Estimates of ozone return dates from Chemistry-Climate Model Initiative simulations
2018
>We analyse simulations performed for the Chemistry-Climate Model Initiative (CCMI) to estimate the return dates of the
stratospheric
ozone layerfrom depletion caused by anthropogenic
stratosphericchlorine 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
ozonerecovery. 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
ozonecolumn. 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
ozonereturn 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
ozonewill return to 1980 values in 2049 (with a 1 σ uncertainty of 2043–2055). At Southern Hemisphere mid-latitudes column
ozoneis 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
OzoneAssessment 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
ozoneto 1980 values, around 2040, while the other half do not reach the 1980 value. All models show a negative trend in tropical total column
ozonetowards the end of the 21st century. The CCMI models generally agree in their simulation of the time evolution of
stratosphericchlorine and bromine, which are the main drivers of
ozoneloss and recovery. However, there are a few outliers which show that the multi-model mean results for
ozonerecovery are not as tightly constrained as possible. Throughout the
stratospherethe spread of
ozonereturn 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
ozoneand chlorine, especially for the large Antarctic depletion. Comparisons of total column
ozonebetween the models is affected by different predictions of the evolution of
tropospheric ozonewithin the same scenario, presumably due to differing treatment of tropospheric chemistry. Therefore, for many scenarios, clear conclusions can only be drawn for
stratospheric
ozonecolumns rather than the total column. As noted by previous studies, the timing of
ozonerecovery is affected by the evolution of N 2 O and CH 4 . However, quantifying the effect in the simulations analysed here is limited by the few realisations available for these experiments compared to
internal modelvariability. The large increase in N 2 O given in RCP 6.0 extends the
ozonereturn globally by ∼ 15 years relative to N 2 O fixed at 1960 abundances, mainly because it allows tropical column
ozoneto be depleted. The effect in extratropical latitudes is much smaller. The large increase in CH 4 given in the RCP 8.5 scenario compared to RCP 6.0 also lengthens
ozonereturn by ∼ 15 years, again mainly through its impact in the tropics. Overall, our estimates of
ozonereturn 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
ozonerecovery. Efforts need to continue to ensure that models used for assessment purposes accurately represent
stratosphericchemistry and the prescribed scenarios of
ozone-depletingsubstances, and only those models are used to calculate return dates. For future assessments of single forcing or combined effects of CO 2 , CH 4 , and N 2 O on the
stratosphericcolumn
ozonereturn 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.
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