3.2.2.4 Scenarios for air pollutants and other radiative substances

Sulphur dioxide emission scenarios

Sulphur emissions are relevant for climate change modelling as they contribute to the formation of aerosols, which affect precipitation patterns and, taken together, reduce radiative forcing. Sulphur emissions also contribute to regional and local air pollution. Global sulphur dioxide emissions have grown approximately in parallel with the increase in fossil fuel use (Smith et al., 2001, 2004; Stern, 2005). However, since around the late 1970s, the growth in emissions has slowed considerably (Grübler, 2002). Implementation of emissions controls, a shift to lower sulphur fuels in most industrialized countries, and the economic transition process in Eastern Europe and the Former Soviet Union have contributed to the lowering of global sulphur emissions (Smith et al., 2001). Conversely, with accelerated economic development, the growth of sulphur emissions in many parts of Asia has been high in recent decades, although growth rates have moderated recently (Streets et al., 2000; Stern, 2005; Cofala et al., 2006; Smith et al., 2004). A review of the recent literature indicates that there is some uncertainty concerning present global anthropogenic sulphur emissions, with estimates for the year 2000 ranging between 55.2 MtS (Stern, 2005), 57.5 MtS (Cofala et al., 2006) and 62 MtS (Smith et al., 2004).^[7]

Many empirical studies have explored the relationship between sulphur emissions and related drivers, such as economic development (see for example, Smith et al., 2004). The main driving factors that have been identified are increasing income, changes in the energy mix, and a greater focus on air pollution abatement (as a consequence of increasing affluence). Together, these factors may result in an inverted U-shaped pattern of SO₂ emissions, where emissions increase during early stages of industrialization, peak and then fall at higher levels of income, following a Kuznets curve (World Bank, 1992). This general trend is also apparent in most of the recent emissions scenarios in the literature.

Over time, new scenarios have generally produced lower SO₂ emissions projections. A comprehensive comparison of the SRES and more recent sulphur-emission scenarios is given in Van Vuuren and O’Neill (2006). Figure 3.12 illustrates that the resulting spread of sulphur emissions over the medium term (up to the year 2050) is predominantly due to the varying assumptions about the timing of future emissions control, particularly in developing countries^[8]. Scenarios at the lower boundary assume the rapid introduction of sulphur-control technologies on a global scale, and hence, a reversal of historical trends and declining emissions in the initial years of the scenario. Conversely, the upper boundaries of emissions are characterized by a rapid increase over coming decades, primarily driven by the increasing use of coal and oil at relatively low levels of sulphur control (SRES A1 and A2).

Figure 3.12: Sulphur dioxide emission scenarios.

Notes: Thick coloured lines depict the four SRES marker scenarios and the black dashed lines show the median, 5^th and 95^th percentile of the frequency distribution for the full ensemble of all 40 SRES scenarios. The blue area (and the thin dashed lines in blue) illustrates individual scenarios and the range of Smith et al. (2004). Dotted lines indicate the minimum and maximum of sulphur emissions scenarios developed pre-SRES.

The comparison shows that overall the SRES scenarios are fairly consistent with recent projections concerning the long-term uncertainty range (Smith et al., 2004; see Figure 3.12). However, the emissions peak over the short-term of some high emissions scenarios in SRES, which lie above the upper boundary estimates of the recent scenarios. There are two main reasons for this difference. First, recent sulphur inventories for the year 2000 have shifted downward. Second, and perhaps more importantly, new information on present and planned sulphur legislation in some developing countries, such as India (Carmichael et al., 2002) and China (Streets et al., 2001) has become available. Anticipating this change in legislation, recent scenarios project sulphur emissions to peak earlier and at lower levels compared to the SRES. Also the lower boundary projections of the recent literature have shifted downward slightly compared to the SRES scenario.

NO_x emission scenarios

The most important sources of NO_x emissions are fossil fuel combustion and industrial processes, which combined with other sources such as natural and anthropogenic soil release, biomass burning, lightning, and atmospheric processes, amount to around 25 MtN per year. Considerable uncertainties exist, particularly around the natural sources (Prather et al., 1995; Olivier et al., 1998; Olivier and Berdowski, 2001; Cofala et al. (2006). Fossil fuel combustion in the electric power and transport sectors is the largest source of NO_x, with emissions largely being related to the combustion practice. In recent years, emissions from fossil fuel use in North America and Europe are either constant or declining. Emissions have been increasing in most parts of Asia and other developing parts of the world, mainly due to the growing transport sector (Cofala et al., 2006; Smith, 2005; WBCSD, 2004). However in the longer term, most studies project that NO_x emissions in developing countries will saturate and eventually decline, following the trend in the developed world. However, the pace of this trend is uncertain. Emissions are projected to peak in the developing world as early as 2015 (WBCSD, 2004, focusing on the transport sector) and, in worst cases, around the end of this century (see the high emissions projection of Smith, 2005).

