Emissions Scenarios - IPCC

Summary Discussions and Recommendations
MtN by 2100 in SRES compared to 5.4 to 10.8 MtN in the IS92
scenarios. (Note that natm-al sources are excluded in this
comparison.)
6.3.2.3. Halocarbons and Halogenated Compounds
The emissions of halocarbons (chlorofluorocarbons (CFCs),
hydrochlorofluorocarbons (HCFCs), halons, methylbromide,
and hydrofluorocarbons (HFCs)) and other halogenated
compounds (polyfluorocaitons (PFCs) and sulfur hexafluoride
(SFg)) across the SRES scenarios are described in detail on a
substance-by-substance basis in Chapter 5 and Fenhaim (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 ai, 1992). For the other gas species, most notably
for CFC and HCFC substitutes, a simple methodology of
developing different emissions trajectories consistent with the
aggregate SRES scenario driving force assumptions
(population, GDP, etc.) was developed. Scenarios are equally
further differentiated as to assumed future technological
change and control rates for these gases, varied across the
scenarios consistently within the interpretation of the SRES
storylines presented in Chapter 4. The literature, as well as the
scenario methodology and data, are documented in more detail
in Fenhann (2000) and are summarized in Chapter 5.
Second, different assumptions about CFC applications as well
as substitute candidates were developed. These were initially
based on Kroeze and Reijnders (1992) and information given
in Midgley and McCulloch (1999), but updated with the most
recent information from the Joint IPCC/TEAP Expert Meeting
on Options for the Limitation of Emissions of HFCs and PFCs
(WMO/UNEP, 1999) as described below. An important
assumption, on the basis of the latest information from the
industry, is that relatively few Montreal gases will be replaced
fully by HFCs. Current indications are that substitution rates of
CFCs by HFCs will be less than 50% (McCulloch and
Midgley, 1998). In Fenhann (2000) a further technological
development is assumed that would result in about 25% of the
CFCs ultimately being substituted by HFCs (see Table 5-9 in
Chapter 5). This low percentage not only reflects the
introduction of non-HFC substitutes, but also the notion that
smaller amounts of halocarbons will be used in many
applications when changing to HFCs (efficiency gains with
technological change). A general assumption is that the present
trend, not to substitute with high GWP substances (including
PFCs and SFg), will continue. As a result of this assumption,
the emissions reported here may be underestimates. This
substitution approach is used in all four scenarios, and the
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technological options adopted are those known at present.
Further substhution away from HFCs is assumed to require a
climate policy and is therefore not considered in SRES
scenarios. Policy measures that may indirectly induce lower
halocai-bon emissions in the scenarios are adopted for reasons
other than climate change. For one scenario (A2) no
reductions were assumed, whereas in the other scenarios
intermediary reduction rates and levels were assumed.
Expressed in HFC-134a equivalents (based on SAR
equivalents), HFCs in the SRES scenarios range between 843
and 2123 kt HFC-134a equivalent by 2100, compared to 1188
to 2375 kt HFC-134a equivalent in IS92. The range of
emissions of HFCs in the SRES scenario is initiaUy generally
lower than in earlier IPCC scenarios because of new insights
about the availability of alternatives to HFCs as replacements
for substances controlled by the Montreal Protocol. In two of
the four scenarios in the report, HFC emissions increase
rapidly in the second half of the 2P' century, while in two
others the growth of emissions is significantly slowed down or
reversed in that period.
Aggregating all the different halocarbons (CFCs, HCFCs,
HFCs) as well as halogenated compounds (PFCs and SFg) into
MtC-equivalents (using SAR GWPs) indicates a range between
386 and 1096 MtC-equivalent by 2100 for the SRES scenarios.
This compares (see Table 6-2b) with a range of 746 to 875
MtC-equivalent for IS92 (which, however, does not include
PFCs and SFg). (The comparable SRES range, excluding PFCs
and SFg, is between 299 and 753 MtC-equivalent by 2100.)
The scenarios presented here indicate a wider range of
uncertainty compared to IS92, particularly toward lower
emissions (because of the technological and substitution
reasons discussed above).
The effect on climate of each of the substances aggregated to
MtC-equivalents given in Table 6-2b varies greatly, because of
differences in both atmospheric lifetime and the radiative effect
per molecule of each gas. The net effect on climate of these
substances is best determined by a calculation of their radiative
forcing - which is the amount by which these gases enhance
the anthropogenic greenhouse effect. The net radiative effect of
all halocarbons, PFCs, and SFg from 1990 to 2100, including a
current estimate of the radiative effect of stratospheric ozone
depletion and subsequent recovery, ranges from 6% to 9% of
the total radiative forcing from all GHGs and SO^. Preliminary
calculations indicate that the net radiafive effect of PFCs and
SFg in SRES scenarios will be no greater, relative to total
anthropogenic forcing, by 2100 than it is at present.
6.3.3. Sulfur Dioxide Emissions
Emissions of sulfur portray even more dynamic pattems in
time and space than the COj emissions shown in Figures 6-5
and 6-6. Factors other than climate change (namely regional
and local air quality, and transformations in the structure of the
energy system and end use) intervene to limit future emissions.
Figure 6-10 shows the range of global sulfur emissions for all