How has ozone depletion affected the radiative forcing of the climate system

Observations and modelling show that ozone depletion has acted to cool the stratosphere, which in turn can contribute to cooling of the troposphere and surface. The warming of the climate by ODSs and the cooling associated with ozone depletion are two distinct mechanisms that are governed by different physical processes and feedbacks and for which there are quite different levels of scientific understanding. For the purposes of this report, we follow IPCC (2001)11 and assume that the observed depletion is caused entirely by ODSs and that the ozone radiative forcing can be considered to be an indirect effect due to ODSs. Gases containing bromine (such as halons) are particularly effective ozone depleters and have a larger contribution to the indirect effect on a per-molecule basis than other ozone-depleting gases such as the CFCs. [1.1, 1.2, 1.3 and 1.5]

11 IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA, 944 pp.

The best estimate of the negative indirect radiative forcing associated with ozone depletion over the period 1970-2000 is -0.15 ± 0.10 W m-2, where the large uncertainty is determined by the range of model estimates and arises mainly because of uncertainties in the detailed vertical distribution of ozone depletion. This indirect effect is very likely12 to be smaller in magnitude than the positive direct radiative forcing due to ODSs alone (0.32 ± 0.03 W m-2 ) which is far better understood . If some fraction of the observed global ozone changes were not attributable to ODSs the magnitude of this indirect effect would be reduced. [1.5]

A limited number of global climate modelling and statistical studies suggest that ozone depletion is one mechanism that may affect patterns of climate variability which are important for tropospheric circulation and temperatures in both hemispheres. In particular, the significant depletion of stratospheric ozone occurring in the Antarctic region is likely to have affected stratospheric circulation, and consequently the troposphere. There are indications that the Antarctic ozone hole has contributed to the cooling observed over the Antarctic plateau and to warming in the region of the Antarctic Peninsula. [1.3]

The relative contributions of different types of gas to positive direct and negative indirect radiative forcing are shown in Figure TS-6. However, the warming and cooling effects produced by direct and indirect radiative forcing do not simply offset one another because the spatial and seasonal distributions of the effects on surface climate are different. [1.2, 1.5 and Box 1.4]

Figure TS-6. Radiative forcing due to changes in halocarbons from 1750 to 2000. [Based on Table 1.1]

2.7 What factors are expected to control ozone in the coming century? Will it 'recover' to pre-ozone-hole values? Has the recovery already begun?

Global ozone recovery is expected to follow decreases in chlorine and bromine loading in the stratosphere as ODS concentrations decline due to reductions in their emissions. While this is expected to be the dominant factor in ozone recovery, emissions of other GHGs (such as CO2, methane and nitrous oxide) can affect both tropospheric and stratospheric chemistry and climate, and will have some effect on ozone recovery. [1.3 and 1.4]

Figure TS-7 shows a range of predictions for changes in stratospheric ozone for the latitude range 60°S-60°N from two-dimensional photochemical models together with comparable ground-based and satellite measurements taken up to 2003. Such computer simulations show global ozone recovery occurring gradually as halogen gas emissions and concentrations decrease. However, the time of recovery varies significantly, depending on assumptions made about future climate and future composition of the atmosphere, and consequently remains quite uncertain. [1.4 and Box 1.7]

Future temperature changes related to GHG emissions are expected to enhance stratospheric ozone depletion in some parts of the stratosphere and decrease it in others. Increases in CO2 concentration are expected to cool the stratosphere, which is known to reduce the rates of gasphase ozone destruction in much of the stratosphere and thereby increase ozone concentrations at altitudes above about 25 km. In contrast, lower temperatures could decrease ozone concentrations at lower altitudes. While the latter effect is expected to be most important in the Arctic in late winter to early spring, it may be small compared with other

12 In this report the following words have been used where appropriate to indicate judgmental estimates of confidence: very likely (90-99% chance); likely (66-90% chance); unlikely (10-33% chance); and very unlikely (1-10% chance).

