Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.2.5.1 Changes in extreme temperatures, heatwaves and drought ====

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It is ''very likely'' that most land areas have experienced a decrease in the number of cold days and nights, and an increase in the number of warm days and unusually hot nights (Orlowsky and Seneviratne 2012 <sup>[[#fn:r193|193]]</sup> ; Seneviratne et al. 2012 <sup>[[#fn:r194|194]]</sup> ; Mishra et al. 2015 <sup>[[#fn:r195|195]]</sup> ; Ye et al. 2018 <sup>[[#fn:r196|196]]</sup> ). Although there is no consensus definition of heatwaves, as some heatwave indices have relative thresholds and others absolute thresholds, trends between indices of the same type show that recent heat-related events have been made more frequent or more intense due to anthropogenic GHG emissions in most land regions (Lewis and Karoly 2013 <sup>[[#fn:r197|197]]</sup> ; Smith et al. 2013b <sup>[[#fn:r198|198]]</sup> ; Scherer and Diffenbaugh 2014 <sup>[[#fn:r199|199]]</sup> ; Fischer and Knutti 2015 <sup>[[#fn:r200|200]]</sup> ; Ceccherini et al. 2016 <sup>[[#fn:r201|201]]</sup> ; King et al. 2016 <sup>[[#fn:r202|202]]</sup> ; Bador et al. 2016 <sup>[[#fn:r203|203]]</sup> ; Stott et al. 2016 <sup>[[#fn:r204|204]]</sup> ; King 2017 <sup>[[#fn:r205|205]]</sup> ; Hoegh-Guldberg et al. 2018 <sup>[[#fn:r206|206]]</sup> ). Globally, 50–80% of the land fraction is projected to experience significantly more intense hot extremes than historically recorded (Fischer and Knutti 2014 <sup>[[#fn:r207|207]]</sup> ; Diffenbaugh et al. 2015 <sup>[[#fn:r208|208]]</sup> ; Seneviratne et al. 2016 <sup>[[#fn:r209|209]]</sup> ). There is ''high confidence'' that heatwaves will increase in frequency, intensity and duration into the 21st century (Russo et al. 2016 <sup>[[#fn:r210|210]]</sup> ; Ceccherini et al. 2017 <sup>[[#fn:r211|211]]</sup> ; Herrera-Estrada and Sheffield 2017 <sup>[[#fn:r212|212]]</sup> ) and under high emission scenarios, heatwaves by the end of the century may become extremely long (more than 60 consecutive days) and frequent (once every two years) in Europe, North America, South America, Africa, Indonesia, the Middle East, South and Southeast Asia and Australia (Rusticucci 2012 <sup>[[#fn:r213|213]]</sup> ; Cowan et al. 2014 <sup>[[#fn:r214|214]]</sup> ; Russo et al. 2014 <sup>[[#fn:r215|215]]</sup> ; Scherer and Diffenbaugh 2014 <sup>[[#fn:r216|216]]</sup> ; Pal and Eltahir 2016 <sup>[[#fn:r217|217]]</sup> ; Rusticucci et al. 2016 <sup>[[#fn:r218|218]]</sup> ; Schär 2016 <sup>[[#fn:r219|219]]</sup> ; Teng et al. 2016 <sup>[[#fn:r220|220]]</sup> ; Dosio 2017 <sup>[[#fn:r221|221]]</sup> ; Mora et al. 2017 <sup>[[#fn:r222|222]]</sup> ; Dosio et al. 2018 <sup>[[#fn:r223|223]]</sup> ; Lehner et al. 2018 <sup>[[#fn:r224|224]]</sup> ; Lhotka et al. 2018 <sup>[[#fn:r225|225]]</sup> ; Lopez et al. 2018 <sup>[[#fn:r226|226]]</sup> ; Tabari and Willems 2018 <sup>[[#fn:r227|227]]</sup> ). Furthermore, unusual heatwave conditions today will occur regularly by 2040 under the RCP 8.5 scenario (Russo et al. 2016 <sup>[[#fn:r228|228]]</sup> ). The intensity of heat events may be modulated by land cover and soil characteristics (Miralles et al. 2014 <sup>[[#fn:r229|229]]</sup> ; Lemordant et al. 2016 <sup>[[#fn:r230|230]]</sup> ; Ramarao et al. 2016 <sup>[[#fn:r231|231]]</sup> ). Where temperature increase results in decreased soil moisture, latent heat flux is reduced while sensible heat fluxes are increased, allowing surface air temperature to rise further. However, this feedback may be diminished if the land surface is irrigated through enhanced evapotranspiration (Mueller et al. 2015 <sup>[[#fn:r232|232]]</sup> ; Siebert et al. 2017 <sup>[[#fn:r233|233]]</sup> ) (Section 2.5.2.2).

