Q. J. R. Meteorol. SOC. (2001), 127, pp. 959-974

Three-dimensional chemical model simulations of the ozone layer: 20 15-55 By JOHN AUSTIN*, NEAL BUTCHART and JEFFREY KNIGHT

Met OfJice, VK

(Received 20 March 2000; revised 27 November 2000)

SUMMARY A stratospheric-chemistry model coupled to a general-circulation model is used to investigate chemistry-

climate coupling processes and their influence on ozone. Simulations commence on 1 March in each of the years 2014, 2024, 2034, 2044 and 2054, and consist of a 4-month spin-up period, followed by a I-year integration. Projected values of halogen amounts and greenhouse gases are imposed on the model. During the period 2 0 1 4 54, ozone generally increases but by 2054 has still not returned to 1980 conditions. In Antarctica, spring ozone recovers temporarily in the 2024 integration but the ozone hole deepens significantly again in the 2034 and 2044 integrations before finally disappearing in the 2054 integration. The results suggest that the deepening of the Antarctic ozone hole in the model in 2034 and 2044, despite a reduction in halogen loading, is due to enhanced cooling due to increased greenhouse gases. Model-predicted temperature and ultraviolet (UV) changes are also investigated. It is found that recovery of ozone during the period of the simulations gives rise to reduced stratospheric temperature decreases and UV levels are still slightly higher in general than in previous calculations for 1980.

KEY WORDS: Chemistry Climate Stratosphere Ultraviolet

1. INTRODUCTION

Stratospheric ozone loss has been observed virtually globally since the beginning of the satellite era (e.g. Stolarski et al. 1991). Its connection with the release of man- made halogenated compounds is also now well established (e.g. WMO 1999, chapter 7). Accordingly, international agreements are now in place to limit these compounds (e.g. WMO 1999, chapter 11) and ozone is expected to return to approximately 1980 levels by the second half of the twenty-first century (WMO 1999, chapter 12). Increasing greenhouse gas (GHG) concentrations could also contribute to severe ozone loss in, for example, the Arctic (Austin et al. 1992) if the chlorine loading were to remain as high as in the 1990s. This is due to the increased amounts of polar stratospheric clouds (PSCs), on the surface of which chemical reactions can take place. For the expected changes in GHG amounts and halogen loading Shindell et al. (1998) predicted large future ozone decreases for the years 2010-19. For the latter part of the twenty-first century, Waibel et al. (1999) predicted severe Arctic ozone loss, assuming temperature changes computed by Shindell et al. (1998). In their mechanism, sedimentation of PSC particles produced by the enhanced cooling reduced the gas-phase total reactive nitrogen, allowing chlorine chemistry to proceed faster.

With current computer resources and the quality of climate models currently avail- able, these processes cannot all be incorporated into a single model simulation covering continuously the period from, for example, 1980 to 2050. Consequently, a number of approximations are often made. Shindell et al. (1998) used a low-resolution climate model, and highly simplified chemical and ozone transport schemes. With these simplifi- cations they were able to complete a continuous simulation from 1959 to 2070. Another approach is to have a more accurate treatment of dynamics and chemistry but shorter integrations, providing snapshots of atmospheric behaviour. This approach was adopted in our previous work (Austin et al. 2000) by completing 1-year integrations of a general- circulation model (GCM) spread at 5-1 5-year intervals over the period 1979-2015 with the model ozone coupled to the radiation scheme. This had the advantage of including * Corresponding author: Met Office, London Road, Bracknell, Berkshire RG12 2SZ, UK. @ Crown copyright, 2001.

