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

== 8.1 Introduction ==

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=== 8.1.1 Scope and Overview ===

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==== 8.1.1.1 Importance of Water for Human Societies and Ecosystems ====

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Water is vital to all life on Earth. Seventy-one percent of the Earth is covered by water, with saline ocean water accounting for around 97% of total water availability (Figure 8.1). Terrestrial freshwater represents less than 2% of all water on Earth, and the remainder (around 1 –2 %) is primarily made up of saline groundwater and saline lakes ( [[#Durack--2015|Durack, 2015]] ; [[#Abbott--2019|Abbott et al., 2019]] ). Ice sheets, glaciers and snow pack account for approximately 96% of all freshwater, with less than 4% of freshwater considered easily accessible and available for essential ecosystem functioning and human society’s water resource needs (Durack, 2015; [[#Abbott--2019|Abbott et al., 2019]] ). This very small fraction of freshwater represents a total volume of about 835,000 km <sup>3</sup> , mostly contained in groundwater (630,000 km <sup>3</sup> ), the remaining 205,000 km <sup>3</sup> being stored in lakes, rivers, wetlands and soils (Abbott et al., 2019). Although the natural cycling rate of this amount is theoretically enough to meet global human and ecosystem needs, there are large geographical and seasonal differences that influence the availability of freshwater to meet regional demands.

Freshwater is the most essential natural resource on the planet (Mekonnen and Hoekstra, 2016; [[#Djehdian--2019|Djehdian et al., 2019]] ) and underpins almost all Sustainable Development Goals (SDGs), which require access to adequate and safe resources for drinking and sanitation (SDG 6) and many other purposes. Freshwater supports a range of human activities from irrigation to industrial processes including the generation of hydro-electricity and the cooling of thermoelectric power plants (Bates et al., 2008; [[#Schewe--2014|Schewe et al., 2014]] ). These activities require sufficient quantities of freshwater that can be drawn from rivers, lakes, groundwater stores, and in some cases, desalinated sea water (Schewe et al., 2014). Recent estimates of global water pools and fluxes suggest that half of global river discharge is redistributed each year by human water use (Abbott et al., 2019). This emphasizes the need to consider both anthropogenic climate change and direct human influences, such as population increase or migration, economic development, urbanization, and land use change, when planning water-related mitigation or adaptation strategies ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ).

Water scarcity occurs when there are insufficient freshwater resources to meet water demands, although water problems may also arise from water quality issues or from economic and institutional barriers (AR6 WGII Chapter 4). This affects the preservation of environmental flows that ultimately influence ecosystem functioning and services (Schewe et al., 2014; [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ; [[#Djehdian--2019|Djehdian et al., 2019]] ). As such, water availability is a major constraint on human society’s ability to meet the future food and energy needs of a growing population (D’Odorico et al., 2018). Water plays a key role in the production of energy, including hydro-electricity, bioenergy, and the extraction of unconventional fossil fuels (Schewe et al., 2014; [[#D’Odorico--2018|D’Odorico et al., 2018]] ; [[#Djehdian--2019|Djehdian et al., 2019]] ). These dependencies have resulted in increasing competition for water between the food and energy sectors. Pressures on this ‘food-energy-water nexus’ are further compounded by increasing globalization, which can transfer large-scale water demands to other regions of the world, raising serious concerns about local food and water security in regions that are highly dependent on agricultural exports or imports (D’Odorico et al., 2018).

The consequences of climate change on terrestrial ecosystems and human societies are primarily experienced through changes to the global water cycle (JiménezCisneros et al., 2014). Changes in the quantity and seasonality of water due to climate change have long been recognized by IPCC and global development agencies as heavily influencing the food security and economic prosperity of many countries, particularly in the arid and semi-arid areas of the world including Asia, Africa, Australia, Latin America, the Mediterranean, and small island developing states ( [[#Bates--2008|Bates et al., 2008]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). Having too much or too little water increases the likelihood of flooding and drought, as precipitation variability increases in a warming climate (Stockeret al., 2013; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). Climate change poses a threat to both regional water availability and global water security. Changes in precipitation and glacier runoff and snowmelt influence other hydroclimate variables like surface and subsurface runoff, and groundwater recharge, which are critical to the water, food and energy security of many regions (Oki andKanae, 2006; [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ).

Currently, around four billion people live under conditions of severe freshwater scarcity for at least one month of the year, with half a billion people in the world facing severe water scarcity all year round (Mekonnen and Hoekstra, 2016). The AR5 WGII reported that approximately 80% of the world’s population already suffers from high levels of threat to water security ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). Given the vulnerability of the planet’s freshwater resources and the role of climate change in intensifying adverse impacts on human societies and ecosystems (Hoegh-Guldberg et al., 2018; [[#IPCC--2018|IPCC, 2018]] ), this chapter evaluates advances in the theoretical, observational and model based understanding of the global water cycle made since AR5 (IPCC, 2013) and AR6 Special Reports.

