Emissions Scenarios - IPCC

74 Background and Ovei-view
1990 and 2100. 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 AIB and B2 marker scenarios, also have different
cumulative COj emissions and radiative forcing, but very
similar COj emissions in 2100. In contrast, scenarios that are
in the same category of cumulative emissions can have
fundamentally different driving forces and different CO,,
emissions in 2100, but very similar cumulative emissions and
radiative forcing. 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 future emissions in future assessments of climate
change. There is no single "best guess" or central scenario.
The SRES emissions scenaiios also have different emissions
for other GHGs and chemically active species such as carbon
monoxide, nitrogen oxides, and volatile organic hydrocarbons.
The emissions of other gases follow dynamic patterns much
like those shown in Figure 1-6 for emissions. Further
details about GHG emissions are given in Chapter 5. Emissions
of sulfur aerosol precursors portray even more dynamic
patterns in time and space than the CO, emissions shown in
Figure 1-6. Factors other than climate change, namely regional
and local air quality, and transformations in the structure of the
energy system and end use intervene to limit future emissions.
In view of the significant adverse impacts, SOj emissions in
the scenarios are increasingly controlled outside countries of
the OECD. As such the SRES scenarios reflect both recent
legislation in North America and in Europe and recent policy
initiatives in a number of developing countries aimed at
reducing SOj emissions (reviewed in more detail in Chapters 3
and 5). As a result, in the second half of the 2F' century both
the trends and regional patterns of SO^ emissions evolve
differently from those of CO, emissions in the SRES scenarios.
Emissions outside OECD90 rise initially, most notably in
ASIA, and compensate for declining OECD90 emissions. Over
die long term, however, SO, emissions decline throughout the
world, but the timing and magnitude vary across the scenaiios.
One important implication of this varying pattern of SOj
emissions is that the historically important, but uncertain
negative radiative forcing of sulfate aerosols may decline in the
very long run.
An important feature of the SRES scenarios is their
implications for radiative forcing. A vigorous increase of
global SOj emissions during the next few decades across most
of the scenarios followed by a decline thereafter will lead to a
coolmg effect that will differ from the effect that results from
the continuously increasing SOj emissions in the IS92
scenarios. On one hand, the reduction in global SOj emissions
reduces the role of sulfate aerosols in determining future
climate toward the end of the 21" century and therefore reduces
one aspect of uncertainty about future climate change (because
the precise forcing effect of sulfate aerosols is highly
uncertain). On the other hand, uncertainty increases because of
the diversity in spatial patterns of SO2 emissions in the
scenarios. Future assessments of possible climate change need
to account for these different spatial and temporal dynamics of
GHG and SOj emissions, and they need to cover the whole
range of radiative forcing associated with the scenarios.
1.8. Structure of the Report
The report consists of six chapters and 11 appendices. After
this introductory chapter. Chapters 2 and 3 present the scenario
literature review and analysis. Chapters 4 and 5 describe the
new SRES scenarios. Chapter 6 summarizes the main findings,
and the appendices present the méthodologie approach and
statistical background material.
Chapter 2 presents the assessment of anthropogenic GHG
emissions scenarios and their main driving forces based on an
extensive literature review. It describes the unique scenario
database developed for this study, which contains over 400
global and regional scenarios. The chapter presents the range
of emissions from the scenarios in the literature with associated
statistics such, as medians, percentiles and histograms. The
main scenario driving forces are analyzed in the same way,
from population and economic development to energy.
Chapter 3 reviews the main driving forces of past and possible
future anthropogenic GHG emissions. These include
demographic, economic and social development, changes in
resources and technology, agriculture and land-use change, and
policy issues other than those related to climate. The
relationships and possible interactions among the driving
forces are highly complex and heterogeneous. The focus of the
chapter is to provide an overview of the main driving forces
and their possible relationships that are particularly relevant for
the SRES scenarios.
Chapter 4 presents the naiTative scenario storylines and the
quantification of the main scenario driving forces with the six
SRES IA models. First, an overview of the four storylines is
given which describes their main characteristics, relationships
and implications. Then, the 40 scenario quantifications of the
four storylines with the six models are presented. For each
storyline one scenario is designated as a representative marker
scenario. Together the 40 scenarios span the range of scenario
driving forces and their relationships presented in the previous
two chapters.
Chapter 5 documents anthropogenic GHG and SO2 emissions
for the 40 SRES scenarios highlighting the four marker
scenarios. First, COj emissions are presented, followed by
other GHGs, and by the assessment of indhrect effects and
aerosols. Together the 40 scenarios span the emissions ranges
from the literature and the four marker scenarios jointly