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

==== 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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