6.3.2.2. Nitrous Oxide Emissions

Even more than for CH₄ , the assumed future food supply will be a key determinant of future N₂O emissions. Size, age structure, and regional spread of the global population will be reflected in the emissions trajectories, together with assumptions on diets and improvements in agricultural practices. Again, as for CH₄ in the SRES scenarios (see Section 5.4.1 in Chapter 5), continued growth of N₂O emissions emerges only in the A2 scenario, largely because of high population growth. In the other three marker scenarios, emissions peak and then decline sooner or later in the course of the 21 st century. Importantly, as the largest anthropogenic source of N₂O (cultivated soils) is already very uncertain in the base year, all future emissions trajectories are affected by large uncertainties, especially if calculated with different models, as is the case in this SRES report. Therefore, the writing team recommends further research into the sources and modeling of long-term N₂O emissions. Uncertainty ranges are correspondingly large, and are sometimes asymmetric. For example, while the range in 2100 reported in all A1 scenarios is between 5 and 10 MtN (7 MtN in the A1B marker), the A2 marker reports 17 MtN in 2100. Other A2 scenarios report emissions that fall within the range reported for A1 (from 8 to 19 MtN in 2100). Thus, different model representations of processes that lead to N₂O emissions and uncertainties in source strength can outweigh easily any underlying differences between individual scenarios in terms of population growth, economic development, etc. Different assumptions with respect to future crop productivity, agricultural practices, and associated emission factors, especially in the very populous regions of the world, explain the very different global emission levels even for otherwise shared main scenario drivers. Hence, the SRES scenarios extend the uncertainty range of future emissions significantly toward higher emissions (4.8 to 20.2 MtN by 2100 in SRES compared to 5.4 to 10.8 MtN in the IS92 scenarios. (Note that natural sources are excluded in this comparison.)

6.3.2.3. Halocarbons and Halogenated Compounds

The emissions of halocarbons (chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methylbromide, and hydrofluorocarbons (HFCs)) and other halogenated compounds (polyfluorocarbons (PFCs) and sulfur hexafluoride (SF₆)) across the SRES scenarios are described in detail on a substance-by-substance basis in Chapter 5 and Fenhann (2000). However, none of the six SRES models has its own projections for emissions of ozone depleting substances (ODSs), their detailed driving forces, and their substitutes. Hence, a different approach for scenario generation was adopted.

First, for ODSs, an external scenario, the Montreal Protocol scenario (A3, maximum allowed production) from WMO/UNEP (1998) is used as direct input to SRES. In this scenario corresponding emissions decline to zero by 2100 as a result of international environmental agreements, a development not yet anticipated in some of the IS92 scenarios (Pepper et al., 1992). For the other gas species, most notably for CFC and HCFC substitutes, a simple methodology of developing different emissions trajectories consistent with the aggregate SRES scenario driving force assumptions (population, GDP, etc.) was developed. Scenarios are equally further differentiated as to assumed future technological change and control rates for these gases, varied across the scenarios consistently within the interpretation of the SRES storylines presented in Chapter 4. The literature, as well as the scenario methodology and data, are documented in more detail in Fenhann (2000) and are summarized in Chapter 5.

Second, different assumptions about CFC applications as well as substitute candidates were developed. These were initially based on Kroeze and Reijnders (1992) and information given in Midgley and McCulloch (1999), but updated with the most recent information from the Joint IPCC/TEAP Expert Meeting on Options for the Limitation of Emissions of HFCs and PFCs (WMO/UNEP, 1999) as described below. An important assumption, on the basis of the latest information from the industry, is that relatively few Montreal gases will be replaced fully by HFCs. Current indications are that substitution rates of CFCs by HFCs will be less than 50% (McCulloch and Midgley, 1998). In Fenhann (2000) a further technological development is assumed that would result in about 25% of the CFCs ultimately being substituted by HFCs (see Table 5-9 in Chapter 5). This low percentage not only reflects the introduction of non-HFC substitutes, but also the notion that smaller amounts of halocarbons will be used in many applications when changing to HFCs (efficiency gains with technological change). A general assumption is that the present trend, not to substitute with high GWP substances (including PFCs and SF₆), will continue. As a result of this assumption, the emissions reported here may be underestimates. This substitution approach is used in all four scenarios, and the technological options adopted are those known at present. Further substitution away from HFCs is assumed to require a climate policy and is therefore not considered in SRES scenarios. Policy measures that may indirectly induce lower halocarbon emissions in the scenarios are adopted for reasons other than climate change. For one scenario (A2) no reductions were assumed, whereas in the other scenarios intermediary reduction rates and levels were assumed. Expressed in HFC-134a equivalents (based on SAR equivalents), HFCs in the SRES scenarios range between 843 and 2123 kt HFC-134a equivalent by 2100, compared to 1188 to 2375 kt HFC-134a equivalent in IS92. The range of emissions of HFCs in the SRES scenario is initially generally lower than in earlier IPCC scenarios because of new insights about the availability of alternatives to HFCs as replacements for substances controlled by the Montreal Protocol. In two of the four scenarios in the report, HFC emissions increase rapidly in the second half of the 21st century, while in two others the growth of emissions is significantly slowed down or reversed in that period.

Aggregating all the different halocarbons (CFCs, HCFCs, HFCs) as well as halogenated compounds (PFCs and SF₆) into MtC-equivalents (using SAR GWPs) indicates a range between 386 and 1096 MtC-equivalent by 2100 for the SRES scenarios. This compares (see Table 6-2b) with a range of 746 to 875 MtC-equivalent for IS92 (which, however, does not include PFCs and SF₆). (The comparable SRES range, excluding PFCs and SF₆, is between 299 and 753 MtC-equivalent by 2100.) The scenarios presented here indicate a wider range of uncertainty compared to IS92, particularly toward lower emissions (because of the technological and substitution reasons discussed above).

