IPCC Special Report on Emissions Scenarios

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1. Introduction and Background

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 GHG emissions are directly affected by non-climate change policies designed for a wide range of other purpose.

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, sulfur dioxide (SO2 ), carbon monoxide (CO), nitrogen oxides (NOx ), and non-methane volatile organic compounds (NMVOCs), see Table 1-1 in Chapter 1. It covers most of the range of GHG emissions compared with the published scenario literature. For example, emissions of carbon dioxide (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.

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 that is also captured by the SRES scenarios. The use of scenarios in this report addresses the uncertainties related to known factors. Uncertainties related to unknown factors can of course never be persuasively captured by any approach. As the prediction of future anthropogenic GHG emissions is impossible, alternative GHG emissions scenarios become a major tool for analyzing potential long-range developments of the socio-economic system and corresponding emission sources.

Emissions scenarios are a central component of any assessment of climate change. GHG and SO2 emissions are the basic input for determining future climate patterns with simple climate models, as well as with complex general circulation models (GCMs). Possible climate change, 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. The new set of emissions scenarios is intended for use in future IPCC assessments and by wider scientific and policymaking communities for analyzing the effects of future GHG emissions and for developing mitigation and adaptation measures and policies.

2. Emissions Scenarios and Their Purposes

Scenarios are images of the future, or alternative futures. They are neither predictions nor forecasts. Rather, each scenario is one alternative image of how the future might unfold (see Chapter 1 and Chapter 4 for more detail). As such they enhance our understanding of how systems behave, evolve and interact. They are useful tools for scientific assessments, learning about complex systems behavior and for policymaking and assist in climate change analysis, including climate modeling and the assessment of impacts, adaptation and mitigation.

Future levels of global GHG emissions are a product of very complex, ill-understood dynamic systems, driven by forces such as population growth, socio-economic development, and technological progress among others, thus making long-term predictions about emissions virtually impossible. However, near-term policies may have profound long-term climate impacts. Consequently, policy makers need a summary of what is understood about possible future GHG emissions, and given the uncertainties in both emissions models and our understanding of key driving forces, scenarios are an appropriate tool for summarizing both current understanding and current uncertainties.

GHG emissions scenarios are usually based on an internally consistent and reproducible set of assumptions about the key relationships and driving forces of change, which are derived from our understanding of both history and the current situation. Often these scenarios are formulated with the help of formal models. Sometimes GHG emissions scenarios are less quantitative and more descriptive, and in a few cases they do not involve any formal analysis and are expressed in qualitative terms. The SRES scenarios involve both qualitative and quantitative components; they have a narrative part called "storylines" and a number of corresponding quantitative scenarios for each storyline. SRES scenarios can be viewed as a linking tool that integrates qualitative narratives or stories about the future and quantitative formulations based on different formal modeling approaches. Although no scenarios are value free, the SRES scenarios are descriptive and are not intended to be desirable or undesirable in their own right. They have been built as descriptions of plausible alternative futures, rather than preferred developments.

However, developing scenarios for a period of one hundred years is a relatively new field. This is not only because of large scientific uncertainties and data inadequacies. For example, within the 21st century technological discontinuities should be expected, and possibly major shifts in societal values and in the balance of geopolitical power. The study of past trends over such long periods is hampered by the fact that most databases are incomplete if we go back much further than 50 years. Given these gaps in our data, methods, and understanding, scenarios are the best way to integrate demographic, economic, societal, and technological knowledge with our understanding of ecological systems to evaluate sources and sinks of GHG emissions. Scenarios as an integration tool in the assessment of climate change allow a role for intuition, analysis, and synthesis, and thus we turn to scenarios in this report to take advantage of those features to aid the assessment of future climate change, impacts, vulnerabilities, adaptation, and mitigation. Since the scenarios focus on the century time scale, tools have been used that have been developed for this purpose. These tools are less suitable for analysis of near-term developments and this report does not intend to provide reliable projections for the near term.

3. Review of Past IPCC Emissions Scenarios

The IPCC developed sets of emissions scenarios in 1990 and 1992. The six IS92 scenarios developed in 1992 (Leggett et al., 1992; Pepper et al., 1992), have been used very widely in climate change assessments. In 1995 the IPCC formally evaluated the 1992 scenarios and found that they were innovative at the time of their publication, path-breaking in their coverage of the full range of GHG emissions and useful for the purpose of driving atmospheric and climate models (Alcamo et al., 1995). Specifically, their global carbon emissions spanned most of the range of other scenarios identified in the literature at that time.

The review also identified a number of weaknesses. These included the limited range of carbon intensities of energy (carbon emissions per unit energy) and the absence of any scenario with significant closure in the income gap between developed and developing countries, even after a full century (Parikh, 1992). Furthermore, rapid growth of sulfur emissions in the IS92 scenarios had been questioned on the basis that they did not reflect recent legislation in Japan, Europe, and North America and that in general regional and local air quality concerns might prompt limits on future sulfur emissions.

An important recommendation of the 1995 IPCC review was that, given the degree of uncertainty about future climate change, analysts should use the full range of IS92 emissions as input to climate models rather than a single scenario. This is in stark contrast to the actual use of one scenario from the set, the IS92a scenario, as the reference scenario in numerous studies. The review concluded that the mere fact of the IS92a being an intermediate, or central, CO2 emissions scenario at the global level at that time does not equate it with being the most likely scenario. Indeed, the conclusion was that there was no objective basis on which to assign likelihood to any of the scenarios. Furthermore, the IS92a scenario was shown to be "central" for only a few of its salient characteristics such as global population growth, global economic development and global CO2 emissions. In other ways, IS92a was found not to be central with respect to the published literature, particularly in some of its regional assumptions and emissions. The same is the case with the new set of SRES scenarios, as is shown below.

The new set of SRES scenarios presented here is designed to respond to the IS92 weaknesses identified in the 1995 IPCC scenario evaluation and to incorporate advances in the state of the art since 1992. As in the case of the IS92 scenario series, also in this new set of SRES scenarios there is no single central case with respect to all characteristics that are relevant for different uses of emissions scenarios and there is no objective way to assign likelihood to any of the scenarios. Hence there is no "best guess" or "business-as-usual" scenario.

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