Emissions Scenarios - IPCC

Summary Discussions and Recommendations 315
Box 6-3 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 different groups - the three scenario families A2, Bl, and B2, plus four groups within
the Al scenario family, two of which (AlC and AIG) have been combined into one fossil-intensive group AlFI m the
Summary for Policymakers; see also footnote 2. The four Al groups are distinguished by their technological emphasis -
on coal (AlC), oil and gas (AIG), non-fossil energy sources (AIT), or a balance across all sources (Al).
• Tire scenarios are grouped into four categories of cumulative CO^ 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 scenaiios 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 Al 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 IS 92 scenarios while, at the same time, they jointly
cover a wide range of GHG and SO, 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 2P' century, while
emissions of SOj, which have a cooling effect on the atmosphere, are significantly lower than in IS92.
• Altemative combinations of main scenario driving forces can lead to similar levels of GHG emissions by the end of the
2P' 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 6-4: Recommendations for Consideration by the User Communities
The writing team recommends that the SRES scenarios be the mani basis for the assessment of future emissions and their
driving forces in the Third Assessment Report (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 Working Group I (WGI), for the models that will be used in the assessment of climate change impacts
by Working Group II (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 scenario families A2, Bl, and B2, plus the groups within the Al scenario family, and four cumulative emissions
categories were developed as the smallest subsets of SïŒS scenarios that capture the range of uncertahities 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 scenario groups. For mitigation analysis, variation ui 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
(Box 6.4 continues)