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

=== 11.6.4 Detection and Attribution, Event Attribution ===

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==== 11.6.4.1 Precipitation Deficits ====

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There are only two AR6 regions where there is at least ''medium confidence'' that human-induced climate change has contributed to changes in meteorological droughts ( [[#11.9|Section 11.9]] ). In South-Western South America, there is ''medium confidence'' that human-induced climate change has contributed to an increase in meteorological droughts ( [[#Boisier--2016|Boisier et al., 2016]] ; [[#Garreaud--2020|Garreaud et al., 2020]] ), while in Northern Europe, there is ''medium confidence'' that it has contributed to a decrease in meteorological droughts ( [[#11.9|Section 11.9]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ). In other AR6 regions, there is inconclusive evidence in the attribution of long-term trends, but a human contribution to single meteorological events or sub-regional trends has been identified in some instances ( [[#11.9|Section 11.9]] ; see also below). In the Mediterranean region, some studies have identified a precipitation decline or increase in meteorological drought probability for time frames since the early or mid 20th century, and a possible human contribution to these trends ( [[#Hoerling--2012|Hoerling et al., 2012]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ; [[#Knutson--2018|Knutson and Zeng, 2018]] ), also on sub-regional scale in Syria from 1930 to 2010 ( [[#Kelley--2015|Kelley et al., 2015]] ). On the contrary, other studies have not identified precipitation and meteorological drought trends in the region for the long term ( [[#Camuffo--2013|Camuffo et al., 2013]] ; [[#Paulo--2016|Paulo et al., 2016]] ; [[#Vicente-Serrano--2021|Vicente-Serrano et al., 2021]] ) and also from the mid 20th century ( [[#Norrant--2006|Norrant and Douguédroit, 2006]] ; [[#Stagge--2017|Stagge et al., 2017]] ). There is evidence of substantial internal variability in long-term precipitation trends in the region ( [[#11.6.2.1|Section 11.6.2.1]] ), which limits the attribution of human influence on variability and trends of meteorological droughts from observational records ( [[#Kelley--2012|Kelley et al., 2012]] ; [[#Peña-Angulo--2020b|Peña-Angulo et al., 2020b]] ). In addition, there are important sub-regional trends showing mixed signals ( [[#11.9|Section 11.9]] ; [[#MedECC--2020|MedECC, 2020]] ). The evidence thus leads to an assessment of ''low confidence'' in the attribution of observed short-term changes in meteorological droughts in the region ( [[#11.9|Section 11.9]] ). In North America, the human influence on precipitation deficits is complex ( [[#Wehner--2017|Wehner et al., 2017]] ), with ''low confidence'' in the attribution of long-term changes in meteorological drought in AR6 regions ( [[#11.9|Section 11.9]] ; [[#Lehner--2018|Lehner et al., 2018]] ). In Africa there is ''low confidence'' that human influence has contributed to the observed long-term meteorological drought increase in Western Africa (Sections 11.9 and 10.6.2). There is ''low confidence'' in the attribution of the observed increasing trends in meteorological drought in East Southern Africa, but evidence that human-induced climate change has affected recent meteorological drought events in the region ( [[#11.9|Section 11.9]] ).

