Often these scenarios are formulated with the help of formal models. Such scenarios specify the future emissions of GHGs in quantitative terms and, if fully documented, they are also reproducible. 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. Figure 1-1 illustrates the interrelated nature of these alternative scenario formulations. Although no scenarios are value free, it is often useful to distinguish between normative and descriptive scenarios.
Descriptive scenarios are evolutionary and open-ended, exploring paths into the future.
The SRES scenarios are descriptive and should not be construed as desirable or undesirable in their own right. They are built as descriptions of possible, rather than preferred, developments. They represent pertinent, plausible, alternative futures. Their pertinence is derived from the need for policy makers and climate-change modelers to have a basis for assessing the implications of future possible paths for GHG and SO 2 emissions, and the possible response strategies. Good scenarios are challenging and court controversy, since not everybody is comfortable with every scenario, but used intelligently they allow policies and strategies to be designed in a more robust way.