Emissions Scenarios - IPCC

An Overview of
Scenarios
drivmg forces (population, GDP) and order of magnitude of
GHG emissions. Subsequently, additional scenarios within
each scenario family were developed to explore the sensitivity
of adoptmg alternative quantitative scenario input assumptions
on future GHG emissions. As a result the markers are not
necessarily the median or mean of a scenario family (nor
would it be possible to construct such a median or mean
scenario by taking all salient scenario characteristics and
regional results into account). The markers are simply those
scenarios considered by the SRES writing team as illustrative
of a particular storyline. They are not singled out as more
likely than alternative quantitative interpretations of a
particular scenario family and its underlying storyline.
Perhaps they may be best described as "first among equals."
However, as a result of time and resource limitations the
marker scenarios have received the closest scrutiny from the
entire writing team and through the SRES open process
compared to other scenario quantifications. The marker
scenarios are also the SRES scenarios most intensively tested
m terms of leproducibility. As a rule, at least four different
models were used in attempts to replicate the model
quantification of a particular marker scenario. Available time
and resources did not allow a similar exercise to be conducted
for all SRES scenarios, albeit a more limited effort was
devoted to reproduce the Al scenario groups (next to the Al
marker) with different models.
To enable a comparison of the resultant GHG emissions, the
writing team decided to define a subset of harmonized
scenarios within each family that share common main scenario
driving-force assumptions, such as population or GDP growth.
Two harmonization criteria were developed (see also Section
4.4.1). This procedure and the harmonization criteria were
adopted in a joint agreement among the six SRES modeling
teams.
"Fully harmonized" scenarios are those that share important
driving force variables, including population, GDP, and final
energy use for each of the four SRES regions and the world
(according to the quantitative criteria listed in Table 4-1). Fully
harmonized scenaiios by definition include the respective
marker scenario. From 40 scenarios 11 are classified as
scenarios with "full harmonization." For each scenario family
at least two scenarios are harmonized using the most restiictive
criteria. This also applies to the scenario groups within the Al
scenario family, which correspondingly has the highest number
of fully harmonized scenarios. This subset of "fully
harmonized" scenarios serves to provide a better
correspondence between the development of the tliree main
driving forces and the resultant GHG emissions. The "fully
harmonized" scenarios thus demonstrate the degree by which a
particular marker scenario is reproducible by alternative
modeling approaches. Therefore, "fully harmonized" scenaiios
are not independent from each other within a particular
scenario family (or scenario group in case of Al).
"Globally hannonized" scenarios are those that share global
population and GDP profiles within the agreed upon bounds of
5% and 10%, respectively, over the period 1990-2100^ (see
Table 4-1). Altogether 26'^ scenarios are categorized into this
category and can be considered to capture the main global
development characteristics over time for each respective
scenario family and storyline. Again these 26 scenarios are not
independent from each other, constituting seven distinct
scenario groups (see also footnote 1).
Thus, there are three different types of scenarios within each
family;
• One marker and a set of "fully harmonized" scenarios
that attempt to replicate the marker scenario
quantification.
• A set of "globally harmonized" scenaiios.
• A set of non-harmonized scenarios.
In addition, two illustrative scenarios have been selected in the
Summary for Policymakers (SPM) from the additional Al
scenario groups (see also footnote 1).
For the sake of simplicity, the teirn "harmonized" is used
herein to describe global harmonization of population and
GDP growth. "Fully harmonized" scenarios, for which the
main objective is to assess the reproducibility of particular
marker scenario quantifications and any remaining uncertainty
in GHG emissions from internal model parametrizations, are
referred to in the text where appropriate. Figure 4-3 shows the
SRES scenarios as a set consisting of subsets that correspond
to the four families. The Al family is in addition divided into
different groups of scenarios. The detailed descriptions of
inputs and outputs (other than GHG emissions) of the SRES
markei scenarios, other harmonized scenarios, and other
scenarios are presented in Section 4.4 (see Appendix VII for
further numeric detail). The emissions of GHGs and other
radiatively important species of gases are described ш Chapter
5 (more detail is again presented in Appendix VII).
Table 4-2 summarizes the main characteristics of the four
SRES scenario families and their scenario groups, and gives an
overview about the numbet of scenarios that were developed
for each scenario group (see also Table 4-1 and Section 4.4.1).
4.3. Scenario Storylines
Although each of the four SRES storylines can unfold only if
certain values are emphasized more than others, no explicit
Deviauons withm each 10-year time period are not considered.
"* AdditionaUy, three scenarios (A2-AIM. A2-MiniCAM, and B2-
MuuCAM) deviate only slighdy from the global
hannomzaUon
criteria for between two to three time steps. Hence these scenarios can
be considered as 'almost' harmonized and comparable with the olhei
harmonized scenarios.