Attribution studies for recent meteorological drought events are available for various regions. In Western and Central Europe, a multi-method and multi-model attribution study on the 2015 Central European drought did not find conclusive evidence for whether human-induced climate change was a driver of the rainfall deficit, as the results depended on model and method used ( [[#Hauser--2017|Hauser et al., 2017]] ). In the Mediterranean region, a human contribution was found in the case of the 2014 meteorological drought in the southern Levant based on a single-model study ( [[#Bergaoui--2015|Bergaoui et al., 2015]] ). In Africa, there is some evidence of a contribution of human emissions to single meteorological drought events, such as the 2015–2017 southern African drought ( [[#Funk--2018a|Funk et al., 2018a]] ; [[#Yuan--2018a|Yuan et al., 2018a]] ; [[#Pascale--2020|Pascale et al., 2020]] ), and the three-year (2015–2017) drought in the western Cape Town region of South Africa ( [[#Otto--2018c|Otto et al., 2018c]] ). An attributable signal was not found in droughts that occurred in different years with different spatial extents in the last decade in North and South Eastern Africa ( [[#Marthews--2015|Marthews et al., 2015]] ; [[#Uhe--2017|Uhe et al., 2017]] ; [[#Otto--2018a|Otto et al., 2018a]] ; [[#Philip--2018b|Philip et al., 2018b]] ; [[#Kew--2021|Kew et al., 2021]] ). However, an attributable increase in 2011 long rain failure was identified ( [[#Lott--2013|Lott et al., 2013]] ). Further studies have attributed some African meteorological drought events to large-scale modes of variability, such as the strong 2015 El Niño (Box 11.4; [[#Philip--2018b|Philip et al., 2018b]] ) and increased SSTs overall ( [[#Funk--2015a|Funk et al., 2015a]] , 2018b). Natural variability was dominant in the California droughts of 2011–2012 to 2013–2014 ( [[#Seager--2015a|Seager et al., 2015a]] ). In Asia, no climate change signal was found in the record dry spell over Singapore and Malaysia in 2014 ( [[#Mcbride--2015|Mcbride et al., 2015]] ) or the drought in central south-west Asia in 2013–2014 ( [[#Barlow--2015|Barlow and Hoell, 2015]] ). Nevertheless, the South East Asia drought of 2015 has been attributed to anthropogenic warming effects ( [[#Shiogama--2020|Shiogama et al., 2020]] ). Recent droughts occurring in South America, specifically in the southern Amazon region in 2010 ( [[#Shiogama--2013|Shiogama et al., 2013]] ) and in north-east South America in 2014 ( [[#Otto--2015b|Otto et al., 2015b]] ) and 2016 ( [[#Martins--2018|Martins et al., 2018]] ) were not attributed to anthropogenic climate change. Nevertheless, the central Chile drought between 2010 and 2018 has been suggested to be partly associated to global warming ( [[#Boisier--2016|Boisier et al., 2016]] ; [[#Garreaud--2020|Garreaud et al., 2020]] ). The 2013 New Zealand meteorological drought was attributed to human influence by Harrington et al. (2014, 2016) based on fully coupled CMIP5 models, but no corresponding change in the dry end of simulated precipitation from a stand-alone atmospheric model was found by [[#Angélil--2017|Angélil et al. (2017)]] .

Event attribution studies also highlight a complex interplay of anthropogenic and non-anthropogenic climatological factors for some events. For example, anthropogenic warming contributed to the 2014 drought in North Eastern Africa by increasing east African and west Pacific temperatures, and increasing the gradient between standardized western and central Pacific SSTs, causing reduced rainfall ( [[#Funk--2015a|Funk et al., 2015a]] ). As different methodologies, models and data sources have been used for the attribution of precipitation deficits, [[#Angélil--2017|Angélil et al. (2017)]] re-examined several events using a single analytical approach and climate model and observational datasets. Their results showed a disagreement in the original anthropogenic attribution in a number of precipitation deficit events, which increased uncertainty in the attribution of meteorological droughts events.

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==== 11.6.4.2 Soil Moisture Deficits ====

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There is a growing number of studies on the detection and attribution of long-term changes in soil moisture deficits. [[#Mueller--2016|Mueller and Zhang (2016)]] concluded that anthropogenic forcing contributed significantly to soil moisture drying in the warm season in the Northern Hemisphere from 1951 to 2005 and also led to an increase in the land surface area affected by soil moisture deficits, which can be reproduced by CMIP5 models only if anthropogenic forcings are involved. [[#Gu--2019b|Gu et al. (2019b)]] similarly identified a global-scale soil moisture drying tendency in land surface model data from the Global Land Data Assimilation System 2 over the time frame 1948–2005, which was attributed to anthropogenic forcing based on evaluation with CMIP5 models using optimal fingerprinting. [[#Padrón--2019|Padrón et al. (2019)]] analysed long-term reconstructed and CMIP5 simulated dry season water availability, defined as precipitation minus ET (i.e., equivalent to soil moisture and runoff availability), also related to agricultural and ecological droughts. They found an intensification of dry-season precipitation minus evapotranspiration deficits over a predominant fraction of the land area in the last three decades, which can only be explained by anthropogenic forcing and is mostly related to increases in ET. Similarly, [[#Williams--2020|Williams et al. (2020)]] concluded that human-induced climate change contributed to the strong soil moisture deficits recorded in the last two decades in Western North America through VPD increases associated with higher air temperatures and lower air humidity. There are few studies analysing the attribution of particular episodes of soil moisture deficits to anthropogenic influence. Nevertheless, the available modelling studies coincide in supporting an anthropogenic attribution associated with more extreme temperatures, exacerbating AED and increasing ET, and thus depleting soil moisture, as observed in southern Europe in 2017 ( [[#García-Herrera--2019|García-Herrera et al., 2019]] ) and in Australia in 2018 ( [[#Lewis--2020|Lewis et al., 2020]] ) and 2019 ( [[#van%20Oldenborgh--2021|van Oldenborgh et al., 2021]] ), the latter event having strong implications in the propagation of widespread megafires ( [[#Nolan--2020|Nolan et al., 2020]] ).

