Emissions Scenarios - IPCC

28 Technical Summary
Figure TS-2: Schematic illustration of SRES scenarios. The
four scenario "families" are shown, very simplistically, as
branches of a two-dimensional tree. In reality, the four
scenario families share a space of a much higher
dimensionality given the numerous assumptions needed to
define any given scenario in a particular modeling approach.
The schematic diagram illustrates that the scenarios build on
the main driving forces of GHG emissions. Each scenario
family is based on a common specification of some of the
main driving forces. The Al storyline branches out into four
groups of scenarios to illustrate that alternative development
paths are possible within one scenario family. Two of these
groups were merged in the SPM.
Box TS-1: The Main Characteristics of the Four SRES Storylines and Scenario Families
By 2100 the world will have changed in ways that are hard to imagine - as hard 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 population growth, 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.
• The Al storyline and scenario family describes a future world of very rapid economic growth, low population growth,
and the rapid introduction of new and more efficient technologies. Major underiying 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 Al scenario family develops into four groups that describe alternative directions of
technological change in the energy system. Two of the fossil-intensive groups were merged in the SPM.
• 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 high population
growth. Economic development is primarily regionally oriented and per capita economic growth and technological
change are more fragmented and slower than in other storylines.
• The Bl storyline and scenario family describes a convergent world with the same low population growth as in the Al
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 moderate population growth, intermediate levels of economic
development, and less rapid and more diverse technological change than in the Bl and Al storylines. While the scenario
is also oriented toward environmental protection and social equity, it focuses on local and regional levels.
After determining the basic features and driving forces for each of the four storylines, the team began modeling and quantifying
the storylines. This resulted in 40 scenarios, each of which constitutes an alternative inteфretation and quantification of a
storyline. All the inteфretations and quantifications associated with a single storyline are called a scenario family (see Chapter
1 for terminology and Chapter 4 for further details).