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==== 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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=== 8.1.2 Summary of Water Cycle Changes From AR5 and Special Reports ===

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This Report is the first IPCC assessment to include a chapter specifically dedicated to providing an integrated assessment of the global water cycle changes, by building on many chapters from previous reports. This section summarizes observed and projected water cycle changes reported in AR5( [[#IPCC--2013|IPCC, 2013]] ) and in the recent IPCC Special Reports on Global Warming of 1.5°C (SR1.5), the Ocean and Cryosphere in a Changing Climate (SROCC), and Climate Change and Land (SRCCL).

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==== 8.1.2.1 Summary of Observed and Projected Water Cycle Changes from AR5 ====

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Based on long-term observational evidence ( [[#Hartmann--2013|Hartmann et al., 2013]] ), AR5 concluded it was ''likely'' that anthropogenic influence has affected the water cycle since the 1960s ( [[#IPCC--2018|IPCC, 2018]] ). Detectable human influence on changes to the water cycle were found in atmospheric moisture content ( ''medium confidence'' ), global-scale changes of precipitation over land ( ''medium confidence'' ), intensification of heavy precipitation events over land regions where sufficient data networks exist ( ''medium confidence'' ), and ''very likely'' changes to ocean salinity through its connection with evaporation minus precipitation change patterns (Sections 2.5, 2.6, 3.3, 7.6, 10.3 and 10.4; [[#Stocker--2013|Stocker et al., 2013]] ). The AR5 also reported that it is ''very likely'' that global surface air specific humidity increased since the 1970s. There was ''low confidence'' in the observations of global-scale cloud variability and trends, ''medium confidence'' in reductions of pan-evaporation, and ''medium confidence'' in the non-monotonic changes of global evapotranspiration since the 1980s. In terms of streamflow and runoff, AR5 identified that there is ''low confidence'' in the observed increasing trends of global river discharge during the 20th century. Similarly, AR5 concluded that there is ''low confidence'' in any global-scale observed trend in drought or dryness (lack of rainfall) since the mid-20th century. Yet, the frequency and intensity of drought ''likely'' increased in the Mediterranean and West Africa, while they ''likely'' decreased in central North America and north-western Australia since 1950.

Water cycle projections in AR5 ( [[#Collins--2013|Collins et al., 2013]] ) were considered primarily in terms of water vapour, precipitation, surface evaporation, runoff, and snowpack. Globally-averaged precipitation was projected to increase with global warming with ''virtual certainty'' ( [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary and [[IPCC:Wg1:Chapter:Chapter-12#12.4.1.1|Section 12.4.1.1]] ). Regionally, precipitation in some areas of the tropics and polar regions could increase by more than 50% by the end of the 21st century under the RCP8.5 emissions scenario, while precipitation in large areas of the subtropics could decrease by 30% or more (AR5 FAQ 12.2, Figure 12.22). Overall, the contrast of annual mean precipitation between dry and wet regions and between dry and wet seasons (‘wet get wetter, dry get drier’) was projected to increase over most of the globe with ''high confidence'' ( [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.2|Section 12.4.5.2]] ). Globally, the frequency of intense precipitation events was projected to increase while the frequency of all precipitation events was projected to decrease, leading to the contradictory-seeming projection of a simultaneous increase in both droughts and floods (12.2 and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.5|Section 12.4.5.5]] in AR5 WGI). Surface evaporation change was projected to be positive over most of the ocean and to generally follow the pattern of precipitation change over land [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary, and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.4|Section 12.4.5.4]] ). Near-surface relative humidity reductions over many land areas were projected to be ''likely'' , with ''medium confidence'' ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.1|Section 12.4.5.1]] ). General decreases in soil moisture in present-day dry regions were considered ''likely ,'' and projected with ''medium confidence'' under the RCP8.5 scenario ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.3|Section 12.4.5.3]] ). Soil moisture drying in the Mediterranean, south-west USA and southern African regions was considered ''likely'' , with ''high confidence'' by the end of this century under the RCP8.5 scenario ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.3|Section 12.4.5.3]] ). Projections for annual runoff included both decreases and increases. Decreases in Northern Hemisphere snow cover were assessed as ''very likely'' with continued global warming ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.6.2|Section 12.4.6.2]] ). As temperatures increase, snow accumulation was projected to begin later in the year and melting to start earlier, with related changes in snowmelt-driven river flows (FAQ 12.2 and [[IPCC:Wg1:Chapter:Chapter-12#12.4.6.2|Section 12.4.6.2]] in AR5 WGI). In terms of the potential for abrupt change in components of the water cycle, long-term droughts and monsoonal circulation were identified as potentially undergoing rapid changes, but the assessment was reported with ''low confidence'' (Sections 12.5.5.8.1 and 12.5.5.8.2, and Table 12.4).

