The Montreal Protocol is an internationally accepted agreement whereby nations agree to control the production of ozone-depleting substances. Many of the chemicals that release chlorine atoms into the stratosphere, and deplete stratospheric O₃, are also greenhouse gases, so they are discussed briefly here. Detailed assessment of the current observations, trends, lifetimes, and emissions for substances covered by the protocol are in WMO (Kurylo and Rodriguez, 1999; Prinn and Zander, 1999). The ozone-depleting gases with the largest potential to influence climate are CFC-11 (CFCl₃), CFC-12 (CF₂Cl₂), and CFC-113 (CF₂ClCFCl₂). It is now clear from measurements in polar firn air that there are no natural sources of these compounds (Butler et al., 1999). Surface measurements of these compounds show that their growth rates continue to decline. Growth rates are slightly negative for CFC-11 and CFC-113 (Montzka et al., 1996a, 1999; Prinn et al., 2000); see Figure 4.6. CFC-12 increased by 4 ppt/yr during 1995 to 1996, down from about 12 ppt/yr in the late 1980s, see Figure 4.7). Methyl chloroform (CH₃CCl₃) has decreased dramatically since the Montreal Protocol was invoked, due to its relatively short lifetime (about 5 years) and the rapidity with which emissions were phased out. Its decline was 13 ppt/yr during the period 1995 to 1996 (Prinn et al., 1998, 2000). The halon abundances are small relative to the CFCs, and will never become large if the Montreal Protocol is adhered to. Atmospheric measurements show that growth rates of halon-1301 and halon-2402 decreased in response to the Montreal Protocol, but halon-1211 continues to increase at rates larger than expected based on industrial emissions data (Butler et al., 1998a; Fraser et al., 1999; Montzka et al., 1999).

The depletion of stratospheric ozone over the past three decades has been substantial. Between 60°S and 60°N it averaged about 2%/decade. A thorough review of the direct and possible indirect effects of stratospheric ozone depletion are given in WMO (Granier and Shine, 1999). The depletion of O₃ (and its radiative forcing) is expected to follow the weighted halogen abundance in the stratosphere. Therefore, both will reach a maximum in about 2000 before starting to recover; however, detection of stratospheric O₃ recovery is not expected much before 2010 (Jackman et al., 1996; Hofmann and Pyle, 1999). Methyl chloroform has been the main driver of the rapid turnaround in stratospheric chlorine during the late 1990s (Montzka et al., 1999; Prinn et al., 2000), and further recovery will rely on the more slowly declining abundances of CFC-11 and -12, and halons (Fraser et al., 1999; Montzka et al., 1999). It is expected that stratospheric ozone depletion due to halogens will recover during the next 50 to 100 years (Hofmann and Pyle, 1999). In the short run, climatic changes, such as cooling in the northern winter stratosphere, may enhance ozone depletion, but over the next century, the major uncertainties in stratospheric ozone lie with (i) the magnitude of future consumption of ozone-depleting substances by developing countries (Fraser and Prather, 1999; Montzka et al., 1999), (ii) the projected abundances of CH₄ and N₂O, and (iii) the projected climate change impacts on stratospheric temperatures and circulation.

Figure 4.4: Abundance of CF4 (ppt) over the last 200 years as measured in tropospheric air (open diamonds), stratospheric air (small filled diamonds), and ice cores (open squares) (Harnisch et al., 1996; 1999).	Figure 4.6: Global mean CFC-11 (CFCl3 ) tropospheric abundance (ppt) from 1950 to 1998 based on smoothed measurements and emission models (Prinn et al., 2000). CFC-11’s radiative forcing is shown on the right axis.
Figure 4.5: Abundance of SF6 (ppt) measured at Cape Grim, Tasmania since 1978 (Maiss et al., 1996; Maiss and Brenninkmeijer, 1998). Cape Grim values are about 3% lower than global averages.	Figure 4.7: Global mean CFC-12 (CF2Cl2 ) tropospheric abundance (ppt) from 1950 to 1998 based on smoothed measurements and emission models (Prinn et al., 2000). CFC-12’s radiative forcing is shown on the right axis.

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