Elimination of the Antarctic ozone hole:
The phase-out schedule of the amended Montreal Protocol, if fully complied with by all nations and if there are no continued uses of HCFCs, affords the opportunity to return to stratospheric chlorine abundances of 2 ppbv sometime between the middle and the end of the next century. This is the level at which the Antarctic ozone hole appeared in the late 1970s and hence is about the level that is thought to be necessary (other conditions assumed constant, including bromine loading) to eliminate the ozone hole.
Such levels could never have been reached under the provisions of the original 1987 Protocol.
Future levels of ozone:
Even if the control measures of the amended Montreal Protocol (London, 1990 were to be implemented by all nations, the current abundance of stratospheric chlorine (3.3-3.5 ppbv) is estimated to increase during the next several years, reaching a peak of about 4.1 ppbv around the turn of the century. With these increases, the additional middle-latitude ozone losses during the l990s are expected to be comparable to those observed during the 1980s, and there is the possibility of incurring widespread losses in the Arctic.
Reducing these expected and possible ozone losses requires further limitations on the emissions of chlorine- and bromine-containing compounds.
Approaches to limiting future levels of ozone depletion:
Lowering the peak and hastening the subsequent decline of chlorine and bromine levels can be accomplished in a variety of ways, including an accelerated phase-out of controlled substances and limitations on currently uncontrolled halocarbons.
A significant reduction in peak chlorine loading (a few tenths of a ppbv) can be achieved with accelerated phase-out schedules of CFCs, carbon tetrachloride and methyl chloroform. Even stringent controls on HCFC-22 would not significantly reduce peak chlorine loading (at most 0.03 ppbv, especially when ODP weighted), but do hasten the decline of chlorine. Specifically actions should include: a global phase-out in the emissions of long-lived chlorofluorocarbons, methylchloroform and carbon tetrachloride as soon as possible (100% compliance is essential); not-in-kind substitution of CFCs wherever practical; the substitution of long-lived CFCs with HCFCs having the shortest possible lifetimes, hence low values of ozone depletion potentials (remember that all HCFCs are not equal: those with short atmospheric lifetimes (1-5 years) pose a significantly lower threat to the ozone layer than those with moderately long lifetimes (greater than 15 years); a phase-out of halocarbon substitutes (HCFCs) sometime early in the next century (phase-out date should depend upon the atmospheric lifetime of the substitute), and possible emission rate limitations; and the recycling of HCFCs to the maximum extent possible.
A three-year acceleration of the phase-out schedule for the halons would reduce peak bromine loading by about 1 pptv. If the anthropogenic sources of methyl bromide are significant and their emissions can be reduced, then each 10% reduction in methyl bromide would rapidly result in a decrease in stratospheric bromine of 1.5 pptv, which is equivalent to a reduction in stratospheric chlorine of 0.045 to 0.18 ppbv. This gain is comparable to that of a three-year acceleration of the scheduled phase-out of the CFCs.