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

===== 2.3.1.3.2 Surface humidity =====

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The AR5 reported ''very likely'' widespread increases in near-surface air specific humidity since the 1970s, abating from around 2000 to 2012 ( ''medium confidence'' ). This abatement resulted in a recent decline in relative humidity over the land.

Near surface humidity has been monitored using in-situ data (e.g., NOCSv2.0; [[#Berry--2011|Berry and Kent, 2011]] ), satellite-derived estimations (e.g., HOAPS3, [[#Liman--2018|Liman et al., 2018]] ; J-OFURO3, [[#Tomita--2019|Tomita et al., 2019]] ), global gridded products such as HadISDH ( [[#Willett--2014|Willett et al., 2014]] , 2020), and reanalyses (e.g., ERA5, JRA-55 and 20CRv3). In-situ based humidity products suffer from uncertainties over poorly sampled regions particularly in the SH ( [[#Berry--2011|Berry and Kent, 2011]] ; [[#Kent--2014|Kent et al., 2014]] ; [[#Willett--2014|Willett et al., 2014]] ). There is general consensus in the inter-annual variability and sign of trends implying ''high confidence'' in increasing specific humidity since the 1970s and decreasing relative humidity since 2000, particularly over land ( [[#Simmons--2010|Simmons et al., 2010]] ; [[#Willett--2014|Willett et al., 2014]] , 2020). Since 2012, specific humidity over land and ocean has remained well above the 1973–2019 average and reached record or near-record values (Figure 2.13b), with the strong 2015–2016 El Niño event boosting surface moisture levels ( [[#Byrne--2018|Byrne and O’Gorman, 2018]] ). The abatement from around 2000 to 2012 reported in AR5 has not persisted. This is consistent with increases in total column water vapour ( [[#2.3.1.3.3|Section 2.3.1.3.3]] ) and a resumption of rapid warming in surface temperatures ( [[#2.3.1.1.3|Section 2.3.1.1.3]] ). The global averaged relative humidity however has remained depressed since 2000 (Figure 2.13d; [[#Simmons--2010|Simmons et al., 2010]] ; [[#Willett--2014|Willett et al., 2014]] , 2020; [[#Dunn--2017|Dunn et al., 2017]] ; [[#Vicente-Serrano--2018|Vicente-Serrano et al., 2018]] ).

Since 1973, increases in specific humidity have been widespread and significant across the majority of the land and ocean regions where observations are available (Figure 2.13a). In contrast, trends in relative humidity show distinct spatial patterns with generally increasing trends over the higher latitudes and the tropics and generally decreasing trends over the sub-tropics and mid-latitudes, particularly over land areas (Figure 2.13c). Near-surface specific humidity over the oceans has increased since the 1970s according to several in-situ, satellite and reanalysis data records ( [[#Kent--2014|Kent et al., 2014]] ; [[#Robertson--2020|Robertson et al., 2020]] ; [[#Willett--2020|Willett et al., 2020]] ). According to the HadISDH product, increases in specific humidity and decreases in relative humidity are significant particularly over the NH mid-latitudes (Figure 2.13a,c). Poor data coverage over the SH south of 20°S does not allow for the robust assessment of trends. Sources of uncertainty include the initial measurement accuracy, homogenization over land, observational height at ships and instrument bias adjustment over ocean, and sparse spatio-temporal sampling ( [[#Prytherch--2015|Prytherch et al., 2015]] ; [[#Roberts--2019|Roberts et al., 2019]] ; [[#Willett--2020|Willett et al., 2020]] ).

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[[File:fac3527940cee5a3edb1f1f7ea1185f3 IPCC_AR6_WGI_Figure_2_13.png]]

'''Figure 2.13''' '''|''' '''Changes in surface humidity. (a)''' Trends in surface specific humidity over 1973–2019. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] ; ‘×’ marks denote non-significant trends). '''(b)''' Global average surface specific humidity annual anomalies (1981–2010 base period). '''(c)''' as (a) but for the relative humidity. '''(d)''' as (b) but for the global average surface relative humidity annual anomalies. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, observations since the 1970s show a ''very likely'' increase in near surface specific humidity over both land and oceans. A ''very likely'' decrease in relative humidity has occurred over much of the global land area since 2000, particularly over mid-latitude regions of the NH, with increases at northern high latitudes.

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