Emissions Scenarios - IPCC

68 Background and Overview
the open process. The choice of the marlcers was based on
extensive discussion of:
• range of emissions across all of marker scenarios;
• which of the initial quantifications (by the modelers)
reflected the storyline;
• preference of some of the modeling teams and features
of specific models;
• use of different models for the four markers.
As a result the markers are not necessarily the median or mean
of the scenario family, but are those scenarios considered by
the SRES writing team as illustrative of a particular storyline.
These scenarios have received the closest scrutiny of the entire
writing team and via the SRES open process compared to other
scenario quantifications. The marker scenarios are also those
SRES scenarios that have been most intensively tested in terms
of reproducibility. As a rule, different modeling teams have
attempted to replicate the model quantification of a particular
marker scenario. Available time and resources have not
allowed a similar exercise to be conducted for all SRES
scenarios, although some effort was devoted to reproduce the
four scenario groups^ that constitute different interpretations of
one of the four storylines (see Figure 1-4) with different
models. Additional versions of the preliminary marker
scenaiios by different modeling teams and other scenarios that
give alternative quantitative interpretations of the four
storylines constitute the final set of 40 SRES scenarios. This
also means that the 40 scenarios are not independent of each
other as they are all based on four storylines. However,
differences in modeling approaches have meant that not all of
the scenarios provide estimates for all the direct and indirect
GHG emissions for all the sources and sectors. The four SRES
marker scenarios cover all the relevant gas species and
emission categories comprehensively
The four marker scenarios were posted on the IPCC web site
(sres.ciesin.org) in June 1998, and the open scenario review
process through the IPCC web site lasted until January 1999.
The submissions invited through the open process and web site
fell into three categories (see Appendix VI):
• additional scenarios published in the reviewed
literature that had not been included in the scenario
database (see Appendix V);
• new scenarios based on the SRES marker scenarios;
and
• general suggestions to improve the work of the SRES
writing team as posted on the web site (preferably
based on referenced literature).
The submissions were used to revise the marker scenarios and
to develop additional alternatives within each of the four
scenario families. The result is a more complete, refined set of
^ Please note that in the Summary for Policymakers, two of these
groups were merged into one. See also the endnote in Box 1-2.
40 new scenarios that reflects the broad spectrum of modeling
approaches and regional perspectives. The preliminary marker
scenarios posted on the web site were provided also to climate
modelers, with the approval of the IPCC Bureau.
1.7. SRES Emissions Scenarios
1.7.1. Literature Review and A nalysis
The first step in formulation of the SRES emissions scenarios
was to review both the published scenario literature and the
development of the scenario database accessible through the
web site (www-cger.nies.go.jp/cger-e/db/ipcc.html). Chapters
2 and 3 give a more detailed description of the literature review
and analysis. Figure 1-3 shows the global energy-related CO2
emission paths from the database as "spaghetti" curves for the
period to 2100 against the background of the historical
emissions from 1900 to 1990. These curves are plotted against
an index on the vertical axis rather than as absolute values
because of the large differences and discrepancies for the
values assumed for the base yeai- 1990. These sometimes arise
from genuine differences among the scenarios (e.g., different
data sources, definitions) and sometimes from different base
years assumed in the analysis or in altemative calibrations.
The differences among the scenarios in the specification of the
base year illustrate the large genuine scientific and data
uncertainty that surrounds emissions and their main driving
forces captured in the scenarios. The literature includes
scenarios with additional climate polices, which are sometimes
referred to as mitigation or intervention scenarios. There are
many ambiguities associated with the classification of
emissions scenarios into those that include additional climate
initiatives and those that do not. Many cannot be classified in
this way on basis of the information available from the
database. Figure 1-3 indicates the ranges of emissions in 2100
from scenarios that apparently include additional climate
initiatives (designated as intervention emissions range), those
that do not (non-intervention) and those that cannot be
assigned to either of these two categories (non-classified). This
classification is based on the subjective evaluation of the
scenarios in the database by the members of the writing team
and is explained in Chapter 2. The range of the whole sample
of scenarios has significant overlap with the range of those that
cannot be classified and they share virtually the same median
(15.7 and 15.2 GtC in 2100, respectively) but the non-classified
scenarios do not cover the high part of the range. Also, the
range of the scenarios that apparently do not include climate
polices (non-intervention) has considerable overlap with the
other two ranges (lower bound is higher) but with a
significantly higher median (of 21.3 GtC in 2100).
' The 1990 emissions from energy production and use are estimated by
Marland et al. (1994) at 5.9 GtC excluding cement production. The 1990
base year values in the scenarios reviewed range from 4.8
(CETA/EMF14, Scenario MAGICC CO2) to 6.4 GtC (ICAM2/EMF14);
see Dowlatabadi and Kandlikar 1995; Peck andTeisberg, 1995.