Emissions Scenarios - IPCC

Summary Discussions and Recommendations 295
6.1. Introduction and Baclcground
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)^. 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/ cgere/db/ipcc.html).
One of the recommendations of the writing
team is that IPCC or a similar intemational 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 quaUtative 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 altemative
futures. As an integration tool in the assessment of climate
change they allow a role for intuition, analysis, and synthesis;
thus we tum 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 altemative scenarios (see Chapters 1
and 4 for more detail).
' Included are anthropogenic emissions of carbon dioxide (CO^),
methane (CH^), nitrous oxide (NjO), hydrofluorcarbons (HFCs),
perfluorocarbons (PFCs), sulfur hexafluoride (SFg),
hydrochlorofluorocarbons (HCFCs), chlorofluorocarbons (CFCs),
sulfur dioxide (80^), carbon monoxide (CO), nitrogen oxides (NO,,),
and non-methane volatile organic compounds (NMVOCs).
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
altemative 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 inherentiy
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
compficating the process with too many altematives. 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 tities of the storylines have been kept simple
- Al, A2, Bl, 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 altemative 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.