There have been very few global scenarios for NO_x emissions since the earlier IS92 scenarios and the SRES. An important characteristic of these (baseline) scenarios is that they consider air pollution legislation (in the absence of any climate policy). Some scenarios, such as those by Bouwman and van Vuuren (1999) and Collins et al. (2000) often use IS92a as a ‘loose’ baseline, with new abatement policies added. Many scenarios report rising NO_x emissions up to the 2020s (Figure 3.13), with the lower boundary given by the short-term Cofala et al. (2006) reference scenario, projecting emissions to stay at about present levels for the next two to three decades. In the most recent longer-term scenarios (Smith, 2005), NO_x emissions range between 32 MtN and 47 MtN by 2020, which corresponds to an increase in emissions of around 6–50% compared to 2000. The long-term spread is considerably larger, ranging from 9 MtN to 74 MtN by 2100 (see Figure 3.13). The majority of the SRES scenarios (70%) lie within the range of the new Smith (2005) scenarios. However, the upper and lower boundaries of the range of the recent projections have shifted downward compared to the SRES.

Figure 3.13: NO_x emission scenarios.

Notes: Thick coloured lines depict the four SRES marker scenarios and the black dashed lines show the median, 5^th and 95^th percentile of the frequency distribution for the full ensemble of all 40 SRES scenarios. The blue area illus-trates the range of the recent Smith (2005) projections.

Emission scenarios for black and organic carbon

Black and organic carbon emissions (BC and OC) are mainly formed by incomplete combustion, as well as from gaseous precursors (Penner et al., 1993; Gray and Cass, 1998). The main sources of BC and OC emissions include fossil fuel combustion in industry, power generation, traffic and residential sectors, as well as biomass and agriculture waste burning. Natural sources, such as forest fires and savannah burning, are other major contributors. There has recently been some research suggesting that carbonaceous aerosols may contribute to global warming (Hansen et al., 2000; Andrae, 2001; Jacobson, 2001; Ramaswamy et al., 2001). However, the uncertainty concerning the effects of BC and OC on the change in radiative forcing and hence global warming is still high (see Jacobson, 2001; and Penner et al., 2004).

In the past, BC and OC emissions have been poorly represented in economic and systems engineering models due to unavailability of data. For example, in the IPCC’s Third Assessment Report, BC and OC estimates were developed by using CO emissions (IPCC, 2001b). One of the main reasons for this has been the lack of adequate global inventories for different emission sources. However, some detailed global and regional emission inventories of BC and OC have recently become available (Table 3.3). In addition, some detailed regional inventories are also available including Streets et al. (2003) and Kupiainen and Klimont (2004). While many of these are comprehensive with regard to detail, considerable uncertainty still exists in the inventories, mainly due to the variety in combustion techniques for different fuels as well as measurement techniques. In order to represent these uncertainties, some studies, such as Bond et al. (2004), provide high, low and ‘best-guess’ values.

Table 3.3: Emission inventories for black and organic carbon (Tg/yr).

Note: ^a) Emissions from fossil-fuel use

The development in the inventories has resulted in the possibility of estimating future BC and OC emissions. Streets et al. (2004) use the fuel-use information and technological change in the SRES scenarios to develop estimates of BC and OC emissions from both contained combustion as well as natural sources for all the SRES scenarios until 2050. Rao et al. (2005) and Smith and Wigley (2006) estimate BC and OC emissions until 2100 for two IPCC SRES scenarios, with an assumption of increasing affluence leading to an additional premium on local air quality. Liousse et al. (2005) use the fuel-mix and other detail in various energy scenarios and obtain corresponding BC and OC emissions.

The inclusion of technological development is an important factor in estimating future BC and OC emissions because, even though absolute fossil fuel use may increase, a combination of economic growth, increased environmental consciousness, technology development and legislation could imply decreased pollutant emissions (Figure 3.14). Liousse et al. (2005) neglect the effects of technological change leading to much higher emission estimates for BC emissions in the long-term in some cases, as compared to other studies such as Streets et al. (2004), Rao et al. (2005) and Smith and Wigley (2006), all of which show declining emissions in the long-term. Another important factor that Rao et al. (2005) also account for is current and proposed environmental legislation. This suggests the necessity for technology-rich frameworks that capture structural and technological change, as well as policy dynamics in the energy system in order to estimate future BC and OC emissions.

Figure 3.14: Total black carbon (left panel) and organic carbon (right panel) emission estimates in scenarios from different studies.

Notes: Rao et al., (2005) include emissions from contained combustion only. Liousse et al. (2005), A2 not included as 2100 value of 100 Tg lies way above range.

Both Streets et al. (2004) and Rao et al. (2005) show a general decline in BC and OC emissions in developed countries, as well as in regions such as East Asia (including China). In other developing regions, such as Africa and South Asia, slower technology penetration rates lead to much lower emission reductions. There is a large decline in emissions from the residential sector in the developing countries, due to the gradual replacement of traditional fuels and technologies with more efficient ones. Transport-related emissions in both industrialized and developing countries decline in the long-term due to stringent regulations, technology improvements and fuel switching.

To summarize, an important feature of the recent scenario literature is the long-term decline in BC/OC emission intensities per unit of energy use (or economic activity). The majority of the above studies thus indicate that the long-term BC and OC emissions might be decoupled from the trajectory of CO₂ emissions.