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==== 8.1.2.2 Key Findings of AR6 Special Reports ====

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The SR1.5 assessed the impacts of global warming of 1.5°C above pre-industrial levels. The dominant human influence on observed global warming and related water cycle changes was confirmed. Further evidence that anthropogenic global warming has caused an increase in the frequency, intensity and/or amount of heavy precipitation events at the global scale ( ''medium confidence'' ), as well as in drought occurrence in the Mediterranean region ( ''medium confidence'' ) was also reported. [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] of SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) highlights that each half degree of additional global warming influences the climate response. Heavy precipitation shows a global tendency to increase more at 2°C compared to 1.5°C, though there is ''low confidence'' in projected regional differences in heavy precipitation at 1.5°C compared to 2°C global warming, except at high latitudes or at high altitude where there is ''medium confidence'' . A key finding is that ‘limiting global warming to 1.5°C compared to 2°C would approximately halve the proportion of the world population expected to suffer water scarcity, although there is considerable variability between regions ( ''medium confidence'' )’ (SR1.5). This is consistent with greater adverse impacts found at 2°C compared to 1.5°C for a number of dryness or drought indices (Schleussner et al., 2016; [[#Lehner--2017|Lehner et al., 2017]] ; [[#Greve--2018|Greve et al., 2018]] ). There is also ''medium confidence'' that land areas with increased runoff and exposure to flood hazards will increase more at 2°C compared to 1.5°C of global warming.

The Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) provides a comprehensive assessment of recent and projected changes, specifically in snow and ice-covered areas that form a key component of the water cycle in high-elevation and high-latitude areas. High mountain regions have experienced significant warming since the early 20th century, resulting in reduced snowpack on average (Marty et al., 2017), with glaciers retreating globally since the mid-20th century (Marzeion et al., 2018; [[#Zemp--2019|Zemp et al., 2019]] ). Glacier shrinkage and snow cover changes have led to changes (both increases and decreases) in streamflow in many mountain regions in recent decades (Milner et al., 2017). Permafrost regions have undergone degradation and ground-ice loss due to recent warming (Lu et al., 2017). Glacier mass loss is projected to continue through the 21st century under all scenarios. In high mountain areas, low-elevation snow cover is also projected to decrease, regardless of emissions scenario. Widespread permafrost thaw is projected to continue through this century and beyond. River runoff in snow- or glacier-fed basins is projected to increase in winter and to decrease in summer (and in the annual mean) by 2100. In the oceans, the Atlantic Meridional Overturning Circulation (AMOC) will ''very likely'' weaken over the 21st century under all emissions scenarios (SROCC), with potential effects on atmospheric circulation and the water cycle at the regional scale (see also [[#8.6|Section 8.6]] ).

The Special Report on climate change, desertification, land degradation, sustainable management, food security, and greenhouse gas (GHG) fluxes in terrestrial ecosystems (SRCCL) has clear connections with the water cycle. This Report indicates that since 1850–1900, land surface temperature has risen nearly twice as much as global surface temperature ( ''high confidence'' ), with an increase in dry climates ( ''high confidence'' ). Land surface processes modulate the likelihood, intensity and duration of many extreme events including droughts ( ''medium confidence'' ) and heavy precipitation ( ''medium confidence'' ). The direction and magnitude of hydrological changes induced by land use change and land surface feedbacks vary with location and season ( ''high confidence'' ). Desertification exacerbates climate change through feedbacks involving vegetation cover, greenhouse gases and mineral dust aerosol ( ''high confidence'' ). Urbanization increases extreme rainfall events over or downwind of cities ( ''medium confidence'' ). Intensification of rainy events increase their consequences on land degradation.

