The storylines were essentially complete by January 1998. After determining the basic features and driving forces for each of the four storylines, the six modeling groups represented on the writing team (on a voluntary basis) began quantifying them. The six modeling groups that quantified the storylines are listed in Box TS-2. Each model quantification of a storyline constitutes a scenario, and all scenarios derived from one storyline constitute a scenario family. The six models are representative of different approaches to modeling emissions scenarios and different integrated assessment (IA) frameworks

The storylines were essentially complete by January 1998. After determining the basic features and driving forces for each of the four storylines, the six modeling groups represented on the writing team (on a voluntary basis) began quantifying them. The six modeling groups that quantified the storylines are listed in Box TS-2. Each model quantification of a storyline constitutes a scenario, and all scenarios derived from one storyline constitute a scenario family. 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 assure participation of modeling groups around the world and especially from developing countries in any future scenario development and assessment efforts. Clearly, this would also require resources specifically directed at assisting 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 the recommendations of the last IPCC scenario evaluation (Alcamo et al., 1995).

The six models have different regional aggregations. The writing team decided to group the various global regions into four "macro-regions" common to all the different regional aggregations across the six models. Box TS-3 indicates that the four macro-regions (see Appendix III) are broadly consistent with the allocation of countries in the United Nations Framework Convention on Climate Change (UNFCCC, 1992), although the correspondence is not exact because of changes in the countries listed in Annex I of UNFCCC (1997).

All the qualitative and quantitative features of scenarios belonging to the same family were set to conform to the corresponding features of the underlying storyline. Together, 26 scenarios were "harmonized" to share agreed common assumptions about global population and GDP (gross domestic product) development (a few that also share common population, GDP, and final energy trajectories at the level of the four SRES macro-regions are called "fully harmonized," see Section 4.1. in Chapter 4). Thus, the harmonized scenarios are not independent within each of the four families. However, scenarios within each family vary quite substantially in such characteristics as the assumptions about availability of fossil-fuel resources, the rate of energy-efficiency improvements, the extent of renewable-energy development, and, hence, resultant GHG emissions. Thus, after the modeling teams had quantified the key driving forces and made an effort to harmonize them with the storylines by adjusting control parameters, there still remained diversity in the assumptions about the driving forces and in the resultant emissions (see Chapter 4).

The remaining 14 scenarios adopted alternative interpretations of the four scenario storylines to explore additional scenario uncertainties beyond differences in methodologic approaches, such as different rates of economic growth and variations in population projections. These variations reflect the "modeling teams' choice" of alternative but plausible global and regional development compared to "harmonized" scenarios and also stem from the differences in the underlying modeling approaches. Each of the 40 quantifications of one of the storylines constitutes a SRES scenario. This approach generated a large variation and richness in different scenario quantifications, often with overlapping ranges of main driving forces and GHG emissions across the four families.

In addition, the A1 scenario family branched out into four distinct scenario groups. They are based on four alternative technological developments in future energy systems, from carbon-intensive development to decarbonization. Similar storyline variations were considered for other scenario families, but they did not result in genuine scenario groupings within the respective families. This further increased richness in different GHG and SO₂ emissions paths, because this variation in the structure of the future energy systems in itself resulted in a range of emissions almost as large as that generated through the variation of other main driving forces such as population and economic development. It should be noted that future energy systems variations could be applied to the other storylines, but they may evolve differently from those in A1. They have been introduced into the A1 storyline because of its "high growth with high technology" nature, where differences in alternative technology developments translate into large differences in future GHG emission levels. Altogether the 40 SRES scenarios fall into seven groups: the three scenario families, A2, B1, and B2, plus four groups within the A1 scenario. In the SPM, two of these groups, the coal and gas and oil intensive groups, were merged into one fossil-intensive group, leading to six groups.

As in the case of the storylines, no single scenario - whether it represents a modeler's choice or harmonized assumptions - was treated as being more or less "probable" than the others belonging to the same family. Initially, for each storyline, one modeling group was given principal responsibility, and the quantification produced by that group is referred to as the "marker" scenario for that storyline. The four preliminary marker scenarios were used in 1998 to solicit comments during the "open process" and as input for climate modelers in accordance with a decision of the IPCC Bureau in 1998. The four marker scenarios were posted on the SRES web site (http://sres.ciesin.org/) in June 1998 and were subsequently revised to account for comments and suggestions received through this open scenario review process that lasted until January 1999. In addition to many revisions, the marker scenarios were also harmonized along with the other 26 scenarios that adopted common assumptions for the main driving forces within the four respective families. The choice of the markers was based on extensive discussion of:

• range of emissions across all of marker scenarios;
• which of the initial quantifications (by the modelers) reflected the storyline best;
• preference of some of the modeling teams and features of specific models;
• use of four different models for the four markers.

As a result the markers were not intended to be the median or mean scenarios from their respective families. Indeed, in general it proved impossible to develop scenarios in which all relevant characteristics match mean or median values. Thus, marker scenarios are no more or less likely than any other scenarios, but are those scenarios considered by the SRES writing team as illustrative of a particular storyline. These scenarios have received the closest scrutiny of the entire writing team and via the SRES open process compared to other scenario quantifications. The marker scenarios are also those SRES scenarios that have been most intensively tested in terms of reproducibility. As a rule, different modeling teams have attempted to replicate the model quantification of marker scenarios. Available time and resources have not allowed a similar exercise to be conducted for all SRES scenarios, although some effort was devoted to reproduce the four scenario groups (merged into three in the SPM) that constitute different interpretations of the A1 storyline with different models.

Additional scenarios using the same harmonized assumptions as the marker scenarios developed by different modeling teams and other scenarios that give alternative quantitative interpretations of the four storylines constitute the final set of 40 SRES scenarios. This also means that the 40 scenarios are not independent of each other as they are all based on four storylines and subdivided into seven scenario groups (after merging two groups, six in the SPM) that share many common assumptions. In addition to many revisions of the marker and other harmonized scenarios, other alternative scenarios were formulated by the six modeling teams within each of the four scenario families. The result is a more complete, refined set of 40 emissions scenarios that reflects the broad spectrum of modeling approaches and regional perspectives. However, differences in modeling approaches have meant that not all of the scenarios provide estimates for all the direct and indirect GHG emissions for all the sources and sectors. In addition to the marker scenarios, two scenarios were also selected in the SPM to illustrate the alternative energy systems developments in the A1 family. Hence, this report has an illustrative scenario for each of the six scenario groups in the SPM. The four SRES marker scenarios and the two illustrative scenarios (selected in the SPM) cover all the relevant gas species and emission categories comprehensively and thus constitute the smallest set of independent and fully documented SRES scenarios.

The scenario groups and cumulative emissions categories were developed as the smallest subsets of SRES scenarios that capture the range of uncertainties associated with driving forces and emissions. Together, the four markers and the two additional illustrative scenarios selected in the SPM from the A1 scenario groups constitute the set of SRES scenarios that reflects the uncertainty ranges in the emissions and their driving forces. Furthermore, the writing team recommends that, to the extent possible, these scenarios, but at least the four markers and the two additional illustrative scenarios selected in the SPM, should always be used together, and that, in general, no individual scenario should be singled out for any purpose. Multiple baselines and overlapping emissions ranges have important implications for making policy analysis, e.g., similar policies might have different impacts in different scenarios. Combination of policies might shape the future development in the direction of certain scenarios.

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