Emissions Scenarios - IPCC

An Overview of Scenarios 169
4.1. Introduction
In Chapter 4 the main characteristics of the scenarios
developed for the Special Report on Emissions Scenarios
(SRES scenarios) are presented. These scenarios cover a wide
range of driving forces from demographic to social and
economic developments, and they encompass a wide range of
future greenhouse gas (GHG) emissions (see Chapter 5).
Chapters 2 and 3 provide an overview and assessment of the
scenario literature, the main driving forces of future GHG
emissions, and their relationships. How the driving forces are
combined to produce a set of scenarios that cover the ranges of
GHG emissions from the literature is described in this chapter.
Chapters 2 and 3 demonstrate the large uncertainty in the
literature that surrounds both future emissions and the possible
developments of their underlying driving forces. The
uncertainties range from inadequate scientiñc understanding of
the problems, through data gaps or lack of data, to the inherent
unceitainties of future events in general. Hence altemative
scenarios are used to describe the range of possible future
emissions.
The SRES approach involved the development of a set of four
ahemative scenario "families" (see Chapter 1, Section 1.7.2).
Each family of SRES scenarios includes a descriptive part
(called a "storyline") and a number of altemative interpretations
and quantifications of each storyline developed by six different
modeling approaches (see also Box 1-1 on terminology). Each
storyline describes a demographic, social, economic,
technological, and policy future for each of the scenario
families. Within each family different scenarios explore
variations of global and regional developments and their
implications for GHG, ozone precursors, and sulfur emissions.
Each of these scenarios is consistent with the broad framework
specified by the storyline of the scenario family.
Each storyline is basically a short "history" of a possible future
development expressed as a combination of key scenario
characteristics. These descriptions are styHzed and designed to
facilitate specification and further interpretation of scenario
quantifications. The storylines identify particular dynamics,
visible in the world today, that might have important influences
on future GHG emissions. They deliberately explore what
might happen if social, economic, technical, and policy
developments take a particular direction at the global level;
they also pay attention to regional differences and interactions,
especially between developing and industiialized countries.
The broad consensus among the SRES writing team is that the
current literature analysis suggests the future is inherently
unpredictable and so views will differ as to which of the
storylines and representative scenarios could be more or less
likely. Therefore, the development of a single "best guess" or
"business-as-usual" scenario is neither desirable nor possible.
Nor should the storylines and scenarios be taken as policy
recommendations. The storylines represent the playing out of
certain social, economic, technological, and environmental
paradigms, which will be viewed positively by some people
and negatively by others. The SRES writing team decided on
four storylines - an even number helps to avoid the impression
of a "central" or "most likely" case. The team wanted more
than two storylines to help illustrate that the future depends on
many different underlying dynamics; the team wanted no more
than four to avoid complicating the process with too many
alternatives. The scenarios cover a wide range of, but not all
possible, futures. In particular, it was decided that possible
"suфrises" would not be considered and that there would be no
"disaster" scenarios. The team decided to carry out sensitivity
tests within some of the storylines by considering altemative
scenarios with different fossil-fuel reserves, rates of economic
growth, or rates of technical change. These sensitivity analyses
resulted in groups of scenarios within a given scenario family
and alternative scenario interpretations within a scenario group
or family (see Section 4.2 below for a description of scenario
terminology and taxonomy).
The tities of the storylines are deliberately simple - Al, A2,
Bl, and B2. There is no particular order among the storytines
(they are listed alphabetically). Figure 4-1 shows that the SRES
scenarios build on the main driving forces of GHG emissions.
Each scenario family is based on a common specification of the
main driving forces.
All four storylines and scenario families describe future worlds
that are generally more affluent compared to the current
situation. They range from very rapid economic growth and
technologic change to high levels of environmental protection,
from low-to-high global populations, and from high-to-low
GHG emissions. Perhaps more importantiy, all the storylines
describe dynamic changes and transitions in generally different
directions. The storylines do not include specific climatechange
policies, but they do include numerous other socioeconomic
developments and non-climate environmental
policies. As time progresses, the storylines diverge from each
other in many of their characteristic features. In this way they
span the relevant range of GHG emissions and different
combinations of their main sources.
After the basic features and driving forces for each of the four
storylines had been determined, the team quantified the
storylines into individual scenarios with the help of formal
(computer) models. While the writing team and the modeling
groups included experts from around the world, all six
modeling groups are based in Europe, North America, and
Japan. As indicated above, each model quantification of a
storyline constitutes a scenario, and all scenarios of one
storyline constitute a "scenario family." The six models are
representative of different approaches to emissions-scenario
modeling and different integrated assessment frameworks in
the literature, and include so-called top-down and bottom-up
models. The use of different models reflects the SRES Terms
of Reference call for méthodologie pluralism and for an open
process (see Appendix I). The number and type of models
chosen in the open process was on a voluntary basis. In January
1997 the Intergovernmental Panel on Climate Change Working