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).

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