The 1994 IPCC review also offered recommendations about the process by which a new set of scenarios might be prepared. It recommended that the IPCC or another suitable organization act as an "umbrella" under which different groups could develop comparable, comprehensive emissions scenarios. They further recommended that the process for developing scenarios should draw on increasing experience in scenario harmonization and model calculations, and that it should emphasize:

• openness to broad participation by the research (and stakeholder) community, particularly from developing countries and countries with economies in transition;
• extensive documentation of modeling assumptions, inputs and outputs;
• pluralism and diversity of groups, approaches and methods, although the final set of scenarios should be harmonized;
• comparability across the scenarios that necessitates standardized reporting conventions for model inputs and outputs; and
• harmonization of emission scenarios in the set to provide common benchmarks for scenarios from different modeling groups.

Further recommendations included wide dissemination of the scenarios to countries, international organizations, non-governmental organizations and the scientific community. As part of this effort, a central archive should be established to make available the results of new scenarios to any group. The archive should also make available some aspects of the models and input assumptions used to derive the scenarios. In addition, special efforts are needed to improve the capabilities of researchers to analyze and develop scenarios, especially in developing countries and countries with economies in transition.

As described at the beginning of this chapter, IPCC WGIII appointed the SRES writing team in January 1997, and the team reached early consensus on the four major components of an overall work program (outlined in Section 1.1). The SRES team worked in close collaboration with colleagues on the IPCC Task Group on Climate Scenarios for Impact Assessment (TGCIA) and with colleagues from all three IPCC working groups. As is evident from the four components of the work program, it was agreed that the process be an open one with no "official" model and no exclusive "expert teams." High priority was given to wide participation so that any research group capable of preparing scenarios for any region could participate. In 1997 the IPCC advertised in a number of relevant scientific journals and other publications to solicit wide participation in the process. All global modeling teams and regional modelers were invited and encouraged to participate. In this way, researchers with local expertise from both developing and developed regions could contribute to the global exercise even if their own research was exclusively regional. To facilitate participation and improve the usefulness of the new scenarios, the open-process web site mentioned above was created. The open process provided a wide access to preliminary marker (see below) SRES scenario results and greatly facilitated coordination among the writing team. It also provided feedback about the needs of those who would use the final scenarios, and suggestions for improvements. The open process also served to document all relevant results and associated assumptions for the preliminary scenarios developed by the participating modeling groups.

Four storylines were developed by the whole writing team in an iterative process that identified driving forces, key uncertainties, and quantitative scenario families. The team was fortunate to have a number of skilled practitioners in scenario building. The process of quantifying the four storylines deserves some elaboration. The storylines were essentially complete by January 1998, at which time the modeling groups represented on the writing team began to quantify them. 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 posted on the web site of the open process. 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;
• preference of some of the modeling teams and features of specific models;
• use of different models for the four markers.

As a result the markers are not necessarily the median or mean of the scenario family, 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 a particular marker scenario. 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 ⁶ that constitute different interpretations of one of the four storylines (see Figure 1-4) with different models. Additional versions of the preliminary marker scenarios 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. 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. The four SRES marker scenarios cover all the relevant gas species and emission categories comprehensively.

The four marker scenarios were posted on the IPCC web site (sres.ciesin.org) in June 1998, and the open scenario review process through the IPCC web site lasted until January 1999. The submissions invited through the open process and web site fell into three categories (see Appendix VI):

• additional scenarios published in the reviewed literature that had not been included in the scenario database (see Appendix V);
• new scenarios based on the SRES marker scenarios; and
• general suggestions to improve the work of the SRES writing team as posted on the web site (preferably based on referenced literature).

The submissions were used to revise the marker scenarios and to develop additional alternatives within each of the four scenario families. The result is a more complete, refined set of 40 new scenarios that reflects the broad spectrum of modeling approaches and regional perspectives. The preliminary marker scenarios posted on the web site were provided also to climate modelers, with the approval of the IPCC Bureau.

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