Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.3.2.2 Stratospheric Ozone ====

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Ninety percent of the total column ozone (TCO) resides in the stratosphere. The chemical lifetime of ozone in the stratosphere ranges from less than a day in the upper stratosphere to several months in the lower stratosphere ( [[#Bekki--2009|Bekki and Lefevre, 2009]] ). Global stratospheric ozone trends based on observations are assessed in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.5.2|Section 2.2.5.2]] ). The CMIP6 model ensemble shows that global TCO has slightly changed from 1850–1960 ( [[#Keeble--2021|Keeble et al., 2021]] ). The rapid decline in the 1970s and 1980s due to halogenated ozone-depleting substances (ODSs, as assessed in [[IPCC:Wg1:Chapter:Chapter-2#2.2.5.2|Section 2.2.5.2]] from observations) until the end of the 1990s, followed by a slight increase since then, is captured by the models ( [[#Keeble--2021|Keeble et al., 2021]] ). Overall, the observed climatology patterns and annual cycle amplitudes are well represented in the CMIP6 ensemble mean. The CMIP6 ensemble overestimates the observed TCO values by up to 6% (10–20 Dobson Units (DU)) globally in the NH and SH mid-latitudes, and in the tropics, but the trend in these regions is well captured between 1960 and 2014. However, there is poor agreement between the individual CMIP6 models in the pre-industrial period and throughout the historical period, with model TCO values spread across a range of about 60 DU. The global stratospheric ozone column decreased by 14.3 ± 8.7 DU from 1850–2014 ( [[#Keeble--2021|Keeble et al., 2021]] ).

Model simulations attribute about half of the observed upper-stratospheric ozone increase after 2000 to the decline of ODS since the late 1990s while the other half of the ozone increase is attributed to the slowing of gas-phase ozone destruction cycles due to cooling of the upper stratosphere by increasing GHGs ( [[#Aschmann--2014|Aschmann et al., 2014]] ; [[#Oberländer-Hayn--2015|Oberländer-Hayn et al., 2015]] ).

In summary, global stratospheric ozone column has decreased from pre-industrial period to present day in response to the ODS-induced ozone rapid decline in the 1970s and 1980s, followed by slow, and still incomplete, recovery. There is ''medium confidence'' that global stratospheric ozone column has changed by 14.3 ± 8.7 DU between 1850 and 2014.

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