6.1. Introduction and Background
The set of 40 emissions scenarios in this Special Report on Emissions Scenarios
(SRES) is based on an extensive assessment of the literature, six alternative
modeling approaches, and an "open process" that solicited wide participation
and feedback from many groups and individuals. The set of scenarios includes
all relevant species of greenhouse gases (GHGs)1
. This chapter provides a summary of the SRES emissions scenarios and compares
them with the previous set of Intergovernmental Panel on Climate Change (IPCC)
IS92 scenarios and the underlying literature.
The first step in the formulation of the scenarios was the review and analysis
of the published literature and the development of the database with more than
400 emissions scenarios (accessible on the web site, www-cger.nies.go.jp/cger-e/db/ipcc.html). One of the recommendations of the writing team is
that IPCC or a similar international institution should maintain such a database
to ensure continuity of knowledge and scientific progress in any future assessments
of GHG scenarios. An equivalent database to document narrative and other qualitative
scenarios would also be very useful for future climate-change assessments. One
difficulty encountered in the analysis of the emissions scenarios is that the
distinction between climate policy scenarios, non-climate policy scenarios,
and other scenarios appeared to be to a degree arbitrary and was often impossible
to make. Therefore, the writing team recommends that an effort should be made
in the future to develop an appropriate emissions scenario classification scheme.
Chapters 2 and 3 give a more detailed
description of the very wide range of future emissions paths, their driving
forces, and their relationships as reflected in the literature; the wide rage
indicates that their possible developments are highly uncertain. The sources
of inherent uncertainties range from data and modeling uncertainties through
to inadequate scientific understanding of the underlying problems.
Scenarios are appropriate tools for dealing with such uncertainty. Scenarios
are images of the future, or alternative futures. As an integration tool in
the assessment of climate change they allow a role for intuition, analysis,
and synthesis; thus we turn to scenarios in this report to take advantage of
these features to aid the assessment of future climate change, impacts, vulnerabilities,
adoption, and mitigation. Scenarios are not predictions. A set of scenarios
can assist in the understanding of possible future developments, and hence the
development of a set of alternative scenarios (see Chapters 1
and 4 for more detail).
The SRES approach involved the development of a set of four alternative scenario
"families" that encompass the 40 scenarios. Each family of SRES scenarios includes
a descriptive part called a "storyline," and a number of alternative interpretations
and quantifications of each storyline developed by six different modeling approaches.
All the interpretations and quantifications of one storyline together are called
a scenario family (see Chapter 1 for terminology). Each
storyline describes a demographic, social, economic, technological, and policy
future for one of these scenario families. Within each family different scenarios
explore variations of global and regional developments and their implications
for GHG and sulfur emissions. Each of these scenarios is consistent with the
broad framework of that scenario family as specified by the storyline. Chapters
4 and 5 give a more detailed description
of the storylines, their quantifications, and the resultant 40 emissions scenarios.
The SRES writing team reached a broad consensus that there could be no "best
guess" scenarios; that the future is inherently unpredictable and that views
will differ as to which storylines could be more likely. There is no "business-as-usual"
scenario. The storylines represent the playing out of certain social, economic,
technological and environmental paradigms that will be viewed positively by
some people and negatively by others. The writing team decided on four storylines
- an even number helps to avoid the impression that there is 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, but no more than
four, as they wanted to avoid complicating the process with too many alternatives.
The scenarios cover a wide range, but not all possible futures. In particular,
it was decided that possible "surprises" would not be considered and that there
would be no "disaster" scenarios.
The storylines describe developments in many different social, economic, technological,
environmental, and policy dimensions. The titles of the storylines have been
kept simple - A1, A2, B1, and B2. There is no particular order among the storylines,
which are listed in Box 6-1 in alphabetic order. The team decided to carry out
sensitivity tests within some of the storylines by considering alternative scenarios
with different fossil-fuel reserves, rates of economic growth, or rates of technological
change within a given scenario family.
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 technological change to high levels of environmental protection,
from low to high global populations, and from high to low GHG emissions. What
is perhaps even more important is that all the storylines describe dynamic changes
and transitions in generally different directions. Although they do not include
additional climate initiatives, none of them are policy free. 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 determining the basic features and driving forces for each of the four
storylines, the team quantified the storylines into individual scenarios with
the help of formal (computer) models. The six modeling groups that quantified
the storylines are listed in Box 6-2. The six models
are representative of different approaches to modeling emissions scenarios and
different integrated assessment (IA) frameworks in the literature and include
so-called top-down and bottom-up models. The writing team recommends that IPCC
or a similar international institution should ensure participation of modeling
groups around the world, and especially those from developing countries, in
future scenario development and assessment efforts. Clearly, this would also
require resources specifically to assist modeling groups from developing countries.
Indeed, a concerted effort was made to engage modeling groups and experts from
developing countries in SRES as a direct response to recommendations of the
IPCC scenario evaluation (Alcamo et al., 1995).
|Box 6-1: The Main Characteristics of the Four SRES Storylines and Scenario
By 2100 the world will have changed in ways that are hard to imagine
- as hard as it would have been at the end of the 19th century to imagine
the changes of the 100 years since. Each storyline assumes a distinctly
different direction for future developments, such that the four storylines
differ in increasingly irreversible ways. Together they describe divergent
futures that encompass a significant portion of the underlying uncertainties
in the main driving forces. They cover a wide range of key "future" characteristics
such as population growth, economic development, and technological change.
For this reason, their plausibility or feasibility should not be considered
solely on the basis of an extrapolation of current economic, technological,
and social trends.
- The A1 storyline and scenario family describes a future world of very
rapid economic growth, low population growth, and the rapid introduction
of new and more efficient technologies. Major underlying themes are
convergence among regions, capacity building and increased cultural
and social interactions, with a substantial reduction in regional differences
in per capita income. The A1 scenario family develops into four groups
that describe alternative directions of technological change in the
energy system. Please note that in the Summary for Policymakers, two
of these groups were merged into one.2
- The A2 storyline and scenario family describes a very heterogeneous
world. The underlying theme is self-reliance and preservation of local
identities. Fertility patterns across regions converge very slowly,
which results in high population growth. Economic development is primarily
regionally oriented and per capita economic growth and technological
change are more fragmented and slower than in other storylines.
- The B1 storyline and scenario family describes a convergent world
with the same low population growth as in the A1 storyline, but with
rapid changes in economic structures toward a service and information
economy, with reductions in material intensity, and the introduction
of clean and resource-efficient technologies. The emphasis is on global
solutions to economic, social, and environmental sustainability, including
improved equity, but without additional climate initiatives.
- The B2 storyline and scenario family describes a world in which the
emphasis is on local solutions to economic, social, and environmental
sustainability. It is a world with moderate population growth, intermediate
levels of economic development, and less rapid and more diverse technological
change than in the B1 and A1 storylines. While the scenario is also
oriented toward environmental protection and social equity, it focuses
on local and regional levels.
After determining the basic features and driving forces for each of the
four storylines, the team began modeling and quantifying the storylines.
This resulted in 40 scenarios, each of which constitutes an alternative
interpretation and quantification of a storyline. All the interpretations
and quantifications associated with a single storyline are called a scenario
"family" (see Chapter 1 for terminology and Chapter
4 for further details).