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

==== 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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