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

==== 8.1.1.2 Overview of the Global Water Cycle in the Climate System ====

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As shown in Figure 8.1, the global water cycle is the continuous, naturally occurring movement of water through the climate system from its liquid, solid and gaseous forms among reservoirs of the ocean, atmosphere, cryosphere and land ( [[#Stocker--2013|Stocker et al., 2013]] ). In the atmosphere, water primarily occurs as a gas (water vapour), but it is also present as ice and liquid water within clouds where it substantially affects Earth’s energy balance (Sections 7.4.2.2 and 7.4.2.4). The water cycle primarily involves the evaporation <sup>[[#footnote-001|1]]</sup> and precipitation of moisture at the Earth’s surface including transpiration associated with biological processes. Water that falls on land as precipitation, supplying soil moisture, groundwater recharge, and river flows, was once evaporated from the ocean or sublimated from ice-covered regions before being transported through the atmosphere as water vapour, or in some areas was generated over land through evapotranspiration (Gimeno et al., 2010; [[#van%20der%20Ent--2013|van der Ent and Savenije, 2013]] ). In addition, the net flux of atmospheric and continental freshwater is a key driver of sea surface salinity, which in turn influences the density and circulation of the ocean (Chapter 9).

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'''Figure 8.1 |''' '''Depiction of the present-day water cycle based on previous assessments ( [[#Trenberth--2011|Trenberth et al., 2011]] ; [[#Rodell--2015|Rodell et al., 2015]] ; [[#Abbott--2019|Abbott et al., 2019]] ) with adjustments for groundwater flows ( [[#Zhou--2019|]] [[#Zhou--2019|]] [[#Zhou--2019|Zhou et al., 2019]] c; [[#Luijendijk--2020|Luijendijk et al., 2020]] ), seasonal snow ( [[#Pulliainen--2020|Pulliainen et al., 2020]] ) and ocean precipitation and evaporation ( [[#Stephens--2012|Stephens et al., 2012]] ; [[#Allan--2020|Allan et al., 2020]] ; [[#Gutenstein--2021|Gutenstein et al., 2021]] ).''' '''The net loss of frozen and liquid water from land to ocean is estimated from Chapter 9, Table 9.5. In the atmosphere, which accounts for only 0.001% of all water on Earth, water primarily occurs as a gas (water vapour), but it is also present as ice and liquid water within clouds. The ocean is the primary water reservoir on Earth: it comprises mostly liquid water across much of the globe but also includes areas covered by ice in polar regions. Liquid freshwater on land forms surface water (lakes, rivers) and, together with soil moisture and mostly unusable groundwater stores, accounts for less than 2% of global water ( [[#Stocker--2013|Stocker et al., 2013]] ). Solid terrestrial water that occurs as ice sheets, glaciers, snow and ice on the surface, and permafrost currently represents nearly 2% of the planet’s water ( [[#Stocker--2013|Stocker et al., 2013]] ). Water that falls as snow in winter provides soil moisture and streamflow after melting, which are essential for human activities and ecosystem functioning. Note that these best estimates do not lead to a perfectly closed global water budget and that this budget has no reason to be closed given the ongoing human influence through both climate change (e.g., melting of ice sheets and glaciers, see Chapter 9) and water use (e.g., groundwater depletion through pumping into fossil aquifers, see Figure 8.10).'''

Understanding the interactions between the water and energy cycles is one of the four core projects of the World Climate Research Programme (WCRP). Latent heat fluxes, released by condensation of atmospheric water vapour and absorbed by evaporative processes, are critical to driving the circulation of the atmosphere on scales ranging from individual thunderstorm cells to the global circulation of the atmosphere (Stocker et al., 2013; [[#Miralles--2019|Miralles et al., 2019]] ). Water vapour is the most important gaseous absorber in the Earth’s atmosphere, playing a key role in the Earth’s radiative budget ( [[#Schneider--2010|Schneider et al., 2010]] ). As atmospheric water vapour content increases with temperature, it has a considerable influence on climate change ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.2.2|Section 7.4.2.2]] ). Additionally, a small fraction of the atmospheric water content is liquid or solid and has a major effect on both solar and longwave radiative fluxes, from the Earth’s surface to the top of the atmosphere. The cloud response to anthropogenic radiative forcings, both in the tropics and in the extratropics ( [[#Zelinka--2020|Zelinka et al., 2020]] ), is therefore also crucial for understanding climate change ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.2.4|Section 7.4.2.4]] ).

The terrestrial water and carbon cycles are also strongly coupled (Cross-Chapter Box 5.1). As atmospheric carbon dioxide (CO <sub>2</sub> ) concentration increases, the physical environment in which plants grow is altered, including the availability of soil moisture necessary for plants’ CO <sub>2</sub> uptake and, potentially, the effectiveness of CO <sub>2</sub> removal techniques to mitigate climate change ( [[IPCC:Wg1:Chapter:Chapter-5#5.6.2.1.2|Section 5.6.2.1.2]] ). Rising surface CO <sub>2</sub> concentrations also modify stomatal (small pores at the leaf surface) regulation as well as the plants’ biomass, thus affecting ecosystem photosynthesis and transpiration rates and leading generally to a net increase in water use efficiency ( [[#Lemordant--2018|Lemordant et al., 2018]] ). These coupled changes have profound implications for the simulation of the carbon and water cycles (Gentine et al. , 2019 ; see also [[IPCC:Wg1:Chapter:Chapter-5#5.4.1|Section 5.4.1]] ), which can be better assessed with the new generation Earth system models, although both the carbon concentration and carbon-climate feedbacks remain highly uncertain over land [[IPCC:Wg1:Chapter:Chapter-5#5.4.5|Section 5.4.5]] ; [[#Arora--2020|Arora et al., 2020]] ). The water constraints on the terrestrial carbon sinks are a matter of debate regarding the feasibility or efficiency of some land-based CO <sub>2</sub> removal and sequestration techniques requested to comply with the Paris Agreement (Section 5.6.2.2.1; Fuss et al. , 2018; Belyazid and Giulia na, 2019) .

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