2.4.3. Sulfur Dioxide Emissions
Figure 2-3: Global SO2 emissions
- historical development and 81 scenarios from the database, shown as
an index (1990 = 1). For comparison, the IS92 scenarios are shown, as
is the range and median of SO2 emissions control (Sc_median, Sc_min,
Sc_max) scenarios (see also Chapter 3). The vertical
bars on the right side of the figure indicate the ranges for the SO2
emissions control (intervention) scenarios and for SO2 non-control
scenarios for 2100, respectively. Data sources: Dignon and Hameed, 1989;
Grübler, 1998; Morita and Lee, 1998.
An overview of global long-term scenarios of SO2 emissions is shown in Figure
2-3. Altogether 81 scenarios in the scenario database report SO2 emissions.
Most scenarios were published after 1995, which indicates the importance of
the influential and innovative SO2 emissions included in the previous IPCC scenario
series IS92 (Pepper et al., 1992). Apparently, they stimulated research
on long-term trends and impacts of SO2 emissions.
The 1990 base-year emissions estimates in the database range from 55 to 91
megatons of sulfur (MtS), a seemingly large difference that can be explained
partially by the different extent of coverage of SO2 emissions in different
models and scenario studies, in addition to uncertainties in emissions factors.
Typically, lower range emissions derive from models that report only (the dominant)
energy sector emissions, higher ranges also include other anthropogenic sources
such as SO2 emissions from metallurgic processes. Differences in 1990 base-year
values across scenario studies and a review of available SO2 emissions inventories
are discussed in more detail in Chapter 3. Indexed to
a common 1990 basis, future SO2 emissions trends reveal a number of remarkable
characteristics. First, contrary to other trends discussed in this chapter,
increases are generally modest; numerous scenarios even depict a long-term decline
in emissions. Thus, SO2 emissions are invariably projected to be decoupled progressively
from their underlying driving forces of increases in population and economic
activity, and hence energy demand. The median across all scenarios indicates
a gradual increase of some 22% over 1990 levels by 2050, and a return to 1990
levels by 2100. Only two scenarios exceed the range of increases in long-term
SO2 emissions spanned by the IS92 scenario series.
A detailed review of long-term global and regional SO2 emissions scenarios
is given in Grübler (1998) and is summarized in Chapter 3.
The most important new finding from the scenario literature is recognition of
the significant impacts of continued unabated high SO2 emissions on human health,
food production, and ecosystems. As a result, scenarios published since 1995
all assume various degrees of SO2 emissions control and interventions to be
implemented in the future, and are thus substantially lower than previous projections,
including the IS92 series. In most of these scenarios, such low levels of SO2
emissions are not simply the result of direct SO2 emissions control measures,
such as flue gas desulfurization. They also result from other interventions
in which SO2 emissions reduction is more a secondary benefit than a primary
goal (e.g., structural changes for various reasons other than SO2 control).
2.4.4. Population Projection Ranges
Population is one of the fundamental driving forces of future emissions. Most
models used to formulate population projections for the emissions scenarios
are taken from the literature and are exogenous inputs. Today three main research
groups project global population - United Nations (UN, 1998, 1999), World Bank
(Bos and Vu, 1994), and IIASA (Lutz et al., 1997). (For more details
see the discussion in Chapter 3.) Most of the "central"
population projections lead to a doubling of global population by 2100 (to about
10 billion people compared to 5.3 billion in 1990). In recent years the central
population projections for the year 2100 have declined somewhat, but are still
in line with a doubling by 2100. For example, the latest UN (1998) medium-low
and medium-high projections indicate a range of between 7.2 and 14.6 billion
people by 2100, with the medium scenario at 10.4 billion. The IIASA central
estimate for 2100 is also 10.4 billion, with a 95% probability that world population
would exceed six and be lower than 17 billion (Lutz et al., 1997).
Figure 2-4: Global population
- historical development and 46 scenarios from the database. Fewer than
46 lines are shown because many of the 46 scenarios use identical population
projections. Only 46 of the scenarios in the database identify their
population projections. Data source: Durand, 1967; Demeny, 1990; UN,
1996; Morita and Lee, 1998.
While all scenarios require some kind of population assumptions, relatively
few are reported explicitly in the SRES database. Figure
2-4 illustrates global population projections in the database. Of the 416
scenarios currently documented, only 46 report their underlying population projections.
This limited number indicates that, even though population is an extremely important
driving force for emissions, it is typically either not reported or not well
explored in most models. For the small sample of population projections, the
range is from about 20 to 6 billion people in 2100, with the central or median
estimates at about 10 billion. Thus, population assumptions in the emissions
scenarios appear to be broadly consistent with the recent population projections,
with the caveat that only a few underlying projections are reported in the database.
Figure 2-4 contrasts the alternative population
projections from the SRES database with historical developments. The long-term
historical population growth rate has been on average about 1% per year during
the past two centuries. Between 1800 and 1900 the global population growth rate
was about 0.5% per year. The average annual growth rate since 1900 has been
1.3%. Between 1990 and 1995 the rate was 1.46%; and since 1995 world population
has been growing at a rate of 1.3% annually (UN, 1998). All scenarios reviewed
here envisage that population growth will slow in the future. The most recent
doubling of the world's population took approximately 40 years. Even the highest
population projections in Figure 2-4 require 70
years or more for the next doubling, while in roughly half of the scenarios
the global population does not double during the 21st century. The highest average
population growth across all projections is 1.2% per year, the lowest is 0.1%
per year, and the median is about 0.7% per year.
Interestingly, the population projections in Figure
2-4 are not evenly distributed across the full range. Instead, they are
grouped into three clusters. The middle cluster is representative of the central
projections, with a range of about 10 to less than 12 billion people by 2100.
The other two clusters mark the highest and the lowest population projections
available in the literature, with between 15 and 20 billion at the high end
and about 6 billion at the low end.
Despite these large ranges among alternative global population projections,
the variation in this factor compared to the base year is the smallest of all
the scenario driving forces considered in this comparison. Compared with 1990
values, the factor increase varies from 3.3 to 1.2.