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

==== 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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