Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.2.5.2 Stratospheric Ozone ====

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The AR5 assessed that it was certain that global stratospheric ozone from the mid-1990s to 2011 was nearly constant and about 3.5% lower than in the reference period 1964–1980. Most of the declines occurred prior to the mid-1990s.

Global annual mean total ozone (Figure 2.7) significantly declined by about 3.5% during the 1980s and the early 1990s and by 2.5% over 60°S–60°N (near-global). Then, during 2000–2017, both global and near-global concentrations increased slightly, but not significantly, all in line with production and consumption limits of ODS regulated under the Montreal Protocol and its amendments. Near-global 2014–2017 mean total ozone is about 2.2% below the pre-ozone depletion 1964–1980 average ( [[#Braesicke--2018|Braesicke et al., 2018]] ). At southern and northern mid-latitudes, declines are 5.5% and 3.0% compared to the 1964–1980 average respectively. Total ozone remained practically unchanged in the tropics ( [[#Braesicke--2018|Braesicke et al., 2018]] ). Emission of ODS started before 1980 and some estimates suggest that as much as 40% of the long-term ozone loss occurred between 1960 and 1980 ( [[#Shepherd--2014|Shepherd et al., 2014]] ), lowering the 1964–1980 baseline values by about 1% (outside the polar regions), a value close to observational uncertainties. The world’s longest record of total ozone measurements from Arosa, Switzerland, initiated in 1926, does not show any substantial long-term changes before about 1980 ( [[#Staehelin--2018|Staehelin et al., 2018]] ).

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'''Figure''' '''2.7 |''' '''Time series of annual mean total column ozone from 1964–2019.''' Values are in Dobson Units (DU), a good proxy for vertically integrated stratospheric ozone. Time series are shown for '''(a)''' near-global domain; '''(b–d)''' three zonal bands; and '''(e)''' polar (60°–90°) total ozone in March (Northern Hemisphere) and October (Southern Hemisphere): the months when polar ozone losses usually are largest. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

ERF depends strongly on the altitude of ozone changes. Two stratospheric regions are mainly responsible for long-term changes outside the polar regions. In the upper stratosphere (35–45 km), there was a strong decline (about 10%) from the start of observations in 1979 up to the mid-1990s and a subsequent increase by about 4% to present (SPARC/IO3C/GAW, 2019). In the lower stratosphere (20–25 km), there also was a statistically significant decline (7–8%) up to the mid-1990s, followed by stabilization or a small further decline ( [[#Ball--2018|Ball et al., 2018]] , 2019), although the natural variability is too strong to make a conclusive statement ( [[#Chipperfield--2018|Chipperfield et al., 2018]] ).

The strongest ozone loss in the stratosphere continues to occur in austral spring over Antarctica (ozone hole) with emergent signs of recovery after 2000 ( [[#Langematz--2018|Langematz et al., 2018]] ). Interannual variability in polar stratospheric ozone is driven by large scale winds and temperatures, and, to a lesser extent, by the stratospheric aerosol loading and the solar cycle. This variability is particularly large in the Arctic, where the largest depletion events, comparable to a typical event in the Antarctic, occurred in 2011 ( [[#Manney--2011|Manney et al., 2011]] ; [[#Langematz--2018|Langematz et al., 2018]] ) and again in 2020 ( [[#Manney--2020|Manney et al., 2020]] ; [[#Grooß--2021|Grooß and Müller, 2021]] ). Further details on trends and ERF can be found in Sections 6.3.2 and 7.3.2.5.

In summary, compared to the 1964–1980 average, stratospheric ozone columns outside polar regions (60°S–60°N) declined by about 2.5% over 1980–1995, and stabilized after 2000, with 2.2% lower values in 2014–2017. Large ozone depletions continue to appear in spring in the Antarctic and, in particularly cold years, also in the Arctic. Model-based estimates disagree on the sign of the ERF due to stratospheric ozone changes, but agree that it is much smaller in magnitude than that due to tropospheric ozone changes (Section 7.3.2.5).

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