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960 J. AUSTIN et al.

most of the important processes, but excluding PSC particle sedimentation and volcanic- aerosol chemistry, at a much lower cost than the continuous 35-year calculation. The disadvantage, however, is that interannual variability of the model is large and that the chosen years may not be representative of the period in general. Also, each simulation has no memory of the previous simulation. In Arctic observations, interannual variability impedes the accurate determination of trends (Chipperfield and Jones 1999) while the fact that high ozone depletion has sometimes, but not always, been followed by low ozone depletion (SORG 1999, Fig. 1.7) suggests that there is little memory from one year to the next in the atmosphere. Therefore, one would expect similar characteristics in model simulations. For the past atmosphere, the results of Austin et al. (2000) agreed reasonably well with observations and, for the future, indicated that some delay in the onset of ozone recovery is likely. This occurred due to the effect of increasing GHG amounts cooling the stratosphere, in qualitative agreement with Shindell et al. (1998). However, this effect was somewhat smaller than in Shindell et al. (1998) and was com- parable to model interannual variability. Thus, the degree of simplification of the model used is likely to have a profound effect on the conclusions reached.

In this paper the behaviour of ozone over the period 2015-55 is investigated using the Met Office Unified Model coupled with a stratospheric-chemistry scheme. The model is integrated for 16 months starting 1 March in each of the years 2014, 2024, 2034, 2044 and 2054, with the first 4 months used as a spin-up period to reduce the impact of the initial conditions. The results are analysed to determine the way in which ozone may recover following the expected reduction in halogen amounts. Because of its importance for human health and the biosphere, the impacts of changes in GHGs and halogen amounts on surface ultraviolet (UV) amounts are also investigated.

2 . MODEL DESCRIPTION AND INTEGRATIONS PERFORMED

The model is identical to that used by Austin et al. (2000) and is the Met Office Unified Model with an upper boundary at 0.1 hPa. The model latitude-longitude resolution is 2.5" by 3.75" and there are 49 vertical levels. The model advects 15 tracers, and incorporates a stratospheric-chemistry subroutine which is coupled to the model radiation scheme via the predicted ozone. The chemical model operated between the tropopause and the lower mesosphere. Elsewhere, all the chemical constituents were advected passively, except for tropospheric ozone which was relaxed towards the initial conditions. At the ground, the constituents were kept fixed and equal to the initial conditions.

Five integrations of approximately 16 months, each starting 1 March, were per- formed with the fully coupled model for conditions appropriate to the periods from 2014/15 to 2054/55 at 10-year intervals. These cover the period of expected recovery in ozone. The results are supplemented with earlier results from the Austin et al. (2000) study for comparison. Meteorological initial conditions were taken from a 60-year in- tegration of a version of the model without chemistry (Butchart et al. 2000), extended by a few years to permit the final integration to be initialized. The concentrations of the well mixed GHGs carbon dioxide (C02), methane (CH4), nitrous oxide (N20) and the chlorofluorocarbons CFC 1 1 and CFC 12 were determined from the Intergovernmental Panel on Climate Change scenario IS92A (Houghton et al. (1996), Table 2.5; see also Butchart et al. (2000), Fig. 1). Sea surface temperatures and sea-ice amounts were taken from a Hadley Centre coupled atmosphere/ocean experiment 'HadCM2' (Mitchell et al. 1995; Johns et al. 1997; Mitchell and Johns 1997).

3D OZONE SIMULATIONS FOR 2015-55 96 1

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Figure 1 . Concentrations of the long-lived species carbon dioxide (COz), methane ( C h ) , total active chlorine ((Iy), total active bromine (Br,), and total odd nitrogen (NO,), as used in the model simulations. The values are relative to 1995 values for which the following concentrations were assumed: C02: 360 parts per million by volume (p.p.m.v.), CH4: 1.73 p.p.m.v., Cl,: 3.1 parts per billion (lo9) by volume (p.p.b.v.), Br,: 15.1 parts per trillion (10l2) by volume (p.p.t.v.), and NO,: 23.4 p.p.b.v. In the radiation scheme the concentrations of COz and CH4 are uniformly mixed throughout the atmosphere. Other values used in the chemistry, are shown at the equator

at the top model level, except for NO, for which the peak value is shown.