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==== 11.6.4.3 Hydrological Deficits ====

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It is often difficult to separate the role of climate trends from changes in land use, water management and demand for changes in hydrological deficits, especially on a regional scale. However, a global study based on a recent multi-model experiment with global hydrological models and covering several AR6 regions suggests a dominant role of anthropogenic radiative forcing for trends in low, mean and high flows, while simulated effects of water and land management do not suffice to reproduce the observed spatial pattern of trends ( [[#Gudmundsson--2021|Gudmundsson et al., 2021]] ). Regional studies also suggest that climate trends have been dominant compared to land use and human water management for explaining trends in hydrological droughts in some regions, for instance in Ethiopia ( [[#Fenta--2017|Fenta et al., 2017]] ), China ( [[#Xie--2015|Xie et al., 2015]] ), and North America for the Missouri and Colorado basins, as well as in California ( [[#Shukla--2015|Shukla et al., 2015]] ; [[#Udall--2017|Udall and Overpeck, 2017]] ; [[#Ficklin--2018|Ficklin et al., 2018]] ; K. [[#Xiao--2018|]] [[#Xiao--2018|Xiao et al., 2018]] ; [[#Glas--2019|Glas et al., 2019]] ; [[#Martin--2020|Martin et al., 2020]] ; [[#Milly--2020|Milly and Dunne, 2020]] ).

In other regions, the influence of human water uses can be more important to explain hydrological drought trends (Y. [[#Liu--2016|]] [[#Liu--2016|Liu et al., 2016]] ; [[#Mohammed--2016|Mohammed and Scholz, 2016]] ). There is ''medium confidence'' that human-induced climate change has contributed to an increase of hydrological droughts in the Mediterranean ( [[#Giuntoli--2013|Giuntoli et al., 2013]] ; [[#Vicente-Serrano--2014|Vicente-Serrano et al., 2014]] ; [[#Gudmundsson--2017|Gudmundsson et al., 2017]] ), but also ''medium confidence'' that changes in land use and terrestrial water management contributed to these trends ( [[#11.9|Section 11.9]] ; [[#Teuling--2019|Teuling et al., 2019]] ; [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ). A global study with a single hydrological model estimated that human water consumption has intensified the magnitude of hydrological droughts by 20–40% over the last 50 years, and that the human water use contribution to hydrological droughts was more important than climatic factors in the Mediterranean, and central USA, as well as in parts of Brazil ( [[#Wada--2013|Wada et al., 2013]] ). However, [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] concluded that the contribution of human water use is smaller than that of anthropogenic climate change to explain spatial differences in the trends of low flows based on a multi-model analysis. There is still ''limited evidence'' and thus ''low confidence'' in assessing these trends at the scale of single regions, with few exceptions ( [[#11.9|Section 11.9]] ).

