It is recommended that a range of SRES scenarios with a variety of assumptions regarding driving forces be used in any analysis. Thus more than one family should be used in most analyses. The six scenario groups - the three scenario families A2, B1, and B2, plus three groups within the A1 scenario family, A1B, A1FI, and A1T - and four cumulative emissions categories were developed as the smallest subsets of SRES scenarios that capture the range of uncertainties associated with driving forces and emissions.
The important uncertainties ranging from driving forces to emissions may be different in different applications - for example climate modeling; assessment of impacts, vulnerability, mitigation, and adaptation options; and policy analysis. Climate modelers may want to cover the range reflected by the cumulative emissions categories. To assess the robustness of options in terms of impacts, vulnerability, and adaptation may require scenarios with similar emissions but different socio-economic characteristics, as reflected by the six scenario groups. For mitigation analysis, variation in both emissions and socio-economic characteristics may be necessary. For analysis at the national or regional scale, the most appropriate scenarios may be those that best reflect specific circumstances and perspectives.
There is no single most likely, "central", or "best-guess" scenario, either with respect to SRES scenarios or to the underlying scenario literature. Probabilities or likelihood are not assigned to individual SRES scenarios. None of the SRES scenarios represents an estimate of a central tendency for all driving forces or emissions, such as the mean or median, and none should be interpreted as such. The distribution of the scenarios provides a useful context for understanding the relative position of a scenario but does not represent the likelihood of its occurrence.
The driving forces and emissions of each SRES scenario should be used together. To avoid internal inconsistencies, components of SRES scenarios should not be mixed. For example, the GHG emissions from one scenario and the SO2 emissions from another scenario, or the population from one and economic development path from another, should not be combined.
While recognizing the inherent uncertainties in long-term projections 10 , the SRES scenarios may provide policymakers with a long-term context for near-term analysis. The modeling tools that have been used to develop these scenarios that focus on the century time scale are less suitable for analysis of near term (a decade or less) developments. When analyzing mitigation and adaptation options, the user should be aware that although no additional climate initiatives are included in the SRES scenarios, various changes have been assumed to occur that would require other interventions, such as those leading to reductions in sulfur emissions and significant penetration of new energy technologies.
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