Driving forces of emissions other than CO2 or those of agriculture or land-use changes are discussed here. The direct GHGs N2O and CH4 are discussed first, followed by the indirect GHGs, which include sulfur and the ozone precursors NOx, CO, and volatile organic compounds (VOCs). Finally, the many various powerful GHGs, including ozone-depleting substances (ODS), are discussed.
The sources and sinks for these gases continue to be highly uncertain. Little research has been carried out to evaluate the influences of socio-economic and technological driving forces on long-term emission trends of these gases. As a rule, future emissions of these gases are included in long-term emission models on the basis of simple relationships to aggregate economic or sector-specific activity drivers, not least because individual source strengths continue to be highly uncertain. Notable exceptions are emissions of sulfur and ODS, which have been more intensively studied in connection with non-climate policy analysis in the domains of regional acidification and stratospheric ozone depletion.
Natural and agricultural soils are the dominant sources of N2O emissions, so future emission levels are governed by the land-use changes and changes in agricultural output and practices discussed in Section 3.5.2. Nevertheless, other sources are also important and are discussed here.
The dominant industrial sources are the production of HNO3 and adipic acid. The key driver for the production of HNO3 is the demand for fertilizer. Hence this emission source is closely related to the agricultural production driving forces discussed in Section 3.5, as well as to improvements in production technologies. Adipic acid, (CH2) 4 (COOH)2 , is a feedstock for nylon production and one of the largest-volume synthetic chemicals produced in the world each year - current annual global production is 1.8 million metric tons (Stevens III, 1993). Production has an associated by-product of 0.3 kg N2O/kg adipic acid for unabated emission, which at present results in a global emission of about 0.4 MtN as N2O annually. Emissions mostly arise in the OECD countries, which accounted for some 95% of global adipic acid production in 1990 (Davis and Kemp, 1991). Fenhann (2000) reviews the (sparse) scenario literature and concludes that future emissions will be determined mostly by two variables - demand growth as a result of growth in economic activity and progressively phased-in emission controls.
By the early 1990s, it was estimated that about one-third of OECD emissions had been abated (Stevens III, 1993). This abatement is an accidental result of the treatment of flue-gases in a reductive furnace (thermal destruction) to reduce NOx emissions, which coincidentally also converts about 99% of the N2O into nitrogen gas (N2). In other regions only about 20% of emissions had been abated by the early 1990s.
Major adipic acid producers worldwide have agreed to substantially reduce N2O emissions by 1996 to 1998. In July 1991 they formed an inter-industry group to share information on old and new technologies developed for N2O abatement, such as improved thermal destruction, conversion into nitric oxide for recycling, and the promising low-temperature N2O catalytic decomposition into N2 currently being developed by DuPont. The introduction of all three technologies could result in a 99% reduction of N2O emissions from adipic acid production (Storey, 1996). They are expected to be introduced at plants owned by Asahi (Japan), BASF and Bayer (Germany), DuPont (US), and Rh�ne-Poulenc (France) (Chemical Week, 1994). After the planned changes, US producers will have abated over 90% of the N2O emissions from adipic acid production. In recent years nylon-6.6 production dropped in the US, Western Europe, and Japan, largely in response to capacity and production in other Asian countries. By 2000 production is expected to recover in these countries (Storey, 1996).
Another major source of N2O is the transport sector. Gasoline vehicles without catalytic converters have very low, sometimes immeasurably small, emissions of N2O. However, vehicles equipped with three-way catalytic converters have N2O emissions that range from 0.01 to 0.1 g/km in new catalysts, and from 0.16 to 0.22 g/km in aging catalysts (IPCC, 1996). Emission levels also depend on precise engine running conditions. At the upper end of the emission range from aging catalysts, N2O emissions contribute around 25% of the in-use global warming impact of driving (Michaelis et al., 1996).
The introduction of catalytic converters as a pollution control measure in the majority of industrialized countries is resulting in a substantial increase in N2O emissions from gasoline vehicles. Several Annex I countries include projections of N2O from this source in their national communications to the UNFCCC, using a variety of projection methods (for example, Environment Canada, 1997; UNFCCC, 1997; VROM, 1997). The projections from these counties differ substantially in the contribution that transport is expected to make to their national N2O emissions in 2020, ranging from about 10% in France to over 25% in Canada. They anticipate that mitigation measures will be much more effective in reducing industrial and agricultural emissions of N2O than mobile source emissions. Indeed, little research has been carried out to identify catalytic converter technologies that result in lower N2O emissions. However, emissions are likely to be lower in countries that require regular emission inspections and replacement of faulty pollution control equipment.
Agricultural and land-use change emission drivers are discussed in Section 3.5.2. The other major sources are from the use of fossil fuels and the disposal of waste, for which the driving forces are briefly reviewed here. The earlier literature is reviewed in Barnes and Edmonds (1990). A more detailed recent literature review is given in Gregory (1998).
Emissions from the extraction, processing, and use of fossil fuels will be driven by future fossil fuel use. CH4 emissions from venting during oil and gas production may decrease because of efforts to reduce them (IGU, 1997b). Flaring and venting volumes from oil and gas operations peaked in 1976 to 1978, but a gradual reduction in volumes of gas flared and vented has occurred over the past 20 years (Boden et al., 1994, Marland et al., 1998; Stern and Kaufmann, 1998). Shell International Ltd. (1998) estimated a reduction in its own emissions from venting by 1 MtCH4 per year to 0.367 MtCH4 in the five years to 1997. The IEA Greenhouse Gases R&D Programme (1997) notes that emission reductions from the oil and gas sector would yield a high economic return. Additionally, new natural gas developments generally use the latest technology and are almost leak free compared to older systems. Taking all these factors into account, it seems plausible that CH4 emissions from the oil and gas sector should fall as the 21st century progresses. Nonetheless, the primary driver (oil and gas production) is likely to expand significantly in the future, depending on resource availability and technological change. A representative range from the literature, for example the scenarios described in Nakicenovic et al. (1998a), indicates substantial uncertainty in which future levels of oil and gas production could range between 130 and some 900 EJ. Assuming a constant emission factor, future CH4 emissions from oil and gas could range from a decline compared to current levels to a fourfold increase. With the more likely assumption of declining emission factors, future emission levels would be somewhat lower than suggested by this range.
