The Intergovernmental Panel on Climate Change (IPCC) decided at its September 1996 plenary session in Mexico City to develop a new set of emissions scenarios (see Appendix I for the Terms of Reference). This Special Report on Emission Scenarios (SRES) describes the new scenarios and how they were developed.

The SRES writing team formulated a set of emissions scenarios. These scenarios cover a wide range of the main driving forces of future emissions, from demographic to technological and economic developments. The scenarios encompass different future developments that might influence greenhouse gas (GHG) sources and sinks, such as alternative structures of energy systems and land-use changes. As required by the terms of reference however, none of the scenarios in the set includes any future policies that explicitly address additional climate change initiatives ¹ although all necessarily encompass various assumed future policies of other types that may indirectly influence GHGs sources and sinks.

The set of SRES emissions scenarios 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 anthropogenic emissions of all relevant GHG species and sulfur dioxide (SO₂), carbon monoxide (CO), nitrogen oxides (NO_x) and non-methane volatile organic hydrocarbons (VOCs), as shown in Table 1-1. It covers most of the range of GHG emissions compared with the published scenario literature. For example, in the SRES scenarios, emissions of CO₂ in 2100 range from more than 40 to less than 6 giga (or billion) tons ² of elemental carbon (GtC), that is, from almost a sevenfold increase to roughly the same emissions level as in 1990.

Emissions scenarios are a central component of any assessment of climate change. Scenarios facilitate the assessment of future developments in complex systems that are either inherently unpredictable or have high scientific uncertainties, and the assessment of future emissions is an essential component of the overall assessment of global climate change by the IPCC.

Emissions of GHGs and SO₂ are the basic input for determining future climate patterns with simple climate models, as well as with complex general circulation models (GCMs). Possible climate changes together with the major driving forces of future emissions, such as demographic patterns, economic development and environmental conditions, provide the basis for the assessment of vulnerability, possible adverse impacts and adaptation strategies and policies to climate change. The major driving forces of future emissions also provide the basis for the assessment of possible mitigation strategies and policies designed to avoid climate change.

Future emissions and the evolution of their underlying driving forces are highly uncertain, as reflected in the very wide range of future emissions paths in the literature. Of the many ways that uncertainties have been classified in the literature (see Box 1-1 in Section 1.2 below), this introduction uses the three categories of Funtowicz and Ravetz (1990): "data uncertainties,""modeling uncertainties" and "completeness uncertainties." This categorization has the advantages of a small number of categories and of clear descriptive titles. Data uncertainties reflect the reality that most historical and base year data sets are neither fully complete nor fully reliable. This is certainly true for data on population, energy consumption, energy efficiency, gross world product, energy resources and reserves, and probably true for every parameter mentioned in this report. Modeling uncertainties refer both to the approximations necessary in any model of complex phenomena like GHG emissions, and to the range of plausible but different modeling approaches, each with its own strengths and weaknesses. Completeness uncertainties encompass, first, relevant factors that can be identified but are nonetheless excluded from an analysis - for example exclusion of criteria other than cost minimization in an energy model, such as energy security, the protection of domestic industries, and free trade. Second, they also include factors that may be relevant but are as essentially unknown to us as jet airplanes were to Thomas Malthus or 3-D seismic techniques in oil exploration were to John D. Rockefeller. The use of scenarios and storylines in this report partially addresses completeness uncertainties related to known factors. Completeness uncertainties related to unknown factors can, of course, never be persuasively captured by any approach.

The IPCC developed sets of emissions scenarios in 1990 (Houghton et al., 1990) and 1992 (Leggett et al., 1992; Pepper et al., 1992). In 1994 the IPCC formally evaluated the 1992 scenario set (Alcamo et al., 1995) and, in 1996, it initiated the effort described in this report. The new set of emissions scenarios is intended for use in future IPCC assessments and by wider scientific and policymaking communities who analyze the effects of future GHG emissions and develop mitigation and adaptation measures and policies. The emissions profiles of the new scenarios can provide inputs for GCMs and simplified models of climate change. The new scenarios also contain information, such as the level of economic activity, rates of technological change and demographic developments in different world regions, required to assess climate-change impacts and vulnerabilities, adaptation strategies and policies. The same kind of information, in conjunction with emissions trajectories, can serve as a benchmark for the evaluation of alternative mitigation measures and policies. Finally, the new set of scenarios may provide a common basis and an integrative element for the Third Assessment Report (TAR).

IPCC Working Group III ( WGIII) appointed the SRES writing team in January 1997. After some adjustments it eventually included more than 50 scientists. Together they represent a broad range of scientific disciplines, regional backgrounds and non-governmental organizations. In particular the team includes representatives of six leading groups with extensive expertise in modeling alternative emissions scenarios. It also includes a number of members who were convening and lead authors in all three earlier IPCC scenario activities (see above). Their expertise and familiarity with earlier IPCC emissions scenario work assured continuity and allowed the SRES effort to build efficiently upon prior work. Appendix II lists the members of the writing team and their affiliations.

The writing team reached a consensus concerning the overall work program. It was agreed that the scenario development process would consist of four major components.

• First, a review of existing global and regional emissions scenarios from the published literature and development of a unique database of 416 global and regional scenarios (accessible at a web site: www-cger.nies.go.jp/cger-e/db/ipcc.html).
• Second, an analysis of the range of the scenarios' main characteristics, their relationships, and their "driving forces" (such as population, economic development, energy consumption, rates of technological change and GHG emissions) and the documentation of the results, some of which are published in the peer-reviewed literature.³
• Third, a formulation of narrative "storylines" to describe the main scenario characteristics, the development of quantitative prototype scenarios by six leading groups representing the main modeling approaches from around the world, and the publication of the prototype scenarios on a specially developed IPCC web site (sres.ciesin.org), on web sites of the modeling teams ⁴, and in the peer-reviewed literature.⁵
• Fourth, an "open" process through the IPCC web site (sres.ciesin.org) that involves feedback from modeling groups and experts worldwide, followed by the IPCC expert and government reviews that were coordinated by four review editors.

Most of the background material and findings of the assessments conducted by the writing team have been documented in this report and in a series of publications including two special issues of international scientific journals, Mitigation and Adaptation Strategies for Global Change ³ and Technological Forecasting and Social Change ^5.

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