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

=== CCP3.3.1 Projected Changes and Risks in Natural Systems ===

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==== CCP3.3.1.1 Temperature ====

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Globally, warming rates have been twice as high in drylands as in humid lands, because the sparse vegetation cover and lower soil moisture of dryland ecosystems amplify temperature and aridity increases ( [[#Huang--2016|Huang et al., 2016]] ). This enhanced warming is expected to continue in the future. Surface warming over drylands is projected to reach ~6.5°C (~3.5°C) under the high Representative Concentration Pathway (RCP) 8.5 (low-moderate RCP4.5) emissions scenario by the end of this century, relative to the historical period (1961–1990) ( [[#Huang--2016|Huang et al., 2016]] ; [[#Huang--2017|Huang et al., 2017]] ). Exploring the spatial variations between the aeolian desertification response in selected climate change scenarios, [[#Wang--2017|Wang et al. (2017)]] reported that temperature rise could trigger aeolian desertification in West Asia, Central China and Mongolia. The number of extremely hot days with temperatures above 40°C is projected to increase considerably across the Arab region by the end of the 21st century ( [[#ESCWA--2017|ESCWA, 2017]] ).

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==== CCP3.3.1.2 Rainfall, Evaporation and Drought ====

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Drylands are highly sensitive to changes in precipitation and evapotranspiration. Potential evapotranspiration (PET) is projected to increase in all regions globally, under all RCPs, as a result of increasing temperatures and surface water vapour deficit ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ). Simulations based on coupled land surface, energy, and water and vegetation models in the Central Sahel showed a strong response of the water budget. Under +2°C and +4°C warming scenarios, decreased evapotranspiration, runoff and drainage were found for all scenarios, except those with the highest precipitation ( [[#Léauthaud--2015|Léauthaud et al., 2015]] ).

Globally, soil moisture declined over the 20th century ( [[#Gu--2019|Gu et al., 2019]] ), a trend that is projected to continue under all emissions scenarios (WGI). Projected drier soils can further amplify aridity through feedbacks with land surface temperature, relative humidity and precipitation ( [[#Berg--2016|Berg et al., 2016]] ).

Drought conditions (frequency, severity and duration) are expected to substantially worsen in global drylands, driven by a higher saturation threshold and more intense and frequent dry spells under rising temperatures ( [[#Liu--2019|Liu et al., 2019]] a; 2019b). In a +1.5°C world, historical 50-year droughts (based on the Standardised Precipitation-Evapotranspiration Index (SPEI)) could occur twice as frequently across 58% of global landmasses relative to the 1976–2005 period, an area that increases to 67% under 2°C warming ( [[#Gu--2020|Gu et al., 2020]] ). Multi-year drought events of magnitudes exceeding historical baselines will increase by 2050 in countries with drylands including Australia, Brazil, Spain, Portugal and the USA (Jenkins and Warren, 2015). The magnitude of drought stress in different regions differs depending on the metric used. Projections based on the PDSI suggest drought stress will increase by more than 70% globally, while a substantially lower estimate of 37% is found when precipitation minus evapotranspiration (P – E) is used ( [[#Swann--2016|Swann et al., 2016]] ). However, the two metrics agree on increasing drought stress in regions with more robust decreases in precipitation, such as southern North America, northeastern South America ( [[IPCC:Wg2:Chapter:Chapter-12#12.3.1.1|Section 12.3.1.1]] ) and southern Europe ( [[IPCC:Wg2:Chapter:Chapter-13#13.1.3|Section 13.1.3]] ; [[#Swann--2016|Swann et al., 2016]] ).

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==== 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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==== CCP3.3.1.4 Dryland Extent ====

