Box SPM-1: The Main Characteristics of the Four SRES Storylines
and Scenario Families.
Figure SPM-1: Schematic illustration
of SRES scenarios. Four qualitative storylines yield four sets of scenarios
called "families": A1, A2, B1, and B2. Altogether 40 SRES scenarios
have been developed by six modeling teams. All are equally valid with
no assigned probabilities of occurrence. The set of scenarios consists
of six scenario groups drawn from the four families: one group each
in A2, B1, B2, and three groups within the A1 family, characterizing
alternative developments of energy technologies: A1FI (fossil fuel intensive),
A1B (balanced), and A1T (predominantly non-fossil fuel). Within each
family and group of scenarios, some share "harmonized" assumptions on
global population, gross world product, and final energy. These are
marked as "HS" for harmonized scenarios. "OS" denotes scenarios that
explore uncertainties in driving forces beyond those of the harmonized
scenarios. The number of scenarios developed within each category is
shown. For each of the six scenario groups an illustrative scenario
(which is always harmonized) is provided. Four illustrative marker scenarios,
one for each scenario family, were used in draft form in the 1998 SRES
open process and are included in revised form in this report. Two additional
illustrative scenarios for the groups A1FI and A1T are also provided
and complete a set of six that illustrate all scenario groups. All are
equally sound.
By 2100 the world will have changed in ways that are difficult
to imagine - as difficult as it would have been at the end of the 19th
century to imagine the changes of the 100 years since. Each storyline
assumes a distinctly different direction for future developments, such
that the four storylines differ in increasingly irreversible ways. Together
they describe divergent futures that encompass a significant portion
of the underlying uncertainties in the main driving forces. They cover
a wide range of key "future" characteristics such as demographic change,
economic development, and technological change. For this reason, their
plausibility or feasibility should not be considered solely on the basis
of an extrapolation of current economic, technological, and social trends.
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The A1 storyline and scenario family describes 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) 3
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- The A2 storyline and scenario family describes 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 global population. Economic
development is primarily regionally oriented and per capita economic
growth and technological change are more fragmented and slower than
in other storylines.
- The B1 storyline and scenario family describes a convergent world
with the same global population that peaks in mid-century and declines
thereafter, as in the A1 storyline, but with rapid changes 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.
- The B2 storyline and scenario family describes 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 toward environmental
protection and social equity, it focuses on local and regional levels.
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Within each scenario family two main types of scenarios were developed -
those with harmonized assumptions about global population, economic growth,
and final energy use and those with alternative quantification of the storyline.
Together, 26 scenarios were harmonized by adopting common assumptions on global
population and gross domestic product (GDP) development. Thus, the harmonized
scenarios in each family are not independent of each other. The remaining 14
scenarios adopted alternative interpretations of the four scenario storylines
to explore additional scenario uncertainties beyond differences in methodologic
approaches. They are also related to each other within each family, even though
they do not share common assumptions about some of the driving forces.
There are six scenario groups that should be considered equally sound that
span a wide range of uncertainty, as required by the Terms of Reference.
These encompass four combinations of demographic change, social and economic
development, and broad technological developments, corresponding to the four
families (A1, A2, B1, B2), each with an illustrative "marker" scenario. Two
of the scenario groups of the A1 family (A1FI, A1T) explicitly explore alternative
energy technology developments, holding the other driving forces constant, each
with an illustrative scenario. Rapid growth leads to high capital turnover rates,
which means that early small differences among scenarios can lead to a large
divergence by 2100. Therefore the A1 family, which has the highest rates of
technological change and economic development, was selected to show this effect.
In accordance with a decision of the IPCC Bureau in 1998 to release draft
scenarios to climate modelers for their input in the Third Assessment Report,
and subsequently to solicit comments during the open process, one marker scenario
was chosen from each of four of the scenario groups based on the storylines.
The choice of the markers was based on which of the initial quantifications
best reflected the storyline, and features of specific models. Marker scenarios
are no more or less likely than any other scenarios, but are 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. Scenarios have also been selected to illustrate the other two
scenario groups. Hence, this report has an illustrative scenario for each of
the six scenario groups.
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