2.4.3. Sulfur Dioxide Emissions

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).

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.

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