TABLE 1. LONG-LIVED TRACER AMOUNTS USED IN THE MODEL SIMULATIONS

Year 1979180 2014115 2024125 2034135 2044145 2054155

Chlorine (p.p.b.v.) 2.01 2.91 2.67 2.42 2.17 1.95 Bromine (p.p.t.v.) 9.8 17.4 15.7 13.9 12.1 10.4 Effective halogen (p.p.b.v.) 2.51 3.78 3.46 3.12 2.78 2.41 NO, (p.p.h.v.) 22.4 24.8 25.6 26.3 27.1 27.9

Parts per billion (lo9) by volume = p.p.b.v, parts per trillion (1OI2) by volume = p.p.t.v. Values are given at the equator for the top model level, except for total odd nitrogen (NO,), for which the peak value is given. Other altitudes and latitude values are scaled in proportion to the 1994/95 values. The effective halogen is given by (CI,) + 50.0 x (Br,).

The chemical initial conditions were taken from two-dimensional model results and adjusted according to the local distribution of equivalent latitude as described in Austin et al. (1997). Total chlorine and total bromine were based on values in WMO (1999), chapter 1 1 , although for consistency with Austin et al. (2000) the chlorine loading was about 10% lower and the bromine amount was about 10% higher. Thus, the effective halogen loading is about 5% lower than currently anticipated in the atmosphere giving an underprediction in the ozone loss of the order of 10%. Active nitrogen was made proportional to the value of N 2 0 used in the model radiation calculation. Concentrations of the key molecules and GHGs are indicated in Table 1 and Fig. 1 for the period 1975- 2055. By 2054/55, the chlorine, bromine and effective halogen loadings are projected to be very similar to those of 1979180 and hence comparisons between the model results for these years is of particular interest.

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Figure 2. Global annual-mean temperature (T) in the chemistry calculations (grey) compared with dynamics- only calculations (black) at the model levels 1 , 10,46 and 100 hPa. Y = year.

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3. ANALYSIS OF RESULTS: TEMPERATURE

(a ) Global average The systematic change in globally and annually averaged temperatures is clearly

apparent in Fig. 2, where results from the transient climate-change experiment (Butchart et al. 2000) are also included. The temperature trends in the dynamics-only and coupled- chemistry calculations are generally similar in the global average, particularly in the middle stratosphere. Differences are slightly larger in the upper stratosphere, where it might be expected that ozone change is more significant. At 46 hPa, the largest difference from the dynamics-only simulation is apparent, although this is smaller than previously reported for the period 1979-2015 (Austin et al. 2000). At 100 hPa the model warms slightly where a cooling has been observed over approximately the last 20 years. It is possible that water-vapour changes in the atmosphere may be responsible for a significant part of the cooling (Forster and Shine 1999) and that this is not properly represented in the GCM. Also, during the period 2009-54 ozone was approximately constant in the tropics (Fig. 6) following a period of significant decrease. Consequently, the change in the net ozone radiative heating was much smaller than in the satellite observational record.

3D OZONE SIMULATIONS FOR 2015-55 963

Figure 3. Left panels: minimum temperature north of 45"N at the 46 hPa level for the simulated northern winters. The shaded regions indicate the observed range in the Met Office data assimilation analyses for 1991192 to 1998/99 UARS (Upper Atmosphere Research Satellite) data (Swinbank and O'Neill 1994). Right panels: minimum temperature south of 45"s at the 46 hPa level for the simulated southern winters. The shaded regions indicate the observed range in the Met Office data assimilation analyses for 1992-99 UARS data. Ten-day running- mean temperatures are plotted. The upper panels give results from the simulations for 1994, 2014 and 2024, the

lower panels are for the years 2034,2044 and 2054.

