Emissions Scenarios - IPCC

Background and Overview 65
interventions and measures in new model runs tliat siiare tlie
same specifications for the other principal driving forces of
future emissions. However, the SRES emissions scenarios
include a host of other policies and measures that are not
directed at reducing sources and increasing sinks of GHGs, but
that nevertheless have an indirect effect on future emissions.
For example, policies directed at achieving greater
environmental protection may also lead to lower emissions of
GHGs. Moreover, afforestation and reforestation measures
increase COj sinks, and a shift to renewable energy sources
reduces the sources of emissions.
Within thi'ee of the broad objectives listed above, the new
SRES emissions scenarios are also intended to meet the
specific needs of three main IPCC user communities:
• Working Group I (WGI), which includes climate
modelers who need future emission trajectories for
GHGs and aerosol precursors as inputs for the GCMs
used to develop climate change scenarios.
• Working Group II (WGII), which analyzes climate
impacts and adaptation policies, first need the climatechange
scenarios produced by WGI's climate modelers.
Second, analysts need to know the socio-economic
changes associated with specific emissions scenarios,
as impacts of climate change on ecosystems and people
depend on many factors. Among these are whether the
people arc numerous or few, rich or poor, free to move
or relatively immobile, and included or excluded from
world markets in technologies, food, etc.
• WGIII, which analyzes potenüal mitigation policies for
climate change, also needs to know the socio-economic
settings against which policy options are to be
evaluated. Are markets open or protected? Are
technological options and economic resources plentiful
or scarce? Are people vulnerable or adaptable?
The interests of these three user groups create certain
requhements that the SRES scenarios attempt to fulfill. For
example, climate modelers and those who analyze climate
impacts need scenaiios on the order of 100 years because of the
long response time of the climate system. At the same time
adaptation-policy analysis tends to be focused more on the
medium-term, around 20 to 50 years. The SRES scenarios
attempt to include enough infonnation and specific details to be
useful to these groups. Spatially explicit emissions and socioeconomic
variables are required for slightly different reasons.
Some emissions, such as the SOj emissions that contribute to
sulfate aerosols, have impacts that vary depending on where they
are emitted. Climate modelers therefore need spatially explicit
emission estimates. Similarly, impacts depend on the geographic
pattems of changing temperatures, rainfall, humidity and cloud
cover, and how these compare to evolving socio-economic
patterns in specific scenarios. Impact modelers therefore need
spatially explicit estimates of, in particular, population growth,
migration, and the economic variables that reflect the expected
adaptability or vulnerability of different populations and regional
economies to different regional climate changes.
Taking the above audiences and purposes into account, the
following more precise specifications for the new SRES
scenarios were developed. The new scenarios should:
• cover the full range of radiatively important gases,
which include direct and indirect GHGs and SO^;
• have sufficient spatial resolution to allow regional
assessments of climate change in the global context;
• cover a wide spectrum of alternative futures to reflect
relevant uncertainties and knowledge gaps;
• use a variety of models to reflect methodological
pluralism and uncertainty;
• incorporate input from a wide range of scientific
disciplines and expertise from non-academic sources
through an open process;
• exclude additional initiatives and policies specifically
designed to reduce climate change;
• cover and describe to the extent possible a range of
policies that could affect climate change although they
are targeted at other issues, for example, reductions in
SOT emissions to limit acid rain;
• cover as much as possible of the range of major
underlying "driving forces" of emissions scenarios
identified in the open literature;
• be transparent with input assumptions, modeling
approaches and results open to external review;
• be reproducible - input data and methodology are
documented adequately enough to allow other
researchers to reproduce the scenarios; and
• be internally consistent - the various input assumptions
and data of the scenarios are internally consistent to the
extent possible.
1.4. Review of Past IPCC Emissions Scenarios
The 1994 IPCC evaluation of the usefulness of the IPCC 1992
(IS92) scenarios found that for the purposes of driving
atmospheric and climate models, the COj emissions trajectories
in these provided a reasonable reflection of variations found in
the open literature. Specifically, their global CO^ emissions
spanned most of the range of other scenarios identified in the
literature. Figure 1-2 shows the global, energy-related and
industrial CO2 emissions of the IS92 scenarios ranging from
very high emissions of 35.8 GtC to very low emissions of 4.6
GtC by 2100 (corresponding to a sixfold increase and a
decline by a third compared to 1990 levels, respectively). The
shaded area in Figure 1-2 indicates the coverage of the IS92
scenarios while the "spaghetti-like" curves indicate other
energy-related emissions scenarios found by the IPCC review
to be representative of the scenarios available in the open
literature at that time (Alcamo et al., 1995). In the open
literature, emissions trajectories for other gases were
extremely thin in many instances, but the IS92 cases were not
dissimilar to these.
Another important recommendation of the 1994 IPCC review
was that, given the degree of uncertainty about future climate