To be most useful for the future work of the IPCC and other scientific assessments, the new reference scenarios must meet several criteria.
- They need to cover all species of gas that have a radiative forcing effect, including greenhouse gases (GHGs) and other pollutants, such as sulfates and particulates.
- They should have sufficient spatial resolution to allow regional disaggregation.
- They need to provide a rich set of alternatives that span a wide spectrum of alternative futures to reflect the salient uncertainties of the future.
- They should use a variety of models for the scenario quantification to reflect methodological pluralism and uncertainty (see terms of reference).
- They should ask for input from a wide constituency through an open process (specified in the terms of reference).
- They should develop scenarios of future GHG emissions without including the effects of new policies specifically designed to reduce climate change: there should be no "mitigation" scenarios in the set (see terms of reference).
The Writing team aims to meet the needs of three main user communities:
- IPCC Working Group I the climate modelers using global circulation models (GCMs) who need GHG emission trajectories to develop their scenarios of climate change;
- IPCC Working Group II analysts of climate impacts and adaptation policy, who need both the climate change scenarios generated by climate modelers and the socio-economic scenarios developed within the SRES to analyze the effects of climate change on ecosystems and human society within the context of alternative future developments not related to climate change;
- IPCC Working Group III analysts of climate mitigation policy, who need scenarios that are rich in economic, technological and social information to be able to evaluate the impacts of specific types of policies and measures on lowering GHG emissions.
These three groups are interested in different time-scales, and different degrees of regional and/or sectoral detail. Climate modelers and analysts of climate impacts need scenarios covering the period over which substantial changes in GHG concentrations are anticipated: on the order of 100 years. Climate modelers need global concentrations of GHG gases and spatially explicit information e.g., on sulfate aerosols (see also {recommendations to users of new scenarios}). Climate impact analysis needs a high degree of spatial resolution of climate impacts and detail of socio-economic background information such as per capita income levels, to assess vulnerability to climate changes. Mitigation analysts are far more interested in detailed economic and technological developments over the next 20-30 years. Many politicians and mitigation policy analysts are interested mainly in the period to 2010, roughly the span of current commitments under the Kyoto Protocol of Parties in Annex I to the UN Framework Convention on Climate Change. Adaptation policy analysis tends to be focused more on the medium-term, which is around 20-50 years.