Emissions Scenarios - IPCC

Technical Summary 47
Box TS-5: Recommendations for Consideration by the User Communities
The writing team recommends that the SRES scenarios be the main basis for the assessment of future emissions and their
driving forces in the TAR. Accordingly, the SRES writing team makes the following recommendations regarding the emissions
scenarios to be used in the atmosphere/ocean general circulation models (A/O GCMs) simmlations for WGI, for the models that
will be used in the assessment of climate change impacts by WGII, and for the mitigation and stabilization assessments by
WGIH:
• It is recommended that a range of SRES scenarios from more than one family be used in any analysis. The scenario
groups - the three scenario families A2, Bl, and B2, plus the groups within the AI scenario family, AIB, AlC & AIG
(combined into AlFI in the SPM), and AIT - 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 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 seven 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 ckcumstances and perspectives.
• There is no single most, likely "central" or "best-guess" scenario, either with respect to other SRES scenarios or to the
underlying scenario literature. Probabilities or likelihoods are not assigned to individual SRES scenarios. None of the
SRES scenarios represents an estimate of a central tendency for all driving forces and emissions, such as the mean or
median, and none should be interpreted as such. The statistics associated with the frequency distributions of SRES
scenarios do not represent the likelihood of their occurrence. The writing team cautions against constructing a central,
"best-estimate" scenario from the SRES scenarios; instead it recommends use of the SRES scenarios as they are.
• Concerning large-scale climate models, the writing team recommends that the minimum set of SRES scenarios should
include the four designated marker scenarios and the two additional illustrative scenarios selected in the SPM from the
scenario groups in the AI family. At the minimum (a) a simulation for one and the same SRES marker scenario should
be performed by every TAR climate model for a given stabilization ceiling, and (b) the set of simulations performed by
the TAR climate models and stabiUzation runs for a given ceiling should include аН four of the SRES marker scenarios
and the two additional illustrative scenarios selected in SPM from the scenario groups in the Al family.
• The driving forces and emissions of each SRES scenario should be used together. To avoid intemal inconsistencies,
components of SRES scenarios should not be mixed. For example, the GHG emissions from one scenario and the SOj
emissions from another scenario, or the population from one and economic development path from another, should not
be combined.
• The SRES scenarios can provide policy makers with a long-term context for near-term decisions. This implies that they
are not necessarily well suited for the analysis of near-term developments. When analyzing mitigation and adaptation
options, the user should be aware that although no additional clunate initiatives are included in the SRES scenarios,
various changes have been assumed to occur that would require other policy interventions.
• All 40 SRES emissions scenarios, their main driving forces, and underlying assumptions should be made widely
available. Depending on resources available the scenario documentation, should e.g., be placed on the web and made
available on a CD-ROM. In addition, the time-dependent geographic distributions of the concentrations of GHGs and
sulfate aerosol burden, together with their corresponding radiative forcings, should also be placed on the web.
Box TS-6: Recommendations for Consideration by the IPCC
• Assure that the SRES scenarios, their main assumptions, and modeling approaches are widely available through a web
site or a CD-ROM.
• Establish a long-term facility for documentation and comparison of emissions scenarios to succeed the SRES open
process. This should include a scenario database and analytic evaluation capabilities and should be regularly maintained.
• An effort should be made in the future to develop an appropriate emissions scenario classification scheme.
• Identify resources for capacity building in the area of emissions scenarios and modeling tools, with a particular emphasis
to involve strong participation from developing countries.
• Promote activities within and outside the IPCC to extend the SRES multi-baseline and multi-model approach in future
assessments of climate change impacts, adaptation, and mitigation.
• Initiate new programs to assess GHG emissions from land use and sources of emissions other than energy-related COj
emissions, to go beyond the effort of SRES, which was limited by tune and resources.
• Initiate new programs to assess future developments of driving forces and GHG emissions for different regions and for
different sectors (taking the set of SRES scenarios as reference for overall global and regional developments) to provide
more regional and sectorial detail than time and resources allowed SRES to achieve.