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==== 11.6.4.4 Atmospheric-based Drought Indices ====

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Different studies using atmospheric-based drought indices suggest an attributable anthropogenic signal, characterized by the increased frequency and severity of droughts ( [[#Cook--2018|Cook et al., 2018]] ), associated to increased AED ( [[#11.6.4.2|Section 11.6.4.2]] ). The majority of studies are based on the PDSI-PM. [[#Williams--2015|Williams et al. (2015)]] and [[#Griffin--2014|Griffin and Anchukaitis (2014)]] concluded that increased AED has had an increased contribution to drought severity over the last decades, and played a dominant role in the intensification of the 2012–2014 drought in California. The same temporal pattern and physical mechanism was stressed by Z. [[#Li--2017|]] [[#Li--2017|]] [[#Li--2017|]] [[#Li--2017|Li et al. (2017)]] in central Asia. [[#Marvel--2019|Marvel et al. (2019)]] compared tree ring-based reconstructions of the PDSI-PM over the past millennium with PDSI-PM estimates based on output from CMIP5 models. The comparisons suggested a contribution of greenhouse gas forcing to the changes since the beginning of the 20th century, although characterized with temporal differences that could be driven by temporal variations in the aerosol forcing. This was in agreement with the dominant external forcings of aridification at global scale between 1950 and 2014 ( [[#Bonfils--2020|Bonfils et al., 2020]] ). In the Mediterranean region, there is ''medium confidence'' of drying attributable to antropogenic forcing as a consequence of the strong AED increase ( [[#Gocic--2014|Gocic and Trajkovic, 2014]] ; [[#Azorin-Molina--2015|Azorin-Molina et al., 2015]] ; [[#Liuzzo--2016|Liuzzo et al., 2016]] ; [[#Maček--2018|Maček et al., 2018]] ), which has enhanced the severity of drought events ( [[#Vicente-Serrano--2014|Vicente-Serrano et al., 2014]] ; [[#Stagge--2017|Stagge et al., 2017]] ; [[#González-Hidalgo--2018|González-Hidalgo et al., 2018]] ). In particular, this effect was identified to be the main driver of the intensification of the 2017 drought that affected south-western Europe, and was attributed to the human forcing ( [[#García-Herrera--2019|García-Herrera et al., 2019]] ). [[#Nangombe--2020|Nangombe et al. (2020)]] and L. [[#Zhang--2020|]] [[#Zhang--2020|]] [[#Zhang--2020|]] [[#Zhang--2020|Zhang et al. (2020)]] concluded from differences between precipitation and AED that anthropogenic forcing contributed to the 2018 droughts that affected southern Africa and south-eastern China, respectively, principally as consequence of the high AED that characterized these two events.

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==== 11.6.4.5 Synthesis for Different Drought Types ====

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The regional evidence on attribution for single AR6 regions generally shows ''low confidence'' for a human contribution to observed trends in meteorological droughts at regional scale, with few exceptions ( [[#11.9|Section 11.9]] ). There is ''medium confidence'' that human influence has contributed to increases in agricultural and ecological droughts in the dry season in some regions and has led to an overall increase in the affected land area. At regional scales, there is ''medium confidence'' in a contribution of human-induced climate change to increases in agricultural and ecological droughts in the Mediterranean and Western North America ( [[#11.9|Section 11.9]] ). There ''is medium confidence'' that human-induced climate change has contributed to an increase in hydrological droughts in the Mediterranean region, but also ''medium confidence'' in contributions from other human influences, including water management and land use ( [[#11.9|Section 11.9]] ). Several meteorological and agricultural and ecological drought events have been attributed to human-induced climate change, even in regions where no long-term changes are detected ( ''medium confidence'' ). However, a lack of attribution to human-induced climate change has also been shown for some events ( ''medi'' ''um confidence'' ).

In summary, human influence has contributed to increases in agricultural and ecological droughts in the dry season in some regions due to increases in evapotranspiration ( ''medium confidence'' ). The increases in evapotranspiration have been driven by increases in atmospheric evaporative demand induced by increased temperature, decreased relative humidity and increased net radiation over affected land areas ( ''high confidence'' ). There is ''low confidence'' that human influence has affected trends in meteorological droughts in most regions, but ''medium confidence'' that they have contributed to the severity of some single events. There is ''medium confidence'' that human-induced climate change has contributed to increasing trends in the probability or intensity of recent agricultural and ecological droughts, leading to an increase of the affected land area. Human-induced climate change has contributed to global-scale change in low flow, but human water management and land-use changes are also important drivers ( ''medi'' ''um confidence'' ).

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