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

===== 2.3.1.3.3 Total column water vapour (TCWV) =====

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The AR5 concluded that total column water vapour (TCWV) ''very likely'' increased since the 1970s, at a rate that was overall consistent with the Clausius-Clapeyron relationship (about 7% per °C) given the observed increase in atmospheric temperature.

Records prior to the instigation of quasi-global coverage by radiosondes require the use of statistical relationships to infer TCWV from historical SST observations or the evaluation of centennial-scale reanalysis products ( [[#Smith--2015|Smith and Arkin, 2015]] ). These approaches reveal two periods of positive trends, one from 1910 to 1940 and the other from 1975 onwards ( [[#Zhang--2013|Zhang et al., 2013]] ; [[#Mieruch--2014|Mieruch et al., 2014]] ; [[#Shi--2018|Shi et al., 2018]] ), concurrent with periods of positive SST trends (Figure 2.11). Potential sources of errors in the SST-based estimation of TCWV include both uncertainties in historical SST and uncertainties in the parameters that define the relationship between the variables ( [[#Smith--2015|Smith and Arkin, 2015]] ). Trends based on 20CRv2c, ERA-20C and ERA-20CM indicate an increase in TCWV over much of the global ocean since the beginning of the 20th century, particularly over the tropics ( [[#Bordi--2015|Bordi et al., 2015]] ; [[#Smith--2015|Smith and Arkin, 2015]] ; [[#Poli--2016|Poli et al., 2016]] ). TCWV trends estimated since the middle of the 20th century from radiosonde observations show significant increases over North America and large portions of Eurasia, while decreases are restricted to Australia, eastern Asia and the Mediterranean region (Y. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ). Overall, there is a significant increase in TCWV over global land areas since 1979 ( [[#Chen--2016|Chen and Liu, 2016]] ).

Since the late 1970s a range of satellite missions permit a quasi-global assessment of TCWV. Several satellite products provide water vapour retrievals based upon distinct spectral domains, in addition to products from radiosondes, reanalyses and GNSS radio occultation. The GEWEX Water Vapour Assessment (G-VAP) provided an intercomparison of several TCWV data records, with global coverage but limited timespan ( [[#Schröder--2018|Schröder et al., 2018]] ). The various global products generally exhibit a positive trend since 1979 (Figure 2.14; [[#Allan--2014|Allan et al., 2014]] ; [[#Mieruch--2014|Mieruch et al., 2014]] ; [[#Schröder--2016|Schröder et al., 2016]] ; J. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ), most evident over the tropics ( [[#Gu--2013|Gu and Adler, 2013]] ; [[#Chen--2016|Chen and Liu, 2016]] ; [[#Mears--2018|Mears et al., 2018]] ; [[#Wang--2020|Wang and Liu, 2020]] ; [[#Salamalikis--2021|Salamalikis et al., 2021]] ). The existence of apparent breakpoints in several products, which are generally coincident with changes in the observing system, lead to trend estimates that are not in line with theoretical expectations imposed by the Clausius-Clapeyron relationship ( [[#Schröder--2019|Schröder et al., 2019]] ), although other factors such as regional moisture divergence/convergence could account for the observed TCWV-temperature scaling. Substantial potential inhomogeneities affect trend estimates based on satellite, reanalysis and merged products in particular over Central Africa, the Sahara and central South America ( [[#Schröder--2016|Schröder et al., 2016]] , 2019; J. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ). Moreover, data gaps in observations from ground-based GNSS receivers and radiosondes lead to ''low confidence'' in TCWV estimation in these regions.

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[[File:100dd2195115423d233bb4cf4b6a71ee IPCC_AR6_WGI_Figure_2_14.png]]

'''Figure 2.1''' '''4 |''' '''Time series of global mean total column water vapour annual anomalies (mm) relative to a 1988–2008 base period.''' Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, positive trends in global total column water vapour are ''very likely'' since 1979 when globally representative direct observations began, although uncertainties associated with changes in the observing system imply ''medium confidence'' in estimation of the trend magnitudes. ''Low confidence'' in longer-term trends arises from uncertainties in the SST-TCWV relationship and current centennial scale reanalyses, particularly during the first half of the 20th century.

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