Emissions Scenarios - IPCC

Summary Discussions and Recommendations 307
different groups of scenarios tliat explore different structures of
future energy systems, from carbon-intensive development
paths to high rates of decarbonization. All groups otherwise
share the same assumptions about the main driving forces (see
Section 6.2.3 and, for further detail, Chapters 4 and 5). This
indicates that different su-uctures of the energy system can lead
to basically the same variation in future emissions as generated
by different combinations of the other main driving forces -
population, economic activities, and energy consumption
levels. The implication is that decarbonization of energy
systems - the shift from carbon-intensive to less carbonintensive
and carbon-free sources of energy - is of similar
importance in determining the future emissions paths as other
driving forces. Sustained decarbonization requires the
development and successful diffusion of new technologies.
Thus investments in new technologies during the coming
decades might have the same order of influence on future
emissions as population growth, economic development, and
levels of energy consumption taken together.
Figure 6-6b shows that CO^ emissions from deforestation peak
in many SRES scenarios after several decades and
subsequently gradually decline. This pattem is consistent with
many scenarios in the literature and can be associated with
slowing population growth and increasing agricultural
productivity. These allow a reversal of cuiTcnt deforestation
trends, leading to eventual CO^ sequestration. Emissions
decline fastest in the Bl family. Only in the A2 family do net
anthropogenic CO2 emissions from land use remain positive
thiough to 2100. As was the case for energy-related emissions,
CO2 emissions related to land-use in the Al family cover the
widest range. The range of land-use emissions across the IS92
scenarios is narrower in comparison.
63.1.2. Four Categories of Cumulative Emissions
This comparison of some of the SRES scenario characteristics
implies that similar future emissions can result from very
different socio-economic developments, and similar
developments of driving forces can result in different future
emissions. Uncertainties in the future development of key
emission driving forces create large uncertainties in future
emissions, even within the same socio-economic development
paths. Therefore, emissions from each scenario family overlap
substantially with emissions from other scenario families.
Figure 6-6 shows this for CO2 emissions. For example,
comparison of the AlB and B2 marker scenarios indicates that
they have similar emissions of about 13.5 and 13.7 GtC by
2100, respectively. The dynamics of the paths, however, are
different so that they have different cumulative CO^ emissions
and different emissions of other GHG gases and SOj.
To facilitate comparisons of emissions and their driving forces
across the scenarios, the writing team grouped them into four
categories of cumulative emissions between 1990 and 2100.
However, any categorization of scenarios based on emissions
of multiple gases is quite difficult. Figure 6-7 shows total COj
emissions from all sources (from Figures 6-6a and b). Most of
the scenarios are shown aggregated into seven groups, the four
Al groups and the other three families. The scenarios that
remain outside the seven groups adopted alternative
inteфretations of the four scenario storylines. The emission
trajectories ("bands") of the seven groups display different
dynamics, from monotonie increases to non-linear trajectories
in which there is a subsequent decline from a maximum. The
dynamics of the individual scenarios are also different across
gasses, sectors, or world regions. This particularly diminishes
the significance of focusing scenario categorization on any
given year, such as 2100. In addition, all gases that contribute
to radiative forcing should be considered, but methods of
combining gases such as the use of global warming potentials
(GWP) are appropriate only for neai-term GHG inventories.*
In light of these difficulties, the classification approach
presented here uses cumulative COj emissions between 1990
and 2100. CO2 is the dominant GHG and cumulative CO2
emissions are expected to be roughly proportional to CO2
radiative forcing over the time scale of a century. According to
the IPCC SAR, "any eventual stabilised concentration is
governed more by the accumulated anthropogenic CO,
emissions from now until the time of stabilisation than by the
way emissions change over the period" (Houghton et ai,
1996). Therefore, the writing team also grouped the scenarios
according to their cumulative emissions (see Figure 6.8).
This categorization can guide comparisons using either
scenarios with different driving forces yet similar emissions, or
scenarios with similar driving forces but different emissions.
This characteristic of SRES scenarios also has very important
implications for the assessment of climate-change impacts,
mitigation, and adaptation strategies. Two future worlds with
fundamentally different characteristic features, such as the AI
and B2 marker scenarios, also have different cumulative CO2
emissions, but very similar CO2 emissions in 2100. In contrast,
scenarios that are in the same category of cumulative emissions
can have fundamentally different driving forces and different
CO2 emissions in 2100, but very similar cumulative emissions.
Presumably, adverse impacts and effective adaptation measures
would vary among the scenarios from different families that
share similar cumulative emissions, but have different
demographic, socio-economic, and technological driving
forces. This is another reason for considering the entire range
of emissions in future assessments of climate change.
Figure 6-9 shows the histogram of cumulative COj emissions
from 1990 to 2100 for the SRES scenarios subdivided into the
four emissions categories. Relative positions of the four marker
scenarios, and the ranges of the four families and the six IS92
scenarios, are marked. The SRES scenarios have a bimodal
In particular, the IPCC WGI Second Assessment Report (SAR)
GWPs are calculated for constant concentrations (Houghton et al.,
1996). In long-term scenanos, concentrations may change
significantly, as do GWP values. It is unclear how to apply GWPs to
long-term scenarios in a meaningful manner. In addition, the GWP
approach is not applicable to gases such as SO^ and ozone precursors.