Editing IPCC:AR6/WGII/Cross-Chapter-Paper-3 (section)

==== CCP3.3.1.3 Aridity ====

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Studies based on the AI (the ratio of annual potential evapotranspiration to precipitation), almost always project conditions of increasing aridity under climate change, and associated widespread expansion of drylands ( [[#Huang--2016|Huang et al., 2016]] ). The limitations of the AI are widely reported ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), with alternative indices that consider different variables, including the Ecohydrological Index, PDSI, Standardised Precipitation Index and SPEI ( [[#Stringer--2021|Stringer et al., 2021]] ). AI projections indicate potentially severe aridification in the Amazon, Australia, Chile, the Mediterranean region, northern, southern and western Africa, southwestern USA and South America ( ''medium confidence'' ) ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Greve--2015|Greve and Seneviratne, 2015]] ; [[#Jones--2016|Jones and Gutzler, 2016]] ; [[#Park--2018|Park et al., 2018]] ). However, the AI does not incorporate potential changes to plant transpiration under increasing CO 2 concentration and therefore overestimates drought conditions and aridity. Additionally, it does not reflect seasonality in rainfall and evapotranspiration, which is important in regions where temperature and actual evapotranspiration are not increasing during the wet season when vegetation growth is occurring. [[#Mirzabaev--2019|Mirzabaev et al. (2019)]] concluded that while aridity will increase in some places ( ''high confidence'' ), there is insufficient evidence to suggest a global change in dryland aridity ( ''medium confidence'' ). Nevertheless, a comparison of several metrics of aridity showed aridity increases for several hotspots such as the Mediterranean region and South Africa ( [[#Greve--2019|Greve et al., 2019]] ). Under RCP8.5, aridity zones could expand by one-quarter of the 1990 area by 2100, increasing to over half of the global terrestrial area ( [[#Huang--2016|Huang et al., 2016]] ; [[#Lickley--2018|Lickley and Solomon, 2018]] ). Lower greenhouse gas emissions, under RCP4.5, could limit expansion to one-tenth of the 1990 area by 2100 ( [[#Huang--2016|Huang et al., 2016]] ). Aridity could expand substantially on all continents except Antarctica ( [[#Huang--2016|Huang et al., 2016]] ), with expansion first manifesting in the Mediterranean region, southern Africa, southern South America and western Australia ( [[#Lickley--2018|Lickley and Solomon, 2018]] ). In the Northern Hemisphere, aridity zones could expand poleward as much as 11° latitude (Rajaud and Noblet-Ducoudré, 2017). By 2100, the population of dryland areas could increase by 700 million people and, under RCP8.5, 3 billion people might live in areas with a 25% or greater increase in aridity ( [[#Lickley--2018|Lickley and Solomon, 2018]] ). Many studies point to an increasing dryland area based on the AI, but there is ''low agreement'' on the actual amount and area of change ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Scheff--2015|Scheff and Frierson, 2015]] ; [[#Huang--2017|Huang et al., 2017]] ). The inconsistency between studies is largely due to the substantial internal climate variability in regional precipitation. Changes in annual precipitation have been shown to range from −30% to 25% over drylands. Consistent changes in precipitation are only found at high latitudes, while total PET is projected to increase over most land areas ( [[#Feng--2013|Feng and Fu, 2013]] ). This leads to more consistent, widespread drying in the tropics, subtropics and mid-latitudes in most models ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Cook--2014|Cook et al., 2014]] ; [[#Scheff--2015|Scheff and Frierson, 2015]] ; [[#Zhao--2015|Zhao and Dai, 2015]] ).

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