Although in general the results suggest that the dynamics-only simulation may be sufficient to determine temperature trends, the time over which such trends are determined is an important factor for the coupled runs. With finer temporal resolution for the latter, it may have been possible to distinguish different types of behaviour for the different periods over which ozone decreased and increased in the model. The overall bias in ozone amounts (section 4) also results in the coupled model being systematically colder than the dynamics-only simulations (Fig. 2).

(6) Local minima in winterhpring The Arctic minimum temperatures north of 45"N at the 46 hPa level (Fig. 3, left

panels) were generally higher than was simulated for 1994/95 (Austin et al. 2000) and remained generally within the climatological envelope for the 1990s but were very variable from year to year. This is important for chemical ozone destruction due to PSCs and suggests that although ozone destruction is generally less likely to occur in the future, because of the downward trend in halogen amounts, occasional severe episodes cannot be discounted while chlorine levels still remain high. For the Antarctic (Fig. 3, right panels) model simulations for the years 2034 and 2044 were colder than 1994 for reasons investigated further in section 4(c).

964 .I. AUSTIN et al.

2034-35 2044-45 2054-55

Figure 4. Globally averaged total ozone in Dobson units (DU) simulated by the model in comparison with 1979/80 Total Ozone Mapping Spectrometer (TOMS), Stolarski (1993) data.

4. ANALYSIS OF RESULTS: OZONE

(a) Ozone global average The simulated global average ozone amounts (Fig. 4) were found to be higher than

for the earlier period simulated by Austin et al. (2000), indicating that some ozone recovery has taken place. The highest ozone occurred in 2044/45, but even then, ozone had still not returned to the levels simulated for 1979/80. Ozone recovery in the model was generally steady, but the 2054/55 results are again somewhat lower than in 1979/80, despite the halogen amounts having returned to the same level as observed in 1980.

(b) Ozone annual average Annual-mean high-latitude total-ozone amounts for the model are shown in Fig. 5

in comparison with Total Ozone Mapping Spectrometer (TOMS) observations. In the northern hemisphere, the model ozone is just over 10% lower than that observed and does not show as marked a decrease between 1980 and 2000. Thereafter, ozone does not change significantly until the year 2045 when an ozone increase occurred, only to be followed by a similar decrease in 2055. Part of the cause of the underprediction in ozone loss between 1980 and 2000 may be due to the addition of 5 K to the temperatures in the heterogeneous chemistry scheme, discussed by Austin et al. (2000), although the fact that the spring Arctic maxima are also lower than observed may indicate that there is an additional lack of transport into the polar regions.

For the southern hemisphere, the results (Fig. 5, right panel) are closer to the observations in 1980 and 2000. Thereafter, a general ozone recovery is predicted, but with occasional relatively low ozone values. By 2054/55 the results have returned to values similar to those simulated in 1979/80. The behaviour of the annual average is strongly affected by the ozone hole, as indicated in subsection (c).

3D OZONE SIMULATIONS FOR 2015-55 965

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Figure 5. Annual-mean total-ozone in Dobson units (DU) between 60"N and 90"N (left panel) and between 60"s and 90"s (right panel) for Total Ozone Mapping Spectrometer data (circles) and for the model calculations

(triangles).

1980 1990 2000 2010 2020 2030 2040 2050 1980 1990 2000 2010 2020 2030 2040 2050 Year Year

Figure 6. Annual-mean total-ozone in Dobson units (DU) between 30"N and 60"N (left panel) and between 30"s and 60"s (right panel) for Total Ozone Mapping Spectrometer data (circles) and for the model calculations

(triangles).

Annual-mean mid-latitude total-ozone amounts are shown in Fig. 6. For the northern hemisphere, the decrease between 1980 and 1995 was somewhat smaller than observed by TOMS, and the model is biased low, as at high latitudes. This could also indicate insufficient transport from the tropics or excessive chemical depletion, perhaps due to the high total odd nitrogen (NO,) values assumed in the model. From the year 2000, the results generally increase until 2045, but then a significant decrease is predicted for 2054/55, reflecting the results obtained for the north polar region. In the southern hemisphere, mid-latitude ozone also increased generally from the year 2000 but relatively low values were obtained in the years 2034/35 and 2044/45, reflecting the behaviour of the ozone hole (section 4(c)). By 2054/55 the ozone results had returned to those simulated for 1979180.

