10. Summary, Conclusions, and Recommendations

In summary, the SRES scenarios lead to the following findings:

Alternative combinations of driving forces can lead to similar levels and structure of energy and land-use patterns, as illustrated by different scenarios and groups. Hence, even for a given scenario outcome (e.g., in terms of GHG emissions) there are alternative combinations of driving forces and pathways that could lead to that outcome. For instance, significant global changes could result from a scenario of high population growth, even if per capita incomes rise only modestly, as well as from a scenario in which a rapid demographic transition (to low population levels) coincides with high rates of income growth and affluence.
Important possibilities for further bifurcations in future development trends exist within one scenario family, even when particular values are adopted for the important scenario driving force variables to illustrate a particular development path. The technology scenario groups in the A1 family illustrate such alternative development paths with similar quantifications of the main driving forces.
Emissions profiles are dynamic across the range of SRES scenarios. They portray trend reversals and indicate possible emissions crossover among different scenarios. They do not represent mere extensions of continuous increase of GHGs and SO₂ emissions into the future. This more complex pattern of future emissions across the range of SRES scenarios, time periods, world regions, and sectors reflects recent scenario literature.
Describing potential future developments involves inherent ambiguities and uncertainties. One and only one possible development path (as alluded to, for instance, in concepts such as "business-as-usual scenario") simply does not exist alone. And even for each alternative development path described by any given scenario, there are numerous combinations of driving forces and numeric values that can be consistent with a particular scenario description. The numeric precision of any model result should not distract from the basic fact that uncertainty abounds. However, the multi-model approach increases the value of the SRES scenario set, since uncertainties in the choice of model input assumptions can be separated more explicitly from the specific model behavior and related modeling uncertainties.
Any scenario has subjective elements and is open to various interpretations. While the writing team as a whole has no preference for any of the scenarios, and has no judgment as to the probability or desirability of different scenarios, the open process and initial reactions to draft versions of this report show that individuals and interest groups do have such judgments. The writing team hopes that this will stimulate an open discussion in the policymaking arena about potential futures and choices that can be made in the context of climate change response. For the scientific community, the SRES scenario exercise has led to the identification of a number of recommendations for future research that can further increase the understanding of potential developments of socio-economic driving forces and their interactions, and the associated GHG emissions. A summary of the main findings and recommendations for potential users of the SRES scenarios is given in Boxes TS-4 and Box TS-5. The writing teams' suggestions for consideration by the IPCC are summarized in Box TS-6.
Finally, the writing team believes that the SRES scenarios largely fulfill all specifications set out in Chapter 1. To support reproducibility, more detailed information than can be included in this report will be made available by individual modeling groups and members of the writing team through other means, such as web sites, peer-reviewed literature, or background documentation, if additional resources can be made available.

In conclusion, Tables TS-2, TS-3, and TS-4 summarize the main characteristics of the scenario groups that constitute the four families, both for the harmonized and for all scenarios. Tables TS-2 and TS-3 summarize the ranges of the primary and secondary scenario driving forces, respectively. Table TS-4 summarizes the emissions of GHGs, SO₂ , and ozone precursor emissions. Together, the three tables provide a concise summary of the new SRES scenarios.

Box TS-4: Main Findings and Implications of SRES Scenarios

The four scenario families each have a narrative storyline and consist of 40 scenarios developed by six modeling groups.
The 40 scenarios cover the full range of GHGs and SO₂ emissions consistent with the underlying range of driving forces from scenario literature.
The 40 SRES scenarios fall into various groups - the three scenario families A2, B1, and B2, plus different groups within the A1 scenario family. The A1 groups are distinguished by their technological emphasis - on coal (A1C), oil and gas (A1G), non-fossil energy sources (A1T), or a balance across all sources (A1B). In the SPM, the A1C and A1G scenario groups are combined into one fossil intensive group A1FI. All scenario groups are equally sound.
The scenarios are also grouped into four categories of cumulative CO₂ emissions, which indicate that scenarios with different driving forces can lead to similar cumulative emissions and those with similar driving forces can branch out into different categories of cumulative emissions.
Four from 40 scenarios are designated as marker scenarios that are characteristic of the four scenarios families. Together with the two additional illustrative scenarios selected from the scenario groups in the A1 family, they capture most of the emissions and driving forces spanned by the full set of the scenarios.
There is no single central or "best guess" scenario, and probabilities or likelihood are not assigned to individual scenarios. Instead, the writing team recommends that the smallest set of scenarios used should include the four designated marker scenarios and the two additional illustrative scenarios selected from the scenario groups in the A1 family.
Distinction between scenarios that envisage stringent environmental policies and those that include direct climate policies was very difficult to make, a difficulty associated with many definitional and other ambiguities.
All scenarios describe futures that are generally more affluent than today. Many of the scenarios envisage a more rapid convergence in per capita income ratios in the world compared to the IS92 scenarios while, at the same time, they jointly cover a wide range of GHGs and SO₂ emissions.
Emissions profiles are more dynamic than the IS92 scenarios, which reflects changes in future emissions trends for some scenarios and GHG species.
The levels of GHG emissions are generally lower than the IS92 levels, especially toward the end of the 21 st century, while emissions of SO , which have a cooling effect on the atmosphere, are significantly lower than in IS92.
Alternative combinations of main scenario driving forces can lead to similar levels of GHG emissions by the end of the 21 st century. Scenarios with different underlying assumptions can result in very similar climate changes.
Technology is at least as important a driving force of GHG emissions as population and economic development across the set of 40 SRES scenarios.

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) simulations 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 WGIII:

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, B1, and B2, plus the groups within the A1 scenario family, A1B, A1C & A1G (combined into A1FI in the SPM), 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 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 circumstances 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 A1 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 stabilization runs for a given ceiling should include all four of the SRES marker scenarios and the two additional illustrative scenarios selected in SPM from the scenario groups in the A1 family.
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.
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 climate 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 CO₂ emissions, to go beyond the effort of SRES, which was limited by time 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.

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