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1980 2000 2020 2040

Year

Figure TS-7. Observed and modelled changes in low- and mid-latitude (60°S — 60°N) de-seasonalised column ozone relative to 1980. The black symbols indicate ground-based measurements, and the coloured symbols various satellite-based data sets. The range of model predictions comes from the use of several different two-dimensional photochemistry models forced with the same halocarbon scenario; some models also allowed for the effect of changing CO2 amounts on stratospheric temperatures. The measurements show that column ozone values between 60°S and 60°N decreased beginning in the early 1980s and the models capture the timing and extent of this decrease quite well. Modelled halogen source gas concentrations decrease in the early 21st century in response to the Montreal Protocol, so that simulated ozone values increase and recover towards pre-1980 values. [Box 1.7]

processes and will slowly decrease with time as the chlorine and bromine loadings decrease. Changes in stratospheric circulation may also occur in association with increases in GHGs and these could either increase or decrease future mid-latitude and polar ozone. The net result of future GHG emissions on global ozone depends upon the combination of these effects and its magnitude and direction are poorly quantified at present. [1.3 and 1.4]

As can be seen from the measurements shown in Figure TS-7, detection of the beginning of ozone recovery is difficult because of the high variability in ozone levels. This variability is due to both meteorological variability and the confounding influence of volcanic eruptions on the ozone layer. As a result, it is not yet possible to state that the beginning of ozone recovery has been unambiguously identified. [1.2, 1.4 and Box 1.7]

Models suggest that minimum levels of Antarctic ozone may already have occurred or should occur within the next few years. Predictions of the timing of a minimum in Arctic ozone are more uncertain due to far greater natural variability in this region, but models suggest that it should occur within the next two decades. An Arctic 'ozone hole' similar to that currently observed over the Antarctic is very unlikely to occur. [1.3 and 1.4]

2.8 How much are CFCs, HCFCs and their possible replacements expected to affect the future radiative forcing of the climate system?

The estimated radiative forcing of HFCs in 2015 is in the range 0.022-0.025 W m-2 based on the SRES emission scenarios and in the range 0.019-0.030 W m-2 based on scenarios from Chapter 11 of this report. The radiative forcing of PFCs in 2015 is about 0.006 W m-2 based on SRES scenarios. These HFC and PFC radiative forcings correspond to about 6-10% and 2% respectively of the total estimated radiative forcing due to CFCs and HCFCs in 2015 (estimated to be 0.297 W m-2 for the baseline scenario). [2.5 and 11.5]

Scenario-based projections of future radiative forcing are shown in Figure TS-8. Such projections over longer time scales become more uncertain due to the growing influences of uncertainties in future technological practices and policies, but the contribution of HFCs may be 0.1 to 0.25 W m-2 by 2100 based upon the range of SRES emission scenarios, while that of PFCs may be 0.02 to 0.04 W m-2 by 2100. [1.5 and 2.5]

Figure TS-8 shows estimates of the separate contributions to future halocarbon radiative forcing from: past emissions (i.e. what is currently in the atmosphere); future emissions of new production assuming no change in current practices (based on the WMO Ab scenario); and emissions from current banks of halocarbons. Two different estimates of this last component are shown, one based on the WMO (2003) Ozone Assessment and the other based on later sections of this report. Although the magnitude of current ODS banks remains uncertain, their contribution to radiative forcing is expected to be comparable with that of HFC emissions to the atmosphere in the next few decades. This indicates that choices with respect to end-of-life options, such as the destruction of currently banked material, can provide significant benefits for the climate system. End-of-life recovery, recycling and destruction practices could also reduce emissions of all newly produced halocarbons and their contribution to radiative forcing shown in Figure TS-8. [1.5]

In addition to reducing atmospheric chlorine loading, actions taken under the Montreal Protocol and its adjustments and amendments have also acted to reduce total CO2-equivalent

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