Figure 7 shows the modelled annual-mean ozone in the latitude range 30"N to 30"s. The reduction in model ozone over the period 1980-2010 has been identified as being due to chlorine chemistry (Austin et al. 2000). Although the model results are lower than observed, they are approximately constant over the period 20 10-55, with the possibility of some further loss towards the end of the period.

966 J. AUSTIN et al.

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240 1980 1990 2000 2010 2020 2030 2040 2050

Year

Figure 7. Annual-mean total ozone in Dobson units (DU) between 30"N and 30"s for Total Ozone Mapping Spectrometer data (circles) and for the model calculations (triangles).

TABLE 2. MINIMUM OZONE (DOBSON UNITS) IN THE MODEL

Northern hemisphere Southern hemisphere Year March April September October

SIMULATIONS

1979180 265 295 2 191230 1901225 1994195 244 276 174 132 2014115 244 279 183 173 2024125 264 279 194 189 2034135 247 287 180 144 2044145 270 286 203 142 2054155 259 282 208 217

The tabulated values are minimum daily values attained through- out the area polewards of 60" in the specified months.

(c) Spring ozone amounts The local spring minima (Table 2) reveal quite different behaviour to the global

average, with the north indicating possible recovery after about 20 14/15. However, the occurrence and timing of stratospheric warnings (including the final warming) leads to a high degree of interannual variability in the amount of chemical ozone depletion, which makes an assessment of future trends in the Arctic very uncertain. The pattern of warnings has a major impact on the chemical transport and the duration of PSCs. Figure 8, for example, shows the calculated surface area density of PSCs averaged over the Arctic winter for each year of the dynamical model simulation of Butchart et al. (2000) together with values from the coupled chemistry-climate model simulations (this work). Note that in these calculations, 5 K has been added to the coupled chemistry-climate model temperatures for consistency with the adjustment in the PSC scheme, whereas the results for the dynamical model use the unadjusted temperatures, as diagnosed in Butchart et al. (2000). In principle, this diagnostic reflects only the chemical impact on ozone, but transport is just as important (Chipperfield and Jones 1999). However, cold winters are usually associated with both lower ozone transport and higher chemical depletion and hence Fig. 8 provides a realistic qualitative indication of model interannual variability in the ozone field.

In the southern spring, model values are given in Table 2 for both 1979 and 1980 (Austin et al. 2000). By 2054, ozone has recovered almost to the modelled 1980 levels, but low ozone occurred in the simulations for 2034 and 2044. These results are shown

3D OZONE SIMULATIONS FOR 2015-55 967

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Figure 8. Surface area density (SAD) averaged over the Arctic winter (p~m~cm-~hemisphere days). Crosses show values from the 60-year simulation without chemistry of Butchart et al. (2000)-their run 'A'. This assumes realistic changes in the greenhouse gases, but contains fixed ozone amounts. Results computed from the coupled

chemistry4imate simulations of this work are shown as the filled circles.

1980 1990 2000 2010 2020 2030 2040 2050 Year Year

Figure 9. Minimum ozone in Dobson units (DU) for each ozone-hole season (left panel). Maximum area within the 220 DU contour (right panel). The circles indicate observations from the Total Ozone Mapping Spectrometer,

the triangles are the model results.

in Fig. 9 together with observations from TOMS for the period 1979-97. Apart from the expected recovery in Antarctic ozone by 2054, the most striking feature is the reappearance of the ozone hole in the 2034 and 2044 simulations. This is in marked contrast to other calculations (e.g. Shindell et al. 1998; WMO 1999, Chapter 12), which suggest a broadly monotonic recovery in Antarctic ozone.

