1.7.3. The Range of SRES Emissions and their Implications
Figure 1-6: Range of global
energy-related and industrial CO2 emissions for the 40 SRES scenarios.
The dashed time-paths depict individual SRES scenarios and the shaded
area the range of scenarios from the SRES database. The median (50th), 5th, and 95th percentiles of the frequency distribution are shown.
The statistics associated with scenarios from the literature do not
imply probability of occurrence (e.g., the frequency distribution of
the scenarios may be influenced by the use of IS92a as a reference for
many subsequent studies). The 40 SRES scenarios are classified into
groups that constitute four scenario families. Jointly the scenarios
span most of the range of the scenarios in the literature. The emissions
profiles are dynamic, ranging from continuous increases to those that
curve through a maximum and then decline. The colored vertical bars
indicate the range of the four SRES scenario families in 2100. The black
vertical bar shows the range of the IS92 scenarios. See also the note
in Box 1.2.
The 40 SRES scenarios cover the full range of GHG and SO2 emissions consistent
with the storylines and underlying ranges of driving forces from studies in
the literature as documented in the SRES database. The four marker scenarios
are characteristic of the four scenario families and jointly capture most of
the ranges of emissions and driving forces spanned by the full set of scenarios.
Figure 1-6 illustrates the range of global energy-related
and industrial CO2 emissions for the 40 SRES scenarios against the background
of all the emissions scenarios in the SRES scenario database shown in Figure
1-3. Figure 1-6 also shows a range of emissions of
the four scenario families.
Figure 1-6 shows that the SRES scenarios cover most of
the range of global energy-related CO2 emissions from the literature, from the
95th percentile at the high end of the distribution down to low emissions just
above the 5th percentile of the distribution. Thus, they only exclude the most
extreme emissions scenarios found in the literature - those situated in the
tails of the distribution. What is perhaps more important is that each of the
four scenario families covers a sizable part of this distribution, which implies
that a similar quantification of driving forces can lead to a wide range of
future emissions. More specifically, a given combination of the main driving
forces is not sufficient to uniquely determine a future emissions path. There
are too many uncertainties. The fact that each of the scenario families covers
a substantial part of the literature range also leads to an overlap in the emissions
ranges of the four families. This implies that a given level of future emissions
can arise from very different combinations of driving forces. This result is
of fundamental importance for the assessment of climate-change impacts and possible
mitigation and adaptation strategies. Thus, it warrants some further discussion.
The emissions paths of the A1 and B2 scenario families perhaps best illustrate
The A1 scenario family has explored variations in energy systems most explicitly
and hence covers the largest part of the scenario distribution shown in Figure
1-6, from the 95th to just above the 10th percentile. The A1 marker (A1B) scenario
represents a structure of the future energy mix, balanced in the sense that
it does not rely too heavily on one particular energy source. The A1 scenario
family includes different groups of scenarios that explore different specific
structures of future energy systems, from carbon-intensive development paths
to high rates of decarbonization as captured by the two illustrative scenarios
that span most of the emissions range for the A1 family. All groups otherwise
share the same assumptions about the main driving forces. This indicates that
different structures of the energy system can lead to basically the same variation
in future emissions as can be generated by different combinations of the other
main driving forces - population, economic activities and energy consumption
levels. The implication is that decarbonization of energy systems - the shift
from carbon-intensive to less carbon-intensive and carbon-free sources of energy
- is of similar importance in determining the future emissions paths as other
driving forces. Sustained decarbonization requires the development and successful
diffusion of new technologies. Thus investments in new technologies during the
coming decades might have the same order of influence on future emissions as
population growth, economic development and levels of energy consumption taken
For example, the comparison of the A1B and B2 marker scenarios indicates that
they have similar emissions of about 13.5 and 13.7 GtC by 2100, respectively.
The dynamics of the paths are different so that they have different cumulative
CO2 emissions. To facilitate such comparisons, the scenarios were grouped into
four categories of cumulative emissions between 1990 and 2100. This categorization
can guide comparisons using either scenarios with different driving forces yet
similar emissions, or scenarios with similar driving forces but different emissions.
This characteristic of SRES scenarios also has very important implications for
the assessment of climate-change impacts, mitigation and adaptation strategies.
Two future worlds with fundamentally different characteristic features, such
as A1B and B2 marker scenarios, also have different cumulative CO2 emissions
and radiative forcing, but very similar CO2 emissions in 2100. In contrast,
scenarios that are in the same category of cumulative emissions can have fundamentally
different driving forces and different CO2 emissions in 2100, but very similar
cumulative emissions and radiative forcing. Presumably, adverse impacts and
effective adaptation measures would vary among the scenarios from different
families that share similar cumulative emissions but have different demographic,
socio-economic and technological driving forces. This is another reason for
considering the entire range of future emissions in future assessments of climate
change. There is no single "best guess" or central scenario.
The SRES emissions scenarios also have different emissions for other GHGs and
chemically active species such as carbon monoxide, nitrogen oxides, and volatile
organic hydrocarbons. The emissions of other gases follow dynamic patterns much
like those shown in Figure 1-6 for CO2 emissions.
Further details about GHG emissions are given in Chapter 5.
