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

==== 8.4.1.2 Water Vapour and Its Transport ====

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Globally, AR5 assessed that by the end of the 21st century, the average quantity of water vapour in the atmosphere could increase by 5–25%, depending on emissions. The AR5 assessed that increases in near-surface specific humidity over land are ''very likely'' , but that it was also ''likely'' that near-surface relative humidity would decrease over many land areas, although with only ''medium confidence'' . In terms of moisture transport, AR5 assessed that it was ''likely'' that moisture transport into the high latitudes would increase and that there was ''high confidence'' that, over the ocean, atmospheric moisture transport from the evaporative regions to the wet regions would increase.

CMIP6 climate models continue to project a steady increase in global mean column-integrated water vapour by around 6 – 13% by 2041 – 2060 and 5 – 32% by 2081 – 2100, depending on scenario (Table 8.1). This is consistent with projected atmospheric warming ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1.2|Section 4.5.1.2]] ) and the Clausius–Clapeyron relationship ( [[#8.2.1|Section 8.2.1]] ) where every degree Celsius of warming is associated with an approximate 7% increase in atmospheric moisture in the lower atmospheric layers where most of the water vapour is concentrated. This increase sustains a positive feedback on anthropogenic global warming ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.2.2|Section 7.4.2.2]] ). In contrast, the response of clouds is much more spatially heterogeneous, microphysically complex, and model-dependent so that the projected cloud feedbacks remain a key uncertainty for constraining climate sensitivity ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.2.4|Section 7.4.2.4]] ).

CMIP6 models project an overall decrease in near-surface relative humidity over land, although with some regional and seasonal variations in their response (Figure 4.26). Regional changes in near-surface humidity over land are dominated by thermodynamic processes and are primarily controlled by moisture transport from the warming ocean ( [[#Chadwick--2016a|Chadwick et al., 2016a]] ). Increases in specific humidity lower than the thermodynamic rate are explained by greater warming over land than ocean and modulated by land – atmosphere feedbacks such as soil moisture and plant stomatal changes ( [[#8.2.2.1|Section 8.2.2.1]] ; [[#Berg--2017|Berg et al., 2017]] ; [[#Douville--2020|Douville et al., 2020]] ). This explains why climate models continue to project a contrasting response of near-surface relative humidity, with a slight and possibly overestimated increase over the oceans and a consistent but possibly underestimated decrease over land ( [[#Byrne--2016|Byrne and O’Gorman, 2016]] ; [[#Douville--2017|Douville and Plazzotta, 2017]] ; R. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ).

While projections of water vapour are well understood due to the constraints of the Clausius–Clapeyron relationship, projections of water vapour transport are complicated regionally by the role of changes in the wind field, which is influenced by a wide variety of factors. Additionally, there has been relatively little general evaluation of moisture transport in models. In CMIP5 models, both the mean and variability of the vertically-integrated moisture transport is projected to increase, largely due to increases in water vapour ( [[#Lavers--2015|Lavers et al., 2015]] ), with substantial regional differences ( [[#Levang--2015|Levang and Schmitt, 2015]] ). Single-model studies have illustrated projected increases in low-altitude moisture transport into convergence regions ( [[#Allan--2014|Allan et al., 2014]] ) and from ocean to land ( [[#Zahn--2013|Zahn and Allan, 2013]] ) that are consistent with present day trends. Increases in moisture transport have been linked to increases in large precipitation accumulations over land ( [[#Norris--2019|Norris et al., 2019]] ). Based on robust physics and supported by modelling studies, it is well understood that moisture transport increases into convergent parts of the atmospheric circulation such as storm systems, the tropical rain belt and high latitudes ( [[#8.2.2.1|Section 8.2.2.1]] ), but changes in atmospheric circulation that are less well understood alter moisture transport regionally ( [[#8.2.2.2|Section 8.2.2.2]] ). Therefore, given the limited examination of moisture transport in models, regional projections should be considered with caution. Changes in moisture transport specifically associated with monsoons, atmospheric rivers, and other specific circulation features are discussed further in the following sections.

In summary, there is ''high confidence'' in continued increases in global mean column integrated water vapour and near-surface specific humidity over land. There is ''medium confidence'' in region and season-dependent decreases in near-surface relative humidity over land, due to the complex physical processes involved ''.'' In general, there will be increases in moisture transport into storm systems, monsoons and high latitudes ( ''medium co'' ''nfidence'' ).

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