In the lower stratosphere, the model zonal-mean temperatures at high latitudes decreased significantly as the GHG concentrations increased (Fig. 10). At 31.6 hPa, with typical water-vapour concentrations of 4.5 parts per million by volume (p.p.m.v.),

968 J. AUSTIN et al.

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the period between June and October, covering the winterkpring period for the years 1994 to 2054.

the ice PSC thermodynamic equilibrium temperature is 185.3 K. However, PSCs are not permitted to form in the model until 5 K below this temperature (Austin et al. 2000) because of model temperature bias and the likely barrier to nucleation at the frost point (Tabazadeh et al. 1997). Consequently, the presence of ice PSCs in the model at this level is approximately indicated by the 180 K isotherm, which increases substantially in extent in the simulations for 2024 and 2034 (Fig. 10, shaded region), and decreases in extent in 2054.

In the model simulations, PSC amounts (Fig. 11) increase markedly at the time of the deepest ozone hole (Fig. 9), in the year 1999. Following this, the reduction in chlorine (Fig. 1) causes less ozone depletion and because of radiative feedbacks, considerably fewer PSCs occur in the year 2014. Thereafter, the increases in GHG concentrations trigger additional PSCs and further ozone loss, despite the gradual reduction in chlorine. Also included in Fig. 11 is the product a! = Cl; x ICE, where C1, is the total reactive chlorine, scaled to 1995 values (see Fig. 11 caption for details). The term Cl; is representative of the ozone destruction rate for fixed PSCs (Solomon 1988) with C1, taken from Fig. 1. ICE takes into account the mean effect of the PSCs during the course of the spring season. The similarity to the right panel in Fig. 9 is striking. In particular, a double peak is reproduced and although the model simulates substantial quantities of PSCs in 2054, a! is similar in value to that in 1979, showing the overriding importance of the lower values of Cl,. The hypothesis that the rapid increase in ice is responsible for the low ozone after 2024 was investigated by repeating the 2034 simulation but with the amount of ice PSCs one half of the thermodynamic equilibrium value (including the 5 K temperature offset). Because of the coupling between the dynamics and chemistry, the simulated winterfspring period was slightly different to that previously obtained, with typically just a 40% reduction in ice PSCs obtained rather than the 50% reduction

3D OZONE SIMULATIONS FOR 2015-55

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0.0 1980 2000 2020 2040 2060

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Figure 11. Pressure-weighted averages of water vapour condensing as ice particles (ICE) and condensing as nitric acid trihydrate particles (NAT) simulated by the model above 46 hPa. The values are averaged over the model domain south of 60"s and for the spring period August, September and October. The solid curve is the quantity a = Cl; x ICE, where C1, is the total reactive chlorine, which qualitatively represents the major ozone destruction term. All quantities have been rescaled relative to 1995 values and for the PSCs are proportional to

the area of surfaces available for reaction.

anticipated. Nonetheless, this increased the minimum from 142 to 171 Dobson units (DU) and the maximum area of the 220 DU total-ozone contour decreased from 15.1 to 10.8 x lo6 km2, confirming the substantial impact that changing the ice concentration has had on ozone.

5 . CLEAR-SKY SURFACE ULTRAVIOLET LEVELS

Ultraviolet amounts were predicted for clear skies using the UV model of Austin et al. (1999) assuming the modelled total-ozone amounts. Rather than providing details of the complete UV spectrum, the calculations were weighted according to the action spectrum for erythema (sunburn). Results are shown in Fig. 12 for mean UV levels in the latitude bands corresponding to high latitudes, mid latitudes and the tropics-see Fig. 12 caption for an explanation of the two quantities calculated. In the northern high latitudes the summer-mean results (upper left panel) decreased over the period 20 14-55 and for 2044/45 indicate lower UV in some months than in the 1979/80 simulation of Austin et al. (2000). In northern mid latitudes UV levels in the mean are still slightly higher than in 1979180 except for April and June in the year 2045. In the tropics the mean values are very similar and are typically 15 mW m-2 higher than in 1979/80.