The effect on climate of each of the substances aggregated to MtC-equivalents given in Table 6-2b varies greatly, because of differences in both atmospheric lifetime and the radiative effect per molecule of each gas. The net effect on climate of these substances is best determined by a calculation of their radiative forcing - which is the amount by which these gases enhance the anthropogenic greenhouse effect. The net radiative effect of all halocarbons, PFCs, and SF₆ from 1990 to 2100, including a current estimate of the radiative effect of stratospheric ozone depletion and subsequent recovery, ranges from 6% to 9% of the total radiative forcing from all GHGs and SO₂ . Preliminary calculations indicate that the net radiative effect of PFCs and SF₆ in SRES scenarios will be no greater, relative to total anthropogenic forcing, by 2100 than it is at present.

6.3.3. Sulfur Dioxide Emissions

Emissions of sulfur portray even more dynamic patterns in time and space than the CO₂ emissions shown in Figures 6-5 and 6-6. 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. Figure 6-10 shows the range of global sulfur emissions for all SRES scenarios and the four markers against the emissions range of the IS92 scenarios, more than 80 scenarios from the literature, and the historical development.

A detailed review of long-term global and regional sulfur emission scenarios is given in Grübler (1998) and summarized in Chapter 3. The most important new finding from the scenario literature is recognition of the significant adverse impacts of sulfur emissions on human health, food production, and ecosystems. As a result, scenarios published since 1995 generally assume various degrees of sulfur controls to be implemented in the future, and thus have projections substantially lower than previous ones, including the IS92 scenario series. Of these, only the two low-demand scenarios IS92c and IS92d fall within the range of more recent long-term sulfur emission scenarios. A related reason for lower sulfur emission projections is the recent tightening of sulfur-control policies in the Organization for Economic Cooperation and Development (OECD) countries, such as the Amendments of the Clean Air Act in the USA and the implementation of the Second European Sulfur Protocol. Such legislative changes were not reflected in previous long-term emission scenarios, as noted in Alcamo et al. (1995) and Houghton et al. (1995). Similar sulfur control initiatives due to local air quality concerns are beginning to impact sulfur emissions also in a number of developing countries in Asia and Latin America (see IEA, 1999; La Rovere and Americano, 1998; Streets and Waldhoff, 2000; for a more detailed discussion see Chapter 3). As a result, the median from recent sulfur scenarios (see Chapter 3) is consequently significantly lower compared to IS92, indicating a continual decline in global sulfur emissions in the long-term. The median and mean of sulfur control scenarios are almost identical. As mentioned above, even the highest range of recent sulfur-control scenarios is significantly below that of comparable, high-demand IS92 scenarios (IS92a, IS92b, IS92e, and IS92f). The scenarios with the lowest ranges project stringent sulfur-control levels that lead to a substantial decline in long-term emissions and a return to emission levels that prevailed at the beginning of the 20th century.

Reflecting recent developments and the literature (reviewed in Chapter 3), it is assumed that sulfur emissions in the SRES scenarios will also be controlled increasingly outside the OECD. As a result, both long-term trends and regional patterns of sulfur emissions evolve differently from carbon emissions in the SRES scenarios. As a general pattern, global sulfur emissions do not rise substantially, and eventually decline, even in absolute terms, during the second half of the 21st century (see also Chapters 2 and 3). The spatial distribution of emissions changes markedly. Emissions in the OECD countries continue their recent declining trend (reflecting the tightening of control measures). Emissions outside the OECD rise initially, most notably in Asia, which compensates for the declining OECD emissions. Over the long term, however, sulfur emissions decline throughout the world, but the timing and magnitude vary across the scenarios tightening of control measures). Emissions outside the OECD rise initially, most notably in Asia, which compensates for the declining OECD emissions. Over the long term, however, sulfur emissions decline throughout the world, but the timing and magnitude vary across the scenarios.

The SRES scenario set brackets global anthropogenic sulfur emissions between 27 and 169 MtS by 2050 and between 11 and 93 MtS by 2100 (see Table 6-2b). The range of emissions for the four markers is smaller. In contrast, the range of the IS92 scenarios (Pepper et al., 1992; Alcamo et al., 1995) is substantially higher, starting at 80 MtS and extending all the way to 200 MtS by 2050 and from 55 to 230 MtS by 2100. The two lowest scenarios, IS92c and IS92d, approach the higher end estimates of the SRES scenarios in 2100, while others are above the SRES range. As mentioned, this difference reflects the expected future consequences of recent policies that aim to achieve a drastic reduction in sulfur emissions in OECD countries, as well as an anticipated gradual introduction of sulfur controls in developing regions in the long-term, as reported in the underlying literature (see Chapter 3). In other words, all SRES scenarios assume sulfur control measures, although the uncertainty in timing and magnitude of implementation is reflected in the variation across different scenarios. Importantly, SRES scenarios assume sulfur controls only and do not assume any additional climate policy measures. Nevertheless, one important implication of this varying pattern of sulfur emissions is that the historically important, but uncertain, negative radiative forcing of sulfate aerosols may decline in the very long run. This view is also confirmed by model calculations reported in Subak et al. (1997) and Nakicenovic et al. (1998), on the basis of recent long-term GHG and sulfur emission scenarios.

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