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Continued from previous page
In 1996, the IPCC began the development of a new set of emissions scenarios,
effectively to update and replace the well-known IS92 scenarios. The approved
new set of scenarios is described in the IPCC Special Report on Emission Scenarios
(SRES) (Nakic´enovic´ et al., 2000; see more complete discussion
of SRES scenarios and forcing in Chapters 3, 4,
5 and 6). Four different narrative
storylines were developed to describe consistently the relationships between
emission driving forces and their evolution and to add context for the scenario
quantification (see Box 9.1). The resulting set of forty
scenarios (thirty-five of which contain data on the full range of gases required
for climate modelling) cover a wide range of the main demographic, economic
and technological driving forces of future greenhouse gas and sulphur emissions.
Each scenario represents a specific quantification of one of the four storylines.
All the scenarios based on the same storyline constitute a scenario “family”.
(See Box 9.1, which briefly describes the main characteristics
of the four SRES storylines and scenario families.) The SRES scenarios do not
include additional climate initiatives, which means that no scenarios are included
that explicitly assume implementation of the UNFCCC or the emissions targets
of the Kyoto Protocol. However, greenhouse gas emissions are directly affected
by non-climate change policies designed for a wide range of other purposes.
Furthermore, government policies can, to varying degrees, influence the greenhouse
gas emission drivers and this influence is broadly reflected in the storylines
and resulting scenarios.
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Box 9.1: The Emissions Scenarios of the Special Report
on Emissions Scenarios (SRES)
A1. The A1 storyline and scenario family describe a future world of very
rapid economic growth, global population that peaks in mid-century and
declines thereafter, and the rapid introduction of new and more efficient
technologies. Major underlying themes are convergence among regions, capacity
building and increased cultural and social interactions, with a substantial
reduction in regional differences in per capita income. The A1 scenario
family develops into three groups that describe alternative directions
of technological change in the energy system. The three A1 groups are
distinguished by their technological emphasis: fossil intensive (A1FI),
non-fossil energy sources (A1T), or a balance across all sources (A1B)
(where balanced is defined as not relying too heavily on one particular
energy source, on the assumption that similar improvement rates apply
to all energy supply and end use technologies).
A2. The A2 storyline and scenario family describe a very heterogeneous
world. The underlying theme is self-reliance and preservation of local
identities. Fertility patterns across regions converge very slowly, which
results in continuously increasing population. Economic development is
primarily regionally oriented and per capita economic growth and technological
change are more fragmented and slower than in other storylines.
B1. The B1 storyline and scenario family describe a convergent world
with the same global population, that peaks in mid-century and declines
thereafter, as in the A1 storyline, but with rapid change in economic
structures toward a service and information economy, with reductions in
material intensity and the introduction of clean and resource-efficient
technologies. The emphasis is on global solutions to economic, social
and environmental sustainability, including improved equity, but without
additional climate initiatives.
B2. The B2 storyline and scenario family describe a world in which the
emphasis is on local solutions to economic, social and environmental sustainability.
It is a world with continuously increasing global population, at a rate
lower than A2, intermediate levels of economic development, and less rapid
and more diverse technological change than in the B1 and A1 storylines.
While the scenario is also oriented towards environmental protection and
social equity, it focuses on local and regional levels.
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Because SRES was not approved until 15 March 2000, it was too late for the
modelling community to incorporate the scenarios into their models and have
the results available in time for this Third Assessment Report. Therefore, in
accordance with a decision of the IPCC Bureau in 1998 to release draft scenarios
to climate modellers (for their input to the Third Assessment Report) one marker
scenario was chosen from each of four of the scenario groups based on the storylines
(A1B, A2, B1 and B2) (Box 9.1). The choice of the markers
was based on which initial quantification best reflected the storyline, and
features of specific models. Marker scenarios are no more or less likely than
any other scenarios but these scenarios have received the closest scrutiny.
Scenarios were also selected later to illustrate the other two scenario groups
(A1FI and A1T), hence there is an illustrative scenario for each of the six
scenario groups. These latter two illustrative scenarios were not selected in
time for AOGCM models to utilise them in this report. In fact, time and computer
resource limitations dictated that most modelling groups could run only A2 and
B2, and results from those integrations are evaluated in this chapter. However,
results for all six illustrative scenarios are shown here using a simple climate
model discussed below. The IS92a scenario is also used in a number of the results
presented in this chapter in order to provide direct comparison with the results
in the SAR.
The final four marker scenarios contained in SRES differ in minor ways from
the draft scenarios used for the AOGCM experiments described in this report.
In order to ascertain the likely effect of differences in the draft and final
SRES scenarios each of the four draft and final marker scenarios were studied
using a simple climate model tuned to the AOGCMs used in this report. For three
of the four marker scenarios (A1B, A2 and B2) temperature change from the draft
and final scenarios are very similar. The primary difference is a change to
the standardised values for 1990 to 2000, which is common to all these scenarios.
This results in a higher forcing early in the period. There are further small
differences in net forcing, but these decrease until, by 2100, differences in
temperature change in the two versions of these scenarios are in the range 1
to 2%. For the B1 scenario, however, temperature change is significantly lower
in the final version, leading to a difference in the temperature change in 2100
of almost 20%, as a result of generally lower emissions across the full range
of greenhouse gases. For descriptions of the simulations, see Section
9.3.1.
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