Extreme levels of UV within each latitude band, relative to the zonal average, i.e. UV - UV(8), where an overbar denotes the zonal average for latitude 8, generally occur in mid-latitude summer (Fig. 12, right-hand panels). Here the solar zenith angle is relatively small while ozone variations are generally larger than in lower latitudes. For example, in the northern hemisphere, the maximum deviation from the zonal average can exceed 80 mW mP2, approximately half the zonal average UV level. In high and mid southern latitudes the recovery of Antarctic ozone is indicated by generally smaller UV differences than were calculated by Austin et al. (2000) for the years 1994-2014. However, the deeper ozone hole in 2034 goes against this general trend in giving rise to high UV levels.

Clear-sky UV calculations have also recently been made by Taalas et al. (2000). Ozone calculations from Austin et al. (2000) and the current work (2044/45 case) were

970 J. AUSTIN et al.

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Figure 12. Clear-sky erythemally weighted ultraviolet (UV) (mW m-*) at noon in the simulations, as a function of month. The left-hand panels indicate the differences between the monthly and regionally averaged W from the results for 1979/80 obtained by Austin et al. (2000). The right-hand panels give the maximum deviation from the daily zonal average attained during each month, within the specified latitude band. The panels are labelled

with the latitude range. Note also the different scales on the ordinates of the graphs.

3D OZONE SIMULATIONS FOR 2015-55 97 1

supplied for this purpose, as well as results from other three-dimensional (3D) model simulations, including that of Shindell et al. (2000). Although Taalas et al. use a more detailed UV model, only a limited set of results are shown, primarily monthly averages for the months April and October averaged over just the polar regions. Bearing in mind the differences in model assumptions (GHGs, halogen amounts etc.) all the model results are broadly in agreement and show no significant trends during the period from 1980 to 2050. However, the Taalas et al. results include no results between about 2015 and 2045. Further, Taalas et al. concentrates on the Shindell et al. (1998) calculations in showing very large local Arctic UV changes (consistent with the ozone changes) but this is not supported by any other model simulation included therein.

6. DISCUSSION AND CONCLUSION

This work has extended previous simulations of the future state of the ozone layer using 1-year integrations of a 3D coupled chemistry-climate model. The most dramatic result was a temporary worsening of the Antarctic ozone hole in the 2034 and 2044 simulations following several periods of ozone recovery. Although previous calculations of the Antarctic ozone hole (Shindell et al. 1998) have recognized the interaction between increased GHGs and increased ozone depletion, they did not explicitly allow for the impact of PSC amount and type on ozone abundance. Consequently they simulated near-monotonic behaviour in Antarctic ozone. In contrast, the results here suggest that the formation of additional ice particles as GHG concentrations increase might slow down the ozone recovery that is expected from decreases in halogen amounts.

In the annual average, for the Arctic and northern mid latitudes, no significant changes in modelled ozone could be discerned above the likely interannual variability on time-scales out to 20.55. For the spring, Waibel et al. (1999) showed that PSC sedimentation (a process absent from our simulations) could increase ozone depletion in the second half of the twenty-first century. However, the temperature trend that

,. they imposed, taken from the Shindell et al. (1998) simulations, was quite substantial, and was at least a factor of three larger than indicated in our GCM (with chemistry). Therefore, the differences between temperature trends of different GCMs need to be resolved. One of the major issues is that internal model variability itself can lead to differences in high-latitude temperature trends predicted by the same model (Butchart et al. 2000). The connection between ozone and temperature is clearly apparent in our results as would be expected in a coupled system. Thus, for example, the trends simulated in the lower stratosphere for the period 1980-2015 were considerably larger than for the period 201 5-2055 reflecting the stabilization of ozone after 2015. However, apart from during Antarctic spring, the model results do not give a clear indication of an ozone increase by the year 2055, suggesting that GHG increases contribute at least a little to ozone losses but perhaps not as much in the Arctic as indicated in previous studies (e.g. Austin et al. 1992; Shindell et al. 1998).

