Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.3.1.2 Water Vapour and Its Transport ====

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The AR5 presented evidence of increases in global near-surface and tropospheric specific humidity since the 1970s but with ''medium confidence'' of a slowing of near-surface moistening trends over land associated with reduced relative humidity since the late 1990s. According to AR5, radiosonde, Global Positioning System (GPS) and satellite observations of tropospheric water vapour indicate ''very likely'' increases at near global scales since the 1970s occurring at a rate that is generally consistent with the Clausius–Clapeyron relation (about 7% °C <sup>–1</sup> at low altitudes) and the observed atmospheric warming ( [[#Hartmann--2013|Hartmann et al., 2013]] ).

Since AR5, it is ''very likely'' that increases in global atmospheric water vapour were observed based on in situ, satellite and reanalysis data (with ''medium confidence'' in the magnitude; [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3|Section 2.3.1.3]] ). Satellite records show increases in upper tropospheric water vapour (constant relative humidity while temperatures have increased) since 1979 ( E.-S. Chung et al. , 2014 ; [[#Blunden--2020|Blunden and Arndt, 2020]] ), to which human influence has ''likely'' contributed ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.2|Section 3.3.2.2]] ). Combined satellite and reanalysis estimates and CMIP6 atmosphere-only simulations (1988–2014) show global mean precipitable water vapour increases of 6.7 ± 0.3 % °C <sup>–1</sup> , very close to the Clausius–Clapeyron rate ( [[#Allan--2020|Allan et al., 2020]] ). Satellite-based products show increases close to the Clausius–Clapeyron rate over the ice-free oceans (about 7 to 9 % °C <sup>–1</sup> ; 1998 – 2008), but reanalysis estimates outside this range ( [[#Schröder--2019|Schröder et al., 2019]] ) are an expected consequence of their changing observing systems ( [[#Allan--2014|Allan et al., 2014]] ; [[#Parracho--2018|Parracho et al., 2018]] ). Increases in precipitable water vapour are found over the central and sub-Arctic based on multiple reanalyses with some corroboration from sparse, in situ data ( [[#Vihma--2016|Vihma et al., 2016]] ; [[#Rinke--2019|Rinke et al., 2019]] ; [[#Nygård--2020|Nygård et al., 2020]] ).

Declining near-surface relative humidity over land areas (e.g., the USA, Mediterranean, South Asia, South America and southern Africa) is evident in surface observations ( [[#Willett--2014|Willett et al., 2014]] , 2020; [[#Dunn--2017|Dunn et al., 2017]] ). This is consistent with a faster rate of warming over land than ocean (Sections 2.3.1.3 and 8.2.2.1; [[#Byrne--2018|Byrne and O’Gorman, 2018]] ). CMIP5 simulations underestimate the observed decreases in relative humidity over much of global land during 1979–2015 ( [[#Douville--2017|Douville and Plazzotta, 2017]] ; [[#Dunn--2017|Dunn et al., 2017]] ) even when observed SSTs are prescribed (–0.05 to –0.25% per decade compared with an observed rate of –0.4 to –0.8% per decade). It is not yet clear if this discrepancy is related to internal variability or can be explained by deficiencies in models ( [[#Vannière--2019|Vannière et al., 2019]] ; [[#Douville--2020|Douville et al., 2020]] ) or observations ( [[#Willett--2014|Willett et al., 2014]] ). Over the NH mid-latitude continents, there is ''medium confidence'' that human influence has contributed to a decrease in near-surface relative humidity in summer (Sections 2.3.1.3 and 3.3.2.3).

Water vapour transport (or convergence) estimates from observations have substantial uncertainties even in regions of high quality radiosonde data. Consequently many studies use reanalyses for water transport estimates instead of instrumental observations. For example, increases in low-level (800 – 1000 hPa) moisture convergence into the tropical wet regime with a smaller outflow increase in the mid-troposphere (400 – 800 hPa) with warming was detected in one reanalysis (ERA-Interim; [[#Allan--2014|Allan et al., 2014]] ). Modelling evidence combined with statistical analysis demonstrate consistency between reanalysis moisture convergence and P–E over land ( [[#Robertson--2016|Robertson et al., 2016]] ). Advances in reanalysis representation of atmospheric moisture and winds in addition to new observational isotope analysis have improved the ability to identify the main sources of water vapour for key continental regions and quantify the relative contributions from moisture advection and recycling (Gimeno et al. , 2012; van Der Ent et al. , 2014; Joseph et al., 2016).

Observed changes in moisture transport can also arise from changes in atmospheric circulation as well as thermodynamics. For instance, moisture transport into the Arctic region estimated from reanalyses datasets is consistent with radiosonde data ( [[#Dufour--2016|Dufour et al., 2016]] ) ''',''' with increases since 1979 linked to atmospheric circulation ( [[#Nygård--2020|Nygård et al., 2020]] ). Moisture transport into the Eurasian Arctic was identified to increase by 2.6% per decade during 1948 – 2008 based on a reanalysis estimate (X. [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al., 2013]] ). More intense moist intrusions associated with atmospheric rivers affecting the Arctic and Europe have been documented since 1979, but with a substantial influence from decadal internal variability ( [[#Ummenhofer--2017|Ummenhofer et al., 2017]] ; [[#Mattingly--2018|Mattingly et al., 2018]] ). A recent strengthening of tropical circulation and associated moisture convergence has been identified since around 2000 for the Amazon region (Arias et al. , 2015; Barichivich et al. , 2018; J.C. Espinoza et al. , 2018; X.Y. Wang et al. , 2018). This was also strenghtened by increased moisture transport from the North Atlantic, driving more abundant latent heat release ( [[#Segura--2020|Segura et al., 2020]] ) and leading to an increased frequency of extreme floods in the northern Amazon ( [[#Barichivich--2018|Barichivich et al., 2018]] ; [[#Heerspink--2020|Heerspink et al., 2020]] ). Overall, increased moisture transport has been linked to increased precipitation over wet tropical land areas ( [[#Gimeno--2020|Gimeno et al., 2020]] ) and to more extreme and persistent wet and dry weather events ( [[#Konapala--2020|Konapala et al., 2020]] ) in many regions worldwide.

In summary, there is ''high confidence'' that human-caused global warming has led to an overall increase in water vapour and moisture transport throughout the troposphere, at least since the mid-1990s. In particular, there is ''high confidence'' that moisture transport into the Arctic has increased but only ''medium confidence'' in the attribution of such a trend to a human influence. There is ''medium confidence'' that human influence has contributed to a decrease in near-surface relative humidity over the Northern Hemisphere mid-latitude continents during summer (see also Sections 2.3.1.3 and 3.3.2.3).

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