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=== 8.1.3 Chapter Motivations, Framing and Preview ===

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The AR5 report was a major step forward in the assessment of the human influence on the Earth’s water cycle, yet regional projections of precipitation and water resources often remained very uncertain for a range of reasons including modelling uncertainty and the large influence of internal variability (Sections 1.4.3 and 8.5.2; Hawkins and Sutton, 2011; Deser et al. , 2012) . Since AR5, longer and more homogeneous observational and reanalysis datasets have been produced along with new ensembles of historical simulations driven by all or individual anthropogenic forcings. These factors, together with improved detection-attribution tools, has enabled a more comprehensive assessment and a better understanding of recent observed water cycle changes, including the competing effects of GHGs and aerosol emissions. New paleoclimate reconstructions have been also developed, particularly from the SH, that were not available at the time of AR5. There have also been advances in modelling clouds, precipitation, surface fluxes, vegetation, snow, floodplains, groundwater and other processes relevant to the water cycle. Convection permitting and cloud-resolving models have been implemented over increasingly large domains and can be used as benchmarks for the evaluation of the current-generation climate models. The added value of increased resolution in global or regional climate models can be also assessed more thoroughly based on dedicated model intercomparison projects (Sections 10.3.3 and 8.5.1). Ongoing research activities on decadal predictions and observational constraints are aimed at narrowing the plausible range of near-term (2021 – 2040) to long-term (2081 – 2100) water cycle changes.

This chapter assesses water cycle changes and considers climate change from the perspective of its effects on water availability (including streamflow and soil moisture, snow mass and glaciers, groundwater, wetlands and lakes) rather than only precipitation. The chapter highlights the sensitivity of the water cycle to multiple drivers and the complexity of its responses, depending on regions, seasons and time scales. Anthropogenic drivers include not only emissions of GHGs but also different species of aerosols, land and water management practices. Emphasis is placed on assessing the full range of projections, including ‘low likelihood, high impact’ climate trajectories such as the potential for abrupt changes in the water cycle.

The chapter starts with theoretical evidence that link small-scale processes and drivers, as well as global energy budget and large-scale circulation constraints to physically-understood changes in the global water cycle ( [[#8.2|Section 8.2]] ). Observed and projected water cycle changes (Sections 8.3 and 8.4, respectively) are assessed in separate sections, but with a parallel structure to facilitate comparison of a specific topic across sections. Projections are primarily assessed on the basis of contrasted emissions scenarios to emphasize the water cycle response to mitigation. Unless otherwise specified, projected anomalies are estimated relative to the 1995 – 2014 baseline climatology and are assessed over 20-year time slices, 2021 – 2040, 2041 – 2060 and 2081 – 2100 for near-, mid- and long-term changes respectively. Beyond multi-model ensemble means, model response uncertainty, the influence of natural climate variability, and the potential non-linearities in the regional water cycle response are also considered ( [[#8.5|Section 8.5]] ). Low likelihood but physically plausible high-impact scenarios are also assessed, especially the potential for abrupt climate change ( [[#8.6|Section 8.6]] ). Final remarks about future studies on water cycle changes ( [[#8.7|Section 8.7]] ) are also provided, and the chapter addresses three frequently asked questions (FAQs) on the water cycle’s sensitivity to land use change (FAQ 8.1), the projected occurrence and severity of floods (FAQ 8.2) and droughts (FAQ 8.3) at the global scale. This chapter outline is summarized with a schematic (Figure 8.2) which also provides a quick guide to the main topics addressed across the different sections.

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[[File:fa5402dc21850f037dc2aef444358485 IPCC_AR6_WGI_Figure_8_2.png]]

'''Figure 8.2 |''' '''Visual guide to Chapter 8.'''

( [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-8 Chapter 8] has multiple links across all AR6 WGI chapters, so necessarily includes references to other chapter subsections and figures. Model evaluation of large-scale circulation and precipitation is mostly covered by Chapter 3, while hydrological extremes are covered by Chapter 11. [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-8 Chapter 8] focuses on key processes relevant to the water cycle and their resolution-dependent representation in models. Observed and projected changes in large-scale circulation and precipitation are primarily assessed in Chapters 2, 3 and 4. Beyond global and regional mean precipitation amounts, [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-8 Chapter 8] also focuses on other precipitation properties (e.g., frequency, intensity and seasonality) and other water cycle variables (evapotranspiration, runoff, soil moisture and aridity, solid and liquid freshwater reservoirs). Key regional phenomena (e.g., tropical overturning circulations, monsoons, extratropical stationary waves and storm tracks, modes of variability and related teleconnections) are also assessed given their major dynamical contribution to regional water cycle changes. Although the biosphere and the cryosphere are key components of the water cycle, a more comprehensive assessment of their responses can be found in Chapters 5 and 9, respectively. Further assessment on regional water cycle changes can be found in Chapters 10 to 12 and in the Atlas. The reader is also referred to the interactive ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] for a more detailed assessment of the range of model biases and responses at the regional scale. Beyond WGI, water is also a major topic for both adaptation and mitigation policies so has strong connections with both WGII and WGIII. Assessment of hydrological impacts at basin and catchment scales, including a broader discussion on adaptation and vulnerability, potential threats to water security, societal responses, improving resilience in water systems and related case studies is provided in WGII (Chapter 4).

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