Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.6.2 Observed Trends ===

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Evidence on observed drought trends was limited at the time of SREX (Chapter 3) and AR5 (Chapter 2). The SREX concluded: ‘There is ''medium confidence'' that since the 1950s some regions of the world have experienced a trend to more intense and longer droughts, in particular in southern Europe and west Africa, but in some regions droughts have become less frequent, less intense, or shorter, for example, in Central North America and north-western Australia.’ The assessment at the time did not distinguish between different drought types. This Chapter includes numerous updates on observed drought trends, associated with extensive new literature and longer datasets since AR5.

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==== 11.6.2.1 Precipitation Deficits ====

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Strong precipitation deficits have been recorded in recent decades in the Amazon (2005, 2010), south-western China (2009–2010), south-western North America (2011–2014), Australia (1997–2009), California (2014), the middle East (2012–2016), Chile (2010–2015), the Great Horn of Africa (2011), among others ( [[#van%20Dijk--2013|van Dijk et al., 2013]] ; [[#Mann--2015|Mann and Gleick, 2015]] ; [[#Rowell--2015|Rowell et al., 2015]] ; [[#Marengo--2016|Marengo and Espinoza, 2016]] ; [[#Dai--2017|Dai and Zhao, 2017]] ; [[#Garreaud--2017|Garreaud et al., 2017]] , 2020; [[#Marengo--2017|Marengo et al., 2017]] ; [[#Brito--2018|Brito et al., 2018]] ; [[#Cook--2018|Cook et al., 2018]] ). Global studies generally show no significant trends in SPI time series ( [[#Orlowsky--2013|Orlowsky and Seneviratne, 2013]] ; [[#Spinoni--2014|Spinoni et al., 2014]] ), and in derived drought frequency and severity data ( [[#Spinoni--2019|Spinoni et al., 2019]] ), with very few regional exceptions ( [[#11.9|Section 11.9]] and Figure 11.17). Long-term decreases in precipitation are found in some AR6 regions in Africa (Central Africa and East Southern Africa), and several regions in South America (North-Eastern South America, South American Monsoon, South-Western South America, and Southern South America) ( [[#11.9|Section 11.9]] ). Evidence of precipitation-based drying trends is also found in Western Africa, consistent with studies based on CDD trends (Figure 11.17; [[#Chaney--2014|Chaney et al., 2014]] ; [[#Donat--2014b|Donat et al., 2014b]] ; [[#Barry--2018|Barry et al., 2018]] ; [[#Dunn--2020|Dunn et al., 2020]] ), however, there is a partial recovery of the rainfall trends since the 1980s in this region ( [[IPCC:Wg1:Chapter:Chapter-10#10.4.2.1|Section 10.4.2.1]] ). Some AR6 regions show a decrease in meteorological drought, including Northern Australia, Central Australia, Northern Europe and Central North America ( [[#11.9|Section 11.9]] ). Other regions either do not show substantial trends in long-term meteorological drought, or they display mixed signals depending on the considered time frame and sub-regions, such as in Southern Australia ( [[#Gallant--2013|Gallant et al., 2013]] ; [[#Delworth--2014|Delworth and Zeng, 2014]] ; [[#Alexander--2017|Alexander and Arblaster, 2017]] ; [[#Spinoni--2019|Spinoni et al., 2019]] ; [[#Dunn--2020|Dunn et al., 2020]] ; [[#Rauniyar--2020|Rauniyar and Power, 2020]] ) and the Mediterranean ( [[#Camuffo--2013|Camuffo et al., 2013]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ; [[#Spinoni--2017|Spinoni et al., 2017]] ; [[#Stagge--2017|Stagge et al., 2017]] ; [[#Caloiero--2018|Caloiero et al., 2018]] ; [[#Peña-Angulo--2020b|Peña-Angulo et al., 2020b]] ; see also [[#11.9|Section 11.9]] and Atlas.8.2).

