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

=== 2.2.5 Other Short-lived Gases ===

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==== 2.2.5.1 Stratospheric Water Vapour ====

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The AR5 assessed ''low confidence'' in stratospheric water vapour (SWV) trends based on substantial seasonal and interannual variability in satellite data from 1992 to 2011. The 1980–2010 record of balloon-borne frost point hygrometer measurements over Boulder, Colorado (40°N), showed an average net increase of 1.0 ± 0.2 ppm (27 ± 6%) in the 16–26 km layer.

Since AR5, bias-adjusted spatially comprehensive SWV measurements by different satellite sensors were merged to form continuous records ( [[#Hegglin--2014|Hegglin et al., 2014]] ; [[#Froidevaux--2015|Froidevaux et al., 2015]] ; [[#Davis--2016|Davis et al., 2016]] ). These indicate no net global increase of SWV in the lower stratosphere since the late 1980s. [[#Hegglin--2014|Hegglin et al. (2014)]] reported a latitudinal dependence of SWV trends and suggested that the upward trend over Boulder should not be considered representative of the global stratosphere, while [[#Lossow--2018|Lossow et al. (2018)]] showed insignificant differences between SWV trends at Boulder and those for the 35–45°N zonal mean from 1980 to 2010 using model simulations and satellite observations.

Recent studies of dynamical influences on SWV ( [[#Eguchi--2015|Eguchi et al., 2015]] ; [[#Evan--2015|Evan et al., 2015]] ; [[#Tao--2015|Tao et al., 2015]] ; [[#Konopka--2016|Konopka et al., 2016]] ; [[#Diallo--2018|Diallo et al., 2018]] ; [[#Garfinkel--2018|Garfinkel et al., 2018]] ) have demonstrated that the quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), Sudden Stratospheric Warming (SSW) events and possibly also Pacific Decadal Variability (PDV; W. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ), can significantly influence SWV abundance and the tropical cold point tropopause temperatures that largely control water vapour entering the stratosphere. It has also been shown that the convective lofting of ice can moisten the lower stratosphere over large regions ( [[#Dessler--2016|Dessler et al., 2016]] ; [[#Anderson--2017|Anderson et al., 2017]] ; [[#Avery--2017|Avery et al., 2017]] ). Near-global observations of SWV have revealed unusually strong and abrupt interannual changes, especially in the tropical lower stratosphere. Between December 2015 and November 2016, the tropical mean SWV anomaly at 82 hPa dropped from 0.9 ± 0.1 ppm to –1.0 ± 0.1 ppm, accompanied by highly anomalous QBO-related dynamics in the tropical stratosphere ( [[#Newman--2016|]] [[#Newman--2016|P.A. Newman et al., 2016]] ; [[#Tweedy--2017|Tweedy et al., 2017]] ) and the transition of ENSO from strong El Niño to La Niña conditions ( [[#Davis--2017|Davis et al., 2017]] ). The tropical mean SWV anomaly then rose sharply to 0.7 ± 0.1 ppm in June 2017 as warm westerlies returned to the tropical lower stratosphere and ENSO neutral conditions prevailed ( [[#Davis--2017|Davis et al., 2017]] ).

In summary, in situ measurements at a single mid-latitude location indicate about a 25% net increase in stratospheric water vapour since 1980, while merged satellite data records since the late 1980s suggest little net change. Recent studies of dynamical influences on SWV have highlighted their substantial roles in driving large interannual variability that complicates trend detection. There thus continues to be ''low confidence'' in trends of SWV over the instrumental period. Disregarding dynamic influences on SWV, an ERF of 0.05 ± 0.05 W m <sup>–2</sup> is estimated for SWV produced by CH <sub>4</sub> oxidation (Section 7.3.2.6), unchanged from AR5.

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==== 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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[[File:51f32fe55a5202814a50fe02a6e8fc7a IPCC_AR6_WGI_Figure_2_7.png]]

'''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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==== 2.2.5.3 Tropospheric Ozone ====

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The AR5 assessed ''medium confidence'' in large-scale increases of tropospheric ozone at rural surface sites across the NH (1970–2010), and in a doubling of European surface ozone during the 20th century, with the increases of surface ozone in the SH being of ''low confidence'' . Surface ozone ''likely'' increased in East Asia, but levelled off or decreased in the eastern USA and western Europe. Free tropospheric trends (1971–2010) from ozonesondes and aircraft showed positive trends in most, but not all, assessed regions, and for most seasons and altitudes. This section focuses on large scale ozone changes; chemical and physical processes and regional changes in tropospheric ozone are assessed in Section 6.3.2.1 and Section 7.3.2.5 assesses radiative forcing.

