Presented in this chapter is the assessment of more than 400 global and regional greenhouse gas (GHG) emissions scenarios based on an extensive literature review. Emissions scenarios provide an important input for the assessment of future climate change. Future anthropogenic GHG emissions depend on numerous driving forces, including population growth, economic development, energy supply and use, land-use patterns, and a host of other human activities. These main driving forces that determine the emissions trajectories in the scenarios often also provide input to assess possible emissions mitigation strategies and possible impacts of unabated emissions. In view of the many different uses, it is not surprising that numerous emissions scenarios are presented in the literature and that the number of regional and global emissions scenarios is growing.

An important characteristic of the scenarios in this Special Report on Emissions Scenarios (SRES) is that they reflect the underlying uncertainty, part of which derives from the range of emissions in the literature. The objective was to encompass the variation within the most important scenario driving forces and emissions, the complexity of possible relationships between driving forces and emissions, and the associated uncertainties that characterize alternative future developments. The SRES scenarios cover most of the range of the GHG emissions scenarios found in the literature, including the International Panel on Climate Change (IPCC) 1992 Scenarios (IS92) series (Leggett et al., 1992). The writing team considered the literature in creating a new set of scenarios. Importantly, however, the literature on existing scenarios provides only a general framework to aid analysis; it is informative, but not determinative.

The literature review consists of four parts:

• documentation of as many as possible of the quantitative global and regional emissions scenarios available both in the open literature and from international activities that involve documentation of submitted scenarios;
• development of a scenario database to document the more than 400 emissions scenarios collected during the literature review;
• evaluation of the ranges and relationships of the main scenario driving forces and the resultant emissions for the scenarios documented in the database; and
• assessment of the scenario submissions received through the SRES "open process."

Central to this assessment of emissions scenarios and their main driving forces is a unique scenario database developed by the National Institute for Environmental Studies (NIES) in Japan for SRES (Morita and Lee, 1998). The database version of 3. April 1998, which is assessed in this chapter, includes 416 different scenarios. The current database version can be accessed through an ftp-site (www-cger.nies.go.jp/cger-e/db/ipcc.html). It is the most comprehensive collection of emissions scenarios in the publicly available literature. It includes most of the recent global and regional scenarios and all of the scenarios used in the latest IPCC evaluation of emissions scenarios (Alcamo et al., 1995). Therefore, the emissions scenarios documented in the database are representative of the literature in general. However, there are a number of ways in which the coverage of the scenarios in the database could be extended in the future. For example, inclusion of long-term emissions scenarios for individual countries, when available, would improve the regional coverage (e.g., Parikh, 1996; Murthy et al., 1997). Also, a large majority of the scenarios report only energy-related carbon dioxide (CO₂ ) emissions (230 scenarios), while only some report non-energy CO₂ and other GHG emissions. This shortcoming of the emissions scenarios in the literature was identified in the last IPCC evaluation (Alcamo et al., 1995).

The scenarios in the database were collected from 171 different literature sources and other scenario-evaluation activities, such as the Energy Modeling Forum (EMF; see Weyant, 1993) and the International Energy Workshop (IEW; see Manne and Schrattenholzer, 1996, 1997). The scenarios span a wide range of assumptions about demographic trends, levels of economic development, energy consumption and efficiency patterns, and other factors. The aim of this chapter is to show how the database can be utilized for the analysis of GHG emissions ranges and their main driving forces. Part of this assessment of the emissions scenarios is based on an earlier publication on the analysis of scenarios documented in the SRES database (Nakic´enovic´ et al., 1998a).

The scenarios in the database display a large range of future GHG emissions. Part of the range can be attributed to the different methods and models used to formulate the scenarios, which include simple spreadsheet models, economic models, and systems-engineering models. However, most of the range results from differences in the input assumptions of the scenarios, in particular those of the main scenario driving forces. In addition, simply to compare alternative emissions levels across different scenarios is not sufficient to shed light on internal consistency, plausibility, and comparability of the assumptions behind the scenarios. Analysis of the underlying driving forces is thus also an important part of the evaluation. This chapter provides an analysis of the main driving forces, such as population growth, economic growth, energy consumption, and energy and carbon intensities. Some of these driving forces are specified as model inputs, and some are derived from model outputs, so it is necessary to determine the assumed relationships among the main driving forces.

Although the scenario database is well suited for the documentation of quantitative scenarios, there is also a significant literature on narrative scenarios. Both scenario types have in common that they are generally carefully constructed descriptions of possible future developments within the bounds of explicit assumptions and circumstances (see Chapter 1 for a more detailed discussion about scenarios). The difference is that the quantitative scenarios are usually developed with the help of formal models so as to assign internally consistent values to the various scenario characteristics.

The SRES scenarios employ both approaches - a storyline that gives a broad, narrative, and qualitative scenario description plus a number of quantifications of each storyline with six different models. Thus, even though both narrative stories and quantitative scenarios are an integral part of the SRES emissions scenarios, the literature review focused on the documentation and the assessment of quantitative scenarios, for two reasons. First, it was not possible to devise a classification system that would allow the documentation of many different forms of narrative scenarios. Second, the SRES objective was to develop a set of numeric emissions scenarios for use in the IPCC and other assessments of climate change. Therefore, in this chapter the focus is only on the literature review of quantitative scenarios. A more detailed discussion of narrative scenarios is given in Chapter 4; it deals with the four SRES storylines and how they are related to recent work in the area of qualitative scenarios.

The literature on quantitative scenarios is large indeed. This assessment is focused on the scenarios that extend at least to 2020, but about 10 scenarios with a shorter time horizon of 2010 are also included in the database. In addition, most of the scenarios have a global coverage, although a few regional scenarios are included to enhance the coverage of some parts of the world. These criteria narrowed considerably the number of global and regional GHG emissions scenarios with sufficient information to be included in the scenario literature review.

This scenario literature review and evaluation is the second undertaken by the IPCC. The first was conducted to evaluate the IS92 set of scenarios in comparison to other GHG emissions scenarios found in the literature (Alcamo et al., 1995)¹ . It was completed in 1994 and included a comprehensive evaluation of GHG emissions and their main driving forces. This second review and evaluation builds upon and extends the earlier IPCC assessment. Consequently, an effort was made in the present review to include especially the GHG emissions scenarios published since the presentation of the IS92 scenarios.

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