Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.3.2.2 Atmospheric Water Vapour ====

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The AR5 concluded that an anthropogenic contribution to increases in specific humidity is found with ''medium confidence'' at and near the surface. A levelling off of atmospheric water vapour over land in the last two decades that needed better understanding, and remaining observational uncertainties, precluded a more confident assessment ( [[#Bindoff--2013|Bindoff et al., 2013]] ). Sections 4.5.1.3 and 8.3.1.4 show that there have been significant advances in the understanding of the processes controlling land surface humidity. In particular, there has been a focus on the role of oceanic moisture transport and land-atmosphere feedbacks in explaining the observed trends in relative humidity.

Water vapour is the most important natural greenhouse gas and its amount is expected to increase in a global warming context leading to further warming. Particularly important are changes in the upper troposphere because there water vapour regulates the strength of the water-vapour feedback (Section 7.4.2.2). CMIP5 models have been shown to have a wet bias in the tropical upper troposphere and a dry bias in the lower troposphere, with the former bias and model spread being larger than the latter ( [[#Jiang--2012|Jiang et al., 2012]] ; [[#Tian--2013|Tian et al., 2013]] ). [[#Tian--2013|Tian et al. (2013)]] also showed that in comparison to the AIRS specific humidity, CMIP5 models have the well-known double Inter-tropical Convergence Zone (ITCZ) bias in the troposphere from 1000 hPa to 300 hPa, especially in the tropical Pacific. Water vapour biases in models are dominated by errors in relative humidity throughout the troposphere, which are in turn closely related to errors in large scale circulation; temperature errors dominate near the tropopause ( [[#Takahashi--2016|Takahashi et al., 2016]] ). Section 7.4.2.2 discusses this topic in more detail for CMIP6 models. However, [[#Schröder--2019|Schröder et al. (2019)]] show that the majority of well-established water vapour records are affected by inhomogeneity issues and thus should be used with caution (see also ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3.3|Section 2.3.1.3.3]] ). A comparison of trends in column water vapour path for 1998–2019 in satellite data, a reanalysis, CMIP5 and CMIP6 simulations averaged over the near-global ocean reveals that while on average model trends are higher than those in observations and a reanalysis, the latter lie within the multi-model range (Figure 3.12).

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[[File:dc96a534bd92c51c5bc548b5a349cacd IPCC_AR6_WGI_Figure_3_12.png]]

Figure 3.12 | '''Total column water vapour trends (% per decade) for the period 1988–2019 averaged over the near-global oceans (50°S–50°N).''' The figure shows satellite data (RSS) and ERA5.1 reanalysis, as well as CMIP5 (blue) and CMIP6 (red) historical simulations. All available ensemble members were used (see [[#3.2|Section 3.2]] ). Fits to the model trend probability distributions were performed with kernel density estimation. Figure is updated from [[#Santer--2007|Santer et al. (2007)]] . Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

The detection and attribution of tropospheric water vapour changes can be traced back to [[#Santer--2007|Santer et al. (2007)]] , who used estimates of atmospheric water vapour from the satellite-based Special Sensor Microwave Imager (SSM/I) and from CMIP3 historical climate simulations. They provided evidence of human-induced moistening of the troposphere, and found that the simulated human fingerprint pattern was detectable at the 5% level by 2002 in water vapour satellite data (from 1988 to 2006). The observed changes matched the historical simulations forced by greenhouse gas changes and other anthropogenic forcings, and not those due to natural variability alone. Then, [[#Santer--2009|Santer et al. (2009)]] repeated this study with CMIP5 models, and found that the detection and attribution conclusions were not sensitive to model quality. These results demonstrate that the human fingerprint is governed by robust and basic physical processes, such as the water vapour feedback. Finally, [[#Chung--2014|Chung et al. (2014)]] extended this line of research by focusing on the global-mean water vapour content in the upper troposphere. Using satellite-based observations and sets of CMIP5 climate simulations run under various climate-forcing options, they showed that the observed moistening trend of the upper troposphere over the 1979–2005 period could not be explained by internal variability alone, but is attributable to a combination of anthropogenic and natural forcings. This increase in water vapour is accompanied by a reduction in mid-tropospheric relative humidity and clouds in the subtropics and mid-latitude in both models and observations related to changes in the Hadley cell ( [[#3.3.3.1.1|Section 3.3.3.1.1]] ; [[#Lau--2015|Lau and Kim, 2015]] ).

[[#Dunn--2017|Dunn et al. (2017)]] confirmed earlier findings that global mean surface relative humidity increased between 1973 and 2000, followed by a steep decline (also reported in [[#Willett--2014|Willett et al., 2014]] ) until 2013, and specific humidity correspondingly increased and then remained approximately constant (see also ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3.2|Section 2.3.1.3.2]] ), with none of the CMIP5 models capturing this behaviour. They noted biases in the mean state of the CMIP5 models’ surface relative humidity (and ascribed the failure to the representation of land surface processes and their response to CO <sub>2</sub> forcing), concluding that these biases preclude any detection and attribution assessment. On the other hand, [[#Byrne--2018|Byrne and O’Gorman (2018)]] showed that the positive trend in specific humidity continued in recent years and can be detected over land and ocean from 1979 to 2016. Moreover, they provided a theory suggesting that the increase in annual surface temperature and specific humidity as well as the decrease in relative humidity observed over land are linked to warming over the neighbouring ocean. They also pointed out that the negative trend in relative humidity over land regions is quite uncertain and requires further investigation. A recent study has also identified an anthropogenically-driven decrease in relative humidity over the Northern Hemisphere mid-latitude continents in summer between 1979 and 2014, which was underestimated by CMIP5 models ( [[#Douville--2017|Douville and Plazzotta, 2017]] ). Furthermore, in a modelling study [[#Douville--2020|Douville et al. (2020)]] showed that this decrease in boreal summer relative humidity over mid-latitudes is related not only to global ocean warming, but also to the physiological effect of CO <sub>2</sub> on plants in the land surface model.

In summary, we assess that it is ''likely'' that human influence has contributed to moistening in the upper troposphere since 1979. Also, there ''is medium confidence'' that human influence contributed to a global increase in annual surface specific humidity, and ''medium confidence'' that it contributed to a decrease in surface relative humidity over mid-latitude Northern Hemisphere continents during summertime.

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