It is widely accepted that anthropogenic emissions of chlorine and bromine species are responsible for the major part of the destruction of the ozone layer in the last two decades. The future atmospheric concentrations of species that deplete the ozone layer (CFCs, HCFCs, halons and related species) depend on the future use of these species, which is strongly regulated by the Montreal Protocol and its amendments.
The extent to which the developed and developing nations comply with the Montreal Protocol and its amendments determines to a large degree the condition of the ozone layer in the 21st century. The WMO ozone assessment reports most times assume that all nations fully comply with the Montreal Protocol and its amendments.
Simulations were performed with various IPCC emission scenarios of CFCs and related species, to study the effects of different levels of compliance with the Montreal Protocol and the effects of various control measures on the future chlorine loading of the atmosphere and on the ozone layer. If 100% of the developed nations and only 70% of the developing nations comply with the London amendments to the Montreal Protocol the estimated chlorine levels in the atmosphere will increase from 3.8 ppbv in 1990 to more than 5 ppbv in 2100. The corresponding ozone columns will decrease by 5% to 8% in 2100 relative to 1990 at mid-latitudes.
If all nations comply fully with the Montreal Protocol and the London amendments the critical 2 ppbv chlorine level will be reached in 2115. By limiting the use of HCFCs this level may be reached in 2060. In anticipation of a 100% compliance for both the developed and developing nations, the future ozone columns will reach the 1990 values between 2031 and 2049, while 1980 values will be reached 50 years later. The maximum depletion (year and depth) of the ozone layer depends only slightly on the future scenario of CFCs and related species.
The speed of recovery of the ozone layer, however, depends heavily on the type of future scenario. Restricting the use of HCFCs, as in the Copenhagen amendments, has a significant effect on the recovery of the ozone layer. Accompanied by a decrease in atmospheric concentration of CFCs and HCFCs, is an increase in concentration of HFCs, from only a few ppbv in 1990 to possibly 5.2 ppbv in 2100. HFCs are not harmful for the ozone layer, but they are greenhouse gases. The estimated radiative forcing in 2100 associated with this increase in HFC concentration is three times larger than the radiative forcing of the CFCs and HCFCs in 1990.
The only limiting measure for methyl bromide agreed upon by the Copenhagen amendments to the Montreal Protocol is a freeze in the use by 1996. If emissions of methyl bromide from soil fumigation were to be eliminated in 1997, then the total anthropogenic decrease in ozone column since 1980 is predicted to be 0.7% less in 2000, 5.6% in 2020 and 9.7% less in 2050, relative to a full compliance with the Montreal Protocol and its amendments. Larger decreases may occur if methyl bromide emissions from exhaust of automobiles are also reduced.
Without any protocol restricting the use of CFCs and related species, the chlorine levels in the atmosphere could have increased from 3.8 ppbv in 1990 to 4.9 ppbv in 2000, 18.5 ppbv in 2050 and 39 ppbv in 2100. The corresponding reductions in the ozone columns at mid-latitude, relative to 1990, range from 4% to 6% in 2000, 45% to 65% in 2050 and 70% to 85% in 2100.
Bromine atoms are approximately 40 times more effective in destroying ozone than chlorine atoms, but considering the abundances in the atmosphere, total anthropogenic chlorine destroys approximately more than ten times as much stratospheric ozone as total anthropogenic bromine does. For the simulations, the RIVM version of the 2-dimensional chemical radiative transport model developed at the University of Cambridge was used.
This model has been extended to include the effects of heterogeneous chemistry on sulphate aerosols. A model validation study shows that the modelled atmospheric concentrations of the CFCs and related species as well as the modelled ozone columns agree well with measurements. The modelled trend in the ozone columns from 1980 to 1990 range from -4% to -6% per decade at mid-latitudes. This is half the total, modelled, anthropogenic decrease from 1900 to 1990 of -8% to -12%.