The concentrations of CH4 in coal seams are low close to the surface, and hence emissions from surface mining are also low (IEA CIAB, 1992). Concentrations at a few hundred meters or deeper can be more significant; releases from these depths are normally associated with underground mining. Emissions per ton of coal mined can vary widely both from country to country and at adjacent mines within a country (IEA Greenhouse Gases R&D Programme, 1996a). CH4 mixed with air in the right proportions is an explosive mixture and a danger to miners. Measures to capture and drain the CH4 are common in many countries - the captured CH4, if of adequate concentration, can be a valuable energy source. The techniques currently used reduce total emissions by about 10%. Many older, deeper coal mines in Europe are being closed, which will reduce emissions. Replacement coal mines tend to be in exporting countries with low cost reserves near the surface, so the emissions will be low. For the future, emissions will depend principally on the proportion of coal production from deep mines and on total coal production.
A representative range of future coal production scenarios given in Nakicenovic et al. (1998a) indicates a very wide range of uncertainty. Future coal production levels could range anywhere from 14 to well over 700 EJ, between a sevenfold decrease to an eightfold increase compared to 1990 levels. Conversely, CH4 capture, either during mining or prior to mining, not only reduces risk to miners but also provides a valuable energy source. Thus, rising levels of CH4 capture for non-climate reasons are likely to characterize the 21st century. This would in particular apply to high coal production scenarios, in which most of the coal will need to come from deep mining once the easily accessible surface mine deposits have become exhausted. Growth in future emissions from coal mining is therefore likely to be substantially lower than growth in coal production.
Domestic and some industrial wastes contain organic matter that emits a combination of CO2 and CH4 on decomposition (IEA Greenhouse Gases R&D Programme, 1996b). If oxygen is present, most of the waste degrades by aerobic micro-organisms and the main product is CO2. If no oxygen is present, different micro-organisms become active and a mixture of CO2 and CH4 is produced. Decay by this mechanism can take months or even years (US EPA, 1994). Traditionally, waste has been dumped in open pits and this is still the main practice in most developing countries. Thus, oxygen is present and the main decay product is CO2. In recent decades, health and local environmental concerns in developed countries have resulted in better waste management, with lined pits and a cap of clay, for example, added regularly over newer dumps. This prevents fresh supplies of oxygen becoming available so the subsequent decay process is anaerobic and CH4 is produced. Williams (1993) notes that landfill sites are complex and highly variable biologic systems and many factors can lead to a wide variability in CH4 production. For the future, increasing wealth and urbanization in developing countries may lead to more managed landfill sites and to more CH4 production. However, the CH4 produced can be captured and utilized as a valuable energy source, or at least flared for pollution and safety reasons; indeed, this is a legal requirement in the USA for large landfills. Future emissions are therefore unlikely to evolve linearly with population growth and waste generation, but the scenario literature is extremely sparse on this subject - the major source remains the previous IS92 scenario series (Pepper et al., 1992).
Different methods are used to treat domestic sewage, some of which involve anaerobic decomposition and the production of CH4. Again, capture and use of some of the CH4 produced limits emissions. For the future, emissions will depend on the extension of sewage treatment in developing countries, the extent to which the techniques used enhance or limit CH4 production, and the extent to which the CH4 produced is captured and used.
Several authors, including Rudd et al. (1993) and Fearnside (1995), note that some hydroelectric schemes result in emissions of CH4 from decaying vegetation trapped by water as the dams fill; these emissions climatically exceed those of a thermopower plant delivering the same electricity. Rosa et al. (1996), Rosa and Sch�ffer (1994), and Gagnon and van de Vate (1997) point out that the two schemes discussed by Rudd et al. (1993) and Fearnside (1995) may be exceptional, with very large reservoir surface areas, a high density of organic matter, and low power output. Gagnon and van de Vate (1997) estimate the combined CH4 and N2O emissions from hydroelectric schemes at 5.5 gC equivalent per kWh compared to a range of 80 to 200 gC equivalent per kWh for a modern fossil power station (Rogner and Khan, 1998); that is, hydroelectric power emits less than 3% and 7%, respectively. While some GHG emissions from new hydroelectric schemes are expected in the future, especially in tropical settings (Galy-Lacaux et al., 1999), in the absence of more comprehensive field data, such schemes are regarded as a lower source of CH4 emissions compared to those of other energy sector or agricultural activities. Hydroelectric power is therefore not treated as a separate emission category in SRES.
In summary, numerous factors could lead to increases in emissions of CH4 in the future, primarily related to the expansion of agricultural production and greater fossil fuel use. Recent studies also identify a number of processes and trends that could reduce CH4 emission factors and hence may lead to reduced emissions in the future. These trends are not yet sufficiently accounted for in the literature, in which CH4 emission factors typically are held constant. The overall consequence is to introduce additional uncertainty into projections, as the future evolution of such emission factors is unclear. However, from the above discussion, the least likely future is one of constant emission factors and the range of future emissions is likely to be lower than those projected in previous scenarios with comparable growth in primary activity drivers.
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