The assessment of climate change dictates a global perspective and a very long time horizon that covers periods of at least a century. As the prediction of future anthropogenic GHG emissions is impossible, alternative GHG emissions scenarios become a major tool for the analysis of potential long-range developments of the socio-economic system and corresponding emission sources.

However, to develop scenarios for a period of 100 years is a relatively new field. Difficulties arise not only from large scientific uncertainties and data inadequacies, but also because people are not trained to think in such time-spans. We are educated in narrow disciplines, and our ability to model complex systems, at the global level, is still in its infancy. For example, within the next 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 because most databases are incomplete if more than 50 years old. Given these gaps in our data, methods and understanding, scenarios are the best way to integrate our demographic, economic, societal and technological knowledge with our understanding of ecologic 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 to take advantage of these features and aid the assessment of future climate change, impacts, vulnerabilities, adaptation and mitigation. Since the scenarios focus on the century time scale, tools are used that have been developed for this purpose. These tools are less suitable for the analysis of near-term developments, so this report does not intend to provide reliable projections for the near term.

The IPCC's 1994 evaluation of its 1992 emissions scenarios identified four principal uses (Alcamo et al., 1995):

• To provide input for evaluating climatic and environmental consequences of alternative future GHG emissions in the absence of specific measures to reduce such emissions or enhance GHG sinks.
• To provide similar input for cases with specific alternative policy interventions to reduce GHG emissions and enhance sinks.
• To provide input for assessing mitigation and adaptation possibilities, and their costs, in different regions and economic sectors.
• To provide input to negotiations of possible agreements to reduce GHG emissions.

The SRES emissions scenarios are intended for the first, third and fourth uses. They do not include any additional (explicit) policies or measures directed at reducing GHG sources and enhancing sinks. Thus, they cannot be directly applied to the second purpose of emissions scenarios. Instead, they could be used as reference cases for the introduction of specific policy interventions and measures in new model runs that share the same specifications for the other principal driving forces of future emissions. However, the SRES emissions scenarios include a host of other policies and measures that are not directed at reducing sources and increasing sinks of GHGs, but that nevertheless have an indirect effect on future emissions. For example, policies directed at achieving greater environmental protection may also lead to lower emissions of GHGs. Moreover, afforestation and reforestation measures increase CO₂ sinks, and a shift to renewable energy sources reduces the sources of emissions.

Within three of the broad objectives listed above, the new SRES emissions scenarios are also intended to meet the specific needs of three main IPCC user communities:

• Working Group I (WGI), which includes climate modelers who need future emission trajectories for GHGs and aerosol precursors as inputs for the GCMs used to develop climate change scenarios.
• Working Group II (WGII), which analyzes climate impacts and adaptation policies, first need the climate-change scenarios produced by WGI's climate modelers. Second, analysts need to know the socio-economic changes associated with specific emissions scenarios, as impacts of climate change on ecosystems and people depend on many factors. Among these are whether the people are numerous or few, rich or poor, free to move or relatively immobile, and included or excluded from world markets in technologies, food, etc.
• WGIII, which analyzes potential mitigation policies for climate change, also needs to know the socio-economic settings against which policy options are to be evaluated. Are markets open or protected? Are technological options and economic resources plentiful or scarce? Are people vulnerable or adaptable?

The interests of these three user groups create certain requirements that the SRES scenarios attempt to fulfill. For example, climate modelers and those who analyze climate impacts need scenarios on the order of 100 years because of the long response time of the climate system. At the same time adaptation-policy analysis tends to be focused more on the medium-term, around 20 to 50 years. The SRES scenarios attempt to include enough information and specific details to be useful to these groups. Spatially explicit emissions and socio-economic variables are required for slightly different reasons. Some emissions, such as the SO₂ emissions that contribute to sulfate aerosols, have impacts that vary depending on where they are emitted. Climate modelers therefore need spatially explicit emission estimates. Similarly, impacts depend on the geographic patterns of changing temperatures, rainfall, humidity and cloud cover, and how these compare to evolving socio-economic patterns in specific scenarios. Impact modelers therefore need spatially explicit estimates of, in particular, population growth, migration, and the economic variables that reflect the expected adaptability or vulnerability of different populations and regional economies to different regional climate changes.

Taking the above audiences and purposes into account, the following more precise specifications for the new SRES scenarios were developed. The new scenarios should:

• cover the full range of radiatively important gases, which include direct and indirect GHGs and SO₂ ;
• have sufficient spatial resolution to allow regional assessments of climate change in the global context;
• cover a wide spectrum of alternative futures to reflect relevant uncertainties and knowledge gaps;
• use a variety of models to reflect methodological pluralism and uncertainty;
• incorporate input from a wide range of scientific disciplines and expertise from non-academic sources through an open process;
• exclude additional initiatives and policies specifically designed to reduce climate change;
• cover and describe to the extent possible a range of policies that could affect climate change although they are targeted at other issues, for example, reductions in SO₂ emissions to limit acid rain;
• cover as much as possible of the range of major underlying "driving forces" of emissions scenarios identified in the open literature;
• be transparent with input assumptions, modeling approaches and results open to external review;
• be reproducible - input data and methodology are documented adequately enough to allow other researchers to reproduce the scenarios; and
• be internally consistent - the various input assumptions and data of the scenarios are internally consistent to the extent possible.

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