2.3.4 Montreal Protocol Gases
The Montreal Protocol on Substances that Deplete the Ozone Layer regulates many radiatively powerful greenhouse gases for the primary purpose of lowering stratospheric chlorine and bromine concentrations. These gases include the CFCs, HCFCs, chlorocarbons, bromocarbons and halons. Observations and global cycles of these gases were reviewed in detail in Chapter 1 of the 2002 Scientific Assessment of Ozone Depletion (WMO, 2003) and in IPCC/TEAP (2005). The discussion here focuses on developments since these reviews and on those gases that contribute most to RF rather than to halogen loading. Using observed 2005 concentrations, the Montreal Protocol gases have contributed 12% (0.320 W m–2) to the direct RF of all LLGHGs and 95% to the halocarbon RF (Table 2.1). This contribution is dominated by the CFCs. The effect of the Montreal Protocol on these gases has been substantial. IPCC/TEAP (2005) concluded that the combined CO2-equivalent emissions of CFCs, HCFCs and HFCs decreased from a peak of about 7.5 GtCO2-eq yr–1 in the late 1980s to about 2.5 GtCO2-eq yr–1 by the year 2000, corresponding to about 10% of that year’s CO2 emissions due to global fossil fuel burning.
Measurements of the CFCs and HCFCs, summarised in Figure 2.6, are available from the AGAGE network (Prinn et al., 2000, 2005b) and the GMD network (Montzka et al., 1999 updated; Thompson et al., 2004). Certain flask measurements are also available from the University of California at Irvine (UCI; Blake et al., 2001 updated) and UEA (Oram et al., 1998; Oram, 1999 updated). Two of the major CFCs (CFC-11 and CFC-113) have both been decreasing in the atmosphere since the mid-1990s. While their emissions have decreased very substantially in response to the Montreal Protocol, their long lifetimes of around 45 and 85 years, respectively, mean that their sinks can reduce their levels by only about 2% and 1% yr–1, respectively. Nevertheless, the effect of the Montreal Protocol has been to substantially reduce the growth of the halocarbon RF, which increased rapidly from 1950 until about 1990. The other major CFC (CFC-12), which is the third most important LLGHG, is finally reaching a plateau in its atmospheric levels (emissions equal loss) and may have peaked in 2003. Its 100-year lifetime means that it can decrease by only about 1% yr–1 even when emissions are zero. The levelling off for CFC-12 and approximately linear downward trends for CFC-11 and CFC-113 continue. Latitudinal gradients of all three are very small and decreasing as expected. The combined CFC and HCFC RF has been slowly declining since 2003. Note that the 1998 concentrations of CFC-11 and CFC-12 were overestimated in Table 6.1 of the TAR. This means that the total halocarbon RF quoted for 2005 in Table 2.1 (0.337 W m–2) is slightly smaller than the 0.34 W m–2 quoted in the TAR, even though the measurements indicate a small 1% rise in the total halocarbon RF since the time of the TAR (Table 2.1).
Figure 2.6. Temporal evolution of the global average dry-air mole fractions (ppt) of the major halogen-containing LLGHGs. These are derived mainly using monthly mean measurements from the AGAGE and NOAA/GMD networks. For clarity, the two network values are averaged with equal weight when both are available. While differences exist, these network measurements agree reasonably well with each other (except for CCl4 (differences of 2 – 4% between networks) and HCFC-142b (differences of 3 – 6% between networks)), and with other measurements where available (see text for references for each gas).
The major solvent, methyl chloroform (CH3CCl3), is of special importance regarding RFs, not because of its small RF (see Table 2.1 and Figure 2.6), but because this gas is widely used to estimate concentrations of OH, which is the major sink species for CH4, HFCs, and HCFCs and a major production mechanism for sulphate, nitrate and some organic aerosols as discussed in Section 2.3.5. The global atmospheric methyl chloroform concentration rose steadily from 1978 to reach a maximum in 1992 (Prinn et al., 2001; Montzka et al., 2003). Since then, concentrations have decreased rapidly, driven by a relatively short lifetime of 4.9 years and phase-out under the Montreal Protocol, to levels in 2003 less than 20% of the levels when AGAGE measurements peaked in 1992 (Prinn et al., 2005a). Emissions of methyl chloroform determined from industry data (McCulloch and Midgley, 2001) may be too small in recent years. The 2000 to 2003 emissions from Europe estimated using surface observations (Reimann et al., 2005) show that 1.2 to 2.3 Gg yr–1 need to be added over this 4-year period to the above industry estimates for Europe. Estimates of European emissions in 2000 exceeding 20 Gg (Krol et al., 2003) are not supported by analyses of the above extensive surface data (Reimann et al., 2005). From multi-year measurements, Li et al. (2005) estimated 2001 to 2002 emissions from the USA of 2.2 Gg yr–1 (or about half of those estimated from more temporally but less geographically limited measurements by Millet and Goldstein, 2004), and suggested that 1996 to 1998 US emissions may be underestimated by an average of about 9.0 Gg yr–1 over this 3-year period. East Asian emissions deduced from aircraft data in 2001 are about 1.7 Gg above industry data (Palmer et al., 2003; see also Yokouchi et al., 2005) while recent Australian and Russian emissions are negligible (Prinn et al., 2001; Hurst et al., 2004).
Carbon tetrachloride (CCl4) is the second most rapidly decreasing atmospheric chlorocarbon after methyl chloroform. Levels peaked in early 1990 and decreased approximately linearly since then (Figure 2.7). Its major use was as a feedstock for CFC manufacturing. Unlike methyl chloroform, a significant inter-hemispheric CCl4 gradient still exists in 2005 in spite of its moderately long lifetime of 20 to 30 years, resulting from a persistence of significant NH emissions.
Figure 2.7. Annual rates of change in the global atmospheric masses of each of the major LLGHGs expressed in common units of GtC yr–1. These rates are computed from their actual annual mass changes in Gt yr–1 (as derived from their observed global and annual average mole fractions presented in Figures 2.3 to 2.6 and discussed in Sections 2.3.1 to 2.3.4) by multiplying them by their GWPs for 100-year time horizons and then dividing by the ratio of the CO2 to carbon (C) masses (44/12). These rates are positive or negative whenever the mole fractions are increasing or decreasing, respectively. Use of these common units provides an approximate way to intercompare the fluxes of LLGHGs, using the same approach employed to intercompare the values of LLGHG emissions under the Kyoto Protocol (see, e.g., Prinn, 2004). Note that the negative indirect RF of CFCs and HCFCs due to stratospheric ozone depletion is not included. The oscillations in the CF4 curve may result partly from truncation in reported mole fractions.
HCFCs of industrial importance have lifetimes in the range of 1.3 to 20 years. Global and regional emissions of the CFCs and HCFCs have been derived from observed concentrations and can be used to check emission inventory estimates. Montzka et al. (2003) and IPCC/TEAP (2005) concluded that global emissions of HCFC-22 rose steadily over the period 1975 to 2000, while those of HCFC-141b and HCFC-142b started increasing quickly in the early 1990s and then began to decrease after 2000.
To provide a direct comparison of the effects on global warming due to the annual changes in each of the non-CO2 greenhouse gases (discussed in Sections 2.3.2, 2.3.3 and 2.3.4) relative to CO2, Figure 2.7 shows these annual changes in atmospheric mass multiplied by the GWP (100-year horizon) for each gas (e.g., Prinn, 2004). By expressing them in this way, the observed changes in all non-CO2 gases in GtC equivalents and the significant roles of CH4, N2O and many halocarbons are very evident. This highlights the importance of considering the full suite of greenhouse gases for RF calculations.