Emissions Scenarios - IPCC

70 Background and Overview
• make it easier to explain the scenarios to tlie various •
user communities by providing a narrative description
of alternative futures that goes beyond quantitative
scenario features;
• make the scenarios more useful, in particular to
analysts who contribute to IPCC WGII and WGIII; the
social, political and technological context described in
the scenario storylines is all-important in analyzing the
effects of policies either to adapt to climate change or
to reduce GHG emissions; and
• provide a guide for additional assumptions to be made
in detailed climate impact and mitigation analyses,
because at present no single model or scenario can
possibly respond to the wide variety of informational
and data needs of the different user communities of
long-term emissions scenarios.
The writing team consciously applied the principle of Occam's
Razor (i.e., the economy of thought, Eatwell et al., 1998). They
sought the minimum number of scenarios that could still serve
as an adequate basis to assess climate change and that would
still challenge policy makers to test possible response strategies
against a significant range of plausible futures. The team
decided on four storylines, as an even number helps to avoid
the impression that there is a "central" or "most likely" case.
The writing team wanted more than two storylines to help to
illustrate that the future depends on many different underlying
dynamics; the team did not want more than four, as it wanted
to avoid complicating the process by too many alternatives.
The scenarios would cover a wide range of - but not all
possible - futures. In particular, there would be no "disaster"
scenarios. None of the scenarios include new explicit climate
policies. The team decided to carry out sensitivity tests within
some of the storylines by considering alternative scenarios with
different fossil-fuel reserves, rates of economic growth, or rates
of technical change.
The storylines describe developments in many different social,
economic, technological, environmental and policy dimensions.
The titles of the storylines have been kept simple: AI, A2, Bl
and B2. There is no particular order among the storylines; they
are listed in the alphabetic and numeric order:
• The AI 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 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 Al scenario family develops into
four groups that describe alternative directions of
technological change in the energy system.**
^ Please note that in the Summary for Policymakers, two of these
groups were merged into one. See also the endnote in Box 1 -2.
•
The A2 storyline and scenario family describes a very
heterogeneous worid. The underlying theme is selfreliance
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 scenaiio 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 sustain ability,
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 В1 and
AI storylines. While the scenario is also oriented
toward environmental protection and social equity, it
focuses on local and regional levels.
Figure 1-4 schematically illustrates the SRES scenarios. It
shows that the scenarios build on the main driving forces of
GHG emissions. Each scenario family is based on a common
specification of the main driving forces. The four scenario
families are illustrated, very simplistically, as branches of a
two-dimensional tree. The two dimensions indicate global
and regional scenario orientation and development and
environmental orientation, respectively. In reality, the four
scenarios share a space of a much higher dimensionality
given the numerous driving forces and other assumptions
needed to define any given scenario in a particular modeling
approach.
After determining the basic features and driving forces for each
of the four storylines, the teams began to model and quantify
them. This resulted in 40 scenarios, each constituting an
alternative inteipretation and quantification of a storyline. All
the interpretations and quantifications associated with a single
storyline are called a scenario "family" (see also Box 1-2 on
terminology and Chapter 4 for further details).
In all, six models were used to generate the 40 scenarios:
• Asian Pacific Integrated Model (AIM) from the
National Institute of Environmental Studies in Japan
(Morita et al., 1994);
• Atmospheric Stabilization Framework Model (ASF)
from ICF Consulting in the USA (Lashof and Tirpak,
1990; Pepper et al., 1992, 1998; Sankovski et ai,
2000);