Drought (IPCC 2013c <sup>[[#fn:r234|234]]</sup> ), including megadroughts of the last century, for example, the Dustbowl drought (Hegerl et al. 2018 <sup>[[#fn:r235|235]]</sup> ) (Chapter 5), is a normal component of climate variability (Hoerling et al. 2010; Dai 2011 <sup>[[#fn:r236|236]]</sup> ) and may be seasonal, multi-year (Van Dijk et al. 2013 <sup>[[#fn:r237|237]]</sup> ) or multi-decadal (Hulme 2001 <sup>[[#fn:r238|238]]</sup> ) with increasing degrees of impact on regional activities. This inter-annual variability is controlled particularity through remote sea surface temperature (SST) forcings, such as the Inter-decadal Pacific Oscillation (IPO) and the Atlantic Multi-decadal Oscillation (AMO), El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), that cause drought as a result of reduced rainfall (Kelley et al. 2015 <sup>[[#fn:r239|239]]</sup> ; Dai 2011 <sup>[[#fn:r240|240]]</sup> ; Hoell et al. 2017 <sup>[[#fn:r241|241]]</sup> ; Espinoza et al. 2018 <sup>[[#fn:r242|242]]</sup> ). In some cases however, large scale SST modes do not fully explain the severity of drought some recent event attribution studies have identified a climate change fingerprint in several regional droughts, for example, the western Amazon (Erfanian et al. 2017 <sup>[[#fn:r243|243]]</sup> ), southern Africa (Funk et al. 2018 <sup>[[#fn:r244|244]]</sup> ; Yuan et al. 2018 <sup>[[#fn:r245|245]]</sup> ), southern Europe and the Mediterranean including North Africa (Kelley et al. 2015 <sup>[[#fn:r246|246]]</sup> ; Wilcox et al. 2018 <sup>[[#fn:r247|247]]</sup> ), parts of North America (Williams et al. 2015 <sup>[[#fn:r248|248]]</sup> ; Mote et al. 2016 <sup>[[#fn:r249|249]]</sup> ), Russia (Otto et al. 2012 <sup>[[#fn:r250|250]]</sup> ), India (Ramarao et al. 2015 <sup>[[#fn:r251|251]]</sup> ) and Australia (Lewis and Karoly 2013 <sup>[[#fn:r252|252]]</sup> ).

Long-term global trends in drought are difficult to determine because of this natural variability, potential deficiencies in drought indices (especially in how evapotranspiration is treated) and the quality and availability of precipitation data (Sheffield et al. 2012 <sup>[[#fn:r253|253]]</sup> ; Dai 2013 <sup>[[#fn:r254|254]]</sup> ; Trenberth et al. 2014 <sup>[[#fn:r255|255]]</sup> ; Nicholls and Seneviratne 2015 <sup>[[#fn:r256|256]]</sup> ; Mukherjee et al. 2018 <sup>[[#fn:r257|257]]</sup> ). However, regional trends in frequency and intensity of drought are evident in several parts of the world, particularly in low latitude land areas, such as the Mediterranean, North Africa and the Middle East (Vicente-Serrano et al. 2014 <sup>[[#fn:r258|258]]</sup> ; Spinoni et al. 2015a <sup>[[#fn:r259|259]]</sup> ; Dai and Zhao 2017 <sup>[[#fn:r260|260]]</sup> ; Páscoa et al. 2017 <sup>[[#fn:r261|261]]</sup> ), many regions of sub-Saharan Africa (Masih et al. 2014 <sup>[[#fn:r262|262]]</sup> ; Dai and Zhao 2017 <sup>[[#fn:r263|263]]</sup> ), central China (Wang et al. 2017e <sup>[[#fn:r264|264]]</sup> ), the southern Amazon (Fu et al. 2013 <sup>[[#fn:r265|265]]</sup> ; Espinoza et al. 2018 <sup>[[#fn:r266|266]]</sup> ), India (Ramarao et al. 2016 <sup>[[#fn:r267|267]]</sup> ), east and south Asia, parts of North America and eastern Australia (Dai and Zhao 2017 <sup>[[#fn:r268|268]]</sup> ). A recent analysis of 4500 meteorological droughts globally found increased drought frequency over the East Coast of the USA, Amazonia and north-eastern Brazil, Patagonia, the Mediterranean region, most of Africa and north-eastern China with decreased drought frequency over northern Argentina, Uruguay and northern Europe (Spinoni et al. 2019 <sup>[[#fn:r269|269]]</sup> ). The study also found drought intensity has become more severe over north-western USA, parts of Patagonia and southern Chile, the Sahel, the Congo River basin, southern Europe, north-eastern China, and south-eastern Australia, whereas the eastern USA, south-eastern Brazil, northern Europe, and central- northern Australia experienced less severe droughts. In addition to the IPCC SR15 assessment of ''medium confidence'' in increased drying over the Mediterranean region (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r270|270]]</sup> ), it is further assessed with ''medium confidence'' that frequency and intensity of droughts in Amazonia, north-eastern Brazil, Patagonia, most of Africa, and north-eastern China has increased.

There is ''low confidence'' in how large-scale modes of variability will respond to a warming climate (Deser et al. 2012 <sup>[[#fn:r271|271]]</sup> ; Liu 2012 <sup>[[#fn:r272|272]]</sup> ; Christensen et al. 2013 <sup>[[#fn:r273|273]]</sup> ; Hegerl et al. 2015 <sup>[[#fn:r274|274]]</sup> ; Newman et al. 2016 <sup>[[#fn:r275|275]]</sup> ). Although, there is evidence for an increased frequency of extreme ENSO events, such as the 1997/98 El Niño and 1988/89 La Niña (Cai et al. 2014a <sup>[[#fn:r276|276]]</sup> , 2015 <sup>[[#fn:r277|277]]</sup> ) and extreme positive phases of the IOD (Christensen et al. 2013 <sup>[[#fn:r278|278]]</sup> ; Cai et al. 2014b <sup>[[#fn:r279|279]]</sup> ). However, the assessment by the SR15 was retained on an increased regional drought risk ( ''medium confidence'' ), specifically over the Mediterranean and South Africa at both 1.5°C and 2°C warming levels compared to present day, with drought risk at 2°C being significantly higher than at 1.5°C (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r280|280]]</sup> ).

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