Emissions Scenarios - IPCC

46 Technical Summary
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
dilïerent 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, SOj, 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 ftill range of GHGs and SOj 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, В1, and B2, plus different groups within
the Al scenario family. The 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 (AlB). In the SPM, the AlC and AIG scenario
groups are combined into one fossil intensive group AlFI. All scenario groups are equally sound.
• The scenarios are also grouped into four categories of cumulative COj emissions, which indicate that scenarios with
different driving forces can lead to similar cumulative emissions and those with similar drivmg forces can branch out mto
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 spaimed 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 sttingent envkonmental policies and those that kiclude direct climate policies
was very difficult to make, a difficulty associated with many definitional and other ambiguities.
• AU 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 SOj 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.
• Alternative 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.