Emissions of sulfur aerosol precursors portray even more dynamic patterns in
time and space than the CO2 emissions shown in Figure
1-6. Factors other than climate change, namely regional and local air quality,
and transformations in the structure of the energy system and end use intervene
to limit future emissions. In view of the significant adverse impacts, SO2 emissions
in the scenarios are increasingly controlled outside countries of the OECD.
As such the SRES scenarios reflect both recent legislation in North America
and in Europe and recent policy initiatives in a number of developing countries
aimed at reducing SO2 emissions (reviewed in more detail in Chapters 3
and 5). As a result, in the second half of the 21 st century
both the trends and regional patterns of SO2 emissions evolve differently from
those of CO2 emissions in the SRES scenarios. Emissions outside OECD90 rise
initially, most notably in ASIA, and compensate for declining OECD90 emissions.
Over the long term, however, SO2 emissions decline throughout the world, but
the timing and magnitude vary across the scenarios. One important implication
of this varying pattern of SO2 emissions is that the historically important,
but uncertain negative radiative forcing of sulfate aerosols may decline in
the very long run.
An important feature of the SRES scenarios is their implications for radiative
forcing. A vigorous increase of global SO2 emissions during the next few decades
across most of the scenarios followed by a decline thereafter will lead to a
cooling effect that will differ from the effect that results from the continuously
increasing SO2 emissions in the IS92 scenarios. On one hand, the reduction in
global SO2 emissions reduces the role of sulfate aerosols in determining future
climate toward the end of the 21st century and therefore reduces one aspect
of uncertainty about future climate change (because the precise forcing effect
of sulfate aerosols is highly uncertain). On the other hand, uncertainty increases
because of the diversity in spatial patterns of SO2 emissions in the scenarios.
Future assessments of possible climate change need to account for these different
spatial and temporal dynamics of GHG and SO2 emissions, and they need to cover
the whole range of radiative forcing associated with the scenarios.
Figure 1-5: Schematic illustration
of SRES scenarios. The set of scenarios consists of four scenario families:
A1, A2, B1 and B2. Scenario family A1 is further subdivided into four
scenario groups: A1C, A1G, A1B and A1T, (see also note below), resulting
in seven scenario groups together with the other three scenario families.
Each family and group consists of a number of scenarios. Some of them
have "harmonized" driving forces and share the same prespecified population
and gross world product (a few that also share common final energy trajectories
are called "fully harmonized"). These are marked as "HS" for harmonized
scenarios. One of the harmonized scenarios, originally posted on the
open-process web site, is called a "marker scenario." All other scenarios
of the same family based on the quantification of the storyline chosen
by the modeling team are marked as "OS." Six modeling groups developed
the set of 40 emissions scenarios. The GHG and SO2 emissions of the
scenarios were standardized to share the same data for 1990 and 2000
on request of the user communities. The time-dependent standardized
emissions were also translated into geographic distributions.
Model: a formal representation of a system that allows quantification
of relevant system variables.
Storyline: a narrative description of a scenario (or a family
of scenarios) highlighting the main scenario characteristics, relationships
between key driving forces, and the dynamics of the scenarios.
Scenario: a description of a potential future, based on a clear
logic and a quantified storyline.
Family: scenarios that have a similar demographic, societal,
economic and technical-change storyline. Four scenario families comprise
the SRES: A1, A2, B1 and B2.
Group: scenarios within a family that reflect a variation of
the storyline. The A1 scenario family includes four groups designated
by A1T, A1C, A1G and A1B (see also note below) that explore alternative
structures of future energy systems. In the Summary for Policymakers,
the A1C and A1G groups have been combined into one "fossil intensive"
A1FI scenario group, thus reducing the number of groups constituting
the A1 scenario family to three. The other three scenario families consist
of one group each.
Category: scenarios are grouped into four categories of cumulative
CO2 emissions between 1990 and 2100: low, medium-low, medium-high, and
high emissions. Each category contains scenarios with a range of different
driving forces yet similar cumulative emissions.
Marker: a scenario that was originally posted on the SRES web
site to represent a given scenario family. A marker is not necessarily
the median or mean scenario.
Illustrative: a scenario that is illustrative for each of the
six scenario groups reflected in the Summary for Policymakers of this
report (after combining A1G and A1C into a single A1FI group). They
include four revised "scenario markers" for the scenario groups A1B,
A2, B1 and B2, and two additional illustrative scenarios for the A1FI
and AIT groups. See also "(Scenario) Groups" and "(Scenario) Markers."
Harmonized: harmonized scenarios within a family share common
assumptions for global population and GDP while fully harmonized scenarios
are within 5% of the population projections specified for the respective
marker scenario, within 10% of the GDP and within 10% of the marker
scenario's final energy consumption.
Standardized: emissions for 1990 and 2000 are indexed to have
the same values.
Other scenarios: scenarios that are not harmonized.
Note: During the approval process of the Summary
for Policymakers at the 5th Session of WGIII of the IPCC from 8-11 March
2000 in Katmandu, Nepal, it was decided to combine the A1C and A1G groups
into one "fossil intensive" group A1FI in contrast to the non-fossil
group A1T, and to select two illustrative scenarios from these two A1
groups to facilitate use by modelers and policy makers. This leads to
six scenario groups that constitute the four scenario families, three
of which are in the A1 family. These six groups all have "illustrative
scenarios," four of which are marker scenarios. All scenarios are equally
sound. See also Figure SPM-1.