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[[File:a97af5c3786b1cd60461310b16197a9a IPCC_AR6_WGI_Figure_11_17.png]]

'''Figure 11.17 |''' '''Observed linear trend for (a) consecutive dry days (CDD) during 1960–2018, (b) standardized precipitation index (SPI) and (c) standardized precipitation-evapotranspiration index (SPEI) dur''' ing 1951–2016. CDD data are from the HadEx3 dataset ( [[#Dunn--2020|Dunn et al., 2020]] ), trend calculation of CDD as in Figure 11.9. Drought severity is estimated using 12-month SPI (SPI-12) and 12-month SPEI (SPEI-12). SPI and SPEI datasets are from [[#Spinoni--2019|Spinoni et al. (2019)]] . The threshold to identify drought episodes was set at -1 SPI/SPEI units. Areas without sufficient data are shown in grey. No overlay indicates regions where the trends are significant at the p = 0.1 level. Crosses indicate regions where trends are not significant. For details on the methods see Supplementary Material 11.SM.2. Further details on data sources and processing are available in the chapter data table (Table 11.SM.9).

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==== 11.6.2.2 Atmospheric Evaporative Demand ====

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In several regions, AED increases have intensified recent drought events ( [[#Williams--2014|Williams et al., 2014]] , 2020; [[#Seager--2015b|Seager et al., 2015b]] ; [[#Basara--2019|Basara et al., 2019]] ; [[#García-Herrera--2019|García-Herrera et al., 2019]] ), enhanced vegetation stress ( [[#Allen--2015|Allen et al., 2015]] ; [[#Sanginés%20de%20Cárcer--2018|Sanginés de Cárcer et al., 2018]] ; [[#Yuan--2019|Yuan et al., 2019]] ), or contributed to the depletion of soil moisture or runoff through enhanced ET ( ''high confidence'' ) ( [[#Teuling--2013|Teuling et al., 2013]] ; [[#Padrón--2020|Padrón et al., 2020]] ). Trends in pan evaporation measurements and Penman-Monteith AED estimates provide an indication of possible trends in the influence of AED on drought. Given the observed global temperature increases (Sections 2.3.1.1 and 11.3) and dominant decrease in relative humidity over land areas ( [[#Simmons--2010|Simmons et al., 2010]] ; [[#Willett--2014|Willett et al., 2014]] ), VPD has increased globally ( [[#Barkhordarian--2019|Barkhordarian et al., 2019]] ; [[#Yuan--2019|Yuan et al., 2019]] ). Pan evaporation has increased as a consequence of VPD changes in several AR6 regions, such as East Asia ( [[#Li--2013|Li et al., 2013]] ; Z. [[#Sun--2018|Sun et al., 2018]] ; M.-Z. [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|Yang et al., 2018]] ), Western and Central Europe ( [[#Mozny--2020|Mozny et al., 2020]] ), the Mediterranean, ( [[#Azorin-Molina--2015|Azorin-Molina et al., 2015]] ) and Central and Southern Australia ( [[#Stephens--2018|Stephens et al., 2018]] ). Nevertheless, there is an important regional variability in observed trends, and in other AR6 regions pan evaporation has decreased – for example, in North Central America ( [[#Breña-Naranjo--2017|Breña-Naranjo et al., 2017]] ) and in the Tibetan Plateau ( [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|C. Zhang et al., 2018]] )). Physical models also show an important regional diversity, with an increase in New Zealand ( [[#Salinger--2014|Salinger and Porteous, 2014]] ) and the Mediterranean ( [[#Gocic--2014|Gocic and Trajkovic, 2014]] ; [[#Azorin-Molina--2015|Azorin-Molina et al., 2015]] ; [[#Piticar--2016|Piticar et al., 2016]] ), a decrease in South Asia ( [[#Jhajharia--2015|Jhajharia et al., 2015]] ), and strong spatial variability in North America ( [[#Seager--2015b|Seager et al., 2015b]] ). This variability is driven by the role of other meteorological variables affecting AED. Changes in solar radiation as a consequence of solar dimming and brightening may affect trends ( [[IPCC:Wg1:Chapter:Chapter-7#7.2.2.2|Section 7.2.2.2]] ; [[#Kambezidis--2012|Kambezidis et al., 2012]] ; [[#Wang--2014|Wang and Yang, 2014]] ; [[#Sanchez-Lorenzo--2015|Sanchez-Lorenzo et al., 2015]] ). Wind speed is also relevant ( [[#McVicar--2012b|McVicar et al., 2012b]] ), and studies suggest a reduction of the wind speed in some regions (Z. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] b) that could compensate the role of the VPD increase. Nevertheless, the VPD trend seems to dominate the overall AED trends, compared to the effects of trends in wind speed and solar radiation ( [[#Wang--2012|Wang et al., 2012]] ; [[#Park%20Williams--2017|Park Williams et al., 2017]] ; [[#Vicente-Serrano--2020a|Vicente-Serrano et al., 2020a]] ).