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Global dryland area (based on the AI) is projected to expand by ~10% by 2100 compared to 1961–1990 under a high emission scenario ( [[#Feng--2013|Feng and Fu, 2013]] ). However, there are significant regional differences in the drivers of dryland expansion and subsequent estimates of change in dryland extent. Subtropical drylands are projected to expand as the climate in these regions shifts from temperate to subtropical and aridity increases in currently sub-humid subtropical regions, resulting in the loss of temperature-controlled seasonal cycles (Schlaepfer et al., 2017). Observed and projected warming and drying trends are most severe in transitional climate regions between dry and wet climates, with some exceptions ( [[#Nkrumah--2019|Nkrumah et al., 2019]] ), which are often highly populated agricultural regions with fragile ecosystems ( [[#Cheng--2016|Cheng and Huang, 2016]] ). In contrast, P – E predicts decreasing drought stress across temperate Asia and central Africa ( [[#Swann--2016|Swann et al., 2016]] ). Expansion of arid regions is anticipated in southwest North America, the northern fringe of Africa, southern Africa and Australia. The main areas of semiarid expansion are expected to occur in the north side of the Mediterranean, southern Africa, and North and South America. In contrast, India, eastern equatorial Africa and other areas of the southern Saharan regions are projected to have shrinking drylands ( [[#Biasutti--2006|Biasutti and Giannini, 2006]] ; [[#Biasutti--2013|Biasutti, 2013]] ; [[#Rowell--2016|Rowell et al., 2016]] ). Future projections may underestimate dryland expansion, since the Coupled Model Intercomparison Project 5 (CMIP5) models underestimate historical warming ( [[#Huang--2016|Huang et al., 2016]] ) and overestimate precipitation over drylands, particularly in the semiarid and dry sub-humid regions ( [[#Ji--2015|Ji et al., 2015]] ). However, estimates vary depending on the metric used ( [[#Swann--2016|Swann et al., 2016]] ; [[#Berg--2017b|Berg et al., 2017b]] ). Studies based on off-line aridity and drought metrics (calculated from model output of precipitation, evapotranspiration or temperature) project strong surface drying trends ( [[#Cook--2014|Cook et al., 2014]] ; [[#Scheff--2015|Scheff and Frierson, 2015]] ; [[#Zhao--2015|Zhao and Dai, 2015]] ), while projections based on total soil water availability from CMIP5 models show weaker and less extensive drying ( [[#Berg--2017a|Berg et al., 2017a]] ). In contrast, projections in southern Africa may overestimate future drying, with systematic rainfall biases being found in the present-day climatology in models that simulate extreme future drying ( [[#Munday--2019|Munday and Washington, 2019]] ). Improvements in projections of future changes in aridity require better understanding of seasonality, land hydrology and the feedbacks between projected soil moisture decrease on land surface temperature, relative humidity and precipitation ( [[#Huang--2016|Huang et al., 2016]] ).

Higher dust emissions are consistent with climate change projections indicating an expansion in the global area of drylands ( [[#Feng--2013|Feng and Fu, 2013]] ; [[#Huang--2016|Huang et al., 2016]] ) and increased drought risk ( [[#Cook--2014|Cook et al., 2014]] ; [[#Xu--2019|Xu et al., 2019]] ), but future trends in dust event frequency and intensity as a result of climate change are uncertain and will vary geographically (Jia, 2019). Combined effects of climate change and anthropogenic activities are projected to increase sand encroachment and extreme dust storms (Omar [[#Asem--2010|Asem and Roy, 2010]] ; [[#Sharratt--2015|Sharratt et al., 2015]] ; [[#Pu--2017|Pu and Ginoux, 2017]] ) as a result of increased aridity, accelerating soil erosion ( [[IPCC:Wg2:Chapter:Chapter-4#4.4.8|Section 4.4.8]] ; [[#Sharratt--2015|Sharratt et al., 2015]] ) and loss of biomass ( [[#Sharratt--2015|Sharratt et al., 2015]] ; [[#Middleton--2017|Middleton and Kang, 2017]] ). Shifts in dust storm timings are also projected in some regions (Hand et al., 2016). Dustiness is projected to increase in the southern US Great Plains in the late 21st century under the RCP8.5 climate change scenario but decrease over the northern Great Plains ( [[#Pu--2017|Pu and Ginoux, 2017]] ). A declining trend in dust emission and transport from the Sahara under RCP8.5 was detected by Evan et al. (2016), but regional climate model experiments conducted by [[#Ji--2018|Ji et al. (2018)]] under the same scenario indicated that overall dust loadings would increase by the end of the 21st century over West Africa. New dust sources may emerge with changing climate conditions, as [[#Bhattachan--2012|Bhattachan et al. (2012)]] indicate for the Kalahari Desert in southern Africa, due to vegetation loss and dune remobilisation. There is overall ''low confidence'' on future atmospheric dust loads at the global and regional scale. Models of future dust emissions are limited by the low accuracy of models of present anthropogenic dust emissions, which range from 10% to 60% of the total atmospheric dust load ( [[#Webb--2018|Webb and Pierre, 2018]] ). A global compilation of data from sedimentary archives (ice cores), remote sensing, airborne sediment sampling and meteorological station data estimated that anthropogenic dust emissions have at least doubled over the past 250 years ( [[#Hooper--2018|Hooper and Marx, 2018]] ). While future emissions of natural dust sources are projected to decrease (Mahowald et al., 2006) or remain stable ( [[#Ashkenazy--2012|Ashkenazy et al., 2012]] ), when sources of human emissions are included, projections of future atmospheric dust loads suggest that emissions may increase ( [[#Stanelle--2014|Stanelle et al., 2014]] ).

The relative contribution of albedo and evapotranspiration to regional trends in surface temperature ( [[#Charney--1975|Charney, 1975]] ) remains unresolved, and may be determined by different mechanisms in different systems, depending on site-specific conditions such as snow coverage, vegetation and soil moisture ( [[#Yu--2017|Yu et al., 2017]] ). For example, the vegetation–albedo feedback mechanism may dominate in the Arctic ( [[#Blok--2011|Blok et al., 2011]] ; [[#te%20Beest--2016|te Beest et al., 2016]] ), while the vegetation–evaporation feedback may drive change in other regions. Actions that increase forest cover across Africa could thus, theoretically, moderate projected future temperature increases ( [[#Wu--2016|Wu et al., 2016]] ; [[#Diba--2018|Diba et al., 2018]] ), but with potentially negative effects on biodiversity (Chapter 2). Soil drying exacerbates atmospheric aridity, which causes more soil drying in a self-reinforcing land–atmosphere feedback that could intensify under RCP8.5 ( [[#Zhou--2019|Zhou et al., 2019]] ).

