Emissions Scenarios - IPCC

An Overview of Scenarios 187
scenario drivers, sucii as population and economic growtti,
technological change, resource availability, land-use changes,
and local and regional environmental policies, are summarized
in this Section (GHG emissions are reported in detail in
Chapter 5). The assumptions are based on the range of driving
forces identified in Chapter 2 and their relationships as
summarized in Chapter 3. For simplicity these drivers are
presented separately, but it is important to keep in mind that the
evolution of these scenario drivers is to a large extent
interrelated, as reflected in the SRES scenarios.
As discussed above, the SRES scenarios were designed to
reflect inherent uncertainties of future developments by
adopting a range of salient input assumptions, but without
attempting to cover the extremes from the scenario literature.
Given the nature of the SRES open process and its multi-model
approach, as well as the need for documented input
assumptions, pubHshed scenario extremes are difficult to
reproduce using altemative model approaches or insufficiently
documented input data. (For instance, many long-term
emission scenarios do not report their underlying population
assumptions (see Chapters 2 and 3), which is especially true for
extreme scenarios that are usually performed within the
context of model sensitivity analysis.)
Compared to the previous IS92 scenario series there are
important similarities, but also important differences. For
instance, three different future population scenarios were
adopted, albeit that the future population levels are somewhat
lower and the range more compressed than those in IS92 this
reflects advances in demographic modeling and population
projections. Conversely, the range of assumptions that concern
resource availability and future technological change is much
wider compared to earlier scenarios, reflecting in particular the
results of the IPCC WGII Second Assessment Report (SAR;
Watson et al., 1996). Another distinguishing characteristic of
the SRES scenarios is an attempt to reflect the most recent
understanding on the relationships between important scenario
driving-force variables. For instance, no scenario combines
low fertility with high mortality assumptions, which reflects
the consensus view from demographers (see Chapter 3).
Equally, all SRES scenarios assume a qualitative relationship
between demographics and social and economic development
trends, which reflects both the literature (see Chapter 3) and the
results of the evaluation of the IS92 scenario series (Alcamo et
al., 1995). All else being equal, fertility and mortafity trends
are thus lower in scenarios with high-income growth
assumptions, but the multidimensionality of the causal
linkages must be recognized and so no particular cause-effect
model is postulated here. Finally, the scenarios also attempt to
reflect recent advances (as reviewed in Chapter 3) in
understanding of the evolution of macro-economic and
material productivity (e.g., their coupling via capital turnover
rates), uncertainties in future levels of "dematerialization"
(reflected in the difference between the Bl and Al scenarios),
and the likely evolution of local and regional environmental
policies (e.g., all scenarios assume various degrees of sulfurcontrol
policies).
The main aspects of translating the storylines into scenario
drivers are summarized below. For each scenario family an
overview of all scenario quantifications is given. Scenarios that
share harmonized input assumptions with the respective
scenario marker in terms of global population and GDP
profiles (see Tables 4-1 and 4-3) are indicated in italics in the
subsequent discussion. Altogether, 26 scenarios in the four
scenario families share similar assumptions about population
and GDP at the global level. The other 14 scenarios either do
not fully comply with the agreed common input assumptions
concerning global population and GDP or explore important
sensitivities of future demographic and economic developments
beyond that described in the 24 scenarios. These sensitivities
include resource availability, technology development, or landuse
changes and describe similar demographic and economic
development pattems as other scenarios within a family, even
if they do not fall within the range suggested by the
harmonization criteria (see Table 4-1). Combined, the SRES
scenario set comprises 40 scenarios grouped into four scenario
families and different scenario groups (see Table 4-3).
Each scenario family is illustrated by a designated marker
scenario. A marker is not necessarily the mean or mode of
comparable scenario quantifications, nor would it be possible
to construct an internally consistent scenario reflecting
medians/modes of all salient scenario characteristics (both in
terms of scenario input assumptions as well as scenario
outcomes, i.e. emissions). Marker scenarios should also not be
inteфreted as being the more likely altemative scenario
quantifications. However, only the four marker scenarios were
subjected to the SRES open process through the SRES website
and they have also received closest scrutiny by the entire
writing team.
4.4.2.1. Al Scenarios
The Al marker scenario (Jiang et al, 2000) was created with
the AIM model, an integrated assessment model developed by
NIES, Japan (see Appendix IV). The Al scenario family is
characterized by;
• An affluent world, with a rapid demographic transition
(declining mortality and fertility rates) and an
increasing degree of intemational development equity.
• Very high productivity and economic growth in all
regions, with a considerable catch-up of developing
countries.
• Comparatively high energy and materials demands,
moderated however by continuous structural change
and the diffusion of more efficient technologies,
consistent with the high productivity growth and capital
tumover rates of the scenario.
The first group of Al scenarios, which includes the AlB
marker, assumes "balanced"^ progress across all resources and
technologies from energy supply to end use, as well as
"balanced" land-use changes. Three other groups of Al
scenarios were identified which describe three altemative