
higher emissions (4.8 to 20.2 MtN by 2100 in SRES compared
to 5.4 to 10.8 MtN in the IS92 scenaiios. (Note that natural
sources are excluded in this comparison.)
9.2.3. Halocarbons and Halogenated Compounds
The emissions of halocarbons (chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs), halons, methylbromide,
and hydrofluorocarbons (HFCs)) and other halogenated
compounds (polyfluorocarbons (PFCs) and sulfur hexafluoride
(SFg)) across the SRES scenarios are described in detail on a
substance-by-substance basis in Chapter 5 and Fenhann (2000).
However, none of the six SRES models has its own projections
for emissions of ozone depleting substances (ODSs), their
detailed driving forces, and their substitutes. Hence, a different
approach for scenario generation was adopted.
First, for ODSs, an external scenario, the Montreal Protocol
scenario (A3, maximum allowed production) from
WMO/UNEP (1998) is used as direct input to SRES. In this
scenario corresponding emissions decline to zero by 2100 as a
result of international environmental agreements, a
development not yet anticipated in some of the IS92 scenarios
(Pepper et al, 1992). For the other gas species, most notably
for CFC and HCFC substitutes, a simple methodology of
developing different emission trajectories consistent with
aggregate SRES scenario driving force assumptions
(population, GDP, etc.) was developed. Scenarios are further
differentiated as to assumed future technological change and
control rates for these gases, varied across the scenarios
consistently with the interpretation of the SRES storylines
presented in Chapter 4 as well as the most recent literature.