The predictions for clear-sky UV levels depend directly on the predicted ozone. In particular, although UV levels are predicted to decrease from the peak levels indicated in Austin et al. (1 999), recovery to 1979/80 levels are not expected by the year 20.54L5.5, except for the Antarctic region. Also in Antarctica, UV does not reduce monotonically with time, and in accordance with the ozone values shows peak values in 2034 larger than those predicted for the year 2014.

Remaining uncertainties in the model simulations presented here, such as the neglect of PSC particle sedimentation and the neglect of water-vapour changes prevent a

972 J . AUSTIN eta!.

clear prediction of the timing of the deepening phase of the ozone hole noted in the simulations for 2034 and 2044. Also, despite many years of research the precise details of the PSC particles remain unclear (Tabazadeh and Toon 1996; Carslaw and Peter 1997; Carslaw et al. 1997; Bertram and Sloan 1998). Further, atmospheric interannual variability can tend to obscure systematic changes in both model results and the atmosphere. However, in the Antarctic ozone hole, this variability appears to be smaller than the trends due to GHG and halogen changes. For example, periods of high temperatures in high latitudes occurred in the simulations for 2024 and 2054 during October (Fig. 5 ) which would have tended to shorten the period of ozone depletion and would have increased the transport of ozone into high latitudes. Performing a linear regression analysis of the minimum total ozone over the period 1979-97, yields a root-mean-square (r.m.s.> deviation from the secular trend of 14 DU in the TOMS data and 15 DU in the model results. This is much smaller than the 65 DU change in modelled minimum ozone between 2024 and 2034. The area of the ozone hole increases approximately steadily for the period 1979-92 after which it increases less rapidly. Hence, the interannual variability may be estimated as the r.m.s. deviation from the linear trend up until 1992, which is about 3 x lo6 km2, a factor of two smaller than the difference in the model results between 2024 and 2034. However, once halogen levels drop to those present in about the middle 1980s, we can expect the interannual variability to increase again because of the ozone transport and or chemical effects of the quasi- biennial oscillation which are likely to be significant under moderate levels of chlorine (Garcia and Solomon 1987; Lait et al. 1989; Butchart and Austin 1996; Shindell et al. 1999). All these effects combined may delay by perhaps 20 years the year at which ozone recovery can be said to be starting (Hoffman 1996; Hoffman et al. 1997). Further, such an ozone recovery may take the form of that published elsewhere (Shindell et al. 1998) of a near-monotonic ozone change with an interannual variability imposed, but with a magnitude perhaps larger than in Shindell et al. Alternatively, our results suggest that the ozone recovery may be highly nonlinear in time as indicated in Fig. 9.

While the reduction in halogen loading is itself uncertain, a secondary impact on ozone recovery occurs through the reaction CH4 + C1+ CH3 (methyl radical) + HCl (hydrochloric acid) which reduces active chlorine. Thus, errors in the anticipated increase in CHq (e.g. Dlugokencky et al. 1998) could slow down the rate of ozone recovery from that indicated here. Changes in other long-lived species (N20, water) also have an impact, although this is generally smaller than the halogen effect.

Finally, GCM studies with chemistry are still fairly new and have a long way to go before the results may be considered completely reliable. It certainly should not be concluded that ozone loss is a ‘solved problem’, until the interactions with GHG increases and the effects of PSC particle sedimentation are better understood.

ACKNOWLEDGEMENTS

This work was supported by the UK Department of Environment, Transport and the Regions (contract number EPG/1/1/83), and the Public Meteorological Service Research and Development Programme. The Met Office data assimilation temperatures were kindly supplied by Penny Connew. We thank Keith Shine and the two referees for their suggestions for improvements to the paper.

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