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==== 11.6.2.3 Soil Moisture Deficits ====

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There are limited long-term measurements of soil moisture from ground observations ( [[#Dorigo--2011|Dorigo et al., 2011]] ; [[#Qiu--2016|Qiu et al., 2016]] ; [[#Quiring--2016|Quiring et al., 2016]] ), which impedes their use in the analysis of trends. Among the few existing observational studies covering at least two decades, several studies have investigated trends in ground soil moisture in East Asia ( [[#11.9|Section 11.9]] ; [[#Chen--2015b|Chen and Sun, 2015b]] ; [[#Liu--2015|Liu et al., 2015]] ; [[#Qiu--2016|Qiu et al., 2016]] ). Alternatively, microwave-based satellite measurements of surface soil moisture have also been used to analyse trends ( [[#Dorigo--2012|Dorigo et al., 2012]] ; [[#Jia--2018|Jia et al., 2018]] ). Although there is regional evidence that microwave-based soil moisture estimates can capture well drying trends in comparison with ground soil moisture observations ( [[#Jia--2018|Jia et al., 2018]] ), there is only ''medium confidence'' in the derived trends, since satellite soil moisture data are affected by inhomogeneities ( [[#Dorigo--2015|Dorigo et al., 2015]] ; [[#Rodell--2018|Rodell et al., 2018]] ; [[#Preimesberger--2021|Preimesberger et al., 2021]] ). Furthermore, microwave-based satellites only sense surface soil moisture, which differs from root-zone soil moisture ( [[#Berg--2017a|Berg et al., 2017a]] ), although relationships can be derived between the two ( [[#Brocca--2011|Brocca et al., 2011]] ). Several studies have also analysed long-term soil moisture time series from observation-driven land-surface or hydrological models, including land-based reanalysis products ( [[#Albergel--2013|Albergel et al., 2013]] ; [[#Jia--2018|Jia et al., 2018]] ; [[#Gu--2019b|Gu et al., 2019b]] ; [[#Markonis--2021|Markonis et al., 2021]] ). Such models have also been used to assess changes in land water availability, estimated as precipitation minus ET, which is equal to the sum of soil moisture and runoff ( [[#Greve--2014|Greve et al., 2014]] ; [[#Padrón--2020|Padrón et al., 2020]] ).

Overall, evidence from global studies suggests that several land regions have been affected by increased soil moisture drying or water balance drying in past decades, despite some spread among products ( [[#Albergel--2013|Albergel et al., 2013]] ; [[#Greve--2014|Greve et al., 2014]] ; [[#Gu--2019b|Gu et al., 2019b]] ; [[#Padrón--2020|Padrón et al., 2020]] ). Drying has not only occurred in dry regions but also in humid regions ( [[#Greve--2014|Greve et al., 2014]] ). Some studies have specifically addressed changes in soil moisture at regional scale ( [[#11.9|Section 11.9]] ). For AR6 regions, several studies suggest an increase in the frequency and areal extent of soil moisture deficits, with examples in East Asia ( [[#Cheng--2015|Cheng et al., 2015]] ; Y. [[#Qin--2015|]] [[#Qin--2015|Qin et al., 2015]] ; [[#Jia--2018|Jia et al., 2018]] ), Western and Central Europe ( [[#Trnka--2015b|Trnka et al., 2015b]] ), and the Mediterranean ( [[#Hanel--2018|Hanel et al., 2018]] ; [[#Moravec--2019|Moravec et al., 2019]] ; [[#Markonis--2021|Markonis et al., 2021]] ). Nonetheless, some analyses also show no long-term trends in soil drying in some AR6 regions – for example, in Eastern North America ( [[#Park%20Williams--2017|Park Williams et al., 2017]] ) and Central North America ( [[#Seager--2019|Seager et al., 2019]] ), as well as in North Eastern Africa ( [[#Kew--2021|Kew et al., 2021]] ). The soil moisture drying trends identified in both global and regional studies are generally related to increases in ET (associated with higher AED) rather than decreases in precipitation, as identified on global land for trends in water balance in the dry season ( [[#Padrón--2020|Padrón et al., 2020]] ), as well as for some regions ( [[#Teuling--2013|Teuling et al., 2013]] ; [[#Cheng--2015|Cheng et al., 2015]] ; [[#Trnka--2015a|Trnka et al., 2015a]] ; [[#van%20Der%20Linden--2019|van Der Linden et al., 2019]] ; X. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ).

