Emissions Scenarios - IPCC

An Overview of Scenarios 235
considerable uncertainty and scenario variability remains. For
instance, features of technological and land-use changes can be
interpreted quite differently within the framework of different
models, even if they conform to the overall conceptual
description and "scenario logic" described in a particular
scenario family. In some instances the broad outlines of
scenario driving forces were not followed entirely in particular
scenario quantifications, but alternative scenario
interpretations were submitted. These highlight important
scenario uncertainties or express scientific disagreements
within the writing team, as for future labor productivity growth
and economic "catch-up" possibilities of currently developing
countries. These alternative scenario interpretations and
different model quantifications are presented here to reflect the
SRES Terms of Reference for an open process and the use of
multiple modeling approaches, even if this necessarily
increases complexity and reduces simplicity and transparency
in discussion of a large number of scenario quantifications.
To guide readers through the different driving-force
assumptions that characterize the various scenarios. Tables 4-2
and 4-3 give an overview of the SRES scenario set. They
classify scenarios that share important input assumptions
(hannonized scenarios share global population and GDP
assumptions) from scenarios that offer alternative
quantifications. Table 4-4 summarizes the main quantitative
scenario descriptors for each of the four SRES scenario
families of the "harmonized" scenario category. Here an
attempt is made to link this information with the resultant
scenario outcomes (emissions) that are discussed in more detail
in Chapter 5.
In Chapter 5, an additional, complementary scenario
classification scheme to that used in this chapter is presented
and focuses on driving forces. Scenarios are classified
according to their cumulative carbon emissions (1990 to 2100,
all sources), the best single quantitative indicator available to
compare emission scenarios that portray widely different
dynamics and different combinations and magnitude of a
variety of emission categories. Four categories of cumulative
emissions, Low (1800
GtC) are presented. Table 4-20 links the scenario overview
from Tables 4-2 and 4-3 with this information to guide readers
through the differences in scenarios.
Table 4-20 indicates that in most cases there is an easily
discernable direct connection between main scenario
characteristics of a paiticular scenario family or scenario group
and the resultant outcomes in terms of cumulative emissions.
For instance, in the high GDP, high energy demand scenario
family AI, all scenarios within the two scenaiio groups that are
fossil fuel and technologies intensive (AIC and AIG combined
into AlFI in the SPM) result in high cumulative carbon
emissions. Conversely, cumulative emissions of the "efficiency
and dematerialization" (without additional climate initiatives)
scenario family Bl are generally in the "low" emissions
category, but two model quantifications indicate medium-low
emissions. For the scenario family B2, outcomes in tenns of
cumulative carbon emissions can also be related clearly to
scenario characteristics. One group of scenarios (which
includes the B2 marker) adopts an incrementalist perspective
of technological change ("dynamics as usual") applied to
medium levels of population and GDP (and resultant energy
demand) and results in medium-low cumulative carbon
emissions. Another group of scenarios explored the sensitivity
of a gradual retum to coal-based technologies (B2C-MARIA,
B2-ASF), in one case combined with higher energy demand
than in the other scenarios (B2High-MiniCAM); and results in
higher cumulative emissions (Medium-High category in Table
4-20).
Equally discernable in Table 4-20 is the wide range in
cumulative carbon emissions that characterize the various
scenario groups within the Al scenario family. By design, the
different scenario groups within this family explored the
implications of different directions of technological change,
ranging from carbon-intensive developments (AIC and AIG,
combined into AlFI in the SPM) to decarbonization (AIT),
with the "balanced" technology development scenario group
taking an intermediary position. Different developments
concerning fossil or non-fossil resource and technology
availability in a less populated but affluent and thus high
energy demand world (such as Al) can lead to widely different
outcomes in terms of cumulative emissions, with a range as
wide as that spanned by all four scenario families together.
Technology can thus be as important a driving force as
population and GDP growth combined. In other words, very
different emissions outcomes are possible for future worlds
that otherwise share similar developments of main driving
forces such as population and economic growth and high rates
of technological change.
However, areas of overlap and uncertainties of scenario
outcomes (cumulative emissions) occur even for scenario
quantifications that share otherwise similar assumptions for the
main scenario drivers. Not surprisingly, differences in
quantifications are largest within the Al "balanced"
technological progress scenario group, which includes the AIB
marker scenario. Most model interpretaflons result in
cumulative carbon emissions within the Medium-High
category (1450-1800 GtC). However, there are also scenario
quantifications in which technological change tilts more in the
direction of the AIC (AI-ASF) or AIT (Al-MARIA) scenario
groups that favor fossil (coal) or post-fossil (nuclear,
renewables, and biomass) technologies, respectively. This
leads to very wide differences in cumulative emissions, from
the Medium-Low through to the High categories. A similar
range of scenario outcomes between Medium-High to High
categories also characterizes the A2 scenario family that
otherwise describes an entirely different world (high
population and comparatively low per capita income compared
to low population with high per capita income for the Al
scenario family; see Table 4-4). Departing from the main
scenario characteristics of the A2 scenario family in terms of
population and income in direction of lower values (such as in