Emissions Scenarios - IPCC

64 Background and Overview
relationships and driving forces of change, which are derived
from our understanding of both history and the entrent
situation. Often these scenarios are formulated with the help of
formal models. Such scenarios specify the future emissions of
GHGs in quantitative terms and, if fully documented, they are
also reproducible. Sometimes GHG emissions scenarios are
less quantitative and more descriptive, and in a few cases they
do not involve any formal analysis and are expressed in
qualitative terms. The SRES scenarios involve both qualitative
and quantitative components; they have a narrative part called
"storylines" and a number of corresponding quantitative
scenarios for each storyline. Figure 1-1 illustrates the
interrelated nature of these altemative scenario formulations.
Although no scenarios are value free, it is often useful to
distinguish between normative and descriptive scenarios.
Normative (or prescriptive) scenarios are explicitly valuesbased
and teleologic, exploring the routes to desired or
undesired endpoints (utopias or dystopias). Descriptive
scenarios are evolutionary and open-ended, exploring paths
into the future. The SRES scenarios are descriptive and should
not be construed as desirable or undesirable in their own right.
They are built as descriptions of possible, rather than preferred,
developments. They represent pertinent, plausible, altemative
futures. Their peitinence is derived from the need for policy
makers and climate-change modelers to have a basis for
assessing the impUcations of future possible paths for GHG
and SO2 emissions, and the possible response strategies. Their
plausibility is based on an extensive review of the emissions
scenarios available in the literature, and has been tested by
altemative modeling approaches, by peer review (including the
"open process" through the IPCC web site), and by the IPCC
review and approval processes. Good scenarios are challenging
and court controversy, since not everybody is comfortable with
every scenario, but used intelligently they allow policies and
strategies to be designed in a more robust way.
1.3. Purposes and Uses of SRES Emissions Scenarios
The assessment of climate change dictates a global perspective
and a very long time horizon that covers periods of at least a
century. As the prediction of future anthropogenic GHG
emissions is impossible, altemative GHG emissions scenarios
become a major tool for the analysis of potential long-range
developments of the socio-economic system and
corresponding emission sources.
However, to develop scenarios for a period of 100 years is a
relatively new field. Difficulties arise not only from large
scientific uncertainties and data inadequacies, but also because
people are not trained to think in such time-spans. We are
educated in narrow disciplines, and our ability to model
complex systems, at the global level, is sfill in its infancy. For
example, within the next century technological discontinuities
should be expected, and possibly major shifts in societal values
and in the balance of geopolitical power. The study of past
trends over such long periods is hampered because most
databases are incomplete if more than 50 years old. Given
these gaps in our data, methods and understanding, scenarios
are the best way to integrate our demographic, economic,
societal and technological knowledge with our understanding
of écologie systems to evaluate sources and sinks of GHG
emissions. Scenarios as an integration tool in the assessment of
climate change allow a role for intuition, analysis and
synthesis, and thus we tum to scenarios to take advantage of
these features and aid the assessment of future climate change,
impacts, vulnerabilities, adaptation and mitigation. Since the
scenarios focus on the century time scale, tools are used that
have been developed for this purpose. These tools are less
suitable for the analysis of near-term developments, so this
report does not intend to provide reliable projections for the
near term.
The IPCC's 1994 evaluation of its 1992 emissions scenarios
identified four principal uses (Alcamo et al., 1995):
• To provide input for evaluating climatic and
environmental consequences of altemative future GHG
emissions in the absence of specific measures to reduce
such emissions or enhance GHG sinks.
• To provide similar input for cases with specific
alternative policy interventions to reduce GHG
emissions and enhance sinks.
• To provide input for assessing mitigation and
adaptation possibilities, and their costs, in different
regions and economic sectors.
• To provide input to negotiations of possible agreements
to reduce GHG emissions.
Figure 1-1: Schematic illustration of altemative scenario
formulations, from narrative storylines to quantitative formal
models.
The SRES emissions scenarios are intended for the first, third
and fourth uses. They do not include any additional (explicit)
policies or measures directed at reducing GHG sources and
enhancing sinks. Thus, they cannot be directly applied to the
second purpose of emissions scenarios. Instead, they could be
used as reference cases for the introduction of specific policy