Prior to 1850 ozone observations do not exist, but a recent analysis using clumped-isotope composition of molecular oxygen ( <sup>18</sup> O <sup>18</sup> O in O <sub>2</sub> ) trapped in polar firn and ice, combined with atmospheric chemistry model simulations, constrains the global tropospheric ozone increase to less than 40% between 1850 and 2005, with most of this increase occurring between 1950 and 1980 ( [[#Yeung--2019|Yeung et al., 2019]] ). Recently, the Tropospheric Ozone Assessment Report identified and evaluated 60 records of surface ozone observations collected at rural locations worldwide between 1896 and 1975, which were based on a range of measurement techniques with potentially large uncertainties ( [[#Tarasick--2019|Tarasick et al., 2019]] ). They found that from the mid-20th century (1930s to the early 1970s) to 1990–2014, rural surface ozone increased by 30–70% across the northern extra-tropics. This is smaller than the 100% 20th-century increase reported in AR5, which relied on far fewer measurement sites, all in Europe. In the northern tropics limited low-elevation historical data (1954–1975) provide no clear indication of surface ozone increases ( [[#Tarasick--2019|Tarasick et al., 2019]] ). However, similar to the northern mid-latitude increases, lower-free tropospheric ozone at Mauna Loa, Hawaii increased by approximately 50% from the late 1950s to present ( [[#Cooper--2020|Cooper et al., 2020]] ). Historical observations are too limited to draw conclusions on surface ozone trends in the SH tropics and mid-latitudes since the mid-20th century, with tropospheric ozone exhibiting little change across Antarctica ( [[#Tarasick--2019|Tarasick et al., 2019]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Based on reliable UV absorption measurements at remote locations (surface and lower troposphere), ozone trends since the mid-1990s varied spatially at northern mid-latitudes, but increased in the northern tropics (2–17%; 1–6 ppbv per decade; ( [[#Cooper--2020|Cooper et al., 2020]] ; [[#Gaudel--2020|Gaudel et al., 2020]] ). Across the SH these more recent observations are too limited to determine zonal trends (e.g., tropics, mid-latitudes, high latitudes).

The earliest observations of free tropospheric ozone (1934–1955) are available from northern mid-latitudes where limited data indicate a tropospheric column ozone increase of 48 ± 30% up to 1990–2012 ( [[#Tarasick--2019|Tarasick et al., 2019]] ). Starting in the 1960s, records from ozonesondes show no significant changes in the free troposphere over the Arctic and mid-latitude regions of Canada, but trends are mainly positive elsewhere in the northern mid-latitudes ( [[#Oltmans--2013|Oltmans et al., 2013]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Tropospheric column and free tropospheric trends since the mid-1990s based on commercial aircraft, ozonesonde observations and satellite retrievals (Figure 2.8b,c), are overwhelmingly positive across the northern mid-latitudes (2–7%; 1–4 ppbv per decade) and tropics (2–14%; 1–5 ppbv per decade), with the largest increases (8–14%; 3–6 ppbv per decade) in the northern tropics in the vicinity of southern Asia and Indonesia. Observations in the SH are limited, but indicate average tropospheric column ozone increases of 2–12% (1–5 ppbv) per decade in the tropics (Figure 2.8c), and weak tropospheric column ozone increases (&lt;5%, &lt;1 ppbv per decade) at mid-latitudes ( [[#Cooper--2020|Cooper et al., 2020]] ). Above Antarctica, mid-tropospheric ozone has increased since the late 20th century ( [[#Oltmans--2013|Oltmans et al., 2013]] ). The total ozone ERF from 1750 to 2019 best estimate is assessed as 0.47 W m <sup>–2</sup> (Section 7.3.2.5) and this is dominated by increases in the troposphere. The underlying modelled global tropospheric ozone column increase ( [[#Skeie--2020|Skeie et al., 2020]] ) from 1850 to 2010 of 40–60%, is somewhat higher than the isotope based upper-limit of [[#Yeung--2019|Yeung et al. (2019)]] . At mid-latitudes (30°–60°N) model increases of 30–40% since the mid-20th century are broadly consistent with observations.

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[[File:29dfc27e28be57444bcf16f98ff974d1 IPCC_AR6_WGI_Figure_2_8.png]]

'''Figure 2.8''' '''|''' '''Surface and tropospheric ozone trends. (a)''' Decadal ozone trends by latitude at 28 remote surface sites and in the lower free troposphere (650 hPa, about 3.5 km) as measured by IAGOS aircraft above 11 regions. All trends are estimated for the time series up to the most recently available year, but begin in 1995 or 1994. Colours indicate significance (p-value) as denoted in the in-line key. See Figure 6.5 for a depiction of these trends globally. '''(b)''' Trends of ozone since 1994 as measured by IAGOS aircraft in 11 regions in the mid-troposphere (700–300 hPa; about 3–9 km) and upper troposphere (about 10–12 km), as measured by IAGOS aircraft and ozonesondes. '''(c)''' Trends of average tropospheric column ozone mixing ratios from the TOST composite ozonesonde product and three composite satellite products based on TOMS, OMI/MLS (Sat1), GOME, SCIAMACHY, OMI, GOME-2A, GOME-2B (Sat2), and GOME, SCIAMACHY, GOME-II (Sat3). Vertical bars indicate the latitude range of each product, while horizontal lines indicate the ''very likely'' uncertainty range. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, ''limited'' available isotopic ''evidence'' constrains the global tropospheric ozone increase to less than 40% between 1850 and 2005 ( ''low confidence'' ). Based on sparse historical surface/low altitude data tropospheric ozone has increased since the mid-20th century by 30–70% across the NH ( ''medium confidence'' ). Surface/low altitude ozone trends since the mid-1990s are variable at northern mid-latitudes, but positive in the tropics [2 to 17% per decade] ( ''high confidence'' ). Since the mid-1990s, free tropospheric ozone has increased by 2–7% per decade in most regions of the northern mid-latitudes, and 2–12% per decade in the sampled regions of the northern and southern tropics ( ''high confidence'' ). Limited coverage by surface observations precludes identification of zonal trends in the SH, while observations of tropospheric column ozone indicate increases of less than 5% per decade at southern mid-latitudes ( ''medium confidence'' ).

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