Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.3.1.2 Extreme Heat ====

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Impacts and risk assessments utilize a large variety of indices and approaches tailored to evaluate heat impacts on human health ( [[#Sanderson--2017|Sanderson et al., 2017]] ; [[#Gao--2018|]] [[#Gao--2018|C. Gao et al., 2018]] ; [[#McGregor--2018|McGregor and Vanos, 2018]] ; [[#Staiger--2019|Staiger et al., 2019]] ; J. [[#Zhu--2019|]] [[#Zhu--2019|Zhu et al., 2019]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ). A mixture of simple and complex heat stress indices often combine extreme temperatures and high humidity to capture human health challenges ( [[#Aström--2013|Aström et al., 2013]] ; [[#Chow--2016|Chow et al., 2016]] ; [[#Dahl--2017a|Dahl et al., 2017a]] ; [[#Im--2017|Im et al., 2017]] ; [[#Coffel--2018|Coffel et al., 2018]] ; J. [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ; [[#Vanos--2020|Vanos et al., 2020]] ). Different optimum temperatures and extreme heat thresholds based on local distributions are needed to reflect acclimation of different locations and populations ( [[#Hajat--2014|Hajat et al., 2014]] ; [[#WHO--2014|WHO, 2014]] ; [[#Kinney--2015b|Kinney et al., 2015b]] ; [[#Russo--2015|Russo et al., 2015]] ; [[#Petitti--2016|Petitti et al., 2016]] ; [[#Dosio--2017|Dosio, 2017]] ; [[#Cheng--2018|Cheng et al., 2018]] ; [[#Lay--2018|Lay et al., 2018]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ). Hot and humid heat episodes can be deadly ( [[#Mora--2017|Mora et al., 2017]] ), are associated with elevated hospital intake ( [[#Goldie--2017|Goldie et al., 2017]] ) and lower safety and productivity of outdoor labourers ( [[#Dunne--2013|Dunne et al., 2013]] ; [[#Graff%20Zivin--2014|Graff Zivin and Neidell, 2014]] ; [[#Kjellstrom--2016|Kjellstrom et al., 2016]] ; [[#Pal--2016|Pal and Eltahir, 2016]] ; Y. [[#Zhao--2016|]] [[#Zhao--2016|Zhao et al., 2016]] ; [[#Mora--2017|Mora et al., 2017]] ; [[#Watts--2018|Watts et al., 2018]] ; [[#Orlov--2019|Orlov et al., 2019]] ). Elevated nighttime temperatures prevent the human body from experiencing relief from heat stress ( [[#Zhang--2012|Zhang et al., 2012]] ) and can be tracked over extended periods of sequential day and night heat extremes ( [[#Murage--2017|Murage et al., 2017]] ; [[#Mukherjee--2018|Mukherjee and Mishra, 2018]] ). Extreme heat also exacerbates asthma, respiratory difficulties and response to airborne allergens such as hay fever ( [[#Upperman--2017|Upperman et al., 2017]] ). Extreme heat affects outdoor exercise such as the use of bike-share facilities ( [[#Heaney--2019|Heaney et al., 2019]] ; [[#Vanos--2020|Vanos et al., 2020]] ). Large-scale recreational and sporting events such as marathons and tennis tournaments monitor heat extremes when determining the viability of host cities ( [[#Smith--2016|Smith et al., 2016]] , 2018).

