As a result of time lags in the impact assessment research cycle, impact assessment studies included in this Third Assessment Report (TAR) do not necessarily employ the set of Intergovernmental Panel on Climate Change (IPCC) reference scenarios outlined in Chapter 3. This time lag is unavoidable because it takes almost half a year to define emissions of GHG after setting socioeconomic scenarios. Following that, it usually takes several months to produce local climate change data used in impact assessment studies. Thus, most of the impact studies reported in the TAR are based on the set of IS92 emission scenarios developed for the IPCC in 1992 and included in the SAR (e.g., Parry and Livermore, 1999).

To assist researchers, the IPCC took the initiative to create the IPCC Data Distribution Centre (http://ipcc-ddc.cru.uea.ac.uk/) and posted the SRES scenarios on the Consortium for International Earth Science Information Network (CIESEN) Web site (http://sres.ciesin.org/index.html). The IPCC is responsible for distributing consistent scenarios, including socioeconomic trends and regional climate change data. Consistent use of common scenarios provides a consistent reference for comparing and interpreting the results of different studies.

Vulnerability assessments can be conducted on temporal and spatial scales where the effects of climate change could feed back to GHG emissions and climatic changes. In such cases, there may be reason to ensure that scenarios of climate change that are based on GHG emissions and scenarios of changing social, economic, and technological conditions are consistent. This is essential for global assessments and integrated assessment (see Section 2.4). It may not be as critical for studies of local adaptation where there is little feedback between mitigation and adaptation, particularly over a typical planning horizon of several decades. Downscaling the global reference scenarios to local socioeconomic and political conditions remains a significant methodological challenge.

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