Evidence from observed or observations-derived trends in soil moisture and precipitation minus ET, are combined with evidence from SPEI and PDSI-PM studies to derive regional assessments of changes in agricultural and ecological droughts ( [[#11.9|Section 11.9]] ). This assessment is summarized in [[#11.6.2.6|Section 11.6.2.6]] .

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==== 11.6.2.4 Hydrological Deficits ====

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There is evidence based on streamflow records of increased hydrological droughts in East Asia (D. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ) and southern Africa ( [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ). In areas of Western and Central Europe and Northern Europe, there is no evidence of changes in the severity of hydrological droughts since 1950 based on flow reconstructions ( [[#Caillouet--2017|Caillouet et al., 2017]] ; [[#Barker--2019|Barker et al., 2019]] ) and observations ( [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ). In the Mediterranean region, there is ''high confidence'' in hydrological drought intensification ( [[#11.9|Section 11.9]] ; [[#Giuntoli--2013|Giuntoli et al., 2013]] ; [[#Lorenzo-Lacruz--2013|Lorenzo-Lacruz et al., 2013]] ; [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ). In south-eastern South America there is a decrease in the severity of hydrological droughts ( [[#Rivera--2018|Rivera and Penalba, 2018]] ). In North America, depending on the methods, datasets and study periods, there are differences between studies that suggest an increase ( [[#Shukla--2015|Shukla et al., 2015]] ; [[#Udall--2017|Udall and Overpeck, 2017]] ) versus a decrease in hydrological drought frequency ( [[#Mo--2018|Mo and Lettenmaier, 2018]] ), but in general there is strong spatial variability ( [[#Poshtiri--2016|Poshtiri and Pal, 2016]] ). Streamflow observation reference networks of near-natural catchments have also been used to isolate the effect of climate trends on hydrological drought trends in a few regions, but these show limited trends in Northern Europe and Western and Central Europe ( [[#Stahl--2010|Stahl et al., 2010]] ; [[#Bard--2015|Bard et al., 2015]] ; [[#Harrigan--2018|Harrigan et al., 2018]] ), North America ( [[#Dudley--2020|Dudley et al., 2020]] ) and most of Australia, with the exception of Eastern and Southern Australia (X.S. [[#Zhang--2016|Zhang et al., 2016]] ). Given the low availability of observations, there are few studies analysing trends of drought severity in the groundwater. Nevertheless, some studies suggest a noticeable response of groundwater droughts to climate variability ( [[#Lorenzo-Lacruz--2017|Lorenzo-Lacruz et al., 2017]] ) and increased drought frequency and severity associated with warming, probably as a consequence of enhanced ET induced by higher AED ( [[#Maxwell--2016|Maxwell and Condon, 2016]] ). This is supported by studies in Northern Europe ( [[#Bloomfield--2019|Bloomfield et al., 2019]] ) and North America ( [[#Condon--2020|Condon et al., 2020]] ).

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==== 11.6.2.5 Atmospheric-based Drought Indices ====