Changes to the composition, structure and functioning of natural communities in deserts and dryland ecosystems are key risks resulting from water stress, drought intensity and continued habitat degradation, greater frequency of wildfire, biodiversity loss and the spread of invasive species ( [[#Hurlbert--2019|Hurlbert et al., 2019]] ). Not all these stresses occur at the same time in a particular environment, with some areas more exposed to, for example, wildfire than others, especially in areas with high amounts of dry herbaceous biomass. Grassland composition may shift as C3 plants are replaced by C4 species, which have higher optimal temperatures and higher water use efficiency (although seasonality of precipitation also plays a role) (Knapp et al., 2020). Many desert species have morphological, physiological and/or behavioural adaptations to cope with climatic extremes, including rapid regeneration following droughts ( [[#Boudet--1977|Boudet, 1977]] ; [[#Hiernaux--2006|Hiernaux and Le Houérou, 2006]] ), leaf dropping during the dry season to reduce water loss ( [[#Santos--2014|Santos et al., 2014]] ), alongside long histories of adaptation to climate change ( [[#Brooks--2005|Brooks et al., 2005]] ; [[#Ballouche--2007|Ballouche and Rasse, 2007]] ), while many animals live near their physiological limits ( [[#Vale--2015|Vale and Brito, 2015]] ). Substantial ecological effects may occur when extreme events such as heatwaves or droughts are superimposed on the warming trend, pushing species beyond their physiological and mortality thresholds ( [[#Hoover--2015|Hoover et al., 2015]] ; [[#Harris--2018|Harris et al., 2018]] ).

Climate change increases risks of continued range retractions of Karoo succulents in South Africa (Young et al., 2016), dry argan woodlands in Morocco ( [[#Alba-Sánchez--2015|Alba-Sánchez et al., 2015]] ), epiphytic cacti in Brazil ( [[#Cavalcante--2019|Cavalcante and Duarte, 2019]] ; [[#Cavalcante--2020|Cavalcante et al., 2020]] ) and other plant species exposed to higher aridity. Projected increases in heat and aridity could increase mortality of trees and shrubs in Sonoran Desert ecosystems in the USA ( [[#Munson--2012|Munson et al., 2012]] ; 2016b), reduce sagebrush in arid ecosystems of the western USA ( [[#Renwick--2018|Renwick et al., 2018]] ), and contribute to the replacement of perennial grasses with xeric shrubs in the southwestern USA ( [[#Bestelmeyer--2018|Bestelmeyer et al., 2018]] ). CO 2 fertilization and warmer conditions, combined with changes in timing and availability of moisture, could increase invasive grasses and wildfire in desert ecosystems of Australia and southwestern USA, where wildfire has historically been absent or infrequent ( [[#Abatzoglou--2011|Abatzoglou and Kolden, 2011]] ; Horn and St. Clair, 2017; [[#Klinger--2017|Klinger and Brooks, 2017]] ; [[#Syphard--2017|Syphard et al., 2017]] ). Trends of woody encroachment may continue in some North American and African drylands or at least not reverse ( [[#Higgins--2012|Higgins and Scheiter, 2012]] ; [[#Caracciolo--2016|Caracciolo et al., 2016]] ). Impacts of woody encroachment on drylands may show a slight increase in carbon, but a decline in water and huge negative impacts on biodiversity, with a tendency for open ecosystem species to be most affected ( [[#Archer--2017|Archer et al., 2017]] ). Expansion of grasses into these arid shrublands has the potential to transform them rapidly, especially through the acceleration of the fire cycle ( [[#Bradley--2016|Bradley et al., 2016]] ). While the impact of increased aridity may be offset by changing water use efficiency by plants under high CO 2 concentrations, limiting the expansion of dryland ecosystems ( [[#Swann--2016|Swann et al., 2016]] ; [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ), increased plant growth in response to elevated CO 2 , which results in increased water consumption, may counteract this. Increased water use efficiency is therefore not expected to counterbalance increased evaporative demand (Chapter 8). There is ''medium confidence'' that succulent species will be particularly vulnerable to increased heat and aridity due to reduced physiological performance, loss of seed banks, lower germination rates and increased mortality (Table CCP3.1; [[#Musil--2005|Musil et al., 2005]] ; [[#Aragón-Gastélum--2014|Aragón-Gastélum et al., 2014]] ; 2017; [[#Shryock--2014|Shryock et al., 2014]] ; [[#Martorell--2015|Martorell et al., 2015]] ; [[#Carrillo-Angeles--2016|Carrillo-Angeles et al., 2016]] ; [[#Koźmińska--2019|Koźmińska et al., 2019]] ).

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