Estimates of ozone return dates from Chemistry-Climate Model Initiative simulations

Abstract

Abstract. >We analyse simulations performed for the Chemistry-Climate Model Initiative (CCMI) to estimate the return dates of the stratospheric ozone layer from depletion caused by anthropogenic stratospheric chlorine and bromine. We consider a total of 155 simulations from 20 models, including a range of sensitivity studies which examine the impact of climate change on ozone recovery. For the control simulations (unconstrained by nudging towards analysed meteorology) there is a large spread (±20 DU in the global average) in the predictions of the absolute ozone column. Therefore, the model results need to be adjusted for biases against historical data. Also, the interannual variability in the model results need to be smoothed in order to provide a reasonably narrow estimate of the range of ozone return dates. Consistent with previous studies, but here for a Representative Concentration Pathway (RCP) of 6.0, these new CCMI simulations project that global total column ozone will return to 1980 values in 2049 (with a 1σ uncertainty of 2043–2055). At Southern Hemisphere mid-latitudes column ozone is projected to return to 1980 values in 2045 (2039–2050), and at Northern Hemisphere mid-latitudes in 2032 (2020–2044). In the polar regions, the return dates are 2060 (2055–2066) in the Antarctic in October and 2034 (2025–2043) in the Arctic in March. The earlier return dates in the Northern Hemisphere reflect the larger sensitivity to dynamical changes. Our estimates of return dates are later than those presented in the 2014 Ozone Assessment by approximately 5–17 years, depending on the region, with the previous best estimates often falling outside of our uncertainty range. In the tropics only around half the models predict a return of ozone to 1980 values, around 2040, while the other half do not reach the 1980 value. All models show a negative trend in tropical total column ozone towards the end of the 21st century. The CCMI models generally agree in their simulation of the time evolution of stratospheric chlorine and bromine, which are the main drivers of ozone loss and recovery. However, there are a few outliers which show that the multi-model mean results for ozone recovery are not as tightly constrained as possible. Throughout the stratosphere the spread of ozone return dates to 1980 values between models tends to correlate with the spread of the return of inorganic chlorine to 1980 values. In the upper stratosphere, greenhouse gas-induced cooling speeds up the return by about 10–20 years. In the lower stratosphere, and for the column, there is a more direct link in the timing of the return dates of ozone and chlorine, especially for the large Antarctic depletion. Comparisons of total column ozone between the models is affected by different predictions of the evolution of tropospheric ozone within the same scenario, presumably due to differing treatment of tropospheric chemistry. Therefore, for many scenarios, clear conclusions can only be drawn for stratospheric ozone columns rather than the total column. As noted by previous studies, the timing of ozone recovery is affected by the evolution of N2O and CH4. However, quantifying the effect in the simulations analysed here is limited by the few realisations available for these experiments compared to internal model variability. The large increase in N2O given in RCP 6.0 extends the ozone return globally by ∼ 15 years relative to N2O fixed at 1960 abundances, mainly because it allows tropical column ozone to be depleted. The effect in extratropical latitudes is much smaller. The large increase in CH4 given in the RCP 8.5 scenario compared to RCP 6.0 also lengthens ozone return by ∼ 15 years, again mainly through its impact in the tropics. Overall, our estimates of ozone return dates are uncertain due to both uncertainties in future scenarios, in particular those of greenhouse gases, and uncertainties in models. The scenario uncertainty is small in the short term but increases with time, and becomes large by the end of the century. There are still some model–model differences related to well-known processes which affect ozone recovery. Efforts need to continue to ensure that models used for assessment purposes accurately represent stratospheric chemistry and the prescribed scenarios of ozone-depleting substances, and only those models are used to calculate return dates. For future assessments of single forcing or combined effects of CO2, CH4, and N2O on the stratospheric column ozone return dates, this work suggests that it is more important to have multi-member (at least three) ensembles for each scenario from every established participating model, rather than a large number of individual models.

The Impact of Continuing CFC‐11 Emissions on Stratospheric Ozone

“…However, compared to the large impact due to chlorine, the ozone sensitivity has a relatively small range among the RCP scenarios in Figure b and Table . Relatively weak Antarctic spring ozone sensitivity to GHG loading was also seen in analysis of the Chemistry‐Climate Model Initiative (CCMI) models (Dhomse et al, ).…”

“…For the REF-C2 simulation, the return date in GSFC2D (2046) is delayed compared with GEOSCCM (2032, after smoothing to remove interannual variability). Although GEOSCCM and GSFC2D show a stronger decline and stronger recovery compared with the CCMI MMM, the GSFC2D recovery date of 2046 is within the uncertainty range of the CCMI models (see Dhomse et al, 2018). Note that because of the different future ODS scenarios and inclusion of stratospheric aerosol and solar cycle variations in REF-C2, the GSFC2D return dates for global and Antarctic spring total ozone cited in this appendix are several years earlier than those listed in Table 1 and shown in Figure 2.…”

“…GSFC2D also captures well the decline and recovery of Antarctic spring total and stratospheric column ozone simulated by GEOSCCM ( Figure A3), although both models show a somewhat stronger decline and recovery than the CCMI MMM. For the REF-C2 simulation, the recovery to 1980 total ozone levels occurs in 2062 for both GSFC2D and GEOSCCM, and the CCMI MMM (Dhomse et al, 2018). Note that since the time dependence of Antarctic spring ozone is dominated by the impact of chlorine in the stratosphere, the GSFC2D low bias in tropospheric ozone has a minimal impact on Antarctic total column ozone in Figure A3a.…”

“…The GEOSCCM has a comprehensive tropospheric and stratospheric chemical mechanism (Oman et al, 2016) and performed well in both chemical-and transport-related process evaluations (Douglass et al, 2012(Douglass et al, , 2014Eyring et al, 2006;SPARC CCMVal, 2010;Strahan et al, 2011). For reference, we also show total and stratospheric column ozone comparisons with the SPARC CCMI multimodel mean (MMM) (Dhomse et al, 2018) and observational ozone data where available. GSFC2D has complete stratospheric chemistry but contains a limited subset of tropospheric species (Fleming et al, 2011;Jackman et al, 2016).…”

Recent Trends in Stratospheric Chlorine From Very Short‐Lived Substances

“…It is predominately caused by chlorine and bromine radicals released from long-lived halocarbons, such as chlorofluorocarbons (CFCs) and halons, whose production is now controlled by the UN Montreal Protocol and its amendments (e.g., WMO, 2014). Owing to the Protocol's ongoing success, the atmospheric abundances of total chlorine and bromine are declining (e.g., Carpenter et al, 2014;Froidevaux et al, 2006;Montzka et al, 2003) and the ozone layer is projected to return to pre-1980s levels in the middle to latter half of this century in consequence (e.g., Chipperfield et al, 2017;Dhomse et al, 2018;Eyring et al, 2010;Solomon et al, 2016).…”