Emissions Scenarios - IPCC

Summary Discussions and Recommendations 305
showing emissions of less than 12 GtC (82 scenarios). As
discussed in Chapter 2, the lowest cluster appears to include
many of the intervention scenarios; the second and third
clusters aie most likely the non-intervention cases. The lowest
cluster may have been influenced by many analyses of
stabilizing atmospheric concentrations. The middle cluster
echoes the many analyses that took IS92a as a reference and is
testament to the enormous influence of the IS 92 series on
emissions assessments in general.
The range of CO, and other GHG emissions for the four
marker scenarios is generally somewhat lower than that of the
six 1S92 scenarios.^ However, the IS92 scenarios do not cover
the "middle" range of emissions where the median and the
average of all scenarios in the literature are situated. Adding
the other 36 scenarios to the four SRES markers increases the
covered emissions range beyond the IS92 series at the high end
of the distribution but not at the low end. SRES scenarios stop
short of the lower literature emissions because they are
scenarios without additional climate initiatives (as per the
Terms of References, see Appendix I).
Figure 6-6 illustrates the range of CO2 emissions of the SRES
scenarios against the background of all the IS92 scenarios and
other emissions scenarios from the literature documented in the
SRES scenario database. The shaded areas depict the range of
the scenarios in the database that exceeds the SRES emissions
range. The range of future emissions is very large so that the
highest scenarios envisage more than a sevenfold increase of
global emissions by 2100, while the lowest have emissions
lower than today.
The literature includes scenarios with additional climate
initiatives and policies, which are also referred to as mitigation
or intervention scenarios. As shown in Chapter 2, many
ambiguities are associated with the classification of emissions
scenarios into those that include additional climate initiatives
and those that do not. Many cannot be classified in this way on
basis of the information available from the SRES scenario
database and the published literature.
Figure 6-6a indicates the ranges of emissions from energy and
industry in 2100 from scenarios that apparently include
additional climate inifiatives (designated as intervenfion
emissions range), those that do not (non-intervention), and
those that cannot be assigned to either of these two categories
(non-classified). This classificafion is based on the subjective
evaluation of the scenarios in the database by the members of
the writing team and is explained in Chapter 2. The range of the
whole sample of scenarios has significant overlap with the
range of those that caimot be classified and they shai-e virtually
the same median (15.7 and 15.2 GtC in 2100, respectively), but
the non-classified scenarios do not cover the high part of the
' This is still true when the two illustrative cases in the Al family -
as selected for the Summary for Policymakers, see footnote 2 - are
added.
range. Also, the range of the scenarios that apparenfly do not
include climate polices (non-intervention) has considerable
overlap with the other two ranges (lower bound is slightly
higher), but with a significantly higher median (of 21.3 GtC in
2100).
The median of all energy and industry emissions scenarios
from the literature is 15.7 GtC by 2100. This is lower than the
median of the IS92 set and is lower than the IS92a scenario
often (inappropriately) considered as the "central" scenario.
Again, the distribution of emissions is asymmetric (see the
emissions histogram in Figure 6-5) and the thin tail that
extends above 30 GtC includes only a few scenarios.
Figure 6-6 shows the range of emissions of the four families
(verfical bars next to each of the four marker scenarios), which
illustrate that the scenarios groups by themselves cover a large
portion of the overall scenario distribution. Together, they
cover much of the range of future etnissions, both with respect
to the scenarios in the literature and all SRES scenarios.
Adding all other scenarios increases the covered range. For
example, the SRES scenarios span jointly from the 95"'
percentile to just above the 5"' percentile of the distribution of
energy and industry emissions scenarios from the literature.
This illustrates again that they only exclude the most extreme
emissions scenarios found in the literatme, which are situated
out in the tails of the distribution. What is perhaps more
important is that each of the four scenario families covers a
substantial part of this distribution. This leads to a substantial
overlap in the emissions ranges of the four scenario families. In
other words, a similar quantification of driving forces can lead
to a wide range of future emissions and a given level of future
emissions can result from different combinations of driving
forces. This result is of fundamental importance for the
assessments of cfimate change impacts and possible mitigation
and adaptation strategies. Thus, it warrants some further
discussion.
Another 1пГефге1а11оп is that a given combination of the main
driving forces, such as population and economic growth, is not
sufficient to determine the future emissions paths. Different
modeling approaches and different specifications of other
scenaiio assumptions overshadow the influence of the main
driving forces. A particular combination of driving forces, such
as specified in the Al scenario family, is associated with a
whole range of possible emission paths for energy and
industry. The nature of climate change impacts and adaptation
and mitigation strategies would be fundamentally different
depending on whether emissions are high or low, given a
particular combination of scenario driving forces. Thus, the
implication is that the whole range needs to be considered in
the assessments of climate change, from high emissions and
driving forces to low ones.
The Al scenario family explored variations in energy systems
most explicitly and hence covers the largest part of the scenario
distribution shown in Figures 6-5 and 6-6a, from the 95* to just
above the 10* percentile. The Al scenario family includes