Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.1.2 Climatic and Non-Climatic Drivers of Changes in the Water Cycle ===

<div id="h2-2-siblings" class="h2-siblings"></div>

The water cycle is affected by both climatic and non-climatic factors ( [[#Douville--2021|Douville et al., 2021]] ). Radiative forcing by changes in greenhouse gas (GHG) concentrations, aerosols and surface albedo drives global and regional changes in evaporation and precipitation ( [[#Douville--2021|Douville et al., 2021]] ). A warmer atmosphere holds more moisture, increasing global and regional mean precipitation, and more extreme precipitation ( [[#Allan--2014|Allan et al., 2014]] ; [[#Giorgi--2019|Giorgi et al., 2019]] ; [[#Allan--2020|Allan et al., 2020]] ). Regional precipitation responses vary according to changes in atmospheric circulation. Geographical variation in aerosols drives changes in atmospheric circulation, affecting precipitation patterns such as the Asian monsoon ( [[#Ganguly--2012|Ganguly et al., 2012]] ; [[#Singh--2019|Singh et al., 2019]] ). ( [[#4.2.1|Section 4.2.1]] )

Warming increases glacier melt and is expected to decrease snowfall globally and lead to shorter snow seasons with earlier but less rapid snowmelt. It can also lead to local increases in snowfall intensity ( [[#Allan--2020|Allan et al., 2020]] ). These changes affect the seasonality of river flows in glacier-fed or snow-dominated basins. ( [[#4.2.2|Section 4.2.2]] )

Rising atmospheric CO 2 generally decreases plant transpiration, affecting soil moisture, runoff, stream flows, the return of moisture to the atmosphere and surface temperature ( [[#Skinner--2017|Skinner et al., 2017]] ). However, in some regions, these can be offset by increased leaf area (‘global greening’) driven by elevated CO 2 , land use change, nitrogen deposition and effects of climate change itself (Zhu Z. et al., 2016; Zeng Z. et al., 2018). Increased ozone can impact plant functioning, reducing transpiration ( [[#Arnold--2018|Arnold et al., 2018]] ). ( [[#4.2.1|Section 4.2.1]] )

Direct human interventions include abstraction of surface water and groundwater for drinking, irrigation and other freshwater uses, as well as streamflow impoundment behind dams and large-scale inter-basin transfers ( [[#Zhao--2015|Zhao et al., 2015]] ; [[#Donchyts--2016|Donchyts et al., 2016]] ; [[#McMillan--2016|McMillan et al., 2016]] ; [[#Shumilova--2018|Shumilova et al., 2018]] ). The consequences of these interventions are substantial and are discussed below briefly. In addition, these direct human interventions can change due to various societal and economic factors, including changes in land use and urbanisation (Sections 4.3 and 4.5).

Irrigation can reduce river flows and groundwater levels via abstraction and increase local precipitation ( [[#Alter--2015|Alter et al., 2015]] ; [[#Cook--2015|Cook et al., 2015]] ), alter precipitation remotely through moisture advection ( [[#de%20Vrese--2016|de Vrese et al., 2016]] ) and change the timing of monsoons through land–sea temperature contrasts ( [[#Guimberteau--2012|Guimberteau et al., 2012]] ) (Box 4.3). Land cover change affects ET and precipitation ( [[#Li--2015|Li et al., 2015]] ; [[#Douville--2021|Douville et al., 2021]] ), interception of precipitation by vegetation canopies ( [[#de%20Jong--2007|de Jong and Jetten, 2007]] ), infiltration ( [[#Sun--2018a|Sun et al., 2018a]] ) and runoff ( [[#Bosmans--2017|Bosmans et al., 2017]] ). Land cover impacts on the hydrological cycle are of similar magnitude as human water use ( [[#Bosmans--2017|Bosmans et al., 2017]] ).

Urbanisation decreases land surface permeability ( [[#Choi--2016|Choi et al., 2016]] ), which can increase fast runoff and flooding risks and reduce local rainfall by decreasing moisture return to the atmosphere ( [[#Wang--2018|Wang et al., 2018]] ). But urbanisation can also increase the sensible heat flux driving greater or more extreme precipitation ( [[#Kusaka--2014|Kusaka et al., 2014]] ; [[#Niyogi--2017|Niyogi et al., 2017]] ). ( [[#4.3.4|Section 4.3.4]] )

In summary, radiative forcing by GHG and aerosols drives changes in ET and precipitation at global and regional scales, and the associated warming shifts the balance between frozen and liquid water ( ''high confidence'' ). Rising CO 2 concentrations also affect the water cycle via plant physiological responses affecting transpiration, including via reduced stomatal opening and increased leaf area ( ''high confidence'' regarding the individual processes; ''medium confidence'' regarding their net impact). Land cover changes and urbanisation affect both the climate and land hydrology by altering the exchanges of energy and moisture between the atmosphere and surface ( ''high confidence'' ) and changing the permeability of the land surface. Direct human interventions in river systems and groundwater systems are non-climatic drivers with substantial impacts on the water cycle ( ''high confidence'' ) and have the potential to change as part of societal responses to climate change (Figure 4.2).

<div id="_idContainer026" class="Figure"></div>

[[File:46d780ee64a1bd77a376e0d3ea474b8b IPCC_AR6_WGII_Figure_4_002.png]]

'''Figure 4.2 |''' '''The water cycle, including direct human interventions.''' Water fluxes on land precipitation, land evaporation, river discharge, groundwater recharge and groundwater discharge to the ocean from [[#Douville--2021|Douville et al. (2021)]] . Human water withdrawals for various sectors are shown from [[#Hanasaki--2018|Hanasaki et al. (2018)]] , [[#Sutanudjaja--2018|Sutanudjaja et al. (2018)]] , [[#Burek--2020|Burek et al. (2020)]] , [[#Droppers--2020|Droppers et al. (2020)]] and [[#Müller%20Schmied--2021|Müller Schmied et al. (2021)]] . Green water use ( [[#Abbott--2019|Abbott et al., 2019]] ) refers to the use of soil moisture for agriculture and forestry. Irrigation water use (called blue water) is not included in green water use.

<div id="4.2" class="h1-container"></div>

<span id="observed-changes-in-the-hydrological-cycle-due-to-climate-change"></span>