Short-term exposure of crops to temperatures beyond a critical temperature threshold can lead to lower yields and above a limiting temperature threshold, crops may fail altogether ( [[#Schlenker--2009|Schlenker and Roberts, 2009]] ; [[#Lobell--2012|Lobell et al., 2012]] , 2013; [[#Gourdji--2013|Gourdji et al., 2013]] ; [[#Deryng--2014|Deryng et al., 2014]] ; [[#Schauberger--2017|Schauberger et al., 2017]] ; [[#Tesfaye--2017|Tesfaye et al., 2017]] ; [[#Vogel--2019|Vogel et al., 2019]] ). The exact level of these thresholds depends on species, cultivar and farm management ( [[#Hatfield--2015|Hatfield and Prueger, 2015]] ; [[#Hatfield--2015|Hatfield et al., 2015]] ; [[#Bisbis--2018|Bisbis et al., 2018]] ; [[#Grotjahn--2021|Grotjahn, 2021]] ). The timing of heatwaves is particularly important, as extreme heat is more damaging during critical phenological stages ( [[#Teixeira--2013|Teixeira et al., 2013]] ; [[#Eyshi%20Rezaei--2015|Eyshi Rezaei et al., 2015]] ; [[#Fontana--2015|Fontana et al., 2015]] ; [[#Wang--2017|]] [[#Wang--2017|]] [[#Wang--2017|B. Wang et al., 2017]] ; [[#Mäkinen--2018|Mäkinen et al., 2018]] ). Extreme canopy temperatures, rather than 2 m air temperatures, may be a more robust biophysical indicator of heat impacts on crop production ( [[#Siebert--2017|Siebert et al., 2017]] ). Heat stress indices based upon temperature and humidity determine livestock productivity as well as conception and mortality rates ( [[#Key--2014|Key et al., 2014]] ; [[#Dash--2016|Dash et al., 2016]] ; [[#Pragna--2016|Pragna et al., 2016]] ; [[#Rojas-Downing--2017|Rojas-Downing et al., 2017]] ).

Heat extremes factor in mortality, morbidity and the range of some thermally sensitive ecosystem species ( [[#Smith--2015|Smith and Nagy, 2015]] ; [[#Ratnayake--2019|Ratnayake et al., 2019]] ; [[#Thomsen--2019|Thomsen et al., 2019]] ). Combined heat and drought stress can reduce forest and grassland primary productivity ( [[#Ciais--2005|Ciais et al., 2005]] ; [[#De%20Boeck--2018|De Boeck et al., 2018]] ) and even cause tree mortality at higher extremes ( [[#Teskey--2015|Teskey et al., 2015]] ).

Extreme heat events raise temperatures in buildings and cities already warmed by the urban heat island effect ( [[#Gaffin--2012|Gaffin et al., 2012]] ; [[#Oleson--2018|Oleson et al., 2018]] ; [[#Zhao--2018|Zhao et al., 2018]] ; [[#Mauree--2019|Mauree et al., 2019]] ; Box 10.3) and can induce disruptions in critical infrastructure networks ( [[#Chapman--2013|Chapman et al., 2013]] ). Heat affects transportation infrastructure by warping roads and airport runways ( [[#Chinowsky--2012|Chinowsky and Arndt, 2012]] ) or buckling railways ( [[#Dobney--2010|Dobney et al., 2010]] ; [[#Dépoues--2017|Dépoues, 2017]] ; [[#Chinowsky--2019|Chinowsky et al., 2019]] ), and high temperatures reduce air density leading to aircraft take-off weight restrictions ( [[#Coffel--2017|Coffel et al., 2017]] ; [[#Palko--2017|Palko, 2017]] ; T. [[#Zhou--2018|]] [[#Zhou--2018|Zhou et al., 2018]] ). Heat extremes increase peak cooling demand and challenge transmission and transformer capacity ( [[#Sathaye--2013|Sathaye et al., 2013]] ; [[#Russo--2016|Russo et al., 2016]] ; [[#Craig--2018|Craig et al., 2018]] ; X. [[#Gao--2018|]] [[#Gao--2018|Gao et al., 2018]] ) and may cause transmission lines to sag or fail ( [[#Gupta--2012|Gupta et al., 2012]] ). Thermal and nuclear electricity plants may be challenged when using warmer river waters for cooling or when mixing waste waters back into waterways without causing ecosystem impacts ( [[#Kopytko--2011|Kopytko and Perkins, 2011]] ; [[#van%20Vliet--2016|van Vliet et al., 2016]] ; [[#Tobin--2018|Tobin et al., 2018]] ). Extreme temperature can also reduce photovoltaic panel efficiency ( [[#Jerez--2015|Jerez et al., 2015]] ).

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