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Globally, trends in SPEI-PM and PDSI-PM suggest slightly higher increases of drought frequency and severity in regions affected by drying over the last decades in comparison to the SPI ( [[#Dai--2017|Dai and Zhao, 2017]] ; [[#Spinoni--2019|Spinoni et al., 2019]] ; [[#Song--2020|Song et al., 2020]] ), mainly in regions of Western and Southern Africa, the Mediterranean and East Asia (Figure 11.17), which is consistent with observed soil moisture trends ( [[#11.6.2.3|Section 11.6.2.3]] ). These indices suggest that AED has contributed to increase the severity of agricultural and ecological droughts compared to meteorological droughts ( [[#García-Herrera--2019|García-Herrera et al., 2019]] ; [[#Williams--2020|Williams et al., 2020]] ), reduce soil moisture during the dry season ( [[#Padrón--2020|Padrón et al., 2020]] ), increase plant water stress ( [[#Allen--2015|Allen et al., 2015]] ; [[#Grossiord--2020|Grossiord et al., 2020]] ; [[#Solander--2020|Solander et al., 2020]] ) and trigger more severe forest fires ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ; [[#Turco--2019|Turco et al., 2019]] ; [[#Nolan--2020|Nolan et al., 2020]] ). A number of regional studies based on these drought indices have also shown stronger drying trends in comparison to trends in precipitation-based indices in the following AR6 regions (see also [[#11.9|Section 11.9]] ): NSA (R. [[#Fu--2013|]] [[#Fu--2013|Fu et al., 2013]] ; [[#Marengo--2016|Marengo and Espinoza, 2016]] ), SCA ( [[#Hidalgo--2017|Hidalgo et al., 2017]] ), WCA ( [[#Tabari--2013|Tabari and Aghajanloo, 2013]] ; [[#Sharafati--2020|Sharafati et al., 2020]] ), SAS ( [[#Niranjan%20Kumar--2013|Niranjan Kumar et al., 2013]] ), NEAF ( [[#Zeleke--2017|Zeleke et al., 2017]] ), WSAF ( [[#Edossa--2016|Edossa et al., 2016]] ), NWN and NEN ( [[#Bonsal--2013|Bonsal et al., 2013]] ), EAS ( [[#Yu--2014|Yu et al., 2014]] ; [[#Chen--2015b|Chen and Sun, 2015b]] ; L. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ; [[#Liang--2020|Liang et al., 2020]] ; Z. [[#Wu--2020|]] [[#Wu--2020|Wu et al., 2020]] ) and MED ( [[#Kelley--2015|Kelley et al., 2015]] ; [[#Stagge--2017|Stagge et al., 2017]] ; [[#González-Hidalgo--2018|González-Hidalgo et al., 2018]] ; [[#Mathbout--2018a|Mathbout et al., 2018a]] ).

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==== 11.6.2.6 Synthesis for Different Drought Types ====

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Few AR6 regions show observed increases in meteorological drought ( [[#11.9|Section 11.9]] ), mostly in Africa and South America (NES: ''high confidence'' ; WAF, CAF, ESAF, SAM, SWS, SSA, SAS: ''medium confidence'' ); a few others show a decrease (WSB, ESB, NAU, CAU, NEU, CNA: ''medium confidence'' ). There are stronger signals indicating observed increases in agricultural and ecological drought ( [[#11.9|Section 11.9]] ), which highlights the role of increased ET, driven by increased AED, for these trends (Sections 11.6.2.3 and11.6.2.5). Past increases in agricultural and ecological droughts are found on all continents and several regions (WAF, CAF, WSAF, ESAF, WCA, ECA, EAS, SAU, MED, WCE, NES: ''medium confidence'' ), while decreases are found only in one AR6 region (NAU: ''medium confidence'' ). The more limited availability of datasets makes it more difficult to assess historical trends in hydrological drought at regional scale ( [[#11.9|Section 11.9]] ). Increasing (MED: ''high confidence'' ; WAF, EAS, SAU: ''medium confidence'' ) and decreasing (NEU, SES: ''medium confidence'' ) trends in hydrological droughts have only been observed in a few regions.

In summary, there is ''high confidence'' that AED has increased on average on continents, contributing to increased ET and resulting water stress during periods with precipitation deficits, in particular during dry seasons. There is ''medium confidence'' in increases in precipitation deficits in a few regions of Africa and South America. Based on multiple evidence, there is ''medium confidence'' that agricultural and ecological droughts have increased in several regions on all continents (WAF, CAF, WSAF, ESAF, WCA, ECA, EAS, SAU, MED, WCE, NES: ''medium confidence'' ), while there is only ''medium confidence'' in decreases in one AR6 region (NAU). More severe hydrological droughts are found in fewer regions (MED: ''high confidence'' ; WAF, EAS, SAU: ''mediu'' ''m confidence'' ).

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