This category of GHG emissions comprises a wide basket of different gas species that originate from a multitude of processes. Generally, their common characteristic is that they are released into the atmosphere in comparatively small amounts, but on a molecular basis most of the gases are long-lived, with atmospheric lifetimes up to 50,000 years. Generally they have a strong greenhouse forcing per molecule (see Chapter 5, Table 5-7).
Anthropogenic emissions of gases that cause stratospheric ozone depletion (chlorofluorocarbons (CFCs), hydrochloro-fluorocarbons (HCFCs), halons, methylchloroform, carbon tetrachloride, and methylbromide) are controlled by consumption restrictions (production plus imports minus exports) in the Montreal Protocol. No special SRES scenarios were developed for these gases because their future emission levels (phase out) are primarily policy driven and hence unrelated to scenario variations of important driving-force variables such as population, economic growth, or industrial output. Instead, the Montreal Protocol scenario (A3, maximum allowed production scenario) from the 1998 WMO/UNEP Scientific Assessment of Ozone Depletion is used (WMO/UNEP, 1998).
The procedures for constructing scenarios for hydro-fluorocarbon (HFC), polyfluorocarbon (PFC), and sulfur hexafluoride (SF6) emissions - for which there is an extreme paucity of scenario literature - are based on Fenhann (2000) and are described in greater detail in Chapter 5, Section 5.3.3. In this approach, future total demand for CFCs, HFCs, and other CFC substitutes is estimated on the basis of historical trends. HFC emissions are calculated using an assumed future replacement of CFCs by HFCs and other substitutes. The main drivers for the emissions are population and GDP growth. The sparse literature available (reviewed in Fenhann, 2000) indicates that emissions are related non-linearly to these driving forces, with important possibilities for saturation effects and long-term decoupling between growth in driving force variables and emissions. The emissions have been tuned to agree with emissions scenarios presented at the joint IPCC-TEAP expert meeting (WMO/UNEP, 1999). Material from the March Consulting Group (1999) has also been used.
For PFCs (CF4 and C2F6) the emissions driver is primary aluminum production, which is generally modeled using GDP and a consumption elasticity. Recycling rates are increasingly important, as reflected in the SRES scenarios (see Chapter 5). Aluminum production by the Soederberg process resulted, on average, in the emission of 0.45 kg CF4 per tAl and 0.02 kg C2F6 per tAl in 1998 in Norway. The effect of future technological change on the emissions factor can be assumed to be large, since the costs of modifications in process technology can be offset by the costs of saved energy. A considerable reduction in the emission factors has already taken place and the present emission factor of 0.5 kg CF4 per tAl is expected to fall to 0.15 kg CF4 per tAl at various rates (see Chapter 5). An emission factor for C2F6, 10 times lower than that of CF4 was used in the calculations. The present trend of not replacing CFCs and HCFCs with high global warming compounds like PFCs (or SF6) is also assumed to continue, which might underestimate the effect of future emissions. The only other source included for PFC emissions is semiconductor manufacturing, for which the industry has globally adopted a voluntary agreement to reduce its PFC emissions by 10% in 2010 relative to 1995 levels.
SF6 emissions originate from two main activities - the use of SF6 as a gas insulator in high-voltage electricity equipment, and its use in magnesium foundries, in which SF6 prevents the oxidation of molten magnesium. The driver for the former is electricity demand and for the latter it is future magnesium production, which will depend on GDP and a consumption elasticity. Emission factor reductions over time that result from more careful handling, recovery, recycling, and substitution of SF6 are assumed for both sources. Fenhann (2000) assumes that in low future scenarios SF6 emissions factors decline to one-tenth their present values between 2020 and 2090. In high future scenarios, Fenhann (2000) assumes reduction levels are somewhat lower, ranging from 55% to 90% depending on the region. In the absence of scenario literature, these assumptions are retained here (see Chapter 5). Other applications of SF6 include as a tracer gas in medical surgery and the production of semiconductors, and as an insulator in some windows. However, these sources are assumed to be cause less than 1% of the global emissions.
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