3.6.4. Sulfur Dioxide

Two major sets of driving forces influence future SO₂ emissions:

• Level and structure of energy supply and end-use, and (to a lesser extent) levels of industrial output and process mix.
• The degree of SO₂-control policy intervention assumed (i.e., level of environmental policies implemented to limit SO₂ emissions).

Grübler (1998c) reviewed the literature and empirical evidence, and showed that both clusters of driving forces are linked to the level of economic development. With increasing affluence, energy use per capita rises and its structure changes away from traditional solid fuels (coal, lignite, peat, fuelwood) toward cleaner fuels (gas or electricity) at the point of end-use. This structural shift combined with the greater emphasis on urban air quality that accompanies rising incomes results in a roughly inverted U (IU) pattern of SO₂ emissions and/or concentrations. Emissions rise initially (with growing per capita energy use), pass through a maximum, and decline at higher income levels due to structural change in the end-use fuel mix and also control measures for large point sources. This pattern emerges also from the literature on environmental Kuznets curves (e.g., World Bank, 1992; IIASA-WEC, 1995) and is corroborated by both longitudinal and cross-sectional empirical data reviewed in detail in Grübler (1998c). Historically, the decline in sulfur pollution levels was achieved simply by dispersion of pollutants (tall stacks policy). Subsequently, the actual emissions also started to decline, as a result of both structural change (substitution of solids by gas and electricity as end-use fuels) and sulfur reduction measures (oil product desulfurization and scrubbing of large point sources).

Emissions for 1990 reported in the scenarios reviewed in Chapter 2 and in Grübler (1998c) indicate a range from 55 to 91 MtS. The upper range is explained largely by a lack of complete coverage of SO₂ emission sources in long-term scenario studies and models. Lower values correspond to studies that include only the dominant energy sector emissions (range of 59.7 to 65.4 MtS), and higher estimates also include other sources, most notably metallurgical and from biomass burning. None of the long-term scenario studies appears to include SO₂ emissions from international bunker (shipping) fuels, estimated at 3 ± 1 MtS in 1990 (Olivier et al., 1996; Corbett et al., 1999; Smith et al., 2000). Historical global sulfur emissions estimates are given in Dignon and Hameed (1989).

Grübler (1998c) also argues that SO₂ control and intervention policies in many rapidly industrializing countries (particularly those with high population densities) are highly likely to be phased in more quickly than the historical experience of Europe, North America, Japan, or Korea. This analysis is supported by existing policies and trends in Brazil, China, and India (Shukla et al., 1999; Rosa and Schechtman, 1996; Qian and Zhang, 1998). Most recent SO₂ emission inventory data suggest that since 1990 SO₂ emission growth has significantly slowed in East Asia compared to earlier forecasts, in response to the first SO₂ control measures implemented in China, South Korea and Thailand (Streets and Waldhoff, 2000). Dadi et al. (1998) estimate that in 1995 about 11% (1.5 MtS of a total of 13.5 MtS gross emissions) of China's SO₂ emissions were removed through various control measures.

The evaluation of the IS92 scenarios (Alcamo et al., 1995) concluded that the projected SO₂ emissions in the IS92 scenarios do not reflect recent changes in sulfur-related environmental legislation, in particular the amendments to the Clean Air Act in the USA, and the Second European Sulfur Protocol. Increasingly, many developing countries are adopting sulfur control legislation that ranges from reduction of sulfur contents in oil products (e.g. China, Thailand, and India; see Streets et al., 2000), through a maximum sulfur content in coal (e.g. in China; see Streets and Waldhoff, 2000), to SO₂ controls at coal-fired power plants (e.g. China, South Korea, Thailand; for a review see IEA, 1999). For instance, an estimated 3575 MW of coal-fired electricity China is generated by plants already equipped with sulfur control devices (IEA, 1999).

Since publication of the IS92 scenarios a number of important new sulfur impact studies have become available, and analyzed in particular:

• Implications of acidic deposition levels of high SO₂ emissions scenarios such as IS92a (Amann et al., 1995; Posch et al., 1996).
• Aggregate ecosystems impacts, especially whether critical loads for acidification are exceeded given deposition levels and different buffering capacities of soils (Amann et al., 1995; Posch et al., 1996).
• Direct vegetation damage, particularly on food crops (Fischer and Rosenzweig, 1996).

These studies provide further information on the impacts of high concentrations and deposition of SO₂ emissions, beyond the well-documented impacts on human health, ecosystems productivity, and material damages (for reviews see Crutzen and Graedel, 1986; WHO and UNEP, 1993; WMO, 1997). These studies are particularly important because they document environmental changes of high-emission scenarios by using detailed representations of the numerous non-linear dose-response relationships between emissions, atmospheric concentrations, deposition, ecosystems sensitivity thresholds, and impacts. All recent studies agree that unabated high SO₂ emissions along the lines of IS92a or even above would yield high impacts not only for natural ecosystems and forests, but also for economically important food crops and human health, especially in Asia where emissions growth is projected to be particularly high.

A representative result (based on Amann et al., 1995) is shown in Figure 3-17, which contrasts 1990 European sulfur deposition levels with those of Asia by 2050 in a high SO₂ emission scenario (very close to IS92a). Typically, in such scenarios, SO₂ emissions in Asia alone could surpass current global levels as early as 2020 (Amann et al., 1995; Posch et al., 1996). Sulfur deposition above 5 g/m² per year occurred in Europe in 1990 in the area of the borders of the Czech Republic, Poland, and Germany (the former GDR), often referred to as the "black triangle." In view of its ecological impacts it was officially designated by UNEP as an "ecological disaster zone." In a scenario such as IS92a (or even higher emissions), similar high sulfur deposition would occur by around 2020 over more than half of Eastern China, large parts of southern Korea, and some smaller parts of Thailand and southern Japan.

Fischer and Rosenzweig (1996) assessed the combined impacts of climate change and acidification of agricultural crops in Asia for such a scenario. Their overall conclusion was that the projected likely regional climate change would largely benefit agricultural output in China, whereas it would lower agricultural productivity on the Indian subcontinent (the combined effect of projected temperature and precipitation changes would have differential impacts across various crops and subregions). However, projected high levels of acidic deposition in China would reduce agricultural output to an extent that would more than offset any possible beneficial impacts of regional climate change. This is primarily because sulfur (and nitrogen) deposition, while acting as fertilizer for plant growth at lower deposition levels, negatively affects plant growth at higher deposition levels. Projections in a scenario such as IS92a are that the threshold levels will be surpassed between 2020 and 2050 for all major Asian food crops.

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